General Description; Vacuum And Flow Rate - Edwards EOSi Series Instruction Manual

2. General description

2.1 Vacuum and flow rate

A vacuum is a pressure in a system that is less than the ambient atmospheric pressure. It
can be shown in absolute terms or in effective gauge terms:
▪ mbar(a) - the absolute pressure shows how much the pressure is above the
absolute zero vacuum.
▪ (minus) mbar(e) - the effective or gauge pressure shows how much the pressure is
below the local atmospheric pressure.
Flow rate definitions
The two ways to show the flow rate in a vacuum are:
1. The displacement or volumetric flow rate (Am³/hr)
2. Throughput or mass flow rate
Displacement/volumetric flow rate
For the relevant pressure range, when the 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 decrease in 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 is always higher than the throughput or mass flow rate.
Throughput or mass flow rate
If the volumetric flow does not change with the decrease in pressure level, the number
of molecules in the volume of air can change. The deeper the vacuum, the less number
of molecules in the same volume of air.
The mass flow rate will decrease with a decrease in (absolute) pressure. A flow rate must
be shown at a specified vacuum level when you use throughput or mass flow rate.
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6996022910_B - General description
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