The Bourdon tube bends like a spring due to increasing pressure. If the bulb and the gas inside it are heated, the pressure increases because the gas cannot expand (the relatively small thermal expansion of the tube can be neglected).
Figure: Cut through a gas filled thermometer (gas-in-metal thermometer) The entire system is sealed gas-tight on all sides. The Bourdon tube is connectet to a bulb as the actual measuring probe. In principle, this is a C-shaped bent tube in which an inert gas such as nitrogen, helium or argon is under high pressure. The figure below shows the operating principle of a gas filled thermometer where the pressure measurement is based on the use of a Bourdon tube. Figure: Gas filled thermometer (gas-in-metal thermometer) For an ideal gas, the gas pressure at a constant volume is determined only by the temperature ( isochoric process).
However, the operating principle is the same in both cases: It is not the expansion of the fluid that is measured, but the increase in pressure at a constant volume which is associated by an increase in temperature. In gas filled temperature gauges (also called gas thermometers or gas-in-metal thermometers), a gas is used as the thermometric fluid instead of an liquid as it is in liquid-in-metal thermometers. In gas thermometers, the pressure rise connected with a temperature increase is used for measuring the temperature!
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