Gasthermometer

The gas thermometer is a thermometer expansion, in which the state quantity is observed, the volume of an ideal gas. The temperature measurement is based on measuring of pressure

As for an ideal gas, there is a linear relationship between the change in volume and the temperature eliminates the otherwise necessary calibration due to non-linear change in volume. However, a two-point calibration must be performed as before.

At a gas thermometer, as it is seen on the right picture, you calibration usually performs with ice water and boiling water. The melting point of ice is at standard conditions at 0 ° C while the boiling point of the water rather strongly depends on the air pressure. First, so the air pressure is measured and the corresponding boiling point of water. Then you measure the internal pressure of the gas thermometer once in ice-water and once in boiling water. From the difference in pressure is then obtained of the tension coefficient of the first gas in the thermometer tube:

Here, the boiling point of water, the pressure in the thermometer bulb into ice water and the pressure in the thermometer bulb in boiling water. When there is a correction term is necessary to account for the thermal expansion of the thermometer bulb:

Is the linear expansion coefficient of the thermometer bulb and the ratio of the volume of the capillary tube plus the hose to the pressure gauge and the volume of the vessel. This is important, because the vessel is cooled or heated while tube and hose commonly continue at ambient temperature. is the ambient temperature in Kelvin.

Is the tension coefficient of the gas in the vessel known thermometer, applies to the temperature measured as a function of pressure:

During the measurement of either the pressure or the volume must be kept constant. The areas where a gas thermometer is between about -200 ° C and 800 ° C. However, there are also applications to -273.15 ° C.