Thermowell is the protective sheath into which the stem of an instrument or a sensor used for temperature measurement is inserted.
The following are the functions of the thermowell
1) The thermowell serves to protect the instrument or the sensor from the medium whose temperature or pressure is to be measured. For instance, if a temperature sensor is used to measure the temperature of an acidic solution, it can get corroded by the acidic solution. Hence, the sensor is usually enclosed in a thermowell which is made of material that can resist the corrosion.
2) The thermowell ensures proper heat transfer between the medium and sensor. In media which are fast flowing, the heat transfer between the media and the sensor is vital for accurate measurements. The thermowell is designed in such a manner that it captures the required heat from the media and transfer it to the sensor. The transfer is done through a heat conducting paste in the thermowell.
3)The thermowell also enables easy replacement of the sensor. If the thermowell did not exist, the pipeline would have to be drained of its media, before the sensor is removed. Otherwise, there would be a leakage. The thermowell makes replacement easy.