Measuring and testing – Fluid pressure gauge – Piston
Reexamination Certificate
2001-11-16
2004-03-09
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Fluid pressure gauge
Piston
C073S001650, C073S001680
Reexamination Certificate
active
06701791
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to dead weight piston fluid pressure gauges/calibration standards, and more particularly to high pressure dead weight piston gas pressure gauges/calibration standards.
Dead weight piston pressure measurement/calibration devices are well-known. Such devices ordinarily include a piston supporting a selected number of calibration weights. A “dead weight piston assembly” includes the piston, a bell housing, and the calibration weights. The piston is slidably disposed in a cylinder, in very low frictional relationship to the cylinder. Fluid, which can be gas or liquid the pressure of which is to be defined or measured, is metered into the bottom of the cylinder so as to push the dead weight piston assembly upward. When the force produced on the bottom of the piston by the pressurized fluid equals the weight of the dead weight piston assembly, the dead weight piston assembly “floats” in an equilibrium condition, wherein a downward force exerted by the piston and supported by the pressure is equal to the total mass multiplied by the gravitational constant. The opposing upward force is produced by the pressure Pg of the gas being measured against the effective area of the piston-cylinder assembly.
Generally, the piston supports a hollow, cylindrical bell housing that in turn supports the annular weights to be loaded on the piston. A horizontal annular flange or ledge is attached to and extends outwardly from a lower outer surface of the bell housing, and one or more annular weights typically are loaded on the annular flange. The piston, bell housing, and annular weight assembly is very precisely and symmetrically shaped and balanced. A selected number of the calibration weights are stacked on the ledge for the purpose of precisely establishing the total weight of the dead weight piston assembly. A spin then is imparted to the dead weight piston assembly, which is sufficiently symmetrical about the vertical axis of the piston that the piston spins freely within the cylinder, the outer surface of the piston being lubricated from the walls of the cylinder by a thin layer of fluid, which can be gas or liquid. The known weight of the dead weight piston assembly and the known “effective area” of the “piston-cylinder” are used to precisely compute the pressure of the fluid being supplied to support the dead weight piston assembly in a “free-floating” equilibrium condition between upper and lower stops of the dead weight piston calibration device.
The closest prior art is thought to include (1) commonly assigned U.S. Pat. No. 5,331,838 entitled “DEAD WEIGHT PISTON DRIVE AND CONTROL SYSTEM”, by Delajoud, issued Jul. 26, 1994, (2) the device shown in
FIG. 1
, described below, (3) and the “integrated piston-cylinder metrological modules” used in the assignee's PG 7000 line of piston gauge products. Above-mentioned U.S. Pat. No. 5,331,838 is incorporated herein by reference.
Gas is much less viscous than oil. Consequently, unless the gap between the rotating piston and the cylinder is very small (0.3 to 0.8 microns), using gas as the lubricant in the gap results in the rotating piston not being as well centered within the cylinder as if oil is used as the lubricating fluid. It is extremely difficult to maintain such a small gap at high pressure due to manufacturing constraints and deformation of the piston and cylinder with respect to pressure. Therefore, pressurized oil normally is used to lubricate the gap between the rotating piston and cylinder for high pressure operation. However, there is a need for piston gauges operating at high pressure using gas as the test medium. Due to the difficulty of lubricating the piston-cylinder with gas at high pressure, the conventional approach is to use an oil operated piston gauge combined with an oil to gas interface external from the piston gauge. However, this method adds uncertainty to the value of the gas pressure due to lack of knowledge of the exact level of the oil to gas interface and is impractical to operate due to the need to maintain the oil to gas level when changing the pressure.
Approximately 20 years ago a French company named Desgranges et Huot developed the system shown in “prior art”
FIG. 1
to solve the problems associated with the use of high pressure gas in a piston gauge by “indirectly” lubricating the gap between the piston and cylinder with oil. The main benefits of the system of “prior art”
FIG. 1
are (1) that the piston-cylinder gap can be a larger size that works well with oil piston gauges, (2) that the “drop rate” of the piston is much lower than with gas, since the viscosity of oil is higher than the viscosity of the gas being measured, and (3) that the operation of the piston-cylinder is unaffected by the cleanliness of gas under the piston.
Referring to
FIG. 1
, the pressurized gas to be measured is introduced through passage
43
into volume
42
, and exerts upward force on the bottom of rotating piston
23
, which supports a mass (not shown) supported by piston head
10
. The pressure Pg of the gas to be measured is transmitted through a tube
44
to the top of a small oil reservoir
45
containing lubricating oil
46
.
The bottom of oil reservoir
45
is coupled by a tube
47
through the wall of cylinder
16
between two O rings
48
and
49
into the approximately 1 micron gap between the vertical wall of piston
23
and the wall of cylinder
16
. The top level of the oil
46
in reservoir
45
is located a distance h above the point at which the channel
47
enters the gap, so the pressure of a column of the oil
46
always is added to the gas pressure Pg and ensures that none of the pressurized gas enters into the gap. The distance h is large enough that the head pressure of the oil
46
ensures that the oil pressure is higher than the gas pressure under the piston so that there is a slight flow of oil out the bottom end of the gap as indicated by arrow
50
B, thus preventing any of the high-pressure gas from displacing oil in the gap. The main flow of oil out of the upper end of the gap as indicated by arrow
50
A is produced by the addition of the gas pressure Pg and the head pressure of the oil.
The system of
FIG. 1
provides gas pressure measurements at the relatively low levels of accuracy that were needed 10 to 20 years ago. However, a problem of the system of
FIG. 1
is that in order to change the range of pressures of gas to be measured, it often is necessary to interchange the piston assembly
23
, and the cylinder
16
. To accomplish this interchanging for the device of
FIG. 1
, it is necessary to first remove the piston assembly
23
,
10
, and then remove the cylinder
16
. However, when piston
23
is removed, the head pressure of oil
46
in reservoir
45
causes a relatively large amount of the oil to leak out by flowing through channel
47
into the volume left open by the removal of piston
23
, and the large flow of oil continues after cylinder
16
is removed. That is quite problematic, because the oil in volume
42
, if not purged, may contaminate the gas which is measured next after another cylinder and piston have been installed. But purging the oil from volume
42
is time-consuming and costly, and also can pollute a laboratory environment with oil vapor. When oil
46
leaks or must be drained from reservoir
45
, it may be excessively time-consuming and expensive to refill the reservoir with the special oil which may be required.
A much larger problem associated with the piston gauge shown in
FIG. 1
is that for very high gas pressures, e.g., for Pg greater than approximately 7 MPa (1000 psi), the structure causes deformation of both piston
23
and cylinder
16
. The deformation of the cylinder that occurs is difficult or impossible to model mathematically, so the pressure deformation coefficient of the piston-cylinder assembly cannot be accurately mathematically computed. This makes it difficult or impossible to accurately determine the variation of “effective area” of the piston-cylinder
23
,
16
with respect to pressure. The
Cahill von Hellens & Glazer, P.L.C.
CalAmerica Corp.
Jenkins Jermaine
Lefkowitz Edward
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