Thermowells differ from Industrial "Protection Tubes" in that they are designed to withstand pressure, whereas Industrial Protection Tubes do not (outside a few pounds of pressure). Depending on the fitting, Thermowells can withstand pressures into the thousands of pounds.
As Pyromation describes them, "A thermowell is a pressure-tight receptacle that protects and extends the life of a temperature sensor in processing applications where the sensor is not mechanically or chemically compatible with the process environment. Installed directly into the piping systems, thermowells facilitate sensor replacement in high pressure pipelines and eliminate the need to interrupt the process flow or drain the process system for sensor maintenance functions. The use of standardized thermowells permits simple relocation of sensors throughout a plant."
Pyromation produces thermowells that have a proven track record in a wide range of industries and applications. Their product line includes threaded, Van Stone, socket-weld, weld-in and flanged thermowells. Thermowells are available as separate components or as a part of complete temperature sensor assemblies.