McLeod gage
McLeod gage
A type of instrument used to measure vacuum by application of the principle of Boyle's law. A known volume of a gas whose pressure is to be measured is trapped by raising the level of a fluid (mercury or oil) by means of a plunger, by lifting a reservoir, by using pressure, or by tipping the apparatus. As the fluid level is further raised, the gas is compressed into the capillary tube (see illustration). Obeying Boyle's law, the compressed gas now exerts enough pressure to support a column of fluid high enough to read. Readings are somewhat independent of the composition of the gas under pressure. See Vacuum measurement
