Measuring and testing – Fluid pressure gauge – With pressure and/or temperature compensation
Reexamination Certificate
2000-02-23
2001-11-13
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Fluid pressure gauge
With pressure and/or temperature compensation
C073S720000
Reexamination Certificate
active
06314815
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a pressure sensor of the type comprising strain gauges mounted in respective arms of a Wheatstone bridge.
The field to which the invention relates more particularly is that of pressure sensors usable at very low temperatures, typically at temperatures below the boiling rate of nitrogen, and possibly going down to a few Kelvins only.
BACKGROUND OF THE INVENTION
A well-known problem with strain gauge pressure sensors connected as a Wheatstone bridge is that of the zero point of the bridge drifting as a function of temperature.
To resolve that problem, use is generally made of compensation networks connected to the inputs and/or the outputs of the bridge. Those networks which include at least one temperature-sensitive element, are generally designed to provide linear compensation, given that the temperature drift of strain gauge bridges is substantially linear over ordinary temperature ranges.
An additional difficulty arises when the pressure sensors are used at very low temperatures. The temperature drift of strain gauge bridges then becomes non-linear, and the non-linearity increases with decreasing temperature. This can be seen from
FIG. 1
which shows how the output voltage from a strain gauge bridge made up of thin film nickel-chromium deposits on a silicon substrate varies as a function of temperature, the bridge being at equilibrium (zero output voltage) at a temperature of 22° C.
Ordinary compensation networks become ineffective, particularly since the thermistors used generally also have resistance that becomes very high and virtually infinite when the temperature drops below −40° C. or −50° C.
The problem which the present invention seeks to resolve is that of compensating the non-linearity of temperature drift, and consequently that of “linearizing” such drift at very low temperatures, in particular at temperatures below the boiling point of nitrogen, i.e. below about −196° C.
BRIEF SUMMARY OF THE INVENTION
The problem is resolved by a pressure sensor in which the strain gauge of at least one of the arms of the bridge is in series with a resistor of resistance that is much less than that of the strain gauge and which is connected in parallel with a compensation circuit comprising a resistive element whose resistance varies as a function of temperature so as to influence the resistance of the resistor with which it is connected in parallel over the very low temperature range in a manner that is substantial and that increases with decreasing temperature.
The resistor having the compensation circuit connected in parallel therewith is of resistance that is much less than that of the strain gauge so as to avoid penalizing the magnitude of the operating range and the sensitivity of the bridge. In the present specification, a resistance that is “much less than” another resistance is less than {fraction (1/20)}th or even {fraction (1/100)}th or even less of the resistance of a strain gauge.
According to a feature of the pressure sensor of the invention, the compensation circuit is connected in parallel with a resistor formed by a connection lead connecting the strain gauge to one of the corners of the bridge.
Thus, there is no need to modify the bridge in order to connect the compensation circuit thereto. This avoids any instabilities that could be generated if it were necessary to open the bridge for this purpose.
In addition, the circuit for compensating non-linearity can thus be placed very close to the bridge, thereby exposing it to exactly the same temperature conditions as the bridge.
The strain gauges and the connection leads connecting the strain gauges to the corners of the bridge are preferably constituted by deposits of metal on a substrate.
To compensate non-linearity in the very low temperature range, i.e. below −196° C., the resistive element of the compensation circuit must have a resistance such as to have an influence on the resistance of the resistor with which it is connected in parallel, and to do so increasingly with decreasing temperature. To this end, it is preferable for the ratio of the resistance of the resistive element to that of the resistor on which it is connected in parallel to be no greater than 100 when the temperature drops below −196° C., and to decrease thereafter with decreasing temperature. By way of example, such a resistive element can be constituted by a platinum probe.
REFERENCES:
patent: 3645136 (1972-02-01), Calhoun
patent: 4333349 (1982-06-01), Mallon et al.
patent: 4414853 (1983-11-01), Bryzek
patent: 4911016 (1990-03-01), Miyazaki et al.
patent: 5877423 (1999-03-01), Mattsson
patent: 2 497 346 A (1982-07-01), None
patent: 55 163880 A (1981-03-01), None
patent: 62 121302 A (1987-11-01), None
Aw-Musse Abdullahi
Fuller Benjamin R.
Societe Nationale d'Etude et de Construction de Moteurs d&a
Weingarten, Schurgin Gagnebin & Hayes LLP
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