   # Gas thermometers

There are two main types of gas thermometer, one operating at constant volume and the other at constant pressure. The constant-volume gas thermometer is by far the more widely used and so we will deal with it alone.

The ideal gas equation states that for n moles of a gas: PV = nRT

and therefore for a gas at constant volume V the absolute temperature T is directly proportional to the pressure of the gas P.

A simple form of constant-volume gas thermometer is shown in Figure 1. The gas is enclosed in the bulb B and the pressure recorded by the difference in levels (h) of the mercury columns. The mercury level at R is always adjusted so that it coincides with the mark. The pressure of the gas within the bulb is then given by P = A + h, where A is the atmospheric pressure.

If the atmospheric pressure varies during the experiment allowance must be made for this, since it is the total gas pressure that is measured.

The gas in the bulb can be air, hydrogen, helium or nitrogen, although it is the constant-volume hydrogen gas thermometer that is taken as standard.

The simple form of constant-volume gas thermometer is subject to errors due to changes in volume of the glass and of the mercury (due to temperature variations), to pressure on the bulb and to the exposed column 'dead space', that is, the volume of gas that is outside the region of which the temperature is being measured.

It has the further disadvantages that it is not direct-reading, and that it cannot be used to measure varying temperatures, because gases are such poor conductors of heat.

A more accurate form of constant-volume thermometer has been designed where some of these errors are reduced, the dead space is made as small as possible and the bulb containing the gas is large (1.6 litres).

By using different gas thermometers a wide range of temperatures can be measured:

Hydrogen   -200 oC to +500 oC
Nitrogen   +500 oC to + 1500 oC
Helium     -270 oC to + 1500 oC

These thermometers can be very accurate, to within 0.005 oC from 0 oC to 100 oC, 0.1 oC around 500 oC and to within 2 oC at 1500 oC