What is a Thermowell?

• A. A protective casing for instrumentation cables
• B. A closed tube designed to protect temperature instruments from process conditions. They also allow maintenance to be performed without draining the process fluid.
• C. A temperature sensor made of glass
• D. A filter for gas lines

Answer: (B)

A thermowell is a closed, protective tube placed in a process to house a temperature sensing element. The sensor (like a thermocouple or RTD) sits inside the well, while the process heat transfers through the well wall to the sensor. This design shields the sensor from direct exposure to the fluid, pressure, corrosion, flow, and particulates, reducing damage and wear. It also lets you remove or replace the sensor for maintenance without draining or shutting down the process. Thermowells are typically metal, sized and material-suited to the process, and mounted by flanges or threaded connections.

The other descriptions don’t fit because one describes a protective casing for cables, another names the sensor itself, and another refers to a gas-line filter.