THERMOCOUPLES TECHNICAL CONTENT

The Canary in the Furnace: Ceramic Disks Give Early Alerts of Temperature Changes

/ FEATURED NEWS, MANUFACTURING HEAT TREAT NEWS, MANUFACTURING HEAT TREAT TECH, THERMOCOUPLES TECHNICAL CONTENT, VACUUM FURNACES TECHNICAL CONTENT

The question in many heat treaters’ minds is, “Why would I want more documentation on my furnaces?” TempTABs can act as an early warning sign that further temperature monitoring is necessary.

This Technical Tuesday article was written by Thomas McInnerney and Garrick Ackart of The Edward Orton Jr. Ceramic Foundation, for Heat Treat Today's November 2023 Vacuum Heat Treating print edition.

The Need for User-Friendly Documentation

Thomas McInnerney
Engineering Manager of Pyrometric Products
Edward Orton Jr. Ceramic Foundation.
Source: Edward Orton Jr. Ceramic Foundation

Increased regulations called for in AMS2750G and CQI-9 were, for the most part, driven by the client purchasing the items. With a business climate that can generate a product-liability lawsuit quicker than a rapid quench, clients are trying to protect themselves.

Contact us with your Reader Feedback!

Consequently, most heat treating facilities will perform the necessary and required temperature uniformity surveys (TUSs) as well as thermocouple calibrations. Once the formal TUS is complete, other than the information generated from the control thermocouples, the challenge still exists for the furnace operators to ascertain what happens throughout the furnace between surveys. By passing the last survey but failing the next one, how do you detect that something changed two days after the good survey or two days before the failed survey?

It is true you can run a temperature data logger with an array of thermocouples attached through the furnace to get a complete picture of the furnace performance, but that process includes production interruption, an expenditure of precious manpower, and significant expense in maintaining the data logger. Essentially, we have just defined the need for a cost-effective, user-friendly device to monitor the day-to-day repeatability of the performance of the furnace.

Metals Industry Demands

Garrick Ackart
Marketing and Business Development Manager
Edward Orton Jr. Ceramic Foundation.
Source: Edward Orton Jr. Ceramic Foundation

Driven by the question raised above, The Edward Orton Jr. Ceramic Foundation initiated a development project to provide such a product for the metals industry.

Demands of the metals industry are quite different from those of the ceramic industry. The detection device would have to be able to withstand rapid heat-up schedules, rapid quench, a wide variety of furnace atmospheres (air, nitrogen, hydrogen), and no atmosphere (vacuum), and do all this without introducing contaminants to the products being heat treated — no small challenge for an engineered ceramic product. Following a great deal of consultation and experimentation, Orton developed a product, the TempTAB, that can be used to benchmark and monitor furnace performance in most heat treating applications.

Measuring Dimension: How a TempTAB Works

How does the device work, and how is it made and controlled? The device depends on a constant slope curve of shrinkage versus temperature. When the device is exposed to more temperature and for longer periods of time at peak temperature, the amount of shrinkage increases.

Figure 1. The temperature monitoring system consists of ceramic sensors, a
measuring gauge, and software to convert dimension to temperature.
Source: Edward Orton Jr. Ceramic Foundation

TempTABs are small disks made from exact blends of select ceramic materials prepared in an environment where the processing variables are tightly controlled. The ceramic material is selected based on its predictable shrinkage, which is affected more by temperature than time; even so, holding at or near the peak temperature will have an impact on the final dimension.

Once the TempTAB is out of the furnace, its diameter is measured with a micrometer. The dimension, in millimeters, is entered into an Excel workbook that automatically looks up the equivalent temperature inside the furnace based on the furnace cycle time.

Temperature conversion charts are available with each batch of TempTABs for converting the diameter measurement to temperature. The charts have several columns of data which allow the user to find the data that is best associated with their final furnace cycle hold times (temperatures available for 10-, 30-, 60-, 120-, and 240-minute hold times). The charts are built into the software to allow you to monitor up to nine different locations inside the furnace for up to 360 runs (Figure 2). The software is available free from Orton’s website.

The resulting temperature data generated by the software is graphically displayed in both table and numerical format for easy interpretation. The data can also be copied into other Excel spreadsheets and SPC (Statistical Process Control) programs to be incorporated into existing quality programs.

Figure 2. Orton TempTAB software allows process temperature tracking at a glance.
Source: Edward Orton Jr. Ceramic Foundation

Primary Uses: Early Warning Device & Quality Control

Heat treat companies use these disks as an early warning device and to document that their processes are under control. First, they benchmark their thermal process by running several TempTABs through the heat treat furnace.

After establishing a benchmark with upper and lower control limits, the company will run the disks on a regular schedule, placing them in the same location alongside the parts being treated in the furnace (see process temperatures graphed with TempTABs in Figure 3).

Figure 3. Temperature data is displayed by location and can be copied into existing SPC software.
Source: Edward Orton Jr. Ceramic Foundation

At a glance, the furnace operator, the quality manager, or the general manager can see if the process is under control. The size of each disk indicates if the thermal process is, or is not, within the established control limits.

The case studies that follow demonstrate these primary uses in real-world heat treat.

Case Study #1: Furnace Documentation When You Need It

A manufacturer with in-house heat treating ran TempTABs alongside the thermocouples in one of its required nine-point uniformity surveys with a data logger. After the formal survey, they continued to run disks in each load, monitoring shrinkage of the disks. The heat treating operations wanted to document the thermal treatment of the product in every load. If something did change inside their furnace before the next required survey, the TempTABs would act as an early warning system alerting them that a formal survey may be necessary.

Case Study #2: Developing Backup Facilities/Preparing for Increased Demand

A company specializing in powder metal sintering wanted to duplicate a sintering process of one of their products, currently only being done at a single manufacturing site, at a second location. Initially, they duplicated all the settings in the new location (temperature settings and belt speed) and found that the resultant parts differed from those of the original site.

The company began to consider TempTABs. They liked the idea of having a device that could provide them with furnace temperature readings since they knew that it was an important variable to the quality of their parts. For one year, TempTABs were used daily for process control of the furnace. This use proved that the furnace was consistent and stable.

Since they had developed a benchmark of the disk dimensions yielding good parts, they were able to adjust the new facility settings so their process could be duplicated in the second facility. Within weeks, the powder metal sintering experts could produce products in the new facility consistent with the original facility.

Case Study #3: High-Value Heat Treating

A heat treating facility serving the aerospace industry historically ran nine thermocouples in every load of their batch furnace for bright annealing stainless steels to document furnace performance. The method required using many type S thermocouples and a data collection unit. Labor costs included setting up the thermocouple array and replacing the certified thermocouples. It was expensive and disruptive; they wanted an alternative.

The time needed to replace the TempTABs was minutes and only required one person’s time to place and gather them. After doing a correlation study of at least five runs over a week, the heat treat facility replaced the thermocouples with TempTAB disks. Now, a single operator places TempTABs inside every load so they can gather information at a lower cost. If they see any change in the amount of TempTAB shrinkage, they will run the thermocouple array to see precisely how the temperature profile has changed.

Figure 4. TempTAB wired in place during daily monitoring.
Source: Edward Orton Jr. Ceramic Foundation

About the authors:

Thomas McInnerney is the engineering manager of Pyrometric Products at The Edward Orton Jr. Ceramic Foundation. He received his BS in Ceramic Engineering at The Ohio State University and has been a key leader in the development and application of TempTABs for 22 years.

Garrick Ackart is the Marketing and Business Development manager at The Edward Orton Jr. Ceramic Foundation. He received a Bachelor of Science degree from Alfred University in Ceramic Engineering, an MBA from The Ohio State University, and has more than 25 years of experience in the ceramic and glass industry.

For more information:

Contact Thomas McInnerney at mcinnerney@ortonceramic.com or Garrick Ackart at ackart@ortonceramic.com.

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

The Canary in the Furnace: Ceramic Disks Give Early Alerts of Temperature Changes Read More »

Discover the DNA of Automotive Heat Treat: Thru-Process Temperature Monitoring

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, QUENCHING TECHNICAL CONTENT, THERMOCOUPLES TECHNICAL CONTENT / Leave a Comment

In addressing the challenges of modern automated production flow, thru-process temperature monitoring and process validation strategies provide viable options in the automotive heat treat industry. Could they help your operations?

This Technical Tuesday article was composed by Steve Offley, “Dr. O,” product marketing manager, PhoenixTM. It appears in Heat Treat Today’s August 2023 Automotive Heat Treating print edition.

The Heat Treat Monitoring Goal

Dr. Steve Offley, “Dr. O”
Product Marketing Manager
PhoenixTM
Source: LinkedIn

In any automotive heat treatment process, it is essential that the heat treat application is performed in a controlled and repeatable fashion to achieve the physical material properties of the product. This means the product material experiences the required temperature, time, and processing atmosphere to achieve the desired metallurgical transitions (internal microstructure) to give the product the material properties to perform it’s intended function.

 

When tackling the need to understand how the heat treat process is performing, it is useful to split the task up into two parts: focusing on the furnace technology first, and then introducing the product into the mix.

If we consider the furnace performance, we need to validate that the heat treat technology is capable of providing the desired accurate uniformity of heating over the working volume of the furnace for the desired soak time where the products are placed. This is best achieved by performing a temperature uniformity survey (TUS). The TUS is a key pyrometry requirement of the CQI-9 Heat Treat System Assessment (AIAG) standard applied by many automotive OEMs and suppliers.

Traditionally temperature uniformity surveys are performed using a field test instrument (chart recorder or static data logger) external to the furnace with thermocouples trailing into the furnace heating chamber. Although possible, this technique has many limitations, especially when applying to the increasingly automated semi or continuous operations discussed later in this article.

Thru-process Temperature Profiling — Discover the Heat Treat DNA

When it comes to heat treatment, the TUS operation gives a level of confidence that the furnace technology is in specification. However, it is important to understand the need to focus on what is happening at the real core of the product from a temperature and time perspective. Product temperature profiling, as its name suggests, is the perfect technique. Thermocouples attached to the part, or even embedded within the part, give an accurate record of the product temperature at all points in the process, referred to as a product temperature profile. Such information is helpful to determine process variations from critical factors such as part size, thermal mass, location within the product basket, furnace loading, transfer rate, and changes to heat treat recipe. Product temperature profiling by trailing thermocouples with an external data logger (Figure 1) is possible for a simple batch furnace, but it is not a realistic option for some modern heat treat operations.

Figure 1. Typical TUS survey set-up for a static batch furnace. PhoenixTM PTM4220 External data logger connected directly to a 9 point TUS frame used to measure the temperature uniformity over the volumetric working volume of the furnace.
Source: PhoenixTM

With the industry driving toward fully automated manufacturing, furnace manufacturers are now offering the complete package with full robotic product loading — shuttle transfer systems and modular heat treat phases to either process complete product baskets or one-piece operations.

The thru-process monitoring principle overcomes the problems of trailing thermocouples as the multi-channel data logger (field test instrument) travels into and through the heat treat process protected by a thermal barrier (Figure 2).

Figure 2. PhoenixTM thru-process monitoring system. (1) The thermal barrier protects internal multi-channel data logger, (2) the field test instrument, (3) the product thermal profile view, (4) the temperature uniformity survey (TUS), and (5) short nonexpendable mineral insulated thermocouples.
Source: PhoenixTM

The short thermocouples are fixed to either the product or TUS frame. Temperature data is then transmitted either live to a monitoring PC running profile or the TUS analysis software via a two-way RF (radio frequency) telemetry link or downloaded post run.

Although thru-process temperature monitoring in principle can be applied to most heat treat furnace operations, obviously no one solution will suit all processes, as we know from the phrase, “One size doesn’t fit all.”

For this very reason, unique thermal barrier designs are required to be tailored to the specific demands of the application whether temperature, pressure, atmosphere, or geometry as described in the following section.

Product Profiling and TUS in Continuous Heat Treat Furnaces

Thru-process product temperature profiling and/or surveying of continuous furnace operations, unlike trailing thermocouples, can be performed accurately and safely as part of the conventional production flow allowing true heat treat conditions to be assessed. As shown in Figure 3, surveying of the furnace working zone can be achieved using the plane method. A frame attached to the thermal barrier positions the TUS thermocouples at designated positions relative to the two dimensional working zone (furnace height and width) as defined in the pyrometry standard (CQI-9) during safe passage through the furnace (soak time).

Figures 3. Temperature uniformity survey of a continuous furnace using the plane method applying the PhoenixTM thru-process monitoring system. The data logger travels protected in a thermal barrier mounted on the TUS frame performing a safe TUS at four points across the width, which is impossible with trailing thermocouples.
Source: : Raba Axle, Györ, Hungary

Sealed Gas Carburizing and Oil Quench Monitoring

For traditional sealed gas carburizing where product cooling is performed in an integral oil quench, the historic limitation of thru-process temperature profiling has been the need to bypass the oil quench and wash stations.

In such carburizing processes, the oil quench rate is critical to both the metallurgical composition of the metal and to the elimination of product distortion and quench cracks, and so the need for a monitoring solution has been significant. Regular monitoring of the quench is important as aging of the oil results in decomposition, oxidation, and contamination of the oil, all of which degrade the heat transfer characteristics and quench efficiency.

To address the process challenges, a unique barrier design has been developed that both protects the data logger in the furnace (typically 3 hours at 1700°F/925°C) and during transfer through the oil quench (typically 15 minutes) and final wash station.

Figures 4. PhoenixTM thru-process temperature profiling system monitoring the core temperature of automotive parts in a traditional sealed gas carburizing furnace with integral oil quench. (left) System entering carburizing furnace in product basket. (right) Thermal barrier showing outer structural frame and sacrificial insulation blocks protecting inner sealed thermal barrier housing the data logger.
Source: PhoenixTM

The key to the barrier design is the encasement of a sealed inner barrier (Figure 4) with its own thermal protection with blocks of high-grade sacrificial insulation contained in a robust outer structural frame. The innovative barrier offers complete protection to the data logger allowing product core temperature monitoring for the complete heat treat process under production conditions.

Low Pressure Carburizing with High Pressure Gas Quench

In the current business environment, an attractive alternative to the traditional sealed gas carburizing application for both energy and environmental reasons is low pressure carburizing (LPC). Following the vacuum carburizing process, the product is transferred to a sealed high-pressure gas quench chamber where the product is rapidly gas cooled using typically N2 or Helium at up to 20 bars.

Such technology lends itself to automation with product baskets being transferred by shuttle drives and robot loading mechanisms from chamber to chamber in a semi-continuous fashion. The sequential processing (with stages often being performed in self-contained sealed chambers) can only be monitored by the thru-process approach where the system (thermal barrier protected data logger) is self-contained within the product basket or TUS frame.

In such processes the technical challenge is twofold. The thermal barrier must be capable of protecting against not only heat during the carburizing phase, but also very rapid pressure and temperature changes inflicted by the gas quench. To protect the thermal barrier in the LPC process with gas quench, the barrier construction needs to be able to withstand constant temperature cycling and high gas pressures. The design and construction features include:

  • Metal work: 310 stainless steel to reduce distortion at high temperature combined with internal structural reinforcement
  • Insulation: ultra-high temperature microporous insulation to minimize shrinkage problems
  • Rivets: close pitched copper rivets reduce carbon pick up and maintain strength
  • Lid expansion plate: reduces distortion during rapid temperature changes
  • Catches: heavy duty catches eliminating thread seizure issues
  • Heat sink: internal heat sink to provide additional thermal protection to data logger

During the gas quench, the barrier needs to be protected from Nitrogen N2 (g) or Helium He(g) gas pressures up to 20 bar. Such pressures on the flat top of the barrier would create excessive stress to the metal work and internal insulation or the data logger. Therefore, a separate gas quench deflector is used to protect the barrier. The tapered top plate deflects the gas away from the barrier. The unique design means the plate is supported on either four or six support legs. As it is not in contact with the barrier, no force is applied directly to the barrier and the force is shared between the support legs.

In LPC technology further monitoring challenges are faced by the development of one piece flow furnace designs.

Figures 5. (left) Thermal barrier being loaded into LPC batch furnace with TUS frame as part of temperature uniformity survey. (right) Thermal barrier shown with independent quench deflector providing protection during the high pressure gas quench.
Source: PhoenixTM

New designs incorporate single piece or single product layer tray loading into multiple vertical heat treat chambers followed by auto loading into mobile high pressure quench chamber. Miniturization of each separate heat treat chamber limits the space available to the monitoring system. The TS02-128-1 thermal barrier has been designed specifically for such processes utilizing the compact 6 channel “Sigma” data logger allowing reduction of the footprint of the system to fit the product tray and reduce thermal mass. With a height of only 128 mm/5 inch and customized independent low height quench deflector, the system is suitable for challenging low height furnace chambers and offers 1 hour protection at 1472°F/800°C in a vacuum.

Figure 6. (left) Low profile TUS system (TS02-128-1 thermal barrier six channel Sigma data logger) designed with TUS surveying individual one-piece flow heat treatment LPC furnace chambers (right) Thermal barrier shown with optional low profile gas quench deflector.
Source: PhoenixTM

Rotary Hearth Furnace Monitoring — Solution Reheat of Aluminum Engine Blocks

In modern rotary hearth furnaces (Figure 7), temperature profiling using trailing thermocouples is impossible as the cables would wind up in the furnace transfer mechanism. Due to the central robot loading and unloading and elimination of charging racks/baskets, the use of a conventional thru-process system would also be a challenge.

Figure 7. A modern rotary hearth furnace.
Source: PhoenixTM

To eliminate the loading restrictions, a unique thermal barrier small enough to fit inside the cavity of the engine block and allow automated loading of the complete combined monitoring system and product has been developed. To optimize the thermal performance of the thermal barrier with such tight size constraints, a phased evaporation technology is employed. Thermal protection of the high temperature data logger is provided by an insulated water tank barrier design keeping the operating temperature of the data logger at a safe 212°F/100°C or less. The system allowed BSN Thermoprozesstechnik GmbH in Germany to commission the furnace accurately and efficiently and thereby optimize settings to not only achieve product quality but also ensure energy efficient, cost effective production.

Summary

Thru-process product temperature profiling and surveying provide a versatile, accurate, and safe solution for monitoring increasingly automated, intelligent furnace lines and the means to understand, control, optimize, and certify your heat treat process.

About the author:

Dr. Steve Offley, “Dr. O,” has been the product marketing manager at PhoenixTM for the last five years after a career of over 25 years in temperature monitoring focusing on the heat treatment, paint, and general manufacturing industries. A key aspect of his role is the product management of the innovative PhoenixTM range of thru-process temperature and optical profiling and TUS monitoring system solutions.

For more information:

Contact Steve at Steve.Offley@phoenixtm.com.

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

Discover the DNA of Automotive Heat Treat: Thru-Process Temperature Monitoring Read More »

Continuing Ed — Heat Treat Technical Tuesday Round Up

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, THERMOCOUPLES TECHNICAL CONTENT

OC

Heat Treat Today’s Technical Tuesday feature means that on just about any given Tuesday, there will be an article that aims to educate our heat treating readers — be it in a process, equipment, metals, analysis, critical parts, or more. Enjoy this sampling of Technical Tuesday articles from the past several months.

Case Study: Heat Treat Equipment Meets the Future Industry Today 

Contact us with your Reader Feedback!

Construction and schematic furnace cross-section CMe-T6810-25
Source: SECO/WARWICK

How has one heat treat furnace supplier contended with modern challenges of manufacturing? In this case study about a shift away from traditional forms of heat treat, explore how vacuum furnace technology has more technological horizons to bound. 

Several key features discussed will be the various challenges that characterize modern industry; the differences between historical heat treat furnaces and vacuum furnaces; furnace features that can meet these obstacles; and a close look at what one equipment option from SECO/WARWICK helps. Additionally, explore the case study of a process that resulted in the following assessment: "all technological requirements have been met, obtaining the following indicators of efficiency and consumption of energy factors calculated for the entire load and per unit net weight of the load (700 kg)." 

Read the entire article here.

How Things Work: Thermocouples 

Eric Yeager of Cleveland Electric explaining the 101 of all things thermocouple
Source: Heat Treat Today

How do thermocouples work? How would you tell if you had a bad one? Those ever present temperature monitors are fairly straightforward to use, but when it comes to how it works — and why — things get complicated.  

This transcript Q&A article was published in the print edition last year (2022), but there was too much information to fill the pages. Online, read the full-length interview, including the final conversation about how dissimilar metals create EMF. Included in the discussion is proper care of the T/C and knowledge of when it’s time to replace. 

Read the entire article here.

6 Heat Treat Tech Trends Fulfilled in 2022

Trends in the heat treat industry
Source: Unsplash.com/getty images

What’s “hot” for heat treaters in recent months? The trends are pointing towards streamlining upgrading information systems, more efforts to reduce carbon footprint, and ensuring processes in salt quenching and electricity use are as efficient as they can be. 

Each of the 6 trends included in the article demonstrates that heat treaters are making thoughtful and responsible decisions and purchases. Considerations include care for the environment and methods to help employees share and receive information needed for each job.  

Read more about each of the trends to see what’s happening with equipment purchases and technology decisions and how companies are pushing to make that carbon footprint smaller. 

Read the entire article here.

A Quick Guide to Alloys and Their Medical Applications 

Sneak peak of this medical alloys resource
Source: Heat Treat Today

If you're pining for a medical heat treat quick resource in our "off-season," we have a resource for you. Whether you are a seasoned heat treater of medical application parts or not, you know that the alloy composition of a part will greatly determine the type of heat treat application that is suitable. Before you expand your heat treat capabilities of medical devices, check out this graphic to quickly pin-point what alloys are in high-demand within the medical industry and what end-product they relate to. 

The alloys addressed in this graphic are: titanium, cobalt chromium, niobium, nitinol, copper, and tantalum.  

Read the entire article here.

Resource -- Forging, Quenching, and Integrated Heat Treat: DFIQ Final Report 

Examples of DFIQ equipment
Source: Joe Powell

How much time and energy does it take to bring parts through forging and heat treatment? Have you ever tried to integrating these heat intensive processes? If part design, forging method, and heat treat quenching solutions are considered together, some amazing results can occur. Check out the report findings when the Direct from Forge Intensive Quenching (DFIQTM) was studied. 

Forgings were tested, in three different locations, to see if immediate quenching after forging made a difference in a variety of steel samples. The report shares, “The following material mechanical properties were evaluated: tensile strength, yield strength, elongation, reduction in area and impact strength. Data obtained on the mechanical properties of DFIQ forgings were compared to that of forgings after applying a conventional post-forging heat-treating process.” 

 Read the entire article here.

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

 

Continuing Ed — Heat Treat Technical Tuesday Round Up Read More »

Heat Treat Radio Series for Spring

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, THERMOCOUPLES TECHNICAL CONTENT

OC

The days are getting a little longer, you've saved up some vacation hours, it's time for a break this spring!

Make use of some down time to listen in on a couple of Heat Treat Radio series. Putting in some driving miles, relaxing in the sand, or enjoying a staycation all mean some time to peacefully enjoy some heat treat topics. We've put together an original content piece that lets you listen in on a 3-part series on thermocouples, and a back-to-basics series on heat treat hardening. It's nice to know that there is plenty to listen to; you can just click to play each episode!

Thermocouples 101 with Ed Valykeo and John Niggle

This series gives the opportunity to learn from an expert all about thermocouples. The first episode digs into thermocouple history, types, vocabulary, and other basics. Hear from Ed Valykeo, as he gives some of his own history and then dives into all things thermocouple.

1. Heat Treat Radio #61

Contact us with your Reader Feedback!

The second episode covers thermocouple accuracy and classification. Ed Valykeo continues to review and explain necessary information on how thermocouples are calibrated and used.

2. Heat Treat Radio #62

The final episode in this series gets into discussion with John Niggle about thermocouple insulation types. His review towards the beginning of the episode is helpful, and his discussion of insulation reminds readers that job specifications and requirements are crucial.

3. Heat Treat Radio #64

Metal Hardening 101 

Mark Hemsath sits down with Heat Treat Radio to provide an overview of metal hardening basics. In the first part of the series he provides explanation of what it is, what materials can be hardened, why it has to be done, and more.

1. Heat Treat Radio #49

For the second episode, Mark Hemsath explains five hardening processes: carburizing, nitriding, carbonitriding, ferritic nitrocarburizing, and low pressure carburizing.

2. Heat Treat Radio #54

In this final episode for the metal hardening series, a discussion is presented on newer advances in metal hardening. A call is even put out for new ideas and engineers willing to experiment with some of these advance.

3. Heat Treat Radio #56

As you can see above, this resource provides two series -- each with three parts -- that give a comprehensive look at two fundamental components in the heat treat industry. Both the discussion of thermocouples and the investigation of metal hardening provide educational listening with something for everyone in the form of review as well as maybe some basics that have been neglected or forgotten.

.

Search for heat treat solution providers and suppliers on Heat Treat Buyers Guide.com

 

Heat Treat Radio Series for Spring Read More »

Heat Treat Radio #91: Understanding the ±0.1°F Requirement in AMS2750, with Andrew Bassett

/ CONTROLS TECHNICAL CONTENT, FEATURED NEWS, HEAT TREAT RADIO, INSTRUMENTATION TECHNICAL CONTENT, MANUFACTURING HEAT TREAT, THERMOCOUPLES TECHNICAL CONTENT

Where did the ±0.1°F AMS2750 requirement come from and how should heat treaters approach this specification, an important change that entails major buy-in? Andrew Bassett, president and owner of Aerospace Testing and Pyrometry, was at the AMS2750F meeting. He shares the inside scoop on this topic with Heat Treat Today and what he expects for the future of this standard.

Heat Treat Radio podcast host and Heat Treat Today publisher, Doug Glenn, has written a column on the topic, which you can find here; read it to understand some of the background, questions, and concerns that cloud this issue.

Below, you can watch the video, listen to the podcast by clicking on the audio play button, or read an edited transcript.

 

The following transcript has been edited for your reading enjoyment.

Doug Glenn: Andrew Bassett, president and owner of Aerospace Testing and Pyrometry, Inc., somewhere in eastern Pennsylvania. We don’t know because you’re on the move! What is your new address, now, by the way?

Contact us with your Reader Feedback

Andrew Bassett: We are in Easton, Pennsylvania at 2020 Dayton Drive.

Doug Glenn: Andrew, we want to talk a bit about this ±0.1°F debate that is going on. It was actually precipitated by the column that I wrote that is in the February issue.

I just wanted to talk about that debate, and I know that you’ve been somewhat involved with it. So, if you don’t mind, could you give our listeners a quick background on what we are talking about, this ±0.1°F debate.

Andrew Bassett: To be honest with you, being part of the AMS2750 sub team, one of the questions came up for us during the Rev F rewrite was this 0.1°F readability — wanting to kind of fix this flaw that’s been in the standard ever since the day that AMS2750 came out. With instrumentation, for instance, you have ±2°F (the equivalent would be 1.1°C). At 1.1°C, the question became, If your instrumentation does not show this 0.1 of a degree readability, how can you show compliance to the standards?

Andrew Bassett
President
Aerospace Testing and Pyrometry
Source: DELTA H

Then, it morphed into other issues that we’ve had in the previous revisions where we talk about precise temperature requirements, like for system accuracy testing: You’re allowed a hard number ±3° per Class 2 furnace or 0.3% of reading, whichever is greater. Now, we have this percentage. With anything over 1000°F, you're going to be able to use the percentage of reading to help bring your test into tolerance. In that example, 1100°F, you’re about 3.3 degrees. If your instrumentation doesn’t show this readability, how are you going to prove compliance?

That’s what it all morphed into. Originally, the first draft that we proposed in AMS2750F was that all instrumentation had to have 0.1°F readability. We got some feedback (I don’t know if I want to say “feedback” or "pitchforks and hammers") that this would be cost-prohibitive; most instrumentation doesn't have that readability, and it would be really costly to go out and try to do this. We understood that. But, at the end of the day, we said: The recording device is your permanent record, and so that’s what we’re going to lean on. But we still had a lot of pushback.

We ended up putting a poll out to AMEC and the heat treating industry to see what their opinions were. We said that with the 0.1 readability (when it came to a percentage reading), recording devices would read hard tolerances. So, for instance, an SAT read at 3° would be just that, not "or .3% of reading."

There was a third option that we had put out to the community at large, and it came back as the 0.1° readability for digital recorders, so that’s where we ran with the 0.1° readability.

When it was that big of an issue, we didn’t make the decisions ourselves; we wanted to put it out to the rest of the community. My guess is not everyone really thought the whole thing through yet. Now people are like, ok, well now I need to get this 0.1° readability.

Again, during the meetings, we heard the issues. Is 0.1° going to really make a difference to metal? If you have a load thermocouple that goes in your furnace and it reads 0.1° over the tolerance, does it fail the load? Well, no, metallurgically, we all know that’s not going to happen, but there’s got to be a line in the sand somewhere, so it was drawn at that.

"...that hard line in the sand had to be drawn somewhere..."
Source: Unsplash.com/Willian Justen de Vasconcellos

That’s a little bit of the background of the 0.1° readability.

Doug Glenn: So, basically, we’re in a situation, now, where people are, in fact (and correct me if I’m wrong here),  potentially going to fail SATs or tests on their system because of a 0.1° reading, correct? I mean, it is possible, correct?

Andrew Bassett: Yes. So, when the 0.1° readability came out in Rev F, we gave it a two-year moratorium that with that requirement, you still had two more years. Then, when Rev G came out, exactly two years to the date, we still had a lot of customers coming to us, or a lot of suppliers coming back to us, and saying, “Hey, look, there’s a supply shortage on these types of recorders. We need to buy some time on this.” It ranged from another year to 10 years, and we’re like — whoa, whoa, whoa! You told us, coming down the pike before, maybe you pushed it down the road, whatever, probably Covid put a damper on a lot of people, so we added another year.

So, as of June 30th of 2023, that requirement is going to come into full play now. Like it or not, that’s where the standard sits.

Doug Glenn: So, you’re saying June 30th, 2023?

Andrew Bassett: Yes.

Doug Glenn Alright, that’s good background.

I guess there were several issues that I raised. First off, you’ve already hit on one. I understand the ability to be precise, but in most heat treatment applications, one degree is not going to make a difference, right? So, why do we push for a 0.1° when 1° isn’t even going to make a difference?

Andrew Bassett: We know that, and it’s been discussed that way. But, again, that hard line in the sand had to be drawn somewhere, and that was the direction the community wanted to go with, so we went with that. Yes, we understand that in some metals, 10 degrees is not going to make a difference, but we need to have some sort of line in the sand and that's what was drawn.

Doug Glenn: So, a Class 1. I was thinking the lower number was a tighter furnace. So, a Class 1 (±5), and you’re saying, that’s all the furnace is classified for, right, ±5? So, if you get a reading of 1000°, it could be 1005° or it could be 995°. Then, you’re putting on top of that the whole idea that your temperature reading has got to be down to 0.1°. There just seems to be some disconnect there.

So, that was the first one. You also mentioned the instrumentation. It’s been pointed out to me, by some of the instrumentation people, that their instruments are actually only reading four digits. So up to 99.9 you actually have a point, but if it goes to 1000°, you’re out of digits; you can’t even read that. I mean, they can’t even read that down to a point.

"So, if you get a reading of 1000°, it could be 1005° or it could be 995°."
Source: Unsplash.com/Getty Images

Andrew Bassett: Correct. On the recording side of things, we went away from analog instrumentation. The old chart papers, that’s all gone, and we required the digital recorders with that 0.1° readability, as of June 30th of this year.

Again, the first draft was all instrumentation. That would be your controllers, your overtemps, and we know that limitation. But everyone does have to be aware of it. We still allow for this calibration of ±2 or 0.2%. If you’re doing a calibration, let’s say, on a temperature control on a calibration point at 1600° and the instrument only reads whole numbers, you can use the percentage, but you would have to round it inward. Let’s use 1800°, that would be an easier way to do it. So, I’m allowed ±2 or 3.6° if I’m using the percentage of reading, but if the instrument does not read in decimal points for a controller or overtemp, you would have to round that down to ±3°.

Doug Glenn: ±3, right; the 0.6° is out the window.

Andrew Bassett: Correct. I shouldn’t say we like to bury things in footnotes, but this was an afterthought. In one of the footnotes, in one of the tables, it talks about instrumentation calibration that people need to be aware of.

Doug Glenn: Let’s just do this because I think we’ve got a good sense of what the situation is, currently. Would you care to prognosticate about the future? Do you think this is going to stand? Do you think it will be changed? What do you think? I realize you’re speaking for yourself, here.

Andrew Bassett: I’m conflicted on both sides. I want to help the supply base with this issue but I’m also on the standards committee that writes the standard. I think because we’re so far down the road, right now — this requirement has been out there since June 2022 — I don’t see anything being rolled back on it, at this point. I think if we did roll it back, we have to look at it both ways.

If we did roll this back and say alright, let’s just do away with this 0.1° readability issue, we still have to worry about the people processing in Celsius. Remember, we’re pretty much the only country in the world that processes in Fahrenheit. The rest of the world has been, probably, following these lines all along. If we rolled this back, just think about all the people that made that investment and moved forward on the 0.1° readability and they come back and say, “Wait a minute. We just spent a $100,000 on upgrading our systems and now you’re rolling it back, that’s not fair to us.”

At this point, with the ball already rolling, it would be very interesting to see when Nadcap starts publishing out the audit findings when it comes to the pyrometry and this 0.1° readability to see how many suppliers are being hit on this requirement and that would give us a good indication. If there are a lot of yeses on it then, obviously, a lot of suppliers haven’t gone down this road. My guess is, for the most part, anybody that’s Nadcap accredited in heat treating — and this goes across chemical processing, coatings, and a few other commodities — I think has caught up to this.

Personally, I don’t think this is going to go away; it’s not going to disappear. It’s going to keep going down this road. Maybe, if people are still struggling with getting the types of devices that can have that 0.1° readability, then maybe another year extension on it, but I don’t know where that is right now. I haven’t gotten enough feedback from aerospace customers that say, "Hey, I can’t get the recorder." I mean,

Doug Glenn: I just don’t understand, Andrew, how it’s even physically possible that companies can record something as accurately as 0.1° if the assembly or thermocouple wire is rated at ±2°? How is that even possible that you can want somebody to be accurate down to ±0.1° when the thing is only accurate up to ±2°?

Andrew Bassett: Right, I get that. We can even go a lot further with that and start talking about budgets of uncertainty. If you look at any reputable thermocouple manufacturer or instrument calibration reports that are ISO 17025, they have to list out their measurements of uncertainty, and that gives you only the 98% competence you’re going to be within that accuracy statement.

Yes, I get the whole issue of this .1° readability. There were good intentions were to fix a flaw, and it spiraled. We’ve seen where PLCs and some of these high logic controllers now can show the .1° readability, but they automatically round up at .5°. Are you now violating the other requirements of rounding to E29? Now, I think we’ve closed out the poll in the standard, but you’re right. We were trying to do the right thing. Personally, I don’t think we gave it all that much further thought on that except hey, let’s just make recorders this way and this should be okay.

Doug Glenn: Right. No, that’s good. Let me be clear, and I think most everybody that was involved with the standards are excellent people and they’re trying to do the right thing. There is no dissing on anybody that was doing it. I’m not a furnace guy, right, I’m a publisher — but when I look at it, I’m going: okay, you’re asking somebody to be as accurate as 0.1° on equipment that can only do ±2°. That’s a 4° swing and you’re asking them to be within 0.1°, basically.

Andrew, this has been helpful. It’s been good hearing from you because you’re on the frontline here. You’ve got one foot firmly planted in both camps.

Andrew Bassett: I’m doing my best to stay neutral with it all.

Doug Glenn: Anyhow, I appreciate it, Andrew. You’re a gentleman. Thanks for taking some time with us.

Andrew Bassett: Thanks, Doug. Appreciate it.

About the expert: Andrew Bassett has more than 25 years of experience in the field of calibrations, temperature uniformity surveys, system accuracy testing, as well an expertise in pressure, humidity, and vacuum measurement calibration. Prior to founding Aerospace Testing & Pyrometry, Andrew previously held positions as Vice President of Pyrometry Services and Director of Pyrometry Services for a large commercial heat treater and Vice President and Quality Control Manager for a small family owned business.

For more information: Andrew Bassett at abassett@atp-cal.com or visit http://www.atp-cal.com/

Doug Glenn at Doug@heattreattoday.com

 

Doug Glenn <br> Publisher <br> Heat Treat Today

Doug Glenn
Publisher
Heat Treat Today

To find other Heat Treat Radio episodes, go to www.heattreattoday.com/radio .

Search heat treat equipment and service providers on Heat Treat Buyers Guide.com

 

Heat Treat Radio #91: Understanding the ±0.1°F Requirement in AMS2750, with Andrew Bassett Read More »

¿Cómo elegir el termopar correcto en Tratamientos Térmicos?

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, THERMOCOUPLES TECHNICAL CONTENT

OCLos termopares: elementos indispensables para lograr un acertado tratamiento térmico, pero ¿cómo elegir el más indicado para su necesidad particular? ¿Qué exigen las normas actuales? A continuación una explicación, por Víctor Zacarías, director general de Global Thermal Solutions México, que le ayudará a saber escoger el termopar adecuado.

Palabras clave: Termopar, Tratamiento térmico, Pirometría, Medición y Control de Temperatura, AMS2750, CQI-9

Read the Spanish translation of this article in the version below, or see both the Spanish and the English translation of the piece where it was originally published: Heat Treat Today's February's Air & Atmosphere Furnace Systems print edition.

Si quisieras aportar otros datos interesantes relacionados con los termopares, nuestros editores te invitan a compartirlos para ser publicados en línea en www.heattreattoday.com. Puedes hacerlos llegar a Bethany Leone al correo bethany@heattreattoday.com

Víctor Zacarías
Director General
Global Thermal Solutions México

La norma aeroespacial SAE AMS2750 y las evaluaciones automotrices de AIAG CQI-9, CQI-11, CQI-12, y CQI-29 son los estándares universalmente aceptados para el control de temperatura en operaciones de procesamiento térmico. Entre muchas cosas, describen los requisitos para el uso y control de los termopares empleados en hornos y estufas de proceso. En este artículo te comparto los requisitos de estas normativas para que puedas tomar una decisión correcta al elegir un termopar y de esta manera contar con una medición repetible que te asegure un proceso confiable.

1. Aplicación

Contact us with your reader feedback!

Para la selección apropiada de un termopar para la medición, control y/o registro de la temperatura debes considerar en primer lugar el tipo de proceso previsto. En la elección del termopar adecuado, toma en cuenta algunos factores que pudieran alterar su desempeño como:

  • El rango de temperatura en el que estará en uso
  • El tipo de atmósfera al que estará expuesto
  • Posible interferencia eléctrica
  • La precisión requerida por la especificación aplicable, etc.

En función de lo anterior, las normativas refieren una clasificación específica para los termopares en función de su fabricación y su aplicación final:

a) Termopares base y termopares nobles
b) Termopares desechables y no desechables

2. Tipos de termopar y su aislamiento

2.1 Termopar base o termopar noble

Un termopar base está fabricado de aleaciones básicas como hierro, cromo, níquel, cobre, etc., y constituyen los tipos más comunes en la industria por su versatilidad y costo: los termopares tipo K, E, J, N, y T. Un buen proveedor de sensores te recomendará un termopar de este tipo en función de la aplicación, el rango de temperatura y tu presupuesto (ver Tabla 1).

Tabla 1: Rango de temperatura y uso de los termopares más comunes
Source: GTS México

Por otro lado, un termopar noble está fabricado a partir de metales como platino y rodio: termopares tipo R, S y B. Éstos termopares son más estables a altas temperaturas y mantienen su precisión por mayor tiempo; sin embargo, tienen un costo elevado debido a que se fabrican a partir de metales preciosos. Debido a esta naturaleza, los termopares nobles son la elección preferida para aplicaciones de tratamiento térmico al vacío y procesos de alta temperatura.

2.2 Termopares desechables o no desechables

El segundo criterio de las normativas lo constituye el material con el que se protegen los elementos del termopar.

Los termopares desechables son aquellos cuyos elementos están revestidos por materiales como fibra de vidrio, tejido cerámico o recubrimiento polimérico y generalmente se suministran en forma de carrete o bobina. Esta presentación permite al usuario cortar el cable a la medida y fabricar el termopar al unir los dos alambres de un extremo por torsión o soldadura, lo que los hace ideales por ejemplo para aplicaciones de un solo uso como una prueba TUS o termopares de carga (ver Figura 1).

Figura 1: TUS usando termopar desechable tipo K aislado en fibra cerámica
Source: Trucal, Inc.

En contraste un termopar no desechable normalmente está protegido con aislamiento cerámico o mineral y revestido en su exterior por una carcasa metálica (los elementos no están expuestos en esta configuración), lo que le proporciona un mayor tiempo de vida útil y por eso se prefieren para emplearse como termopares de control o registro (ver Figura 2).

Figura 2: Termopares no desechables tipo N y K de aislamiento mineral
Source: GTS México

Cualquiera que sea la aplicación, cuando se requiere realizar interconexiones de cableado para la instalación del sensor, dichas conexiones se deben realizar usando conectores y terminales estándar como las que se muestran en la Figura 3, ya que tanto AMS2750 como CQI- 9 prohíben el empalme del cableado.

Figura 3: Conectores estándar tipo K
Source: GTS México

3. Calibración

De acuerdo con la normatividad, todos los termopares usados en operaciones de procesamiento térmico deben haber sido calibrados antes de usarse por primera vez. Para ello, el usuario del termopar debe asegurarse de contar con calibraciones trazables al laboratorio nacional como lo es el NIST en Estados Unidos o su equivalente en México (CENAM).

Las normas de pirometría defi nen los rangos aceptables de error para los termopares en función de su aplicación fi nal: 1) termopares patrón, 2) termopares de prueba (SAT y TUS), 3) termopares de control y registro y 4) termopares de carga. La Tabla 2 describe los máximos errores permitidos a elegir dependiendo del uso del sensor.

Tabla 2: Precisión requerida para sensores de temperatura según AMS2750 y CQI-9
Source: GTS México

Una vez instalado el termopar, el responsable de la operación de tratamiento térmico tiene que deberá documentar la fecha en la que éste entra en servicio, ya que la norma establece un tiempo de vida útil de un sensor en función de la aplicación del mismo.

Al recibir el reporte/certifi cado del termopar, el usuario debe revisar el contenido del documento, pues las normas también definen de manera específi ca la información mínima que debe aparecer en un informe de calibración, que incluye pero no se limita a:

1. Lecturas de prueba
2. Lecturas observadas
3. Factores de corrección
4. Fuente de los datos
5. Acreditación del laboratorio
6. Método de calibración empleado

El certifi cado de calibración puede amparar termopares individuales o un grupo de termopares fabricados a partir del mismo lote (carrete).

Es muy importante observar que tanto AMS2750 como CQI-9 requieren que todas las calibraciones sean realizadas por organismos acreditados en la norma ISO/IEC 17025, por lo que siempre recomiendo que revises el certifi cado de acreditación antes de seleccionar a tu proveedor.

4. En Resumen

Si alguna vez has comprado el termopar equivocado, se lo molesto que puede resultar. Por lo tanto aquí te comparto un resumen para seleccionar el sensor adecuado para su aplicación en 5 sencillos pasos:

1. Define el tipo de termopar: base ( K, T, J, E , N, y M) o noble (S, R, y B)
2. Define el tipo de aislamiento que requieres: fibra textil, polímero, cerámico, metálico, etc.
3. Especifi ca el rango exacto de temperatura en el que operará el sensor
4. Especifi ca el uso del sensor: termopar patrón (estándar), termopar para SAT/TUS, termopar de control / carga
5. Solicita el certifi cado de calibración conforme a la normativa aplicable (AMS2750 o CQI-9)

 

Referencias

ASTM International. ASTM E230, Standard Specification for Temperature-Electromotive Force (emf) Tables for Standardized Thermocouples, Rev. 2017.

Automotive Industry Action Group. CQI-9 Special Process: Heat Treat System Assessment, 4th Edition. June 2020

International Organization for Standardization. ISO/IEC 17025, General Requirements for the Competence of Testing and Calibration Laboratories, 3rd Edition. 2017.

Nadcap AC7102/8 Audit Criteria for Pyrometry, Rev. A, 2021

SAE Aerospace. Aerospace Material Specifi cation AMS2750: Pyrometry, Rev. G, 2022.

 

Sobre el autor: Víctor Zacarías es ingeniero metalúrgico egresado de la Universidad Autónoma de Querétaro con estudios en Gerencia Estratégica por parte del Tec de Monterrey. Con más de 15 años de experiencia en la gestión de tratamientos térmicos, actualmente es director general de Global Thermal Solutions México. Víctor ha realizado numerosos cursos, talleres y evaluaciones en México, Estados Unidos, Brasil, Argentina y Costa Rica y ha participado en el Grupo de Trabajo de Tratamiento Térmico de AIAG (CQI-9) y en el Comité de Ingeniería de Materiales Aeroespaciales de SAE.

Contact/Contacto Victor: victor@globalthermalsolutions.com

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

 

¿Cómo elegir el termopar correcto en Tratamientos Térmicos? Read More »

How To Choose the Right Thermocouple in Heat Treatment

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, THERMOCOUPLES TECHNICAL CONTENT

OC

Thermocouples: You can’t accurately heat treat without them. But how can you choose the best one for your needs? What do current regulations require? Read this helpful explanation, by Víctor Zacarías, managing director of Global Thermal Solutions Mexico, to find out how to choose the right thermocouple.

Keywords: Thermocouple, Heat Treatment, Pyrometry, Temperature Measurement and Control, AMS2750, CQI-9

Read the English version of the article below, or find the Spanish translation when you click the flag above right!

This Technical Tuesday article, first published in English and Spanish translations, is found in Heat Treat Today's February's Air & Atmosphere Furnace Systems print edition.

If you have any facts of your own about thermocouples, our editors would be interested in sharing them online at www.heattreattoday.com. Email Bethany Leone at bethany@heattreattoday.com with your own trivia!

Víctor Zacarías
Managing director 
Global Thermal Solutions México

The SAE AMS2750 aerospace standard and the AIAG CQI-9, CQI-11, CQI-12, and CQI-29 automotive assessments are the universally accepted standards for temperature control in thermal processing operations. Among many things, they describe the requirements for the use and control of thermocouples used in process ovens and furnaces. In this article you will find the requirements of these regulations so that you can make a correct decision when choosing a thermocouple, and thus have a repeatable measurement that ensures a reliable process.

1. Application

Contact us with your Reader Feedback!

For the appropriate selection of a thermocouple for the control and/or recording of temperature, you must first take into account the type of process. In choosing the right thermocouple, consider some factors that could alter its performance, such as:

  • The temperature range at which it will be in use
  • The type of atmosphere to which it will be exposed
  • Possible electrical interference
  • The accuracy required by the applicable specification, etc.

Based on the above, existing regulations refer to a specific classification for thermocouples based on their manufacture and final application. These classifications are:
a) Base thermocouples and noble thermocouples
b) Expendable and non-expendable thermocouples

2. Types of Thermocouples and Their Insulation

2.1 Base Thermocouple or Noble Thermocouple

A base thermocouple is made of basic alloys such as iron, chrome, nickel, copper, etc., and they are the most common types in the industry due to their versatility and cost. Base thermocouples are types K, E, J, N, and T. A good supplier of sensors will recommend a thermocouple based on the application, the temperature range, and your budget (see Table 1).

 

Table 1: Temperature range and application of most common thermocouples
Source: GTS México

On the other hand, a noble thermocouple is made from metals such as platinum and rhodium: types R, S, and B thermocouples. These thermocouples are more stable at high temperatures and maintain their accuracy for a longer time. However, they have the highest cost since they are made from precious metals. Due to this nature, noble thermocouples are the preferred choice for vacuum heat treatment applications and high temperature processes.

2.2 Expendable or Non-expendable Thermocouples

The second criteria from the regulations are the material which protects the elements of the thermocouple.

Expendable thermocouples are those whose elements are covered by materials such as fiberglass, ceramic fabric, or polymeric coating and are generally provided in the form of a spool. This form allows the user to cut the cable to size and manufacture the thermocouple by joining the two wires by twisting or welding, making them ideal for single use applications such as a TUS test or charging thermocouples, for example (see Figure 1).

Figure 1: TUS using type K expendable thermocouple insulated in ceramic fiber
Source: Trucal, Inc.

In contrast, a nonexpendable thermocouple is normally protected with ceramic or mineral insulation and covered on the outside by a metallic sheath (the elements are not exposed in this configuration), which gives it a longer useful life. Therefore, it is preferred for use as a control or recording thermocouple (see Figure 2).

Figure 2: Non-expendable type N and K mineral insulated thermocouples
Source: GTS México

Whatever the application, when wiring interconnections are required for sensor installation, these connections must be made using standard connectors and terminals such as those shown in Figure 3, as both AMS2750 and CQI-9 prohibit the wiring splice.

Figure 3: Standard type K connectors
Source: GTS México

3. Calibration

According to regulations, all thermocouples used in the heat treatment operation must have been calibrated before being used for the first time. The user of the thermocouple must ensure that they have calibrations traceable to a national laboratory such as the NIST in the United States or its equivalent in Mexico (CENAM).

Pyrometry standards defi ne the acceptable error ranges for thermocouples depending on their final application. These categories for final application include: standard thermocouples, test thermocouples (SAT and TUS), control and recording thermocouples, and load thermocouples (see Table 2). Table 2 describes the maximum errors allowed to be selected depending on the use of the sensor.

Table 2: Accuracy required for temperature sensors according to AMS2750 and CQI-9
Source: GTS México

Once the thermocouple is installed, the person responsible for the heat treatment operation must document the date on which it comes into service, since the regulations establish the life of a sensor based on its application.

When receiving the report/certificate of the thermocouple, the user must review the content of the document, since the standards specifically define the minimum information that shall appear in a calibration report, which includes but is not limited to:

1. Test readings
2. Actual readings
3. Correction factors
4. Data source
5. Laboratory accreditation
6. Calibration method used

The calibration certificate can cover individual thermocouples or a group of thermocouples manufactured from the same lot (spool).

It is very important to note that both AMS2750 and CQI-9 require all calibrations to be conducted by ISO/IEC 17025 accredited organizations, so ensure that you review the accreditation certificate before selecting your supplier.

4. In Summary

If you’ve ever bought the wrong thermocouple, you know how annoying it can be. Therefore, here is a quick guide to select the right sensor for your application in five easy steps:

1. Define the type of thermocouple: base (K, T, J, E, N, and M) or noble (S, R, and B)
2. Define the type of insulation you require: textile fiber, polymer, ceramic, metallic, etc.
3. Specify the exact temperature range in which the sensor will operate
4. Specify the use of the sensor: standard thermocouple, SAT/TUS thermocouple, control/load thermocouple
5. Request the calibration certificate in accordance with the applicable regulations (AMS2750 or CQI-9)

 

References

ASTM International. ASTM E230, Standard Specification for Temperature-Electromotive Force (emf) Tables for Standardized Thermocouples, Rev. 2017.

Automotive Industry Action Group. CQI-9 Special Process: Heat Treat System Assessment, 4th Edition. June 2020.

International Organization for Standardization. ISO/IEC 17025, General Requirements for the Competence of Testing and Calibration Laboratories, 3rd Edition. 2017.

Nadcap AC7102/8 Audit Criteria for Pyrometry, Rev. A, 2021

SAE Aerospace. Aerospace Material Specifi cation AMS2750: Pyrometry, Rev. G, 2022.

 

About the Author: Víctor Zacarías is a metallurgical engineer from the University of Queretaro with studies in Strategic Management from Tec de Monterrey. With over 15 years of experience in Heat Treatment Management, he is currently the managing director of Global Thermal Solutions México. He has conducted numerous courses, workshops, and assessments in México, the United States, Brazil, Argentina, and Costa Rica. He has been a member of the AIAG Heat Treat Work Group (CQI-9 committee) and the SAE Aerospace Materials Engineering Committee.

Contact Víctor at victor@globalthermalsolutions.com

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

 

How To Choose the Right Thermocouple in Heat Treatment Read More »

Thermocouple Trivia by the Dozen

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, THERMOCOUPLES TECHNICAL CONTENT

OC

Thermocouples are ubiquitous. Whether you are 20 days or 20 years into the industry, you know the essential role they play in making sure heat treat processes are running efficiently, accurately, and dependably. This quick trivia questionnaire will test your thermocouple knowledge on a dozen either obscure or obvious facts about thermocouples.

Los termopares son ubicuos. Sin importar que tu experiencia en la industria sea de 20 días o 20 años, conoces bien el papel esencial que juegan en asegurar que los procesos de tratamiento térmico avancen de manera eficiente, precisa y confiable...

Take the English version of the quiz below, or find the Spanish translation when you click the flag above right!

If you have any facts of your own about thermocouples, our editors would be interested in sharing them online at www.heattreattoday.com. Email Bethany Leone at bethany@heattreattoday.com with your own trivia!

This Technical Tuesday article, first published in English and Spanish translations, is found in Heat Treat Today's November 2022 Vacuum print edition. 

Thermocouple Trivia

1. What thermocouple type potentially has the longest life (but is also the most expensive)?

(a) Type K (Chromel-Alumel)

(b) Type N (Nicrosil-Nisil)

(c) Type R (Platinum-13% Rhodium)

(d) Type J (Iron-Constantan)

2. What is something you might find at home that uses a thermocouple to control its temperature?

Contact us with your Reader Feedback!

(a) Your oven

(b) Your toaster

(c) Your water heater

(d) All of the above

 

3. What do you need to know when purchasing thermocouples for your heat treat furnace or oven?

(a) The length of the thermocouple

(b) The process application you are running

(c) The type of thermocouple best suited for the

(d) All of the above

4. Who was Thomas Johann Seebeck?

(a) The person credited with describing the scientific theory behind thermocouples

(b) An advocate for the elimination of thermocouples in furnaces and ovens

(c) A German physicist who was responsible for helping develop rockets for the United States

(d) None of the above

5. What would be the best thermocouple to use to control the temperature of an oil quench tank?

(a) Type R (platinum — 13% rhodium)

(b) Type S (platinum — 10% rhodium)

(c) Type K (chromel-alumel)

(d) Type J (iron-constantan)

6. Why use an over temperature (aka "excess temperature") device on your furnace or oven?

(a) For better process control, it is always helpful to have more than one thermocouple in the furnace/oven

(b) To prevent the furnace temperature from running away and damaging the equipment

(c) An obsolete device no longer required by NFPA 86

(d) A method of ensuring the process being run in the furnace stays close to the set point temperature

7. How are thermocouples used in the heat treat industry?

(a)temperature control devices

(b) As part of a safety system designed to prevent the furnace/oven from running away and damaging itself

(c) To ensure that temperature, the most important process parameter, is maintained within limits necessary to successfully run a heat treat process

(d) All of the above

8. Why use type K versus type N thermocouples?

(a) Because type K has better accuracy

(b) Because type K has better temperature limits

(c) Because type K is more expensive

(d) None of the above

9. Thermocouples produce what type of voltage?

(a) PPM (parts per million)

(b) EMF (electromotive force)

(c) EMP (electromagnetic pulse)

(d) mV (millivolt)

10. What are some of the most common reasons why a thermocouple “drifts” or fails in a heat treat furnace or oven?

(a) Age

(b) Running at temperatures higher than its rated use temperature

(c) The wrong thermocouple type is used

(d) All of the above

11. What is a common problem seen in thermocouples that fail in service?

(a) Green rot (oxidation of chromium)

(b) Metal dusting (aka "catastrophic carburization")

(c) Grain growth

(d) All of the above

12. Complete the sentence: Types S, R, and B noble metal thermocouples are generally specified for use . . .

(a) . . . when temperatures exceed the upper recommended operating temperatures of base metal thermocouples.

(b) . . . after failing compliance on three SATs .

(c) . . . if the furnace only processes automotive parts.

(d) . . . to safeguard against low temperature readings in large loads.

Trivia Key

Compare your answers with the key on page 26 . How did you stack up in thermocouple knowledge? See where your skills measure up in the scale below.

Learn more about thermocouples in the interview between Doug Glenn and Eric Yeager on page 16 or check out the reference list below.

 

References

Alexander, Colleen Stroud, et al. “Application of Ribbon Burners to the Flame Treatment of Polypropylene Films.” Platinum Thermocouple - an Overview | ScienceDirect Topics, 20 June 2008, https://www.sciencedirect.com/topics/engineering/platinum-thermocouple.

“Introduction to Thermocouples.” A Perfect Alliance Between Expertise and Know-How, RDC Control, 16 Dec. 2017, https://rdccontrol.com/thermocouples/thermocouples-101/introduction-to-thermocouples/.

Nash, William, and Eric Yeager. “Industrial Heating Magazine: How Long Should My Thermocouple Last?” Cleveland Electric Laboratories, 13 Sept. 2021, https://clevelandelectriclabs.com/industrial-heating-magazine-how-long-should-my-thermocouple-last/.

REOTemp Instruments. Thermocouple, 2011, https://www.thermocoupleinfo.com/.

Staff, Editorial. “Thermocouples Green Rot Effect.” Inst Tools, 20 Nov. 2019, https://instrumentationtools.com/thermocouples-green-rot-effect/.

“Thomas Johann Seebeck.” Editors of Encyclopaedia, Encyclopaedia Britannica, Encyclopaedia Britannica, Inc., 5 Apr. 2022, https://www.britannica.com/biography/Thomas-Johann-Seebeck.

“What Are Thermocouples Used for?” Enercorp Instruments What Are Thermocouples Used for Comments, 2020, https://enercorp.com/what-are-thermocouples-used-for/.

Heat Treat Today would also like to thank the following for their expert input: Dan Herring, The Heat Treat Doctor® at The HERRING GROUP, Inc.; Hank Prusinski, Summit Aerospace Products Corp.; and Andrew Bassett, Aerospace Testing and Pyrometry.

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

 

Thermocouple Trivia by the Dozen Read More »

Termopares: Doce datos menudos

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, THERMOCOUPLES TECHNICAL CONTENT

OCLos termopares son ubicuos. Sin importar que tu experiencia en la industria sea de 20 días o 20 años, conoces bien el papel esencial que juegan en asegurar que los procesos de tratamiento térmico avancen de manera eficiente, precisa y confiable. Este breve cuestionario evaluará tu conocimiento de los termopares en una docena de datos entre obvios y triviales.

Si quisieras aportar otros datos interesantes relacionados con los termopares, nuestros editores te invitan a compartirlos para ser publicados en línea en www.heattreattoday.com. Puedes hacerlos llegar a Bethany Leone al correo bethany@heattreattoday.com

Thermocouples are ubiquitous. Whether you are 20 days or 20 years into the industry, you know the essential role they play in making sure heat treat processes are running efficiently, accurately, and dependably. This quick trivia questionnaire will test your thermocouple knowledge on a dozen either obscure or obvious facts about thermocouples.

Take the Spanish translation of this quiz in the version below, or see both the Spanish and the English translation of the quiz where it was originally published: Heat Treat Today's November 2022 Vacuum Furnace print edition.

Traducido por: Shawna Blair

Datos varios de los termopares

1. ¿Cuál es el tipo de termopar que más larga vida puede llegar a tener (aunque también es el más costoso)?

(a) Tipo K (chromel-alumel)

(b) N (nicrosil-nisil)

(c) Tipo R (platino-13% rodio)

(d) Tipo J (hierro-constantan)

 

2. ¿Cuál de estos electrodomésticos que podrías tener en casa utiliza un termopar para controlar la temperatura?

Contact us with your Reader Feedback!

(a)  El horno

(b)  La tostadora

(c)  El calentador de agua

(d) Todas las anteriores

 

3. ¿Qué debes saber a la hora de comprar termopares para tu horno de tratamiento térmico?

(a) La longitud del termopar

(b) La aplicación propuesta del proceso a realizar

(c) El tipo de termopar que mejor se adapta a la aplicación

(d) Todas las anteriores

 

4. ¿Quién fue Thomas Johann Seebeck?

(a) La persona a la que se le atribuye la teoría científica en la que se fundamentan los termopares

(b) La persona que abogó por la eliminación de los termopares en hornos

(c) Un físico alemán responsable de apoyar en el desarrollo de cohetes para los Estados Unidos

(d) Ninguna de las anteriores

 

5. ¿Cuál termopar sería el más indicado para controlar la temperatura de un tanque para temple en aceite?

(a) Tipo R (platino-13% rodio)

(b) Tipo S (platino-10% rodio)

(c) Tipo  K (chromel-alumel)

(d) Tipo J (hierro-constantan)

 

6. ¿Por qué motivo se  implementaría en un horno un dispositivo de protección contra temperatura en exceso, o ¨sobre¨ temperatura?

(a)  Para lograr un mejor control del proceso es favorable utilizar en el horno o caldera más de un termopar

(b)  Serviría para  impedir que la temperatura del horno se disparara ocasionando daños al equipo

(c) Un dispositivo obsoleto que la norma NFPA 86 ya no exige

(d)  Permitiría asegurar que el proceso que se adelante en el horno se mantenga cercano al punto de temperatura establecido

 

7. ¿Cómo se utilizan hoy en día los termopares en la industria del tratamiento térmico?

(a)  Como dispositivos de control de temperatura

(b)  Como parte de un sistema de seguridad diseñado para evitar que la temperatura del horno se dispare ocasionando que el horno se destruya

(c)  Como mecanismo que asegura que la temperatura, el parámetro más importante de un proceso de tratamiento térmico, no se salga de los límites indicados para lograr un resultado exitoso

(d) Todas las anteriores

 

8. ¿Por qué motivo se utilizaría un termopar tipo K en lugar de uno tipo N?

(a) Porque el tipo K es más exacto

(b) Porque el tipo K tiene mejores límites de temperatura

(c) Porque el tipo K es más costoso

(d) Ninguna de las anteriores

 

9. ¿Qué tipo de voltaje generan los termopares?

(a) PPM (parte por millón)

(b) EMF (fuerza electromotriz)

(c) EMP (pulso electromagnético)

(d) mV (milivoltios)

 

10. ¿Cuáles son algunas de las causas más comunes de que la calibración del termopar de un  horno o caldera de tratamiento térmico se desvíe o falle?

(a) Edad

(b) Manejo a temperaturas superiores al límite recomendado

(c) Utilización del termopar equivocado

(d) Todas las anteriores

 

11. ¿Qué problema comúnmente se observa en los termopares que fallan en el uso?

(a) Moho verde (oxidación de cromo)

(b) Metal dusting (carburización catastrófica)

(c) Crecimiento de grano

(d) Todas las anteriores

 

12. Complete la frase: Los termopares de metales nobles Tipo S, R y B por lo general se especifican para uso…

(a) . . . en casos en los que las temperaturas superan la máxima recomendada para operar los termopares de metales base.

(b) . . . luego de caer en incumplimiento en tres pruebas SAT (prueba de exactitud del sistema, por sus siglas en inglés).

(c) . . . cuando la caldera solo se usa para procesos de piezas automovilísticas.

(d) . . . para prevenir que se baje demasiado la temperatura en cargas grandes.

 

Clave de Doce datos menudos

Compara tus respuestas de la página 27 con la clave a continuación. ¿Cómo te fue en conocimiento de termopares? Califi ca tushabilidades de acuerdo a la escala que encontrarás líneas abajo.

Para aprender más acerca de los termopares, lee la entrevista entre Doug Glenn y Eric Yeager en la página 16, o revisa la lista de obras referenciadas al fi nal de esta página.

Referencias

[1] Alexander, Colleen Stroud, et al. “Application of Ribbon Burners to the Flame Treatment of Polypropylene Films.” Platinum Thermocouple - An Overview [“Aplicación de quemadores de cinta al fl ameado de película de polipropileno.” Termopar de platino - un resumen.] | ScienceDirect Topics, 20 June 2008, https://www.sciencedirect.com/topics/engineering/platinum-thermocouple.
[2] “Introduction to Thermocouples.” A Perfect Alliance Between Expertise and Know-How [¨Introducción a Termopares. Una alianza perfecta entre la experticia y el conocimiento¨], RDC Control, 16 Dec. 2017, https://rdccontrol.com/thermocouples/thermocouples-101/introduction-to-thermocouples/.
[3] Nash, William, and Eric Yeager. “Industrial Heating Magazine: How Long Should My Thermocouple Last?” [¨Revista de Calentamiento Industrial: ¿Cuánto debería durar mi termopar?¨] Cleveland Electric Laboratories, 13 Sept. 2021, https://clevelandelectriclabs.com/industrial-heating-magazine-how-long-should-my-thermocouple-last/.
[4] Staff , Editorial. “Thermocouples Green Rot Eff ect.” [¨Efecto moho verde en termopares¨] Inst Tools, 20 Nov. 2019, https://instrumentationtools.com/thermocouples-green-rot-effect/.
[5] REOTemp Instruments. Thermocouple [Termopar], 2011, https://www.thermocoupleinfo.com/.
[6] “Thomas Johann Seebeck.” Editors of Encyclopaedia, Encyclopaedia Britannica, Encyclopaedia Britannica, Inc., 5 Apr. 2022, https://www.britannica.com/biography/Thomas-Johann-Seebeck.
[7] “What Are Thermocouples Used for?” [¨¿Para qué se utilizan los termopares?¨] Enercorp Instruments What Are Thermocouples Used for Comments, 2020, https://enercorp.com/what-are-thermocouples-used-for/.
Heat Treat Today agradece la colaboración de estos expertos: Dan Herring, The Heat Treat Doctor® del HERRING GROUP, Inc.; Hank Prusinski, Summit Aerospace Products Corp.; y Andrew Bassett, Aerospace Testing and Pyrometry.

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

 

Termopares: Doce datos menudos Read More »

How Things Work: Thermocouples

/ FEATURED NEWS, MANUFACTURING HEAT TREAT, THERMOCOUPLES TECHNICAL CONTENT

OCHeat Treat Today is launching a How Things Work periodic content series. The first topic is the basics of thermocouples. Thermocouples are the bread and butter of the heat treating world. How many of the following questions are news to you? Take a deep dive into the topic and read this question and answer session between Doug Glenn, publisher and founder of Heat Treat Today, and Eric Yeager, director of Corporate Quality at Cleveland Electric Laboratories.

This Technical Tuesday discussion on thermocouple basics will be published in Heat Treat Today's November 2022 Vacuum Heat Treating digital edition.

What is a thermocouple?

Doug Glenn (DG): In this industry, and I suppose in a lot of industries, they often refer to thermocouples as T/Cs.

Let’s start off with one of the very most basic questions: What is a thermocouple?

Eric Yeager (EY): A thermocouple is a device that measures temperature. It contains no moving parts, has no power source and it does not contain any hazardous materials like liquid mercury or anything like that.

DG: Right. That’s interesting you say that, and it’s actually good that you say that, because some of our residential consumer thermometers (which a thermocouple is kind of like a thermometer in one sense) do have hazardous materials like mercury.

EY: Absolutely, absolutely.

How does a thermocouple tell temperature?

DG: So, there are no moving parts or anything of that sort. How, exactly, does a thermocouple tell the temperature?

EY: All metals that exist, when introduced to a temperature gradient (so, if you had the length of metal A and you introduce it to a temperature gradient, which would be a difference from one end to the other) will produce a microvoltage. That microvoltage is the potential that is known as the "absolute Seebeck effect" and that’s the basis on which the single thermocouple element functions.

DG: So, when you say the single thermocouple element, what do you mean by that?

Eric Yeager
Director of Corporate Quality
Cleveland Electric Laboratories

EY: That would be one leg — either your positive leg or your negative leg — or it could be any actual wire that exists, and as long as you introduce a temperature gradient, it will produce some microvoltage. With thermocouples, there are set standards for what those materials are manufactured from, but any wire will create a microvoltage or an EMF output.

DG: So, let’s say we took a copper wire from our house, and we put one end on top of a candle (just for heat’s sake); you’re saying that within the span of that wire, there is going to be a voltage of some sort.

EY: Correct. And that’s actually called the "absolute Seebeck effect" or EMF.

DG: EMF, electromotive force. And Seebeck, if I understand correctly, he was the guy that discovered this stuff, right?

EY: He’s one of them. Peltier was involved and I think a gentleman named Thompson. But it was all around the same time — they kind of all collaborated with one another.

DG: You mentioned, with a thermocouple, if you have a section of wire material, add heat to one end, there’s going to be a voltage of some sort, a millivoltage in this case, a very small voltage, but a voltage, nonetheless. But you mentioned one leg. Explain more about the one leg; because, typically, isn’t there just one piece of wire in there?

EY: Right, correct. A thermocouple consists of two dissimilar metals, two dissimilar wires. For example, in a type K thermocouple, one leg would be chromel and the other leg would be alumel, and when you join those two dissimilar metals together, the net voltage between the two combined dissimilar metals is what is used to measure the output of the thermocouple. [blockquote author="Eric Yeager" style="1"]This conversion of thermal energy to electrical energy is known as the Seebeck effect.[/blockquote]

DG: So, let’s say you stick a piece of copper wire over a candle that’s burning at 400 degrees, or whatever the candle might be burning at, you’re going to get a certain voltage across there or within the wire.

EY: Along the length of that wire, yes.

DG: So, if the temperature of that candle is twice the temperature (let’s say you double the temperature of the candle) the voltage across the length of that wire is now different, yes?

EY: It’s proportional. So, the greater amount of heat energy you apply, the greater amount of EMF will be generated.

DG: And that wire, typically, for the useful life of the wire, does not change? It’s always the same? If it’s at a 100 or 1000 °F, that voltage is one; if it’s 2000, it’s that; it doesn’t ever dissipate over time, does it?

EY: No. It only degrades when a contaminate is introduced to the material.

DG: Gotcha. Because it then prevents the flow, I assume.

EY: Correct. And it’s not as pure. So, that’s one of the effects as you see something that’s called "drift" over time, over use.

Why do dissimilar materials/metals produce a millivolt signal?

DG: Now, you said, though, that in a type K, and I know that in almost all thermocouples we’ve got two dissimilar metals. If one wire can tell you an output of the voltage, why do you need two dissimilar metals in order to get a different type of voltage?

EY: It’s basically the sum of the two materials; combine the voltage generated from the entire length of the wire of the two thermal conductors.

You have to have a signal path. You have to have a source for your voltage to start and a voltage for it to end into your instrumentation. You have to have some way to read that temperature gradient and it’s typically done with two dissimilar metals to create a greater and more stable EMF.

When a lot of the cable or wire manufacturers create, say, a melt of chromel, they test that, and actually test it against a pure platinum wire so as to return the voltage back to the instrument to measure the actual EMF for the single leg output.

How important is the joining of these dissimilar metals?

DG: Now, you talked about the joining of the two dissimilar metals. How exactly how does that need to be done? Can they be welded together, and if they’re welded together, doesn’t the metal that’s used in the weld mess it up? And does it have to be just at a point, or can it be along a length that they are joined together?

Eric Yeager
Source: LinkedIn

EY: It’s important to have the purest, most secure junction when joining the two dissimilar metals. It’s typically done by welding the metals together without adding any filler material. That’s especially important when you have something that has a very low EMF output, which is like your noble metal thermocouples. That’s where purity is essential. Loose connections from twisted or crimped junctions also might cause intermittencies under thermal expansion and affect the thermocouple output signal.

DG: So, typically, they are welded together without a filler; they’re just welded together.

EY: Correct. You just bring a TIG torch in, give it a quick zap, and it melts the two wires together. Once you get that nice little joint or junction, you can run and complete the assembly.

DG: Okay. We already talked about why there are different millivolt readings at different temperatures, because basically it’s the difference in the heat.

EY: Correct. As the temperature increases, there’s a direct correlation to the microvoltage output from that particular wire or wire pair.

DG: And I asked about how important are the joining of these materials. Typically, you don’t want it over a wide section, right? Does it matter if it’s a spot weld, instead? What would happen if you had one that was an inch or two inches long? Is that a big deal?

EY: It’s best to keep it as small and concise as possible, because it could form a heat sink later on when you’re in application; typically you just want a small nice round junction. For example, you want the junction to be about twice the diameter of the single thermal element. So, if it was a 20 thousandths-diameter wire, you want it 40 thousandths in diameter.

Thermocouples welded to a workload; wouldn’t that weld introduce some “interference” in the millivolt signal?

DG:  Aren’t some T/Cs welded? I think I’ve heard that sometimes they’ll take thermocouple wire that will be joined and then welded to, or in some way applied right to, a load. If you were applying it directly to a workload, wouldn't that extra metal kind of mess up the millivolt?

EY: You would think so, but as long as they’re kept as close as possible, and the workpiece that you’re welding to is kept isothermal or actually uniform in temperature between the two welded junctions, it won’t have a detrimental effect on the thermoelectric output. [blocktext align="right"]But you want to make sure that the workpiece is uniform in temperature because you have a temperature gradient across where those two junctions are welded to the material, and it can have a slight effect.[/blocktext] That’s essential to basically ensure that your workpiece is isothermal.

DG: What do you mean by isothermal?

EY: Uniform in temperature across the entire workpiece between the welded beads. The workpiece will become the welded bead, but it won’t create any additional EMF output to the combination because it’s the combination of the length — it measures the temperature across the entire length of the wire not necessarily at the bead.

It’s kind of a common misconception that the bead creates all the EMF, but it’s actually along the length of the wire.

DG: It is along the length of the wire. I always thought that the temperature was measured basically at the bead, at the joint.

EY: Well, that’s where it starts, but it’s combined along the length of the wire.

In the heat treating world, what is the most popular T/C and what are the materials from which it is made?

DG: So, in the heat treat world, what’s the most popular T/C and what are the materials it’s made from?

EY: I would say it’s definitely the type K and those two materials are chromel and alumel as we previously discussed. It’s probably the most popular due to the low cost and the wide temperature range capability. Basically, you can go from 32°F all the way up to 2450°F. It won’t last very long at those temperatures, but it’s the most common and the most versatile. I would say type K is the most popular.

How long do type K thermocouples last in a furnace/application?

DG: The factors: you were talking about them not lasting all that long. This is probably a loaded question, but if you’re in an average heat treat application, what’s a typical lifespan of a type K?

EY: To be honest with you, that’s the question that everybody wants to know. And truthfully, it depends on the application. It depends on thermal cycling, it depends on how well the thermocouple thermoelements are protected from the environment, for example, whatever protection tube you put it in, if it’s an MGO, or an exposed bead. All of those things are contributing factors. Really, it’s very, very application dependent. For example, I’ve seen type K control thermocouples last for 5 years but that’s basically at a stable temperature without any thermocycling and a constant, nice, clean environment. But I’ve seen units that get consumed rapidly at the elevated temperatures, like I mentioned, 2450°F. They don’t last very long there but they do measure.

DG: So, the undesirable conditions for those things would be a lot of thermocycling up and down, so, it’s going to fail faster, I assume?

EY: Correct. And temperature of course: the higher temperature, the greater degradation in the material. That pretty much stands for any thermocouple type.

DG: I want to ask a couple questions that aren’t on here just because I’m curious about this. A lot of times, you’ll have the spot weld where you put them together, that’s called the bead?

EY: Yes. Or junction. Either/or.

DG: So, the bead or the junction — that’s obviously bare wire, right? Assuming we’re actually using to put it on a workpiece. You’ve got the bead and then you’ve got, obviously, a little bit of bare wire at least. Is the rest of that wire covered or is it often not covered?

EY: It must be covered because it could short somewhere along the length of the wire. It could be either a soft wire insulation, like a ceramic fiber or a REFRASIL® or even a fiberglass-type insulation depending upon the temperatures.[blockquote author="Eric Yeager" style="1"]What I actually prefer is an MGO-style thermocouple where it has a metallic outer sheath surrounded by a magnesium oxide insulator that prevents it from shorting out.[/blockquote] So, for example, if you just ran straight wire and had any kind of airflow or thermal expansion, it could short out somewhere along the length of the wire. Basically, a thermocouple will measure from the closest measuring junction to the instrumentation. Therefore, if it’s shorted out, you’d get a false reading.

DG: So, if you had it attached to the load and it runs over here but it touches something else just before it goes out to the outside of the furnace or whatever, you’re going to measure that spot closest to the temperature wall, so it doesn’t give you anything on the load.

EY: What’s very common is people will run the software thermocouples through a door of a furnace where it closes on the door, that’s where it shorts out.

What are some of the factors that will affect the longevity of a T/C? What is the most common cause of failure?

DG: What are the most common causes of failure? Did you have any others besides that we just talked about the door one?

EY: For control thermocouples, like your type R, S, or B, those are subject to contaminates more than the other types. They’re more susceptible to carbon, graphite, silica, and those type of things. So, when you have an assembly like that, like a control thermocouple in a furnace, you have to ensure that it’s properly protected from the environment to which it’s exposed to allow it to have the greatest longevity. There are different sheath materials that you can put the thermocouples in: alumina, it could be silicon carbide tubes, all kinds of different varieties.

DG: You want to keep the environment, the atmosphere out of it and all that good stuff.

EY: Real quick, Doug: You mentioned control thermocouples. If you had like a type R or S control thermocouple and it was exposed to something that was going to contaminate it, what typically happens when a thermocouple fails? The EMF output of the thermocouple is degraded. What that would actually cause is it would cause your furnace to call for more heat because the EMF was degraded. Even if it’s a few degrees, that might cause an overtemp condition when you have very tight requirements on a thermal process.

DG: Right. And then, hopefully, your overtemp thermocouple would kick in and say, “Wait a minute!”

EY: Yes, that’s exactly right. Hopefully, you don’t have it set too high.

How can you tell when your T/C is going bad? Drift, etc.?

DG: How can you tell when your T/C is going bad and could you talk about drift?

EY: The best way to determine if your thermocouple is going bad is to perform regular system accuracy tests. Those tests, will allow you to track the lifecycle of the thermocouples and determine when they begin to drift and when it’s time to remove them from service. Unfortunately, when a thermocouple drifts, there is not adjustment knob on it; you can’t fix it. Once it starts going, it goes, and you just have to replace the assembly.

When thermocouples drift, they typically drift negative. They will see less of a temperature due to the contaminates getting into the material and altering the EMF output of the thermocouple. So, your control will essentially ask for more heat, and that’s where you end up having the problem. That’s why it’s essential to perform your SATs and maybe set up a little PM schedule for your system to know that you're experiencing "x" many life cycles out of the thermocouples before they fall out of your requirements, and so maybe every "x" months you have to replace the assemblies and install new ones.

Because of the drift, the best thing you can do is perform a system accuracy test with a thermocouple that has not been subject to long exposure at temperature.

Dissimilar metals and EMF?

DG: I want to go back to the two-wire thing because I don’t quite understand that. I’m not an engineer guy so see if you can explain. You’ve got the one wire that has an EMF in it, but I still don’t quite get why we use dissimilar metals to create the EMF.

EY: The summation of the voltage between the two thermocouples that provides the set EMF. The set EMF, is determined by the international temperature scale ITS-90 scale; that sets all the microvoltages for the thermocouples. It’s designed as a paired thermocouple group not as a single element. With a single element, you really would not have a good way to return the signal to your instrument.

Both wires conduct the voltage back to the instrument; one is a positive and one is a negative. Since it is a direct current (DC) voltage, one leg provides the negative path and one leg provides the positive path.

DG: Ok, so there’s a millivoltage signal being sent back to the instrument, which is reading that millivolt and then converting it based on what type of thermocouple is out there; and it’s recording that reading and turning it into a temperature.

 

About our expert:

Eric Yeager is the director of Corporate Quality at Cleveland Electric Laboratories. He's been with Cleveland Electric Labs for 17 years and is working on year 18. In that time, he has been director of quality and runs their accredited thermocouple calibration laboratory. Eric is involved with ASTM and is a subcommittee chairman for E2011, which is the calibration section of the thermocouple standards. He also was technical consultant on some of the rewrite of the latest AMS2750.

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

 

How Things Work: Thermocouples Read More »