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Glossary of Terms - All Terms

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Glossary Subsets


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


• 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

Variable Frequency Drive (VFD)

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

Vapor Pressure

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


• 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.


• 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.

Voltage Drop

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


The measurement of the pressure of the electric power.


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

Viscosity Index

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