One of the most important accessories for any temperature-sensing element is a pressure-tight sheath
known as a thermowell. This may be thought of as a thermally conductive protrusion into a process
vessel or pipe25 allowing a temperature-sensitive instrument to detect process temperature without
opening a hole in the vessel or pipe. Thermowells are critically important for installations where
the temperature element (RTD, thermocouple, thermometer, etc.) must be replaceable without
de-pressurizing the process.