Designing Thermocouples for Response Rate

Considerable effort has been put into the study of the transient response of thermocouples. One reason for this is the growing interest in temperature-sensitive controls for jet engines. The advantage of a temperature-sensitive control is obvious — it controls by the variables which requires control. The disadvantages are chiefly in the sensing element. An ideal sensing element would be instantly aware if any change in gas temperature, and would follow accurately the temperature no matter how rapidly it changed. Unfortunately no such ideal sensing element is available. Anything which has mass requires a finite time to change its temperature, the length of time depending on its heat capacity and on how fast heat is being added to it. In terms of a thermocouple, or any other immersion element, this means that if the temperature of the gas is changing the thermocouple will “lag” and not follow the change exactly. This lag is important in control work since the control is not aware of a change in gas temperature until the signal from the sensing element changes. The lag is also important in analyzing transient temperature records made on a test engine. The recorded trace represents thermocouple temperature, not gas temperature. Due to the “lag” there may be considerable difference between gas temperature and thermocouple temperature.