Gas Filled Temperature gauges work on the bourdon tube principle. The bourdon tube is a flattened tube made of a material such as phosphor bronze. This tube is filled with gas. The tube is connected to the stem of the gauge. 
The gas filled temperature gauge can have a capillary tube with the help of which the gauge can be located away from the sensing point.
When the temperature of the the medium rises, the gas in the stem and the bourdon tube expands due to the temperature. The pressure inside the tube rises and the bourdon tube unwinds. This movement is communicated to the pointer by means of a rack and pinion.
The pointer moves across a graduated scale which indicates the temperature.
Gas filled temperature gauges are used in all types of industries such as petrochemical industries, thermal and hydro power stations, chemical industries and iron and steel industries.