Thermowells are tubular fittings used to protect temperature sensors installed in industrial processes. A thermowell consists of a tube closed at one end and mounted in the process stream. A temperature sensor such as a thermometer, thermocouple or resistance temperature detector is inserted in the open end of the tube, which is usually in the open air outside the process piping or vessel and any thermal insulation. Thermodynamically, the process fluid transfers heat to the thermowell wall, which in turn transfers heat to the sensor. Since more mass is present with a sensor-well assembly than with a probe directly immersed into the process, the sensor's response to process temperature changes is slowed by the addition of the well. If the sensor fails, it can be easily replaced without draining the vessel or piping. Since the mass of the thermowell must be heated to the process temperature, and since the walls of the thermowell conduct heat out of the process, sensor accuracy and responsiveness is negatively impacted by the addition of a thermowell.
To be representative of the average temperature of fluid, the thermowell must extend a few per cent of the inside diameter of the process pipe or vessel (Normally 1/3 rd of the pipe inside diameter). A rule of thumb that is sufficient for most industrial applications (approximately 1% accuracy) is to ensure that the thermowell projects 5 times its own diameter into the process plus the length of its sensing element. So, for a grounded thermocouple spring-loaded into a thermowell with a 1-inch-diameter shank and a tip thickness of .25 inch, a typical immersion length would equal 5.5 inches (1 inch OD x 5 + .25 inch well tip thickness +.25 inch sensor wall thickness = 5.5 inches).
A thermowell is typically machined from solid bar stock and gun-drilled to ensure a long straight bore that closely approximates the diameter of the installed sensor (ex: a .260-inch bore matching a .250-inch sensor). A thermowell is typically mounted into the process stream by way of a threaded, welded, sanitary cap or flanged process connection. The temperature sensor such as a thermometer, thermocouple or resistance temperature detector is inserted in the open end of the thermowell and typically spring-loaded to ensure that the outside tip of the temperature sensor is in metal to metal contact with the inside tip of the thermowell.