Fluke RUSKA 2470 - Manual (Page 20)

RUSKA 2470
Users Manual
2-8
For good work, a piston pressure gauge should be provided with an index mark for
associating the reference of the piston with other planes of interest within a system. The
design of this index will vary with the design and manufacture of the instrument, it may
be in the form or an index rod with scribed lines on it, an index groove on the column of
the instrument, or, other type of fixed indicator. Not only does the mark serve to establish
fixed values of pressure differences through a system, it indicates a position of the piston
with respect to the cylinder at which calibration and subsequent use should be conducted.
If the piston is tapered, it is important to maintain a uniform float position for both
calibration and use. This Position is referred to as the “Mid-Float” position as it
represents the middle of the calibrated range of the Piston/Cylinder.
In normal operation, the system is pressurized until the piston is in a floating position
slightly above the index mark. After a period of time, the piston and its load will sink to
the line at which time the conditions within the system are stable. If there is a question as
to the error that may be produced by accepting a float position that is too high or too low,
the error will be equivalent to a fluid head of the same height as the error in the float
position. This statement assumes that the piston is uniform in area over this length.
Crossfloating
It was mentioned earlier that some piston pressure gauges must be calibrated against a
standard gauge. In the jargon of the laboratory, this process is called crossfloating. When
crossfloating one gauge against another, the two are connected together and brought to a
common balance at various pressures. The balancing operation is identical with that
employed on an equal-arm balance where the mass of one object is compared to another.
In each instance the operator must decide when the balance is complete. In a crossfloat,
the two gauges are considered to be in balance when the sink rate of each is normal for
that particular pressure. At this condition there is no pressure drop in the connecting line,
and consequently no movement of the pressure medium. The condition can be difficult to
recognize, particularly if there is no means of amplification in the method of observing.
The precision of the comparison will depend directly upon the ability of the operator to
judge the degree to which the balance is complete. This procedure is repeated for several
pressures, and the values of areas obtained are plotted against the nominal pressure for
each point. A least-squares line is fitted to the plots as the best estimate value of the area
at any pressure.
There are two accepted methods for determining the balance of the two pressures. First,
the sink rates can be observed and graphed using high sensitivity sensors. Second, a
sensitive null-pressure transducer can be interposed which will display small pressure
differences directly.
When using a suitable amplifying device, the scatter in the plotted areas from a good
quality piston gauge should not exceed a few parts per million.
Bibliography
1. Bridgman, P. W., The Physics of High Pressure, G. Bell & Sons, London, 1952.
2. Cross, J. L., "Reduction of Data for Piston Gauge Pressure Measurements". NBS
Monograph 65 (1963).
3. Dadson, R. S., "The Accurate Measurement of High Pressures and the Precise
Calibration of Pressure Balances", Proc. Conf. Thermodynamic and Transport
Properties of Fluids, London, pp. 32-42, 1957, Institute of Mechanical Engineers.
4. "Design and Test of Standards of Mass", NBS Circular No. 3 (Dec., 1918), Included
in NBS Handbook 77, Volume III.
5. Johnson, D. P., J. L. Cross, J. D. Hill, and H. A. Bowman, "Elastic distortion Error in
the Dead Weight Piston Gauge", Ind. Engineering Chem., 40, 2046 (Dec., 1957).