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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. A closed tube with filled air and mercury is used to keep pressure constant, then on varying temperatures the volume of air varies as well, hence, a calibrated reading can be noted.In gas thermometers, the pressure rise connected with a temperature increase is used for measuring the temperature! Such processes involving the constant pressure are said to be isobaric processes. In the case of a constant pressure gas thermometer, the fact that on keeping pressure constant, volume of the given mass of gas varies linearly with the absolute temperature. Using these temperatures can be determined in a constant volume gas thermometer.
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But, it should be noted that the pressure and the temperature of the gas should be such that its form remains gaseous. The temperature of the gas its pressure will increase linearly, and on decreasing the temperature of the gas its pressure will also decrease linearly.
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Now that we have learned the principle of gas thermometers and understood what absolute zero is, let us learn about different kinds of gas thermometers.