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General description
2.1 What is vacuum and how is flow rate understood
What is vacuum and how to denote
A vacuum is any pressure in a system that is below the ambient atmospheric pressure. It can be denoted in absolute
terms or in effective (gauge) terms:
mbar(a) - Absolute pressure - denotes how much the pressure is above absolute zero vacuum.
(minus) mbar(e) - The Effective or Gauge pressure - denotes how much the pressure is below local atmospheric
pressure
Typical DZS claw pump working range 600 mbar(a) (450 Torr(a)) – 150 mbar(a) (112.5 Torr(a))
Atmospheric pressure at sea level is roughly 1 bar or 1000 mbar (750 Torr). A typical working range for the DZS
claw pumps is 600 mbar(a) (450 Torr(a)) to 150 mbar(a) (112.5 Torr(a)). From the illustration it can be seen clearly
that this range is also equivalent to - 400 mbar(e) (-300 Torr(e)) to - 850 mbar(e) (-637.6 Torr(e)).
It is important to understand which type of reference is required before selecting a pressure instrument for
measuring the vacuum. It must be noted that the distinction doesn't matter for a pressure difference (delta P; for
example, pressure loss), since it is always the result of subtracting 2 pressures (whether stated as absolute or
effective pressures).
Flow rate definitions
There are two common but different ways to denote flow rate in vacuum. The first one is based on the displacement
or volumetric flow rate and the second one is based on the throughput or mass flow rate. Atlas Copco vacuum
pumps use volumetric flow rate to denote performance, the unit being Am³/hr.
Displacement/volumetric flow rate
For the relevant pressure range, when a DZS claw pump operates at quasi constant motor speed (rotations per
minute) and since the compression chambers have fixed dimensions, the same volume of air is pumped from inlet
to outlet with falling pressure level. Over the relevant pressure range, this makes the volumetric flow rate quasi-
independent of the vacuum level. It is the expression of the flow rate inside the piping at the governing vacuum level
(Am³/hr), and always higher than the throughput in mass flow rate.
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