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RUSKA 2470
Users Manual
2-6
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 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).
glg02.bmp
Figure 2-4. Reference Plane Determination
When a pressure for the piston pressure gauge is calculated, the value obtained is valid at
the reference plane. The pressure at any other plane in the system may be obtained by
multiplying the distance of the other plane from the reference plane by the pressure
gradient and adding (or subtracting) this value to that observed at the piston reference
plane.