GB1605179A - Pressure sensing - Google Patents

Description

(54) PRESSURE SENSING (71) We, ROBERTSHAW SKIL LIMITED, a British Company, of Greenhey Place, East Gillibrands, Skelmersdale, Lancashire WN8 9SB, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The invention relates to pressure sensing.

Our co-pending application no. 16735/76 (Serial No. 1572814) concerns a pressure sensor comprising a member movable or deformable by pressure and housed for connection at one side to a first fluid medium whose pressure is to be sensed and at its other side to a second fluid medium displaceable by the member to produce a detectable fluid level change in a communicating passageway or chamber, and a preferred differential pressure sensor comprises two chambers each equipped with a diaphragm or other member movable or deformable in response to the application of pressurised fluid to one side thereof, and a fluid-accommodating interconnection between the other sides of the diaphragms or other members forming a closed fluid accommodating volume sealed from the pressure fluids to be sensed, there being in said interconnection a movable discontinuity between the fluid communicating with respective ones of said diaphragms or other members, and means for detecting movement of said discontinuity.

The present invention concerns a modification of this arrangement wherein a further liquid between those liquid volumes moved by the respective diaphragms or other pressure responsive means as a discontinuity therebetween is associated with separate sensing of the interfaces of the further liquid and the two liquid volumes moved by pressure changes. It is then preferred that the further liquid volume be sufficient to ensure that separate interface sensing is facilitated, say by occupying an interconnection between two containers or chambers from the diaphragms or the like and of sufficient volume always to contain the diagphragm contacting liquid.

Although just as possible to utilise the wholly exterior sensing of liquid interfaces specifically indicated in application no.

16735/76, (Serial No. 1572814) it is preferred for embodiments of this invention to utilise sensor members or probes extending within the container or chambers and through the maximum variation of interface position.

Advantages arise from "absolute" detection independent of ambient condition induced changes in liquid or assembly expansion and/or variations of compressibility of the "liquids" which could, or course, be emulsions, so long as the same conditions apply at both interfaces.

In general, the same liquids will be used adjacent each pressure responsive member with a different and immiscible liquid between them to form the desired two interfaces, which may have the characteristics mentioned in our copending application, for example different dielectric constants for capacitive sensing, different resistivities for inductive sensing, or, particularly for exterior sensing, different optical characteristics such as opacity or colour, or even a physical barrier of prescribed and detectable characteristic.

One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic illustration of a pressure sensor; Figure 2 is a circuit diagram of a differential capacitive sensing circuit.

Figure 3 is a circuit diagram of a circuit for varying the supply voltage according to the sum of the probe capacitances; and Figure 4 is a circuit diagram of an alternative differential capacitance sensing circuit.

In Figure 1, two pressure sensing chambers 40 and 42 each have connection 44 and 46, respectively, to pressurised fluid to be sensed, say one to each side of a restrication in a flow pipe for that fluid. The pressure sensing chambers 40 and 42 include diaphragms 48 and 50 offering a seal to the fluid to be measured.

The other sides of the diaphragms 48 and 50 communicate with other fluid accommodating containers 52 and 54, respectively, each shown with a predetermined amount 56, 58 of liquid therein that will move according to diaphragm movement. The two containers 52 and 54 are interconnected at 60 above the maximum level for the liquid volumes 56, 58 and the space between those liquid volumes is filled with a second liquid 62 immiscible therewith.

Thus, differences of pressure at the diaphragms 48 and 50 will cause equal and opposite movements of interfaces 64 and 66 between the liquids 56, 58 and 62 in the containers 52 and 54, respectively. These could be sensed externally after the manner indicated in the above-mentioned application, but are shown to be sensed by probes 68 and 70 in a manner similar to that shown in our cofiled application nos. 50433/77 and 50435/77, again with a differential parameter, preferably capacitance, measuring circuit 72.

One suitable form for the circuit 72 is shown in Figure 2 as comprising a matched pair of monostable circuits 74, 76 commonly available in a single package and crosscoupled as a multivibrator with inputs 78, 80 from the two probe capacitances 68C and 70C connected between ground rail 82 and, via resistors 84, supply rail 86. As one monostable ends its timing cycle the other is triggered so that the overall multivibrator has a mark-space ratio corresponding to the ratio of probe capacitances. Multivibrator output line 88 is shown connected via resistor 90 and capacitor 92 to the ground rail, and any difference between the probe capacitances is represented by an output voltage Vo taken as an offset between the voltage resulting from charging of the capacitor 92 and half the supply voltage from resistor divider 94, 96.

The voltage for capacitor 92 is related to the supply voltage by the ratio of one probe capacitance (68C) to the sum of the probe capacitances, and the output voltage is related to the supply voltage by half the ratio of the difference to the sum of the probe capacitances.

The output voltage Vo will thus be zero for equal probe capacitances and will be unaffected by anything affecting both capacitances equally, but will be non-zero for even small differences of the two probe capacitances and proportionately thereto.

For direct and constant proportionality of the output voltage to the probe capacitance difference, compensation of supply voltage may be needed for small variations in the sum of their capacitances due to ambient temperature changes, evaporation, etc. Figure 3 shows a suitable circuit for varying the supply voltage according to the sum of the probe capacitances, specifically in accordance with the total period of the two multivibrators 74 and 76 of Figure 2.

In Figure 3, a monostable 98 has its time constant set by resistor 100 and capacitor 102 so as always to allow time for capacitor 104 to fully discharge. The multivibrator 98 is connected via line 106 to be triggered by the output of either of the monostables 74 and 76, and its output 108 controls transistor 110 bridging capacitor 104 and thus charging and discharging of capacitor 104. Clearly, the more frequently the capacitor 104 is discharged, the lower is the average voltage developed across it. Resistor 112 and capacitor 114 are shown for averaging and smoothing this voltage which, after amplification at 116, is used to control transistor 118 to give an appropriate supply voltage at 120.

Other differential capacitance arrangements may be used, for example a capacitance bridge circuit as shown in Figure 4 where a constant voltage and constant frequency alternating cur- rent input at 124 is impressed on the probe cap acitances 68C and 70C. The bridge is not rebalanced when the capacitances change and an output voltage varying linearly with capacitance is required, so, as capacitor impedance is inversely proportional to capacitance, current rather than voltage is shown using a pair of current-to-voltage converters 126 and 128 in series with the probe capacitances 68C and 70C, respectively. The difference between the outputs of converters 126, 128 is applied via lines 130, 132 and resistors 134, 136 to a difference aplifier 138 providing an output at 140 for application to an alternating current voltmeter.

If external sensing were used as in our copending application no. 16735/76 (Serial No.

1572814) for each interface 64, 66, similar circuity could be used and the interfaces themselves may be replaced by a solid or viscous plug slidable in each chamber or even in their interconnection, fluid containing capsules, or even a third fluid immiscible with both of the other fluids, in which case the internal probes could still be used.

WHAT WE CLAIM IS: 1. A pressure sensor comprising two chambers each equipped with a member movable or deformable in response to the application of pressurised fluid to one side thereof and containing a fluid medium displaceable by that member, a fluid-accommodating interconnection between the other sides of the movable or deformable members and forming a closed fluid-accommodating volume sealed from the pressurised fluids to be sensed, there being in said interconnection and a part of each said chamber a further fluid medium with movable discontinuities between it and the fluids displaceable directly by said members, and means for detecting movement of each said discontinuity.

2. A pressure sensor according to Claim 1, wherein equal amounts of the same fluid are provided to be moved by each said member.

3. A pressure sensor according to Claim 1 or Claim 2, wherein the further fluid is immiscible with the or each fluid to be moved by the said member.

4. A pressure sensor as claimed in Claim 1, 2 or 3, wherein each discontinuity comprises an interface between immiscible liquids.

5. A pressure sensor according to Claim 1, 2 or 3, wherein each discontinuity is a relatively small volume of liquid immiscible with the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   between those liquid volumes is filled with a second liquid 62 immiscible therewith.

   Thus, differences of pressure at the diaphragms 48 and 50 will cause equal and opposite movements of interfaces 64 and 66 between the liquids 56, 58 and 62 in the containers 52 and 54, respectively. These could be sensed externally after the manner indicated in the above-mentioned application, but are shown to be sensed by probes 68 and 70 in a manner similar to that shown in our cofiled application nos. 50433/77 and 50435/77, again with a differential parameter, preferably capacitance, measuring circuit 72.

   One suitable form for the circuit 72 is shown in Figure 2 as comprising a matched pair of monostable circuits 74, 76 commonly available in a single package and crosscoupled as a multivibrator with inputs 78, 80 from the two probe capacitances 68C and 70C connected between ground rail 82 and, via resistors 84, supply rail 86. As one monostable ends its timing cycle the other is triggered so that the overall multivibrator has a mark-space ratio corresponding to the ratio of probe capacitances. Multivibrator output line 88 is shown connected via resistor 90 and capacitor 92 to the ground rail, and any difference between the probe capacitances is represented by an output voltage Vo taken as an offset between the voltage resulting from charging of the capacitor 92 and half the supply voltage from resistor divider 94, 96.

   The voltage for capacitor 92 is related to the supply voltage by the ratio of one probe capacitance (68C) to the sum of the probe capacitances, and the output voltage is related to the supply voltage by half the ratio of the difference to the sum of the probe capacitances.

   The output voltage Vo will thus be zero for equal probe capacitances and will be unaffected by anything affecting both capacitances equally, but will be non-zero for even small differences of the two probe capacitances and proportionately thereto.

   For direct and constant proportionality of the output voltage to the probe capacitance difference, compensation of supply voltage may be needed for small variations in the sum of their capacitances due to ambient temperature changes, evaporation, etc. Figure 3 shows a suitable circuit for varying the supply voltage according to the sum of the probe capacitances, specifically in accordance with the total period of the two multivibrators 74 and 76 of Figure 2.

   In Figure 3, a monostable 98 has its time constant set by resistor 100 and capacitor 102 so as always to allow time for capacitor 104 to fully discharge. The multivibrator 98 is connected via line 106 to be triggered by the output of either of the monostables 74 and 76, and its output 108 controls transistor 110 bridging capacitor 104 and thus charging and discharging of capacitor 104. Clearly, the more frequently the capacitor 104 is discharged, the lower is the average voltage developed across it. Resistor 112 and capacitor 114 are shown for averaging and smoothing this voltage which, after amplification at 116, is used to control transistor 118 to give an appropriate supply voltage at 120.

   Other differential capacitance arrangements may be used, for example a capacitance bridge circuit as shown in Figure 4 where a constant voltage and constant frequency alternating cur- rent input at 124 is impressed on the probe cap acitances 68C and 70C. The bridge is not rebalanced when the capacitances change and an output voltage varying linearly with capacitance is required, so, as capacitor impedance is inversely proportional to capacitance, current rather than voltage is shown using a pair of current-to-voltage converters 126 and 128 in series with the probe capacitances 68C and 70C, respectively. The difference between the outputs of converters 126, 128 is applied via lines 130, 132 and resistors 134, 136 to a difference aplifier 138 providing an output at 140 for application to an alternating current voltmeter.

   If external sensing were used as in our copending application no. 16735/76 (Serial No.

   1572814) for each interface 64, 66, similar circuity could be used and the interfaces themselves may be replaced by a solid or viscous plug slidable in each chamber or even in their interconnection, fluid containing capsules, or even a third fluid immiscible with both of the other fluids, in which case the internal probes could still be used.

   WHAT WE CLAIM IS: 1. A pressure sensor comprising two chambers each equipped with a member movable or deformable in response to the application of pressurised fluid to one side thereof and containing a fluid medium displaceable by that member, a fluid-accommodating interconnection between the other sides of the movable or deformable members and forming a closed fluid-accommodating volume sealed from the pressurised fluids to be sensed, there being in said interconnection and a part of each said chamber a further fluid medium with movable discontinuities between it and the fluids displaceable directly by said members, and means for detecting movement of each said discontinuity.
2. 2. A pressure sensor according to Claim 1, wherein equal amounts of the same fluid are provided to be moved by each said member.
3. 3. A pressure sensor according to Claim 1 or Claim 2, wherein the further fluid is immiscible with the or each fluid to be moved by the said member.
4. 4. A pressure sensor as claimed in Claim 1, 2 or 3, wherein each discontinuity comprises an interface between immiscible liquids.
5. 5. A pressure sensor according to Claim 1, 2 or 3, wherein each discontinuity is a relatively small volume of liquid immiscible with the

   displaceable and further fluids.
6. 6. A pressure sensor as claimed in Claim I, 2 or 3, wherein each discontinuity comprises a solid or viscous plug slidable in the chamber or interconnection therebetween.
7. 7. A pressure sensor as claimed in Claim 1, 2 or 3, wherein each discontinuity comprises a capsule of fluid slidable in the interconnection.
8. 8. A pressure sensor as claimed in any preceding claim wherein each discontinuity is between fluid of different dielectric constants, means for detecting each discontinuity comprising capacitive means associated with each chamber or their interconnection.
9. 9. A pressure sensor as claimed in any preceding claim, wherein each discontinuity is between fluids of different resistivities, means for detecting each discontinuity comprising inductive means associated with chambers or their interconnection.
10. 10. A pressure sensor as claimed in any preceding claim, wherein each discontinuity is optical.
11. 11. A pressure sensoras claimed in Claim 10, wherein each discontinuity is between contrasting fluids.
12. 12. A pressure sensor as claimed in Claim 10 or Claim 11, wherein means for detecting movement of each discontinuity comprises transparent sections of the chamber or their interconnections.
13. 13. A pressure sensor as claimed in any preceding claim, wherein each member movable or deformable by pressure is a diaphragm.
14. 14. A pressure sensor according to Claim 1, 2, 3 or 4, wherein the means for detecting includes a probe in each container.
15. 15. A pressure sensor according to Claim 14, wherein the probes are connected to differential capacitance-type measuring means.
16. 16. A pressure sensor substantially as herein described with reference to Figure 1 of the accompanying drawings.
17. 17. A pressure sensor according to Claim 15 or Claim 16 having differential capacitance measuring means substantially as herein described with reference to Figure 1 or Figures 2 and 3 or Figure 4 ofthe accompanying drawings.