Description and operation
SF
6
gas monitoring
L20--001EN/04
3/6
01--2003
E ALSTOM
Pressu re measurement
Effective pressure ?
Absolute pressure ?
Absolute pressure = effective pressure + atmospheric pressure
Pressure Description Diagram
Effective
The pressure of SF
6
gas is mea-
sured by of a standard pressure
gaugewith adeformable diaphragm
actuating an indicating pointer.
One surface of the diaphragm is in
contact with the SF
6
gas, while the
other is in contact with the atmo-
sphere. The difference between the
gas and the atmosphere is thus
measured, taking atmospheric
pressure as a reference. This is the
measurement of the effective pres-
sure of the SF
6
gas.
SF
6
Atmospheric pressure
Standard pressure gauge
Absolute
If the deformable diaphragm, one
surface of which is in contact with
theSF
6
gas, blocks a volume where
a vacuum has been developed, the
pressure gauge measures thepres-
sure difference between that of the
SF
6
gas and the vacuum. Since the
latter is zero, the pressure gauge
measures the absolute pressure of
the gas.
The absolute pressure of the SF
6
gas, independent of atmospheric
pressure, reflects the quantity of
gas introduced into the compart-
ment and hence its density at the
present temperature.
This is measured by means of an
absolute pressure gauge which is
less generally used and more deli-
cate than a effective pressure
gauge. This is the reason why a
ef-
fective pressure gauge is used,
pro-
vision being made for
corrections
required by atmospheric
pressure
variations resulting from
atmo-
spheric disturbances and dif
fer-
ences of elevation
.
SF
6
Vacuum volume
Absolute
pressure gauge
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