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A method of measuring temperatures with gas as the thermometric fluid. Gas thermometry is the primary source of information about a fundamental physical parameter, temperature, over the range from about 3 to 900 K (-454 to 1160°F).
In principle, gas thermometry consists of using the ideal gas law, Eq. (1),
Constant-volume gas thermometry is by far the most commonly used of the gas-thermometry methods. The name is somewhat misleading because no gas bulb truly exhibits constant volume over any substantial temperature range. Several steps are involved: inserting a fixed mass of a working gas into a rigid container or bulb; determining the pressure of the gas at the triple-point temperature of water; heating or cooling the container to a new temperature whose value is to be determined; and measuring the gas pressure at the new temperature.
A typical constant-volume gas thermometer (see illustration) includes a mercury manometer, used for the measurement of pressure; a gas-bulb system; and a gas-handling system. The part-per-million accuracy achievable in manometric pressure measurements is central to the thermometric accuracy of this instrument. The entire manometer is operated in a temperature-controlled environment. The distance between the surface of the mercury in an upper cell and mercury surfaces in two lower cells is measured by the use of wrung stacks of calibrated end gage blocks. Axial holes through the gage blocks permit detection of the quality of the wringing process by measurement of the internal vacuum of the stack. The pressure exerted by the column of mercury is given by the product &rgr;gh, where &rgr; is the density of the mercury, g is the acceleration due to gravity at the manometer, and h is the height of the gage-block stack.
The gas bulb is completely enclosed by a second bulb in which a so-called counterpressure of helium gas equal to the gas-bulb pressure is maintained at all times. The counterpressure gas minimizes pressure-induced changes in the gas-bulb volume and helps to reduce contamination of the working gas from impurities in the gas-bulb thermostat.
It is possible to determine the thermodynamic temperature of a gas bulb by repeatedly adding measured quantities of a working gas to it and measuring the resulting pressures. The repeated measurements at a single temperature are known as an isotherm. The virial equation (3)
Two variations on the technique can be used, the absolute isotherm and the relative isotherm. In absolute isothermal gas thermometry, measured quantities of working gas are introduced into a gas bulb at the unknown temperature from a known volume that is maintained at 273.16 K. In the relative isotherm method, the working gas is added stepwise to a gas bulb while it is maintained at another, more convenient reference temperature. See Low-temperature thermometry, Physical measurement, Temperature measurement