Pneumatic Pressure Gauge
Pressure measuring devices normally measure the difference between applied pressure and atmospheric pressure and thus indicate gauge pressure (or vacuum pressure in the case of a vacuum gauge). The equivalent absolute pressure is determined by relating the gauge pressure to zero pressure as a datum- see Figure 7. In the case of positive pressure above atmosphere, absolute pressure is the sum of the gauge and the atmospheric pressure (normally assumed to be 1 bar). Since pneumatics are mainly concerned with gauge pressure less than 10 bar, the difference between absolute and gauge pressure can be significant.