Basic Hydraulics Practice Test

Pressure needs a reference frame, and in hydraulics you’ll see pressure written in different forms depending on that reference. The unit pounds per square inch (psi) is the basic measure of pressure. But you can specify what that pressure is relative to: absolute pressure (PSIA) means pressure above a perfect vacuum, while gauge pressure (Psig) means pressure above atmospheric pressure. Since psi is the unit, and PSIA and Psig are common ways to express the same physical quantity with different references, pressures are often stated as all of the above. To convert between them, you add or subtract atmospheric pressure (about 14.7 psi at sea level) as needed.