300676782_002_C0
6996 0229 87 Issue A – 05/2017 – © Leybold 10
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 CP / OP 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 CP /OP 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.
Leybold vacuum pumps use volumetric flow rate to denote performance, the unit being Am³/hr.
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