In situ response time testing of thermocouples

Abstract Response time information is important in most applications involving transient temperature measurements with thermocouples. Traditionally, the response time of a thermocouple is measured in a laboratory at a reference condition. The laboratory response time information is useful for comparative evaluation of thermocouples but may have little relationship with the response time for the sensor in service. This is because of the effects of installation and process operating conditions on response time. A method has been developed and validated for response time testing of thermocouples as installed in an operating process. The details are reviewed in this paper. The method is referred to as the Loop Current Step Response Test. The test involves an internal heating of the thermocouple by applying an electrical current to the thermocouple extension wires. The current is then terminated and the thermocouple output is monitored as it returns to ambient temperature. This transient output is analyzed to identify the time constant of the thermocouple. A mathematical transformation is used to convert the internal transient data to the response of the thermocouple for an external temperature perturbation.