Measurement of pressure

This chapter briefly discusses pressure management; there are three categories of pressure measurement: absolute pressure, gauge pressure, and differential pressure. Absolute pressure is the difference between the pressure at a particular point in a fluid and the absolute zero of pressure, that is, a complete vacuum. A barometer is one example of an absolute pressure gauge, because the height of the column of mercury measures the difference between the atmospheric pressure and the “zero” pressure of the Torricellian vacuum that exists above the mercury column. When the pressure-measuring device measures the difference between the unknown pressure and local atmospheric pressure, the measurement is known as gauge pressure. There are three basic methods for measuring pressure. The simplest method involves balancing the unknown pressure against the pressure produced by a column of liquid of known density. The second method involves allowing the unknown pressure to act on a known area and measuring the resultant force either directly or indirectly. The third method involves allowing the unknown pressure to act on an elastic member (of known area) and measuring the resultant stress or strain.