II.

Pressure Gauges

Most gauges record the difference between the fluid pressure and local atmospheric pressure. For small pressure differences, a U-tube manometer is used. It consists of a U-shaped tube with one end connected to the container and the other open to the atmosphere. Filled with a liquid, such as water, oil, or mercury, the difference in the liquid surface levels in the two manometer legs indicates the pressure difference from local atmospheric conditions. For higher pressure differences, a Bourdon gauge, named after the French inventor Eugène Bourdon, is used. This consists of a hollow metal tube with an oval cross section, bent in the shape of a hook. One end of the tube is closed, the other open and connected to the measurement region. If pressure (above local atmospheric pressure) is applied, the oval cross section will become circular, and at the same time the tube will straighten out slightly. The resulting motion of the closed end, proportional to the pressure, can then be measured via a pointer or needle connected to the end through a suitable linkage. Gauges used for recording rapidly fluctuating pressures commonly employ piezoelectric or electrostatic sensing elements that can provide an instantaneous response.

As most pressure gauges measure the difference between the fluid and the local atmospheric pressure, the atmospheric pressure must be added to the gauge pressure to arrive at the true absolute pressure. A negative gauge-pressure reading corresponds to a partial vacuum.

Low gas pressure (down to about 10^-6 mm mercury absolute) can be measured by the so-called McLeod gauge, in which a measured volume of gas at the unknown low pressure is compressed at constant temperature to a much smaller volume, and then the pressure is measured directly with a manometer. The unknown pressure is then calculated from Boyle's law (see Gases). For still lower pressures, various gauges depending on radiation, ionization, or molecular effects are used (see Vacuum Technology).