Abstract
This paper deals with the experimental determination and correlation of the dynamic response of mineral insulated thermocouples. In a given installation the dynamic response is largely dependent on the resistance to heat transfer between the hot junction and the region of which the temperature is to be measured.
This thermal resistance is composed of
the resistance from the sheath to the surroundings—usually a conduction or forced convection problem; at elevated temperatures radiation could be important. the resistance from the thermocouple hot junction to the sheath—a conduction problem complicated by the difficulty of defining the physical properties of the thermocouple materials, the interface resistances, and the geometry.
In many applications the thermocouple is inserted into a pocket to provide protection or support. In this case there is an additional resistance from the sheath to the containing pocket which involves conduction and/or radiation across a gas film and resistance (1) becomes the resistance between the pocket and its surroundings.
The effects of all these resistances have been investigated, but for the application of particular interest to the authors, i.e. the dynamic response in gas flows at temperatures below 400°C, the dominant resistance was found to be resistance (1) above. Emphasis has therefore been placed on determining the influence on the time constant of the following parameters:
Reynolds number Temperature level Direction (heating or cooling) and magnitude of the transient.
A correlation of the data has been obtained which enables the time constant to be predicted for a range of thermocouple and pocket diameters and of gas conditions.
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