De Wit thermometers are gas-actuated temperature gauges for temperature measuring in the process industry in a wide variety of applications.

The temperature responsive system consists of a probe, capillary and a flattened spiral bourdon tube. This sealed system is filled with an inert gas. The measuring principle is based on the volumetric temperature expansion of this gas. When the bulb is heated, the pressure in the systems increases and the spiral bourdon tube unwinds slightly to operate the pointer.