How to Meet the Challenges of the AMS2750E Thermocouple Calibration

Originally posted by Herb Dwyer

October 2018 

Introduction: The AMS2750E is undergoing a revision, but in the meantime, our members have to operate within these requirements of the above existing Aerospace Material Specification. On top of that, the need to sinter and anneal above the maximum temperature of 1250C up to and including 1650C along with the pressure and vacuum related parameters and the atmospheres, is leading to confusion in how to meet these new requirements. The traditional Type K work horse is becoming limited by its maximum working temperature and the related life cycle impact of the higher temperatures. 

Discussion of the key AMS2750E requirements and new material challenges that don’t fit: 

1. The Spec allows calibration of both the loaded and unloaded furnace thermal characteristics. A test by Nanmac in conjunction with a large OEM furnace manufacturer indicated that the load volume and the type of atmosphere will adversely impact the accuracy of the temperatures being measured across the furnace. Even in the unloaded condition these furnace conditions have an adverse impact. 

2. The SAT (System Accuracy Test) and the TUS (Temperature Uniformity Survey) thermocouples along with the operational thermocouples are critical components of the overall quality of the resulting components produced by the supplier. 

3. Assurance of these key elements in the quality process by calibration is not only the key to better product but reduced failure modes in the field. 

4. One of the key factors in the existing AMS2750E is the maximum temperature limit of use for the Type K primarily because its alloys can change over use at the upper end of its temperature limit. 

5. The need to calibrate over the actual temperature ranges that the Type K is used in also limits the potential to improve sintering and annealing processes to address the new material systems that are being used like metal/ceramic matrices. These for example, are being sintered at 1650C. In this instance, the Type K is replaced by other TCs that are capable in this higher temperature range like Type C, Type S, Type B. Some of these are quite expensive as compared to the Type K. Their accuracies can be better than the Type K and in some instances like Type C not as good. 

6. When you need to sinter or anneal above the Type K limits how do you meet that challenge?We have seen process temperatures of 1150C, 1250C, 1450C and up to 1650C.While still a small part of the total market this segment is growing rapidly and offers the MTI members a new avenue of sales and profit growth. 

7. New thermocouple Types like Type B, Type C, Type S and Type R can meet these ranges but they can, in some cases, be costly. How do you address that challenge? 

Using a listed calibration lab per ISO17025 can help you to cover the range of 0 to 1700C, new TC types and offer you a competitive edge in todays changing market. 

1. Presently most calibration facilities in-house and outsourced readily calibrate to 1150C the “normal“ maximum limit on Type Ks and similar Types. 

2. Anticipating the higher limits and requirement for traceability to NIST or similar National Labs, many ISO17025:2015 Labs have added specialized furnaces and more sensitive Data Loggers to offer directly traceable calibrations up to 1700C. 

3. One unexpected outcome is that you can calibrate Type C thermocouples over the range of 300C to 1700 which allows the MTI members a wider range of performance and an alternative to the Type K. 

4. For example, the accuracy of the Type K is normally 0.75% in its temperature range of 0C to 1200C. Per AMS2750E the maximum continued use temperature is limited to 1000C. By calibration, the Type C can demonstrate an accuracy of 0.4% over the higher range offering the MTI members an alternative. Of course, these same labs also calibrate the Type K and others as well; thus offering you a one stop ISO17025 certified Lab that can support your ever moving performance requirements. 

5. One thing that the AMS2750E spec does NOT allow is the extrapolation of calibration data; the best lab(s) uses DIRECT COMPARISON all the way to 1700C thus reducing uncertainty over the range of 0 to 1700C. 

What does this mean to you? 

1. You now have options that allow you to meet the intent of the AMS2750E, enable you to have a quality process that can earn you more business and provides assurance to your customers that your products meet or exceed their requirements of these new challenging applications; 

2. These types of certified laboratories can support both field and in-house calibration facilities and services thus providing you the assurance of quality and potentially reduced costs of surveys. 

A quick story: 

A typical user of both Type K and Type C thermocouples in a sintering application for turbine blades, needed to process these above 1150C and up to 1650C. In this instance, the type K could not be used, an alternative, Type B, had a limited range (750C to 1850C) and did not address the debinding temperatures in the lower ranges (approx. 300C), the Type S had the range and the inherent accuracy but was a very expensive alternative. The user, through research, identified the Type C as a thermocouple that would cover the range of 300C (within the debinding range) and up to 2300C, well over the maximum temperature of 1650C. By using direct comparison calibration over the range of 300 to 1700C they were able to demonstrate accuracy of 0.4 to 0.5% which was better than the Type K and provided more range. Problem solved, happy user, profitable innovation. 

Written by:  Herbert E Dwyer, COO/CTO Nanmac