HYDROLOGICAL SERVICES PTY LTD
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PRESSURE SYSTEM OF HEAD MEASUREMENT
GAS PURGE PRINCIPLE
The behaviour of a gas flowing under a variable back pressure is utilised as an accurate means
of measuring liquid head.
Provided that:
1. The density of the gas is nearly the same as air
2. The velocity of gas is low enough so as not to produce significant pressure drop
down the line then the system pressure will be equal to the back pressure.
This principle is illustrated in the following series of diagrams.
In the first example, the tube exhausts to atmosphere and hence the water manometer indicate
zero
pressure in the system because it is acted on by two equal pressures- the system pressure and the
atmospheric pressure
Fig.1 - Depicts first stage in application of gas purge principle.
Fig.1 also illustrate another feature of all pressure systems, namely the gas endeavours to escape
through any material that it is in contact with to find a lower pressure, but for practical purposes,
the system pressure reflects the lowest of the outlet pressures. In this case, gas endeavours to
pass through the walls of the tubing and fittings, through the water, but elects to take the easier
path to atmosphere, which is in effect a “massive leak”.
Gas Purge Compressor
Issue 7:
16/12/03
© Copyright
HS55 110-014
