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Temperature
Piston pressure gauges are temperature sensitive and must, therefore, be corrected to a
common temperature datum.
Variations in the indicated pressure resulting from changes in temperature arise from the
change in effective area of the piston due to expansion or contractions caused by
temperature changes. The solution is a straightforward application of the thermal
coefficients of the materials of the piston and cylinder. The area corresponding to the new
temperature may be found by substituting the difference in working temperature from the
reference temperature and the thermal coefficient of area expansion in the relation as
follows:
Where:
A
o(t)
A
o(r)
c
Reference Plane of Measurements
The measurement of pressure is linked to gravitational effects on the pressure medium.
Whether in a system containing a gas or a liquid, gravitational forces produce vertical
pressure gradients that are significant and must be evaluated. Fluid pressure gradients and
buoyant forces on the piston of a pressure balance require the assignment of a definite
position at which the relation P = F/A exits.
It is common practice to associate this position directly with the piston as the datum to
which all measurements made with the piston are referenced. It is called the reference
plane of measurement, and its location is determined from the dimensions of the piston.
If the submerged portion of the piston is of uniform cross section, the reference plane is
found to lie conveniently at the lower extremity as shown in Figure 2-4. If, however, the
portion of the piston submerged is not uniform, the reference plane is chosen at a point
where the piston, with its volume unchanged, would terminate if its diameter were
uniform.
The reference plane of the standard is the effective bottom of the measurement piston.
This location can be correlated to the index on the mass stack using the L1 dimension
(found on Calibration Report for the Piston/Cylinder) and the D Dimension (found on
Calibration Report for the Mass set).
General Piston Pressure Gauge Considerations
=
A
A
( )
( )
0
t
0
is the effective area at temperature, t
is the effective area at zero pressure and reference temperature, r
is the coefficient of thermal expansion
[
]
(
)
+
−
1
c
t
r
r
2
Temperature
2-7
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