Panasonic PT-DZ12000U Operating Instructions Manual page 142

Differential Pressure - Difference in pressure between two
points in a system or component.
Displacement - The total volume swept by the repetitive
motion of the pumping element. Displacement per revolution
is determined by size of the pumping chamber(s).
Displacement per minute also depends on compressor
speed. Displacement is meaningful only in positive
displacement compressors.
Efficiency, Volumetric - Ratio of actual capacity to
theoretical displacement multiplied by 100 percent.
Free Air - Air under the atmospheric conditions (including
temperature) at any specific location.
Gauge Pressure (Positive) - The pressure differential above
atmospheric pressure (see pressure gauge).
Gauge Pressure (Negative) - The difference between
pressure remaining in an evacuated system and atmospheric
pressure (see vacuum gauge). Also known as -gauge
vacuum" or "vacuum level." In effect, it is the pressure drop
produced by evacuating the system. Measured in inches of
mercury ("Hg). Caution: It is a potentially misleading term
which must be carefully defined when used; negative
pressure (absolute) doesn't exist.
Maximum Vacuum Rating - Highest vacuum level
recommended for a vacuum pump.
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