What is the purpose of a 'thermowell' when installing a temperature sensor?

The purpose of a thermowell when installing a temperature sensor is to provide a protective barrier between the temperature sensor and the process fluid or environment. A thermowell is a closed-end tube, typically made of metal, that is inserted into a process vessel or pipe. The temperature sensor, such as a thermocouple or resistance temperature detector (RTD), is then inserted into the thermowell. The thermowell protects the sensor from corrosion, erosion, high pressure, and mechanical damage. It allows the sensor to be removed for maintenance or replacement without having to shut down the process or drain the vessel. The thermowell also increases the sensor's response time, as the heat must be transferred through the thermowell wall before it reaches the sensor. However, this increase in response time is usually acceptable in exchange for the added protection. The material and dimensions of the thermowell must be carefully selected to withstand the process conditions and to ensure accurate temperature measurement. The thermowell must be strong enough to withstand the pressure and flow rate of the process fluid, and it must be made of a material that is compatible with the fluid. The thermowell's length and insertion depth must be chosen to minimize errors due to heat conduction along the thermowell.