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Therefore, it follows that in any pressure system that can exhaust to atmosphere, either through
a valve or by a large leak, the system pressure is atmospheric and will not reflect any other
pressure.
Provided air or a gas that has a very similar density to air is used, then the errors from this
source can be neglected. Nitrogen is preferred as it does not promote the formation of aquatic
growth.
The effect of the velocity of the gas can be observed by opening valve A, a higher pressure will
be needed to force a high volume of gas through the line, resulting in a pressure drop between
valve A and the outlet.
Depending force a high volume of gas through the line, resulting in a pressure between valve A
and the outlet. Depending on the location of the manometer connection, the manometer will
reflect a proportion of this increase in pressure.
Fig. 2 – High gas flow
* Refer “Bubbler System Instrumentation for Water Level Measurement”
- Report of Investigation No. 23 state of Illinois, U.S.A
Head To Pressure Conversion
As we are not seeking to measure atmospheric pressure, but liquid head, how is the principle
applied ?
Simply by using two physical characteristics.
1. The tendency of the gas under pressure to “seek” a lower pressure.
2. The ability of a gas to “pass through” liquids.
The first characteristic has already been demonstrated . The second characteristic can be
appreciated by blowing through a hollow tube (straw) that is immersed in water. No bubble will
appear until the pressure in the tube is sufficient to expel all the water from the tube.
Gas Purge Compressor
Issue 7:
16/12/03
© Copyright
HS55 110-015
