The analysis of thermocouple time constants as a function of fluid velocity

In steady-state conditions when the fluid temperature is constant, there is no damping and time lag so the temperature measurement can be performed with a high accuracy. But when the fluid temperature is varying rapidly as during the start-up, quite appreciable differences occur between the exact and measured temperature because of the time required for the transfer of heat to the thermocouple placed inside a thermometer pocket. The temperature of the fluid is one of the key parameters affecting the proper operation of thermodynamic cycles, so the precise determination of its value is very important. The speed of the response of control systems to a temperature change is closely related to the time constant of the used thermocouples. The paper presents a significant impact of fluid velocity changes (in this case air) on the value of time constants of thermometers. For this purpose, the experimental study was carried out using sheathed thermometers with different diameters and hot junctions. The time constants determined for various thermometers are compared.