In the eQuaLearn two-day Introduction to Pyrometry course, the entire morning of the first day is dedicated to discussing thermocouples. While there are some limited alternatives, such as Resistance Temperature Detectors (RTD’s) and even optical pyrometers, thermocouples are truly the basis of the thermal processing industry. If it were not for the relatively simple way in which thermocouples provide temperature information, the accuracy and control required for aerospace and medical equipment thermal processing would not be possible.
The AMS2750 specification establishes the requirements for controlling thermocouple usage. There are very specific (some might say complicated) requirements for calibration procedures and frequencies, accuracy of readings, and limits on usage before recalibration or replacement is required. Additionally, there are restrictions specifying when a thermocouple cannot be recalibrated and must be discarded after a specified period of time or number of uses. Those restrictions may also depend on temperature.
With numerous specific requirements, there are multiple opportunities for audit findings. It is necessary that the Process Owner and those involved in the Quality System responsible for thermal processing have a thorough understanding of these requirements and how compliance is maintained and documented. These three articles discuss many of these subjects. Of course, reading a short discussion is not adequate preparation. The Introduction to Pyrometry course is an essential start, but still requires the responsible parties to prepare and understand the requirements and particularly how they relate to the thermocouples in use at the specific organization.
It is important to remember that what is discussed in this article are the requirements of the AMS2750E specification as developed in the eQuaLearn training material and interpreted through my 20 plus years of exposure to Nadcap and AMS2750. I often joke that I should have a sign behind me during the entire course that says “In the Absence of Other Specific Customer Requirements”. It is crucial to be aware of all the customer specifications that may be different from AMS2750. Also, as you read below, please be aware that I am discussing the basic requirements. There are many special cases, so it is important to review AMS2750 when making decisions about your own usage.
So, first some basics.
Thermocouples work because of a property of metals called the thermo-electric effect or Seebeck effect. The electrons in a metal are shared between atoms and are free to move from one atom to another. At higher temperatures, electrons have more energy. So, a wire that is hotter at one end than the other will have an energy difference between the two ends. This results in a voltage difference and in a closed circuit, a current flow. If we take two different metal wires, there will be a difference in that voltage potential between the two wires which we can measure. We can then convert that measurement into a temperature difference between the hot end and the cold one. If we know the temperature at the cold end, we now know the temperature where the wires join.
Thermocouples are classified based on the chemical composition of the two wires. Noble metal thermocouples (commonly Types B, R or S) have wires containing platinum or platinum-rhodium. Base metal thermocouples (commonly Types E, J, K, N and T) do not contain platinum or platinum-rhodium. Noble metal thermocouples are more resistant to exposure to temperatures and atmospheres, but are also significantly more expensive.
Thermocouples are further classified based on the insulation that protects the wires from contact with each other as well as from the atmosphere. The two wires can only be allowed to touch at the measurement tip, otherwise, a false reading will result. Expendable thermocouples are those where each wire is individually wrapped with an insulator and the pair of wires then over-wrapped. Non-expendable thermocouples are those of any other configuration. Expendable thermocouples are usually flexible, procured, and calibrated based on rolls and cut to length. This is helpful guidance, but Nadcap documentation is very specific in that the distinction defining an expendable thermocouple is the individual wire wrap and then over-wrap.
The last factor which is needed to generate requirements for calibration frequency and accuracy and for usage limits is the application. There are requirements for “standard” thermocouples used for calibration, but I will skip over those since most of us do not do our own thermocouple calibrations and look at the others – test thermocouples, working (or system) thermocouples and load thermocouples.
Test thermocouples are those which are used for System Accuracy Tests and for Temperature Uniformity Surveys. Working (or system) thermocouples are those which provide inputs to the controlling, monitoring and recording instruments on the thermal processing equipment. Load thermocouples are those in contact with the actual parts or raw material being processed to provide actual “metal temperature” rather than that of the furnace atmosphere.
Now that we have established these factors (base or noble, expendable or non-expendable, usage), we can look at Table 1 of AMS2750E which gives requirements in terms of frequency of calibration and the required accuracy. I always recommend highlighting your specific usages in Table 1. That makes it easier to concentrate on compliance with your specific requirements without getting caught up in the complications of other situations.
At this point, it is important to mention the very specific calibration requirements applicable to all thermocouples. This is another area where auditors typically identify issues either with compliance or with objective evidence of compliance. Calibration means comparing the thermocouples being checked against known standard thermocouple. The result is a table of “correction factors” which tell the difference between what the thermocouple being tested and the “true” temperature. That difference is called the “error” and the opposite sign value is called the “correction factor”. Adding the value of the correction factor to the measured reading gives the “true temperature”.
AMS2750E requires the use of these correction factors during System Accuracy Testing and Temperature Uniformity Surveys. Some customers require their use in other applications (for example, load thermocouples) and some quality systems elect to use them for many or all applications.
Whether correction factors are being used in practice or not, the thermocouple certifications must be retained and available for auditing. It must show that the thermocouples meet the requirements for accuracy. In addition, (except in the special case of “Fixed Point’ calibration), the temperature points can be no more than 250°F (140°C) apart. The correction factors must cover the entire range over which they are to be used. It would be a clear audit finding if a thermocouple were found to be used at a set point lower than the minimum calibration temperature or higher than the maximum calibration temperature.
Furthermore, AMS2750E is very specific in regards to the content of these calibration reports, as detailed in paragraph 188.8.131.52. It is imperative to ensure your quality system includes review of calibration certificates for compliance of temperatures, accuracy, and other required data. Likewise, ensure that the certificate identifies the standards being used and provides traceability back to the National Institute of Standards and Technology (or appropriate national standards body). And remember that there must be objective evidence that these reviews are being performed.
The second article of this three-part series will discuss the requirements of AMS2750E for calibration frequency and accuracy. The final article will cover recalibration and reuse restrictions.
A complete overview of Nadcap requirements related to Pyrometry is covered in the eQuaLearn Introduction to Pyrometry course. Upcoming Introduction to Pyrometry training locations include Dusseldorf, GER, Wichita, KS, Burlington, CN, and Bordeaux, FRA. For additional information on this training or a complete list of upcoming eQuaLearn courses and locations, please visit www.equalearn.com or email eQuaLearn@p-r-i.org.
Author: Tom Murphy served as the United Technologies member of the Nadcap Heat Treat Task Group from 1996 to 2012 and has been an eQuaLearn instructor since 2012.
Disclaimer: The views and opinions expressed in this article are those of the author and do not necessarily reflect the position or views of the Performance Review Institute or any of its employees or programs.