Types of Pressure Measurements
Absolute pressure is measured relative to a perfect vacuum. An
example is atmospheric pressure. A common unit of measure is pounds per
square inch absolute (psia).
Differential pressure is the difference in pressure between two
points of measurement. This is commonly measured in units of pounds per
square inch differential (psid).
Gauge pressure is measured relative to ambient pressure. Blood
pressure is one example. Common measurement units are pressure per square
inch gauge (psig). Intake manifold vacuum in an automobile engine is an
example of a vacuum gauge measurement (vacuum is negative gauge pressure).
The three types of measurements are shown in Figure 7. Note that the
same sensor may be used for all three types; only the reference is different.
Differential pressures may be measured anywhere in the range—above, below,
and around atmospheric pressure.