filledFilled System, or just “filled thermometers “are those that work on pressure or volume change of a gas or changes in vapor pressure of a liquid.

They have been around a very long time and are almost unique in industrial uses since they do not require power and are considered intrinsically safe in flammable atmosphere situations.

A link to the introductory page at About Temperature & Moisture Sensors A Web Guide, where a description of this type of temperature sensor is maintained along with links to other web resources about them.