The McLeod gauge [2] is a pressure-measuring instrument and laboratory reference standard used to establish gas pressure in the subatmospheric range of 1 mm Hg abs down to 0.1 mm Hg abs.One variation of this instrument is sketched in Figure 9.3 (a) in which tha gauge is connected directly to the low-pressure source. The glass tubing is arranged so that a sample of the gas at an unknown low pressure can be trapped by inverting the gauge from the sensing position, depicted as Figure 9.3 (a), to that of the measuring position, depicted as Figure 9.3(b). In this way, the gas trapped within the capillary is isothermally compressed by a rising
Column of mercury. Boyle’s law is then used to relate the two pressures on either side of the mercury to the distance of travel of the mecury within the capillary. Mecury is the preferred working fluid because of its high density and very low vapor pressure