Process Control and Instrumentation
Thermal wells are used in the temperature measurement to

Reduce measuring lag
Increase the fidelity
Guard against corrosive and oxidising action on thermocouple materials
Increase the sensitivity

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Process Control and Instrumentation
Vapour actuated pressure spring thermometer does not require ambient temperature compensation. Ambient temperature compensation is provided in gas or liquid expansion pressure spring thermometer by

Reducing the volume of the capillary to a minimum
All of these
Reducing the volume of the receiving element to a minimum
Making the volume of thermometer bulb as large as conveniently possible

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