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Glossary of Terms - All Terms

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Glossary Subsets


• The absence of pressure.
• A perfect vacuum is the total absence of pressure.
• A partial vacuum is some condition less than atmospheric pressure.
• Measured in inches of Mercury (in.Hg) with the zero point equal to atm

Vapor Pressure

The pressure at which the liquid and gaseous phases of a fluid are in equilibrium.

Variable Frequency Drive (VFD)

A device to control the speed and efficiency of electric motors.


• The speed of flow through a hydraulic line expressed in feet per second or meters per second.
• The speed of a rotating component measured in revolutions per minute.


A device which controls fluid flow direction, pressure, or flow rate.


• To permit opening of a pressure control valve by opening its pilot port to atmospheric pressure.
• An air breathing device on a fluid reservoir.


The measure of the resistance of a fluid to flow, usually expressed in poise or centipoise. The higher the reading, the thicker the material.

Viscosity Index

A measure of the viscosity-temperature characteristics of a fluid as referred to that of two arbitrary reference fluids.

Voltage Drop

The drop of pressure in an electric circuit due to the resistance of the conductor.


• The size of a space or chamber in cubic units.
• Loosely applied to the output of a pump in gallons per minute (gpm).


The measurement of the pressure of the electric power.