Thermowell is a closed-end reentrant tube designed for insertion of a temperature-sensing element, and provided with means for a pressure-tight attachment to a vessel.

In this instance, the ASTM definition of thermowell leaves a bit to be desired. Thermowells are typically constructed of solid drilled-out bar stock and are designed to protect a temperature sensor from flow, high pressure and harsh environments. Thermowells encase and protect temperature sensors from the harmful effects of the processes into which they are immersed without substantially insulating the temperature sensor (thermocouple, RTD, etc.) from the temperature of the process.

The most common types of thermowells are:

• Threaded : A threaded Thermowell is screwed into the process.
• Socket Weld : A weld in Thermowell is welded directly into the process
• Flanged Welded : A flanged Thermowell has a flange collar which is attached to a mating flange.