How to measure the thermal response time of a K-type thermocouple?

Measuring the thermal response time of a K-type thermocouple is actually quite complex, and different test conditions will produce different results. This is due to the influence of the heat transfer rate of the surrounding medium; a higher heat transfer rate results in a shorter thermal response time.

To ensure comparable thermal response times for thermocouples, national standards stipulate that thermal response time measurements should be conducted in a dedicated water flow test apparatus. The water flow velocity in this apparatus should be maintained at 0.4±0.05m/s, with an initial temperature range of 5-45°C and a temperature step of 40-50°C. During the test, the water temperature should not fluctuate by more than ±1% of the temperature step value. The insertion depth of the tested thermocouple should be 150mm or the designed insertion depth.

Because the thermoelectric potential of a thermocouple is very low near room temperature, thermal response time measurement is difficult. Therefore, national standards stipulate that the thermoelectrode assembly of a K-type thermocouple of the same specifications can be replaced with the original thermoelectrode assembly for testing.

During the test, the time T0.5 required for the thermocouple output to change to a value equivalent to a 50% temperature step change should be recorded. If necessary, the thermal response time T0.1 for a 10% change and the thermal response time T0.9 for a 90% change may be recorded. The recorded thermal response time should be the average of at least three test results from the same experiment, and the deviation of each measurement result from the average value should be within ±10%. In addition, the time required to form a temperature step change should not exceed one-tenth of the T0.5 of the thermocouple being tested. The response time of the recording instrument or meter should not exceed one-tenth of the T0.5 of the thermocouple being tested.