GB1598550A

GB1598550A - Pressure sensing

Info

Publication number: GB1598550A
Application number: GB5043577A
Authority: GB
Original assignee: ROBERTSHAW SKIL Ltd
Current assignee: ROBERTSHAW SKIL Ltd
Priority date: 1978-05-30
Filing date: 1978-05-30
Publication date: 1981-09-23

Description

(54) PRESSURE SENSING (71) We, ROBERTSHAW SKIL UNITED, 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.

Manometers are common for sensing pressure or draught and usually take the form of a generally U-shaped tube containing fluid that is exposed either via one limb to pressure to be measured relative to atmosphere or via both limbs to pressures between which the difference is required. Problems invariably arise in accurately reading such instruments, even with movable markers and vernier scales, as it is a meniscus that is to be judged.

It is an object of this invention to facilitate reading of a manometer type device.

According to the invention there is provided a pressure sensor comprising container means for a flowable substance which container means allows independent pressure access directly to the flowable substance with the flowable substance separating and sealing each such pressure access one from another so that relative pressure variations cause movement of the flowable substance and electrical sensing means for producing a varying signal corresponding to continuous changes in an electrical or magnetic parameter associated with said movement, wherein the container means has or is associ ated with means for accomodating overscale movement of the flowable substance due to pressures beyond those intended to be sensed.

Preferably the flowable substance is a liquid and the means for sensing may be a probe extending through the maximum range of movement of the flowable substance. Such a probe may serve to detect capacitance changes dependent on the extent to which it is covered by the flowable substance. Conductance, resistance and inductance could, of course, be detected by suitable choice of flowable material and construction of probe.

In general, apart from improved "readability" due to obtaining an electrical output rather than relying on direct viewing of a meniscus, embodiments of the invention are capable of far greater sensitivity than a scale reading could provide and may also be used for control purposes as part of an overall electrical/electronic control system.

In a conventional U-tube shape for the container means, suitable probes can be Inserted down one or each limb thereof. Where both limbs have probes, a differential type detection circuit for the parameter concerned will usually be provided in order to take full advantage of the increased sensitivity made possible by the sensing of opposite liquid level variations, thereby facilitating accurate measurement of low pressure or draught.

The limbs of a U-tube need not be straight or vertical in use but may be inclined to increase sensitivity, or bent to give a desired sensitivity variation or output function or characteristics, or even of varied cross-section.

Bending is particularly attractive if the U-tube is of flexible, say plastics, Tubing.

We provide accommodation of over-scale liquid movement which is particularly useful where low draught or positive pressure measurement is involved. Suitable accommodations are for example by upper U-tube limb chambers to take the entire volume of fluid but return it later, preferably automatically on removal of excess pressure, or a cross-connection between one limb above its normal maximum measuring level and the other limb within its normal measuring range, preferably at or below the minimum level, or a "bubble-through" capability by liquid exhaustion of a smaller limb into a larger one, or a liquid filled "trap" in a connection between the two limbs above their maximum levels.

In general, the provision for producing a signal corresponding to changes in an electrical, or even magnetic, parameter related to the flowabl substance and a probe or probes avoids meniscus sighting problems and any reasonable desirec accuracy is usually attainable by electronic sensing circuitry designed or adjusted to the appropriate sensitivity.

Practical implementation of the invention will now be described, by way of example with reference to the accompanying drawings on which: Figure 1 is a diagrammatic illustration of one embodiment; Figures 2, 3, 4 and 5 are diagrammatic illustration of alternative embodiments; Figure 6 shows one differential capacitive measuring circuit; Figure 7 shows a compensation circuit for the measuring circuit of Figure 6; and Figure 8 shows another differential capacitive measuring circuit.

In Figure 1, a pressure measuring device comprises a U-tube 10 containing a measured volume of liquid 12 appropriate to the relative pressure range to be measured. Within each limb 14 of the U-tube 10 is a parameter sensing probe 16 extending into the liquid 12 to an extent corresponding to the desired range of relative pressure measurement. The probes 16 are shown connected to a differential parameter sensing circuit 18 which, for capacitive sensing, may be as described later. The upper part of each limb 14 has an enlargement 20 constituting a chamber capable of accommodating the entire volume of the liquid 12 so that it does not get swept away out of the device if temporary excess relative pressures occur between pressure connections or vents 22.

Figure 2 shows an alternative over-pressure liquid preservation provision in the form of a cross-connection 24 from an upper part of the left-most tube limb to a lower part of the rightmost tube limb so as to afford fluid circulation and accommodation, specifically when the liquid 12 goes to a level in the left-most limb below the cross-connection 24 to equalise limb pressures. The U-tube happens to be shown as of squared form and only one probe 16 is indicated for direct measurement, though two could be provided if desired.

It is also the case that the limbs of the devices, or at least one of the limbs, may not be vertical, say inclined or bent to give an output following a desired characteristic.

Figure 3 shows another embodiment where liquid preservation is by way of an interconnection 26 of the upper parts of both device limbs and includes a liquid filled trap also in the form of a U-tube 28 with liquid accommodation enlargements 30 so that excess pressure fluid can escape through the interconnection.

The embodiments of Figures 4 and 5 are more specific to single probe sensing in that only one narrow bore limb or tube is shown fed from a much larger bore limb, tube or reservoir. In Figure 4 the narrow bore probehousing limb is shown at 32 as being within a larger bore, tube or reservoir 34 for liquid 36.

In Figure 5 the narrow bore, probe-housing limb or tube is shown at 38 exterior to a larger bore tube, limb or reservoir 40. In both cases, if overpressure occurs in the narrow bore tube or limb 32 or 38 liquid will be expelled therefrom into the reservoir and pressurised fluid will bubble harmlessly through.

One suitable form for a differential capacitance measuring circuit 18 is shown in Figure 6 as comprising a matched pair of monostable circuits 74, 76 commonly available in a single package and cross-coupled as a multi-vibrator with inputs from the two probe capacitances 160 connected between ground rail 82 and, via resistors 84, voltage supply rail 86. As one monostable ends its timing cycle the other is triggered so that the overall multi-vibrator has a mark-space ratio corresponding to the ratio or probe capacitances. As only ratios involved the actual capacitance change obtained is not critical and may vary without requiring recalibration. Thus, certain parameters, such as dielectric constant, need not be stable with environment changes or for other reasons so long as both probes are smilarly affected.

Multi-vibrator 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-type divider 94, 96.

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

Variation in total height of the liquid, due to expansion, evaporation, would cause small errors in capacitance ratio. In cases where the liquid height is accurately represented in absolute capacitance values as would be the case for a probe with a stable dielectric coating, the liquid itself being conducting, such errors can be corrected electronically. Thus, reduction of liquid height would emphasise differences in probe capacitances and could be corrected by a reduction of supply voltage. Figure 7 shows a suitable circuit for providing an effective supply voltage that varies with the total period of the two multi-vibrators (determined by probe capacitances) thus giving the required compensation.

In Figure 7, a monstable cirfuit 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 multi-vibrator 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 a transister 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 111 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 8 where constant voltage and constant frequency alternating current input 124 is impressed on the probe capacitances 68C, 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 sensed using a pair of current-to-voltage converters 126 and 128 each in series with one of the probe capacitances 68C, 70C. The difference between the outputs of converters 126, 128 is applied via lines 130, 131 and resistor 134, 136 to a difference amplifier 138 providing an output at 140 for application to an alternating current voltmeter.

WHAT WE CLAIM IS: 1. A pressure sensor comprising container means for a flowable substance which container means allows independent pressure access directly to the flowable substance with the flowable substance separating and sealing each such pressure access one from another so that relative pressure variations cause movement of the flowable substance and electrical sensing means for producing a varying signal corresponding to continuous changes in an electrical or magnetic parameter associated with said movement, wherein the container means has or is associated with means for accommodating overscale movement of the flowable substance due to pressures beyond those intended to be sensed.

2. A pressure sensor according to Claim 1, wherein the flowable substance is a liquid.

3. A pressure sensor according to Claim 1 or Claim 2, wherein the means for sensing is a probe or probes each extending through the maximum range of movement of the flowable substance.

4. A pressure sensor according to Claim 3, wherein said probe or probes is or are for sensing capacitance changes dependent on the extent of coverage thereof by said flowable substance.

5. A pressure sensor according to any preceding Claim, wherein the container means provides a U-shaped container for the flowable substance with pressure access at each end of said U-shape.

6. A pressure sensor according to Claim 5 with Claim 4, having a different said probe in each limbof the U-shaped container.

7. A pressure sensor according to Claim 5 or Claim 6, wherein either or both limbs is or are inclined or bent relative to the other.

8. A pressure sensor according to Claim 5, 6 or 7, wherein the means for accommodating comprises a chamber or chambers communicating with U-tube limbs above normal maximum flowable substance levels and capable of accommodating the entire volume of flowable substance.

9. A pressure sensor according to Claim 5, 6 or 7, wherein the means for accommodating comprises a cross-connection between one limb above its normal maximum measuring level and the other limb within its normal measuring level.

10. A pressure sensor according to Claim 9, wherein the cross-connection is to the other limb below its normal maximum measuring level.

11. A pressure sensor according to Claim 5, 6 or 7, wherein the means for accommodating comprises provision for a "bubble-through" capability by exhaustion of a smaller U-tube limb into a larger one.

12. A pressure sensor according to Claim 5, 6 or 7, wherein the means for accommodating comprises a trap in a connection between the two limbs above their maximum measuring levels.

13. A pressure sensor according to Claim 3 or any one of Claims 4 to 12 as appendant to Claim 3, comprising electronic sensing circuitry connected to said probe or probes.

14. A pressure sensor according to Claim 13 as appendant to Claim 5, wherein the electronic sensing circuitry includes means for deriving a ratio of electrical capacitance measured via probes in the limbs.

15. A pressure sensor according to Claim 14, wherein said means for deriving a ratio includes multi-vibrator means for generating signals with a mark-space ratio representing the ratio of probe capacitances.

16. A pressure sensor according to Claim 15, wherein an output circuit capacitor for the multi-vibrator means provides an output as an offset between its charge voltage level and a proportion of a supply voltage level.

17. A pressure sensor according to Claim 16, wherein the output circuit capacitor provides a charge voltage level related to said supply voltage by the ratio of probe capacitance for one limb to the sum of probe capacitance for both limbs and said output is related to said supply voltage by half the ratio of the difference and sum of the probe capacitances of the two limbs.

18. A pressure sensor according to Claim 16 or Claim 17, comprising means for varying said supply voltage according to the total multivibrator mark-space period.

19. A pressure sensor according to Claim 13 or Claim 14, wherein the electronic sensing circuitry comprises a capacitance bridge circuit.

20. A pressure sensor arranged and adapted to operate substantially as herein described with reference to any one of Figures 1 to 5.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Resistor 112 and capacitor 114 are shown for averaging and smoothing this voltage which, after amplification at 116, is used to control transistor 111 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 8 where constant voltage and constant frequency alternating current input 124 is impressed on the probe capacitances 68C, 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 sensed using a pair of current-to-voltage converters 126 and 128 each in series with one of the probe capacitances 68C, 70C. The difference between the outputs of converters 126, 128 is applied via lines 130, 131 and resistor 134, 136 to a difference amplifier 138 providing an output at 140 for application to an alternating current voltmeter. WHAT WE CLAIM IS:

1. A pressure sensor comprising container means for a flowable substance which container means allows independent pressure access directly to the flowable substance with the flowable substance separating and sealing each such pressure access one from another so that relative pressure variations cause movement of the flowable substance and electrical sensing means for producing a varying signal corresponding to continuous changes in an electrical or magnetic parameter associated with said movement, wherein the container means has or is associated with means for accommodating overscale movement of the flowable substance due to pressures beyond those intended to be sensed.