Gas actuated thermometers are working on the temperature dependend pressure or volume change of a gas. The temperature of a body is defined by the thermic expansion (by means of measurement of volume/pressure change).
The bulb of this type of thermometers is filled with an inert gas. A temperature change causes the system pressure increase or drop. This is measured with a bourton tube pressure system and made visible on the dial.
Inert gas dial thermometer from DRUCK & TEMPERATUR Leitenberger GmbH, Germany, are available for direct reading (without capillary) or for remote reading (with capillary).