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RUSKA 2465A-754

Users Manual

2-4

It can be shown that the effective area of the piston and cylinder assembly is the mean of

the individual areas of the piston and of the cylinder; therefore as the pressure is

increased, the cylinder expands and the effective area becomes greater. The rate of

increase is usually, but not always, a linear function of the applied pressure. The piston

also suffers distortion from the end-loading effects and from the pressure of the fluid, but

to a much lesser extent than the cylinder. It is evident then, that the simple cylinder of

Figure 2-1 would be inadequate for a primary piston pressure gauge unless some means

of predicting the change in area were available.
The increase in the effective area of the simple cylinder is also accompanied by an

increase in the leakage of the fluid past the piston. Indeed, the leakage becomes so great

at some pressures that at some pressure the floating will not be sufficient for an accurate

pressure measurement.
In Figure 2-2, the pressure fluid is allowed to surround the body of the cylinder. The

pressure drop occurs across the cylinder wall near the top of the cylinder at point B, but

in the opposite direction to that of the simple cylinder in Figure 2-1. In consequence, the

elastic distortion is directed toward the piston, tending to decrease the area of the cylinder.
Again, the change in area with changing pressure places a limit on the usefulness of the

cylinder in 2-2 for its primary instrument. But some benefit does result from the use of

this cylinder in the construction of a piston pressure gauge because higher pressures may

be attained without a loss in float time. A small sacrifice is made in the float time at

lower pressures because the total clearance between piston and cylinder must necessarily

be greater at low pressure for the cylinder in 2-2 than for the cylinder in Figure 2-1.
In the controlled-clearance design of Figure 2-3, the cylinder is surrounded by a jacket to

which a secondary fluid pressure system is connected. Adjustment of the secondary, or

jacket, pressure permits the operator to change the clearance between the cylinder and

piston at will. A series of observations involving piston sink rates at various jacket

pressures leads to the empirical determination of the effective area of the assembly.

Throughout the world, the controlled-clearance piston pressure gauge is an accepted

standard of pressure.
Piston pressure gauges having very high resolution may be made by using simple and

re-entrant cylinders. A determination of the distortion coefficients of such gauges may be

made by direct comparison with a controlled-clearance gauge. Most piston pressure

gauges have some elastic distortion, but some, used in the very low pressures, have only

small coefficients and in some instances, correction for distortion may be neglected.
Measurement of pressure with the piston pressure gauge is subject to uncertainties

resulting from effects other than those of elastic distortion. But, it was appropriate that

the subject of elastic distortion be discussed first, since this characteristic is largely

responsible for the various designs that have been developed.
Measurement processes proposed for high accuracy are disturbed by limitations in the

performance of the equipment, by small changes in the environment and by operational

procedures. The disturbances can be reduced to a degree by exercising control of the

apparatus. Some of the disturbances are difficult to control; it is easier to observe their

magnitudes and apply corrections for their effects.
The factors that affect a pressure measurement process when conducted with a piston

pressure gauge are described below. It is important that the operator acquaints himself

with these factors and become accustomed to recognizing their presence. The success of

the measurement will depend upon the degree to which control has been maintained, or to

the completeness by which corrections were applied for these factors.

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