AMS 2750F pyrometric requirements for heat treatments: what’s the comparative SAT

In the previous article we introduced the the AMS 2750 revision F news (Aerospace Material Specifications), the application of which is necessary for NADCAP (National Aerospace and Defense Contractors Accreditation Program) accreditation.
We also analyzed the limitations introduced on the alternate SAT and the conditions under which it is necessary to replace it with the comparative SAT.

What is a comparative SAT?

The comparative SAT (System Accuracy Test) is a comparison, made on the spot, of the difference between the "Not correct" reading of the sensor system of the thermal plant under test (sensor + extension cable + instrument) and the "Correct" reading of test sensor system (test sensor + extension cable + field test tool) after applying the correction factors of the sensor and the test tool.

The comparative SAT also includes the Resident SAT.

The temperature displayed on the plant control and recording systems must be compared with the correct temperature indication of the sensor and the test tool.
The sensor under test of the system under test must include corrections/modifications (offsets), appropriate correction factors, if used in production.
The test can be performed at any temperature within the qualified operating temperature ranges.

For vacuum furnaces with multiple qualified operating temperature ranges, a periodic SAT must be performed in each interval at least once a year.

Operational procedures of the comparative SAT

You start by placing the hot junction (terminal/measuring joint) of the SAT sensor as close as possible to the hot junction (terminal/measuring joint) of the system sensor of the plant under test (control/recording systems).
The distance between the two hot joints must not exceed 76 mm.

In order to reduce the impacts on the production process and improve the repeatability of the control, it is strongly recommended to equip the system with a permanent SAT port, arranged in such a way as to reach the hot junction of the sensor under test from outside the system.
This allows, in addition to performing the comparative SAT test during normal production, also to intervene in case of anomalies during the thermal production process.

In the case of atmosphere systems or vacuum systems the SAT port must be sealed.
The depth of insertion of the SAT sensor must be documented and available to the technician performing the pyrometric test.
All subsequent SAT tests must use the same position/insertion depth used during the initial SAT test.

It is preferable to use integrated sheath solutions, where the process sensors and the SAT port are inserted inside the sheath itself.

In the case of comparative SAT on charge thermocouples, in place of the alternate SAT, the same positioning rules previously illustrated apply (distance between the two hot joints must not exceed 76 mm). Therefore it is necessary to set up the SAT test during production by placing the SAT sensor near the charge sensor.
In the case of use of heat sinks, a double hole must be prepared to house both the charge sensor and the SAT sensor.

The SAT sensor can be "Temporarily" inserted in the SAT port to perform the comparative SAT or it can be a "Resident” SAT sensor subject to the following restrictions:

Resident SAT sensors must be limited to type B, R, S or N sensors for tests performed above 260 °C, and must be “Non Expendable” for temperatures above 538 °C. On the other hand, there are no limitations on temperatures below or equal to 260 °C.
The resident SAT sensor type must be different from that of the sensor under test, as defined in the combinations shown in the table below.

Resisdent SAT sensor	Control sensor or process recording on the plant
Resisdent SAT sensor	B	R	S	N	All other types of sensors
B	No	Yes	Yes	Yes	Yes
R	Yes	No	No	Yes	Yes
S	Yes	No	No	Yes	Yes
N	Yes	Yes	Yes	No	Yes

Table of allowed combinations Resident SAT sensor/Process sensor on board of the plant under test.

The resident SAT sensor must be fixed in its position so as to prevent displacement with respect to the sensor under test.
The position of the sensor must be documented and verified, both during installation and during its replacement.
Alternatively, the resident SAT sensor can be positioned independently from the sensor under test and its position must be verified prior to each test to ensure the position/insertion depth used during the initial SAT test.

It is allowed to use the overtemperature sensor as a resident sensor, provided that it is used for overtemperature and meets the requirements of a SAT sensor as described above.

Finally, the Comparative SAT test must be performed initially and subsequently with the frequency required by tables 14 (parts) or 15 (raw) of the AMS 2750F.
Attention must be paid to the use of the right table, in a few and rare cases table 15 is applicable.
For systems that have a documented "out of service" period, the SAT must be performed on all applicable systems and, only if they are compliant, they can be put back into service.