The thermal response time is relatively complex, and different test conditions will yield different measurement results. This is because it is affected by the heat transfer rate between the thermocouple and the surrounding medium; a higher heat transfer rate results in a shorter thermal response time. To ensure comparability of the thermal response time of thermocouple products, the national standard stipulates that the thermal response time should be measured using a dedicated water flow test device. The water flow velocity of this device should be maintained at 0.4 ± 0.05 m/s, the initial temperature should be in the range of 5-45℃, and the temperature step change should be 40-50℃. During the test, the water temperature change should not exceed ±1% of the temperature step change. The insertion depth of the tested thermocouple should be 150 mm or the designed insertion depth (whichever is smaller, and this should be noted in the test report).
Because this device is quite complex, only a few units currently possess this equipment. Therefore, the national standard allows manufacturers and users to negotiate and use other test methods, but the data provided must include the test conditions.
Since the thermoelectric potential of type B thermocouples is very small near room temperature, the thermal response time is difficult to measure. Therefore, the national standard stipulates that the thermoelectric element assembly of a type S thermocouple of the same specification can be used to replace its own thermoelectric element assembly for testing.
During the test, the time T0.5 for the thermocouple output to reach 50% of the temperature step change should be recorded. If necessary, the thermal response time T0.1 (10% change) and T0.9 (90% change) can also be recorded. The recorded thermal response time should be the average of at least three test results from the same test, and the deviation of each measurement result from the average should be within ±10%. In addition, the time required to create the temperature step change should not exceed one-tenth of the T0.5 of the tested thermocouple. The response time of the recording instrument or meter should not exceed one-tenth of the T0.5 of the tested thermocouple.
