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General Piston Pressure Gauge Considerations
Calculations
2
2-5
•
Temperature of the piston and cylinder.
•
Buoyant effect of the atmosphere upon the masses.
•
Hydraulic and gaseous pressure gradients within the apparatus.
Calculations
For a consolidation of these various corrections, see Appendix A of this manual.
Appendix A contains a Pressure Calculation Worksheet (both SI and English units) with
instructions. The Pressure Calculation Worksheet will step the user through the necessary
corrections as applied to calibrations with a piston pressure gauge.
Measurement of Pressure with the Piston Pressure Gauge
Pressure results from the application of a force onto an area. Numerically, it is the
quotient of the force divided by the area onto which it is applied:
A
F
P
=
Where:
P
represents the pressure
F
represents the force
A
represents the area
Elastic Distortion of the Cylinder
As the pressure is increased within a piston pressure gauge, the resulting stress produces
a temporary and reversible deformation of the cylinder. The net effect is a change in the
effective area of the piston-cylinder combination. If the change in the area is a linear
function of the applied pressure, the relationship may be described by the equation:
( )
2
2 1 0
1 P b P b A A
e
+ + =
Where:
P
is the nominal pressure
e
A
is the effective area at a pressure P
0
A
is the effective area of the piston-cylinder assembly at a reference
pressure level
2 1
& b b
are coefficients of elastic distortion which are determined
experimentally.
Gravity
Since pressure is defined as force per unit area, anything that changes the force applied to
the piston of a piston pressure gauge also changes the pressure produced by that gauge.
Therefore, the effects of gravity on the masses loaded on the piston must be considered.
The gravity correction is usually very significant and must be used during calculations to
achieve the advertised accuracy of the piston pressure gauge.
Confusion has resulted from the English System of units concerning the terms, mass and
weight. The International System of units does not leave room for ambiguity and should
be used whenever possible.
It is recognized that some facilities still operate under the English System of units.
Therefore, this manual provides calibration data and calculation instructions in both the