FEATURE | Process Control & Instrumentation Expendable vs. Nonexpendable Thermocouples: A Detailed Look at AMS 2750D John Popovich and Dale T. Praznik – Furnace Parts LLC; Cleveland, Ohio When it comes to thermal processing, one thing is certain: AMS 2750 rules the world of pyrometry. Since 1980, the date that SAE Aerospace fi rst introduced its governing specifi cation for commercial and captive heat treaters alike, AMS 2750 has become the governing standard for all pyrometric processes connected with aerospace. However, in the 31 years that we have lived with the specifi cation its reach has expanded greatly. Thermocouples and their application are undoubtedly a large part of the most current revision – AMS 2750D. Fig. 3. Beaded nonexpendable thermocouple I n order to understand how tempera-ture sensors are applied in AMS 2750D, it is critical to understand their various uses. According to the specifi fi cation, there are four typical uses for temperature sensors: control sensors, load sensors, system accuracy test (SAT) sen-sors and temperature uniformity sensors. Temperature-Sensor Uses Control sensors, as defi ned in AMS 2750D (8.2.9), are any sensors “connect-ed to the furnace temperature control-ler, which may or may not be recording.” They are installed within, or as close as possible to, the work zone of thermal-pro-cessing equipment. In conjunction with control instrumentation, they control process temperature. Load sensors (8.2.25), on the other hand, are sensors “attached to the pro-duction material or a representation of production material that supply tempera-ture data of the production material to process instrumentation.” They may also be used as control sensors, but they shall not be used to monitor or record any tem-perature above the maximum processing temperature. System accuracy test (SAT) sensors (8.2.60) are “calibrated and traceable … with known deviations, if any, used in system accuracy tests.” System accuracy tests are performed at scheduled intervals, as defi ned by AMS 2750D, to ensure that values being recorded by the process sen-sors are accurate (within acceptable lim-its) to the calibrated SAT sensor in each thermal-processing zone. SAT sensors may be resident, remain-ing in place between tests (only Type N, R or S thermocouples are allowable) and must be of a different type than the sen-sor being tested. Conversely, an SAT sen-sor may also be used as a probe check, or nonresident sensor, temporarily inserted as close as possible to the process ther-mocouple tip (not to exceed 3 inches be-tween sensors) and can be of like type to the process sensor if desired. Finally, temperature uniformity sen-sors (8.2.64) are also “calibrated and traceable … with known deviations that are used for conducting temperature uniformity surveys.” A temperature uni-formity survey (TUS) is a test measur-ing temperature variation in the defi ned Fig. 1. Number of TUS sensors required for batch furnaces, salt baths, controlled-temperature liquid baths, fl uidized-bed furnaces or con-tinuous furnaces tested using the volumetric method Workspace volume less than Number of sensors (1) Classes 1 and 2 (formerly class 1) Number of Classes 3 thru 6 (formerly class 2) Ft 3 per sensor, classes 1 and 2 Ft 3 per sensor, classes 3, 4, 5 & 6 sensors (1) 3 ft 3 (0.085 m 3 ) 5 5 < 1 < 1 225 ft 3 (6.4 m 3 ) 9 9 25 25 300 ft 3 (8.5 m 3 ) 14 12 21 25 400 ft 3 (11 m 3 ) 16 13 25 31 600 ft 3 (17m 3 ) 19 14 32 43 800 ft 3 (23 m 3 ) 21 15 38 53 1000 ft 3 (28 m 3 ) 23 16 43 63 2000 ft 3 (57 m 3 ) 30 20 67 100 3000 ft 3 (85 m 3 ) 35 23 86 130 4000 ft 3 (113 m 3 ) 40 25 100 160 (1) For salt bath furnaces where a single probe is used for the TUS, the above numbers represent sensor locations. (2) For furnace volumes greater than 4000 ft 3 (113 m 3 ) use the appropriate class formula to calculate number of sensors: • Classes 1 & 2: 9 + (1/2 times [the square root of (Furnace volume in ft 3 -225 ft 3 )]) or 9 + [1/2 times (the square root of [35.3 times (Furnace volume in m 3 -6.4 m 3 )])] • Classes 3 thru 6: 9 + (1/4 times [the square root of (Furnace volume in ft 3 -225 ft 3 )]) 9 + [1/4 times (the square root of [35 .3 times (Furnace volume in m 3 -6.4 m 3 )])] (3) For furnace volumes less than 4000 ft 3 (113 m 3 ), it is acceptable to use the appropriate formula or interpolate using the data in Fig. 1. IndustrialHeating.com -December 2010 45
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