For most industrial temperature measurements, the sensing element is never directly exposed to the process. The electronic element such as a thermocouple (TC), resistance temperature detector (RTD) or thermistor is encased inside a metallic enclosure, typically a section of stainless steel tubing closed at one end. This provides protection for the delicate element and provides a practical way to insulate the wire leads.
This approach works because heat is conducted through the metal sheath and eventually reaches the actual sensing element. Often, heat has to flow through multiple barriers, including a thermowell and then the sensor sheath. Even though the enclosures have a slight value as an insulator, it is considered negligible, and the temperature reading will accurately reflect the process.