In the previous article we introduced the
the AMS 2750 revision F news (Aerospace
Material Specifications), the application of which is necessary for
Aerospace and Defense Contractors Accreditation Program) accreditation.
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
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
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.
or process recording on the plant
|All other types of sensors
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
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.
The SAT test allows us to measure the maintenance of the qualification performance
of our plant and consequently the operational continuity.
In the next article we will illustrate the changes of the AMS 2750 revision F forthe plant equipment.
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