GB1572814A

GB1572814A - Pressure sensing

Info

Publication number: GB1572814A
Application number: GB1673576A
Authority: GB
Original assignee: ROBERTSHAW SKIL Ltd
Current assignee: ROBERTSHAW SKIL Ltd
Priority date: 1977-04-07
Filing date: 1977-04-07
Publication date: 1980-08-06

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.

There are requirements, for example in the chemical engineering industry and hydraulic / pneumatic systems engineering,' for accurately measuring pressure drops in fluid flow lines or conduits, for example upstream and downstream of a restrictor or orifice plate interposed in the line or conduit.

Previous methods of sensing such pressure drops have utilised a diaphragm to opposite sides of which upstream and downstream fluid pressures are applied, with movement of the diaphragm in response to a pressure differential being sensed by direct nnections thereto, such as by way of torque-tnbes or capacitive plates mounted to the pressure plate or a mount therefor. Problems arise in relation to making mechanical or electrical connectioos through a chamber body in which the diaphragm is mounted.

It is an object of this invention to provide a pressure sensor that does not require connections to be taken therefrom through a chamber or other enclosure.

According to the invention there is provided 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, a fluid; accomodating interconnection between the other sides of the movable or deformable rnembers and forming a dosed fluid-accomu; dating volume sealed from the pressurised fluids to be sensed, there being in said interconnection a movable discontinuity between the fluid communicating with respective ones of said meatbers, and means for providing output continuously following movement of said discontinuity at least over a range thereof.

Where the interconnection has different fluids communicating with two diaphragms, respectively, those fluids are preferably immiscible liquids with the discontinuity constintted by the interface between those liquids. Preferably, such different fluids have different dielectric constants so that move ment of their interface is readily detected by capacitive means associated with the inter; ooruect,oa.

If the liquids have different resistivities, e.g.

one of the liquids is conductive and the other is not, inductive sensing of their interface may be usei For optically different fluids means may be provided for optically detecting movement of the interface. For example for appropriately contrasting colours of the two liquids, a transparent section of pipe in said interconnection may give a direct visual reading.

The discontinuity could be a comparatively small volume of liquid imuliscible with the contacting fluids, which may then be the same each side of the diaphragm or other member. Alternatively, the discontinuity may be a solid or viscous plug, or a capsule of fluid slidable in the interconnection. The same alternatives regarding sensing of the position of the discontinuity also apply to these alternatives.

The cross-sectional area of the inter conretion where the said fluid level change is detected will determine the range of move, neent of that discontinuity and, if much less than the area of the members for displacing the second fluid gives a desired magnification of file actual movement of the diaphragms or other members.

Although incompressible liquids are to be preferred within the irtercoectior, the use of compressible media for example emulsions, is feasible if desired.

One embodiment of the invention, will now be described, by way of example, with reference to the accompanying drawing showing a dlagrammatic representation of a differential pressure sensing system.

In the drawing, a channel or conduit 10 is shown with a restriction 12 exemplified as an orifice plate. On each side of the restriction pipes 14 and 16 are shown extending to pressure sensing chambers 18 and 20 respectively.

Each of the pressure sensing chambers is indicated as being divided into two parts, subscripted A and B. The sub chambers 18A and 20A are in communication only with the pipes 14 and 16, respectively. The subchambers 18B and 20B are interconnected by a narrow bore pipe 22 and form a closed system.

The partitioning of the chambers 18 and 20 is indicated as being by membrane type diaphragms 24 and 26, respectively, but any other suitable means may be used, such as substantially rigid plates in resilient annular mounts, pistons, bellows, capsules, or other manners movable or deformable by pressure.

The dosed volume between the diaphragms 24 and 26 via the pipe 22 is indicated as being filled with immiscible liquids of different dielectric constants, such as oil and water, having an interface 28 along the pipe 22. In operation different pressures applied through pipes 14 and 16 will cause differential movement of the chamber diaphragms 24 and 26 and a corresponding translation of the interface between the liquids. This translation will be proportionate to pressure difference and a narrow diameter of the pipe 22, at least at the position of the interface 28, will serve to give a suitable amplification of the actual mmremants of tbe diaphragms 24 and 26.

The interface 28 constitutes a detectable discontinuity and detection of its position is conveniendy achieved using capacitance means where the liquids have different di electric constants. Such a capacitive pick-off is indicated purely diagrammatically by two plates 30 and 32, and in practice, may be formed in any suitable way, such as inter leaved coiled foils. A suitable capacitance detector is indicated in block form at 36 and also may take any convenient form. A particularly suitable system, based on one already marketed by the Applicants, may be based on capacitance charging rate with reference capacitor being used to guarantee successive recharging cycles at a minimum rate by triggering momentary discharging for a given voltagelevel. Direct current charging ciruits, or saw-tooth oscillator charging circuits may be used, as indeed may other schemes.

The two chambers may be parts of a single bloct and the intercommunication may be integral with the same block. The above-noted variants fos the discontinuity and sensing may also be used. Valves will also normally be pid in tbe pipes 14 and 16.

A single partitioned chamber with a fluid accomodan.n expansion duct mav be used for direct pressure sensing in relation to the level of fluid expressed by movement of the chamber division. Effectively such a pressure gauge is one side only of the differential system illustrated and the expansion duct may be vented to atmosphere, or evacuated to permit absolute pressure measurement A capacitance detector, such as the block referenced 36, may be designed to derive any desired function of the actual change of capacitance, for example a square root function to give an output or indication that is directly proportional to flow. Alternatively the capacitance pick-off may be physically formed to provide the square root or other function itself.

WHAT WE CLAIM IS : - 1. A pressure sensor comprising two chambers each equipped with a member mov able or deformable in response to the applica tion of pressurised fluid to one side thereof, a fluid-accomodating intercon nertion between the other sides of the movable or deformable members and forming a closed fluid-accomo dating volume sealed from the pressurised fluids to be sensed, there being in said inter connection a movable discontinuity between the fluid communicating with respective orees of said members, and means for providing output continuously following movement of said discontinuity at least over a range te.

2. A pressure sensor as claimed in claim 1 wherein the intelxQnstix is cglLtinue walled.

3. A pressure sensor as claimed in claim 1 wherein the fluid cmss-section bearing on the discontinuity are equal.

4. A pressure sensor as claimed in claim 1 wherein the discontinuity has a constant geometry.

5. A pressure sensor as claimed in claim 1 wherein the discontinuity has a tubular constraint.

6. A pressure sensor as claimed in any of claims 1 to 5, wherein the interconnection has different fluids communicating with the two diaplagms respectively.

7. A pressure sensor as claimed in any of claims 1 to 6, wherein the discontinuity comprises a relatively small volume of liquid imiscible with the fluids communicating with the movable members.

8. A pressure sensor as claimed in any of claims 1 to 5, wherein the discontinuity comprises a solid or viscous plug slidable in the interconnection.

9. A pressure sensor as claimed in any of claims 1 to 6, wherein the discontinuity comprises a capsule of fluid slidable in the interconnecion.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. dlagrammatic representation of a differential pressure sensing system. In the drawing, a channel or conduit 10 is shown with a restriction 12 exemplified as an orifice plate. On each side of the restriction pipes 14 and 16 are shown extending to pressure sensing chambers 18 and 20 respectively. Each of the pressure sensing chambers is indicated as being divided into two parts, subscripted A and B. The sub chambers 18A and 20A are in communication only with the pipes 14 and 16, respectively. The subchambers 18B and 20B are interconnected by a narrow bore pipe 22 and form a closed system. The partitioning of the chambers 18 and 20 is indicated as being by membrane type diaphragms 24 and 26, respectively, but any other suitable means may be used, such as substantially rigid plates in resilient annular mounts, pistons, bellows, capsules, or other manners movable or deformable by pressure. The dosed volume between the diaphragms 24 and 26 via the pipe 22 is indicated as being filled with immiscible liquids of different dielectric constants, such as oil and water, having an interface 28 along the pipe 22. In operation different pressures applied through pipes 14 and 16 will cause differential movement of the chamber diaphragms 24 and 26 and a corresponding translation of the interface between the liquids. This translation will be proportionate to pressure difference and a narrow diameter of the pipe 22, at least at the position of the interface 28, will serve to give a suitable amplification of the actual mmremants of tbe diaphragms 24 and 26. The interface 28 constitutes a detectable discontinuity and detection of its position is conveniendy achieved using capacitance means where the liquids have different di electric constants. Such a capacitive pick-off is indicated purely diagrammatically by two plates 30 and 32, and in practice, may be formed in any suitable way, such as inter leaved coiled foils. A suitable capacitance detector is indicated in block form at 36 and also may take any convenient form. A particularly suitable system, based on one already marketed by the Applicants, may be based on capacitance charging rate with reference capacitor being used to guarantee successive recharging cycles at a minimum rate by triggering momentary discharging for a given voltagelevel. Direct current charging ciruits, or saw-tooth oscillator charging circuits may be used, as indeed may other schemes. The two chambers may be parts of a single bloct and the intercommunication may be integral with the same block. The above-noted variants fos the discontinuity and sensing may also be used. Valves will also normally be pid in tbe pipes 14 and 16. A single partitioned chamber with a fluid accomodan.n expansion duct mav be used for direct pressure sensing in relation to the level of fluid expressed by movement of the chamber division. Effectively such a pressure gauge is one side only of the differential system illustrated and the expansion duct may be vented to atmosphere, or evacuated to permit absolute pressure measurement A capacitance detector, such as the block referenced 36, may be designed to derive any desired function of the actual change of capacitance, for example a square root function to give an output or indication that is directly proportional to flow. Alternatively the capacitance pick-off may be physically formed to provide the square root or other function itself. WHAT WE CLAIM IS : -

1. A pressure sensor comprising two chambers each equipped with a member mov able or deformable in response to the applica tion of pressurised fluid to one side thereof, a fluid-accomodating intercon nertion between the other sides of the movable or deformable members and forming a closed fluid-accomo dating volume sealed from the pressurised fluids to be sensed, there being in said inter connection a movable discontinuity between the fluid communicating with respective orees of said members, and means for providing output continuously following movement of said discontinuity at least over a range te.

10. A pressure sensor as claimed in any one

of claims 1 to 6, wherein the discontinuity comprises an interface beween immiscible liquids.

11. A pressure sensor as claimed in any of claims 1 to 10, wherein the discontinuity is of different dielectric constants, capacitive means for detecting the discontinuity are associated with the interconnection.

12. A pressure sensor as claimed in any of claims 1 to 10, wherein the discontinuity is of different resistivities and means for detecting the discontinuity comprising inductive means are associated with the interconnection.

13. A pressure sensor as claimed in any of claims 1 to 10, wherein the discontinuity is optically detachable.

14. A pressure sensor as claimed in claim 13, wherein the discontinuity is between contrasting fluids.

15. A pressure sensor as claimed in claim 13 or 14, including means for detecting movement of the discontinuity comprising a transparent section of the interconnection passageway.

16. A pressure sensor as claimed in any preceding claim, wherein the or each member movable or deformable by pressure is a diaphragm.

17. A pressure sensor as claimed in any preceding claim wherein the cross-sectional area of the said passageway or chamber where the fluid level change is detected is less than the cross-sectional area of the said member for displacing said second fluid medium.

18. A pressure sensor arranged and adapted to operate substantially as described herein with reference to and as shown in the accompanying drawing.