3. Description
3.1. Vacuum and flow rate
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 the local atmospheric pressure.
Figure 1 Vacuum and flow rate
1. Pressure 2. Absolute vacuum
3. Typical application working range 4. Atmospheric pressure
1. Pressure 2. Absolute vacuum
3. Typical application working range 4. Atmospheric pressure
Atmospheric pressure at sea level is roughly 1 bar(a) or 1000 mbar(a) or
0 bar(e). The typical working range for pump applications is 400 mbar(a) to
100 mbar(a), for example -600 mbar(e) to -900 mbar(e). This operating pressure
range is just indicative. The GVS A vacuum pumps are designed for continuous
operation between atmospheric pressure and their ultimate pressure
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 dif
ference (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 2 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. The vacuum pumps use volumetric
flow rate to denote performance, the unit being actual m³/h.
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