AMS 2750 pyrometric requirements for heat treatments: what's new in revision F

AMS 2750 (Aerospace Material Specifications) is an aerospace international specification which covers the pyrometric requirements for plants used for the heat treatment of metallic materials. Specifically, it covers temperature sensors, instrumentation, plant equipment, correction factors and offsets of instruments, system accuracy testing and temperature uniformity testing. These are necessary to ensure heat treatment of parts or raw materials in accordance with applicable specifications.

The AMS 2750 is now in revision F, was revision E from July 2012.

• The entire document has been reformatted in line with the current AMS, the tables and figures are embedded in the text where they are referenced.
• No edit bars: each section has been reworded or renumbered.
• Compared to revision E, the document is 25% longer, now it has 25 tables instead of 11.
• Big news, the degrees show the decimal: it was +/- 6 °C, now +/- 6.0 °C.
• Most of the changes are textual in order to provide greater clarity with additions, explanations and details through tables and figures.
• Accuracy and requirements are now specified to the first decimal place (e.g. 500.0 °C instead of 500 °C, 1.0 °C instead of 1 °C).

Some changes are immediate, others have a 2-year transition.
For program participants NADCAP (National Aerospace and Defense Contractors Accreditation Program), the application of revision F will be with the publication and application of checklist AC7102/8 in rev. A (forecast: January 2021).

Alternate SAT

One of the most important changes, and with potential impact, is when the load sensor is used during production. The load sensor is typically used in vacuum furnaces with type B instrumentation according to AMS 2750.

The change is on the limitations entered for the Alternate SAT, typically carried out on the load sensors of vacuum furnaces.
SAT stands for the System Accuracy Test.
It is an evaluation of the combined errors of the temperature sensor, extension cable (and connectors) and instrument to ensure compliance with the requirements of the entire pyrometric chain.
More precisely it is the mathematical calculation of the sum of the errors of the furnace sensor (load sensor) and the error of the connector, the extension cable and the channel of the logging instrument.

This test is very simple and has no impact on production, it is possible to perform it during the calibration of the process instruments and detect the system error, from the temperature sensor connector to the logger channel, and add it to that of the load sensor at each sensor change.
In this way with a simple mathematical calculation the SAT difference can be obtained and the conformity of the whole system under test can be defined.

AMS2750F in paragraph 3.4.8.1 requires:

The alternate SAT applies to load sensors used only once (single use).

Therefore the Alternate SAT can be performed on the load sensors used only once (single use).
This obviously penalizes a lot in terms of costs the users who otherwise could use the load sensors for up to 3 months.

This new requirement generated in-depth discussion within the NADCAP PRI (Performance Review Institute) team, in the past few weeks.
It was therefore issued on 14 October 2020 the Auditor Advisory N. HT–20–010 confirming the new requirement of AMS 2750:

Alternate SAT applies to load sensors ONLY when load sensors are used once (single use). When load sensors are used multiple times, the Alternate SAT shall not be used.

Auditees are reminded that they can use any of the three SAT methods defines in AMS2750 Rev. F paragraphs 3.4.7, 3.4.8 or 3.4.9 interchangeably, provided the sensor system meets the requirements of the method implemented.

Therefore, the Alternate SAT MUST be replaced by the Comparative SAT, where load sensors are not replaced with each use.
Obviously the comparative SAT has an impact on the management of the test during production.
This is because during production, at the requested deadline, it is necessary to give instructions to the production which, in coordination with the pyrometry technician, will have to position a SAT sensor within 76 mm from the load sensor and in the same production conditions that will be used for the SAT test, during implant stabilization.

All this is very complex in management and time.

AERONDI, which operates at the Aerospace industries for pyrometry, has prepared, in order to make the task of production less burdensome, heat sink (normally used to house the load sensor, representative of the production) with double hole in order to guarantee the correct positioning of the SAT sensor during the test and at the same time have a better repeatability of the test.

Finally, with the application of AMS 2750 rev. F. the Alternate SAT MUST be replaced by the Comparative SAT, when the number of uses of the load sensors is greater than one.

What is a comparative SAT?
This comparative SAT will be covered in the next article.