Panasonic PT-DZ12000U Operating Instructions Manual page 135

Operating where atmospheric pressure is lower will reduce
the vacuum the pump can produce. An adjusted vacuum
rating for such locations can be determined by multiplying
actual atmospheric pressure by the ratio of the nominal
vacuum rating to standard atmospheric pressure:
Adjusted Vacuum Rating =
Actual Atomospheric Press x Nominal Vacuum Rating
Standard Atmospheric Press
Air Removal Rate -Vacuum pumps are rated according to
their open capacity, which is the volume of air (expressed in
cfm) exhausted when there is no vacuum or pressure load on
the pump. Effectiveness of the vacuum pump in removing air
from the closed system is given by its volumetric efficiency, a
measure of how close the pump comes to delivering its
calculated volume of air.
This equation is applied in two different ways:
• True (or Intake) Volumetric Efficiency -The volume of air
removed during a given time period is converted to an
equivalent volume at the temperature and absolute pressure
existing at the intake.
• Atmospheric Volumetric Efficiency -The volume of air
removed by the pump is converted to an equivalent volume at
standard conditions (14.7 psi and 68 F).
The displacement is the total volume swept by the repetitive
movement of the pumping element during the same time
period (usually one revolution). With various vacuum pumps
having the same displacement, it is the difference in
volumetric efficiencies that accounts for the difference in free
air capacities.