The three-part series on the subject of thermocouples thus far has discussed how thermocouples work, how they are classified, and the requirements for calibration. Also reviewed were the requirements for calibration frequency and accuracy. This third and final article will address recalibration and reuse restrictions.
Thermocouples are the basis of the thermal processing industry. The accuracy and control required for aerospace and medical thermal processing would not be possible if it were not for the relatively simple way in which thermocouples provide temperature information. AMS2750 establishes the specific requirements for controlling thermocouple usage which the Process Owner must understand and the Quality System must address.
Of course, reading a short discussion is not adequate preparation. The eQuaLearn
Introduction to Pyrometry course is an essential start, however, there are also customer specifications which may be different from AMS2750E. As you read below, please be aware that there are many special cases. It is important to always review AMS2750E when making decisions about your own usage.
Turning to Figure 1 on page 13 of AMS2750E, we can see a few patterns that occur. (Remember that Figure 1 groups thermocouples a bit differently than Table 1. It addresses TUS thermocouples, two types of SAT thermocouples (resident and non-resident) and load thermocouples. Resident thermocouples were discussed in the previous article.) Those patterns include:
- Noble metal thermocouples, expendable or non-expendable, in all the applications can ALWAYS be recalibrated (at a maximum period of 6 months).
- Base metal expendable thermocouples in all these applications can NEVER be recalibrated.
- Base metal non-expendable requirements are what we previously saw in Table 1.
Now the question arises and is answered by Figure 1. “Does it simply mean that I can use a base metal thermocouple in applications where recalibration is not allowed for 3 months then throw it away?” Again, if only it were that simple.
There are additional twists (the U formula for SAT and TUS thermocouples, the exceptions for certain TUS thermocouples (that actually allow longer life)), and detailed requirements for load thermocouples. Compliance with these usage restrictions and objective evidence of that compliance is another area of opportunity for audit findings.
The U formula simply sets a limit of 30 on the number of “uses” of expendable base metal SAT and TUS thermocouples. If the temperature is below 1200°F (650°C), a thermal cycle counts as one use. If the temperature is 1200 to 1800°F (650 to 980°C), each thermal cycle counts as 2 uses. Only a single use is permitted above 1800°F (980°C). Documentation of the usage history of each thermocouple is essential to establish compliance.
There is an exception for certain SAT thermocouples. If the specific requirements of that exception are met, including usage only below 1200°F (650°C), the life can be extended to 3 years or 90 uses, whichever comes first.
Load thermocouple usage limits for base metal thermocouples also involve temperature and number of uses, as well as calendar time. Noble metal load thermocouples again can be recalibrated every 6 months (maximum).
Expendable base metal load thermocouples are limited to 90 days from first use or 30 uses if used at or below a set point of 1200°F (650°C). They are allowed only a single use if used above 1200°F (650°C). Remember, they CANNOT be recalibrated.
Non-expendable base metal load thermocouples have more detailed requirements. However, they also cannot be recalibrated. They are restricted to a single use at 2300°F (1260°C) or above. At lower temperatures, the calendar time restriction remains at 90 days, but the number of uses depends on the
maximum temperature of use. Once a thermocouple is used at a higher temperature, the number is restricted to the total number of allowed uses at that higher range even if later usage is at a lower temperature.
For example, a non-expendable base metal load thermocouple used only below 1200°F (650°C) could be used 270 times. If it were first used at 2200°F (1205°C), it could only be used a total of 10 times even if the remaining usages were back to 1200°F (650°C). A thermocouple used first at 1850°F (1010°C) could be used 90 times at that or any lower range; but once used 10 times, it could not be used at 2200°F (1205°C) or above because that would violate the usage limit at the higher temperature. Admittedly this is not easy to describe in text, but looking at AMS2750E and the examples following 3.1.5.3 helps. Remember the phrase “at this or any lower temperature range” should be understood at each line of the requirement. This requires attention to assure compliance and detailed record keeping for the usage of each thermocouple to document that compliance.
Some find it easier to simply restrict all base metal load thermocouples to a single use. This eliminates the risk of miscounting, incorrect assignment of temperature restrictions or gaps in the record. Some find that the economics of replacement attractive versus the record keeping and audit finding risk. Remember, this still requires objective evidence that single usage is controlled. Do not overlook the requirement for a SAT whenever a thermocouple is replaced (or compliance with the Alternate SAT procedures).
It is important to remember the requirements on reuse and recalibration involving condition. There must be a process to inspect for the condition of the measuring junction and of the insulation. If there is damage, salvage is possible. Repair must involve removing the damage, remaking the junction and recalibration (or use of existing roll calibration). Any salvage does not zero out prior usage. For example, if 30 usages are allowed and salvage occurs after 20, only 10 usages remain. Salvage does not reset the life.
I recommend highlighting your specific applications in Table 1 and Figure 1. Base or noble, expendable or non-expendable, specific usage as well as temperature form the basis of determining calibration/replacement frequency, whether recalibration is allowed, and how many uses are permitted if recalibration is not allowed. Highlighting lets you concentrate on your specific applications without trying to keep in mind all the other possibilities. You can then take a hard look at compliance and how you maintain objective evidence of that compliance.
However, it is a good exercise to occasionally look at what alternative approaches could be taken. Most companies made their decisions long ago as to what working or load or SAT or TUS thermocouples made the best choices. Admittedly, changing those choices will often meet resistance. An objective look might show economic advantages of a different choice, once you really understand the AMS2750E requirements, along with economic considerations and customer requirements. Do not overlook what might be called “audit-proofing”. If a particular choice makes compliance easier and minimizes the need for documentation of objective evidence, some additional procurement cost might be justified.
In summary, AMS2750E is the source of basic knowledge for control of thermocouples. It has many requirements, special cases, and exceptions. AMS2750E commands careful attention to requirements and detail record keeping to assure compliance. This three-part series of articles have provided a quick overview of the basics. If you are a heat treat process owner, planner or operator or involved in quality oversight of heat treatment, I strongly recommend the eQuaLearn
Introduction to Pyrometry course for a more detailed discussion of this subject. The course also covers the other major subjects of Pyrometry – Instrumentation, Equipment Classification, System Accuracy Testing and Temperature Uniformity Surveys. Upcoming
Introduction to Pyrometry training locations include Hartford, CT; Wichita, KS; Sheffield, UK; and Troy, MI. Please visit our website
www.eQuaLearn.com or email
eQuaLearn@p-r-i.org for a complete list of all upcoming eQuaLearn course dates and locations.
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. 
