Panasonic PT-DZ12000U Operating Instructions Manual page 143

Non-positive Displacement - (Of a compressor or vacuum
pump). One that uses kinetic energy to create pressure
gradients (slopes) for moving air. This applies to
Regenerative, Axial flow, and Centrifugal pumps.
Open Capacity - The volume of air exhausted per minute
when there is no vacuum or pressure load on the pump,
expressed in cfm.
PSIG - Pounds per square inch gauge-pressure above or
below (vacuum) atmospheric pressure.
Positive Displacement - (Of a compressor or vacuum
pump.) One that moves a specific volume of air for each
cycle of operation.
Pressure Differential - Difference in pressure between two
points in a system or component.
Pressure Drop - Any reduction in pressure from normal
value.
Pressure Gauge - A device that displays the pressure level
in a system. Most gauges use atmospheric pressure as a
reference level and measure the difference between the
actual pressure and atmospheric pressure; the readout is
called "gauge pressure." (A gauge that reads below
atmospheric is called a vacuum gauge).
Rated Capacity (Vacuum) - The cfm of free air exhausted by
a vacuum pump at rated speed. Usually given for vacuums
ranging from 0 "Hg to the maximum vacuum rating.