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General Piston Pressure Gauge Considerations
Measurement of Pressure with the Piston Pressure Gauge 2
2-7
glg03.bmp
Figure 2-5. Head Correction Measurement
()
ghPPP
AIRmH
**−=
Where:
h is the vertical distance between the reference plane of the Standard and
the reference plane of the DUT (Device Under Test)
ρ
air
is the density of the air
ρ
m
is the density of the test media
g is the acceleration due to local gravity
L1 is the vertical distance from the mass loading location to the effective
bottom of the piston.
D is the vertical distance from the mass loading location to the bottom of
the Hanger Mass
Note
For instances where the reference plane of the DUT is LOWER than the
reference plane of the standard, the h is a negative number and therefore
H
P
becomes a negative number.
In addition, gas lubricated piston pressure gauge calculations should account for the fact
that the pressure gradient mentioned in the preceding paragraph changes as system
pressure is changed. This is because the specific gravity of gas varies as a function of
pressure, not remaining approximately constant, as does a hydraulic fluid.