Measuring and testing – Fluid pressure gauge – Electrical
Reexamination Certificate
2003-07-30
2004-10-05
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Fluid pressure gauge
Electrical
C073S719000, C073S725000, C073S734000, C073S753000
Reexamination Certificate
active
06799468
ABSTRACT:
BACKGROUND
Because the rate of heat transfer through a gas is a function of the gas pressure, under certain conditions measurements of heat transfer rates from a heated sensing element to the gas can, with appropriate calibration, be used to determine the gas pressure. This principle is used in the well known Pirani gauge (shown in schematic form in
FIGS. 1
a
and
1
b
), in which heat loss is measured with a Wheatstone bridge network which serves both to heat the sensing element and to measure its resistance.
Referring to
FIG. 1
a
, in a Pirani gauge the pressure sensor consists of a temperature sensitive resistance RS connected as one arm of a Wheatstone bridge. R2 is typically a temperature sensitive resistance designed to have a negligible temperature rise due to the current i
2
. R3 and R4 are typically fixed resistances. RS and typically R2 are exposed to the vacuum environment whose pressure is to be measured.
FIG. 1
b
illustrates an alternative bridge configuration.
Pirani gauges have been operated with constant current i
1
(as shown in U.S. Pat. No. 3,580,081), or with constant voltage across RS. In these methods, an electrical imbalance of the bridge is created which reflects gas pressure. Pirani gauges have also been operated with constant resistance RS (as shown in U.S. Pat. No. 2,938,387). In this mode, the rate at which energy is supplied is varied with changes in gas pressure, so the rate of change in energy supplied reflects changes in gas pressure. Each method of operation has differing advantages and disadvantages, but the following discussion pertains particularly to the constant resistance method and the configuration of
FIG. 1
a.
Voltage V
B
is automatically controlled to maintain the voltage difference between A and C in
FIG. 1
a
at zero volts. When the potential drop from A to C is zero, the bridge is said to be balanced. At bridge balance the following conditions exist:
i
s
=i
2
, (1)
i
4
=i
3
(2)
i
s
RS=i
4
R4, (3)
and i
2
R2=i
3
R3 (4)
Dividing Eq. 3 by Eq. 4 and using Eq. 1 and 2 gives
RS=&bgr;R2 (5) where
β
=
R4
R3
(
6
)
Thus, at bridge balance RS is a constant fraction &bgr; of R2.
To achieve a steady state condition in RS at any given pressure, Eq. 7 must be satisfied:
Electrical power input to RS=Power radiated by RS+Power lost out ends of RS+Power lost to gas by RS (7)
A conventional Pirani gauge is calibrated against several known pressures to determine a relationship between unknown pressure, P
X
, and the power loss to the gas or more conveniently to the bridge voltage. Then, assuming end losses and radiation losses remain constant, the unknown pressure of the gas P
X
may be directly determined by the power lost to the gas or related to the bridge voltage at bridge balance.
Because Pirani gauges may be designed to have wide range and are relatively simple and inexpensive, there is a long-felt need to be able to use these gauges as a substitute for much higher priced gauges such as capacitance manometers and ionization gauges. However, existing designs leave much to be desired for accurate pressure measurement, especially at lower pressures.
Prior to 1977, the upper pressure limit of Pirani gauges was approximately 20 Torr due to the fact that at higher pressures the thermal conductivity of a gas becomes substantially independent of pressure in macroscopic size devices. One of the present inventors helped develop the CONVECTRON® Gauge produced and sold by the assignee (Granville-Phillips Company of Boulder Colo.) since 1977 which utilizes convection cooling of the sensing element to provide enhanced sensitivity from 20 to 1,000 Torr. Hundreds of thousands of CONVECTRON® Gauges have been sold worldwide. Recently several imitations have appeared on the market.
Although the CONVECTRON® Gauge filled an unsatisfied need, it has several disadvantages. It has by necessity large internal dimensions to provide space for convection. Therefore, it is relatively large. Because convection is gravity dependent, pressure measurements at higher pressures depend on the orientation of the sensor axis. Also, because the pressure range where gas conduction cooling is predominant does not neatly overlap the pressure range where convection cooling occurs, the CONVECTRON® Gauge has limited sensitivity from approximately 20 to 200 Torr.
To help avoid these difficulties, microminiature Pirani sensors have been developed which utilize sensor-to-wall spacings on the order of a few microns rather than the much larger spacings, e.g., 0.5 in., previously used. See for example U.S. Pat. Nos. 4,682,503 to Higashi et al. and 5,347,869 to Shie et al. W. J. Alvesteffer et al., in an article appearing in J. Vac. Sci. Technol. A 13(6), November/December 1995, describe the most recent work on Pirani gauges known to the present inventors. Using such small sensor to wall spacings provides a pressure dependent thermal conductivity even at pressures above atmospheric pressure. Thus, such microscopic sensors have good sensitivity from low pressure to above atmospheric pressure and function in any orientation.
There are a number of problems with previous attempts to develop microminiature gauges. Although microminiature sensors provide good sensitivity over a large pressure range independent of orientation, their design is extremely complex and fabrication requires numerous elaborate processing steps in highly specialized equipment costing hundreds of thousands of dollars.
Microminiature sensors suffer from the same type of ambient temperature-caused errors as do macroscopic sensors. All of the heat loss terms in Eq. 7 are dependent on ambient temperature and on sensing element temperature at any given pressure. Thus, any attempt at pressure measurement with a Pirani gauge without temperature correction will be confused by non-pressure dependent power losses caused by changes in ambient temperature. All modern Pirani gauges attempt to correct for the errors caused by ambient temperature changes. A widely used means for correcting for such errors is to use for R2 a temperature sensitive compensating element RC in series with a fixed resistance R, as shown in
FIGS. 1
a
and
1
b.
British Patent GB 2105047A discloses the provision of an additional resistor to provide a potential divider. J. H. Leck, at page 58 of
Pressure Measurement in Vacuum
, Chapman and Hall: London (1964) notes that Hale in 1911 made R2 of the same material and physical dimensions as RS in his Pirani gauge. R2 was sealed off in its own vacuum environment and placed in close proximity to RS. When the pressures at R2 and RS were equal, excellent temperature compensation was achieved. However, at other pressures this means of temperature compensation is not very effective.
To avoid the extra cost and complexity of evacuating and sealing off R2 in a separate bulb, R2 is conventionally placed in the same vacuum environment as RS. By making R2 with a relatively large thermal mass and large thermal losses, self heating of R2 can be made negligible. Leck recommends that R2 be “made in two sections, for example, one of copper and the other Nichrome wire . . . so that the overall temperature coefficient (of R2) just matches that of the Pirani element itself (RS).” According to Leck, this method of temperature compensation has been used by Edwards High Vacuum of Great Britain in the METROVAC® brand gauge. A similar temperature compensation arrangement is used in the CONVECTRON® brand gauge.
However, this technique (using two or more materials in R2 having different temperature coefficients of resistance to approximate the temperature coefficient of RS) is effective only over a narrow range of pressure. In fact the compensation can be made exact only at one, or at most several temperatures as noted in U.S. Pat. No. 4,541,286, which discloses this form of temperature compensation in a Pirani gauge. Also, the inventors have found that configurations with a large thermal mass significantly
Allen Andre
Hamilton Brook Smith & Reynolds P.C.
Helix Technology Corporation
Lefkowitz Edward
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