GB1558770A - Differential pressure transducers - Google Patents

Description

(54) IMPROVEMENTS IN DIFFERENTIAL PRESSURE TRANSDUCERS (71) We. BELL & HOWELL LIMITED.

a British Company, of Lennox Road, Basingstoke, Hampshire, RG22 4AW, 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: This invention relates to differential pressure transducers.

Differential-pressure (d.p.) transducers are used to determine the difference in fluid pressure between two points, and one typical use is in the determination of the difference in line pressure of a liquid across an orifice plate in a flow measuring system.

Thus d.p. transducers can find application in which the applied differential pressure is small relative to the line pressures of the fluids under consideration.

In d.p. transducers to be used with line pressures far in excess of their rated differential pressure, a protective means is required for limiting the maximum differential pressure which can be applied to the differential pressure-responsive sensing element in the event of a collapse of line pressure on one side of the transducer. To achieve this, the transducer can be filled with a low compressibility fluid, so that excess differential pressure beyond the maximum rated differential pressure of the transducer causes a bellows, diaphragm or similar device to operate a valve, which traps a pocket of fluid between the body of the transducer and the sensing element, or isolating diaphragm, or both. Further differential pressure increase causes a pressure rise on both sides of the sensing element or only in a trapped pocket of fluid remote from the sensing element.

Where the sensing element is itself a diaphragm; the sensing element may be made to conform to a shaped support when sub elected to - overpressure, i.e. excess differential pressure beyond the maximum, rated differential pressure of the transducer. This requires extreme care in the profiling and surface finishing of the support to avoid local strains in the sensing element.

According to the invention there is provided a differential pressure transducer for determining the differential pressure between first and second fluids, or first and second parts of a fluid continuum, comprising differential-pressure sensing and measuring means so arranged in a hollow chamber as to define two fluid-tight sub-chambers each having a planar external wall surface, two isolating diaphragms secured around their peripheries to the chamber so that in their unstressed state they lie flat against the two planar wall surfaces respectively, means in each of said walls for providing fluid communication between the internal surface of each of said diaphragms and a corresponding part of the differential-pressure sensing means, and first and second means for applying the first and second fluids to the external surfaces of respective diaphragms, the volume of said chamber, defined in part by the diaphragms, being filled with a low compressibility fluid at a pressure such that each diaphragm is distended slightly from its associated planar wall surface when the differential pressure between said first and second fluids is less than a predetermined magnitude.

One embodiment of a differential-pressure transducer according to the invention will now be described by way of example with reference to the sole figure of the accompanying drawing.

Referring to the figure, there is shown a d.p. transducer 10 for determining the differential pressure between first and second fluids applied, in use, to first and second inlets 12 and 14 respectively in a transducer housing 16 of, for example stainless steel.

A differential-pressure sensing means, or sensing element, in the form of a diaphragm 18 having an associated position or strainsensing device (not shown) such as a straingauge bridge is mounted in a central chamber of a stainless steel body 19 to define two fluid-tight sub-chambers 20, 22. The dia phragm 18, which may be of any suitable material such as stainless steel, and sensing device are conventional and further description is considered unnecessary.

Each sub-chamber 20, 22 has a planar, external wall surface 20a and 22a. As shown the wall surfaces 20a 22a conveniently, but not necessarily oppose each other.

Two isolating diaphragms 24, 26 are fixedly mounted around their periphery to the body 19 so that in their normal, unstressed state they lie flat against the two wall surfaces 20a and 22a respectively and their centre area can move perpendicularly to their plane. The planar faces 20a, 22a of the body 19 against which the diaphragms 24, 26 are mounted are flat and smooth so that the diaphragms may only be distended away from the body 19.

Means, in the form of sintered metal porous plugs 28, 30 fitted in the centre of the wall surfaces 20a, 22a respectively and ducts 32, 34, provide fluid communication between the internal surface 24a, 26a of each diaphragm 24, 26 and a corresponding surface 18a, 18b respectively of the sensing diaphragm 18. The porous plugs 28, 30 are fitted into the centre of the wall surface 20a, 22a so that the top face of the plug is level with its associated wall surface.

The whole of the volume between the isolating diaphragms 24, 26 is filled with a low compressibility fluid, for example, silicone oil. A small additional volume of fluid is then introduced under slight pressure, to distend the isolating diaphragms 24, 26 away from the body 19, by an amount such that when fluids at an excessive differential pressure are applied to inlet ports 12 and 14, the isolating diaphragm on the higher pressure side collapses to lie flat against the corresponding wall surface of the body 19 and the volume of fluid so displaced from between the isolating diaphragm and the body 19 is insufficient to damage the sensing element 18. Further increases in differential pressure simply hold the isolating diaphragm on the higher pressure side firmly against the wall surface of the body 19, without applying any further pressure to the sensing element 18. Because the isolating diaphragm 24 or 26 is returned to its original, flat condition it suffers no local straining, and because its support, the wall surface 20a or 22a of the body 19 is flat, manufacture is cheap and simple.

Where the normal movement of the sensing element 18 is small, typically a few tenths of a thousandth of an inch, the amount by which the isolating diaphragms 24 and 26 must be distended is also small and the need for manufacturing tolerances may impose limitations on the achievable overpressure conditions. Further, the effects of compressibility and thermal expansion of the filling fluid must be small compared to the displacement of the isolating diaphragms 24 and 26. To increase the travel of the isolating diaphragms 24 and 26 without increasing compressibility and thermal effects, the sensing element 18 may be mounted in a rigid holder 36 which may be mounted in a flexible member 38 such as another diaphragm or bellows. The stiffness of the diaphragm 38 may be adjusted to give the required travel of the isolating diaphragms 24 and 26.

Thus there has been described a transducer which eliminates the need for complex shut-off valves and avoids expensive profiling operations. It is applicable to d.p.

transducers in which the isolating diaphragms are used solely to transmit pressure from the process fluid to the fill fluid in the cell and are not mechanically connected to the sensing element.

WHAT WE CLAIM IS: 1. A differential pressure transducer for determining the differential pressure between first and second fluids, or first and second parts of a fluid continuum, comprising differential-pressure sensing and measuring means so arranged in a hollow chamber as to define two fluid-tight sub-chambers each having a planar external wall surface, two isolating diaphragms secured around their peripheries to the chamber so that in their unstressed state they lie flat against the two planar wall surfaces respectively, means in each of said walls for providing fluid com municataion between the internal surface of each of said diaphragms and a corresponding part of the differential-pressure sensing means, and first and second means for applying the first and second fluids to the external surfaces of respective diaphragms, the volume of said chamber, defined in part by the diaphragms, being filled with a low compressibility fluid at a pressure such that each diaphragm is distended slightly from its associated planar wall surface when the differential pressure between said first and second fluids is less than a predetermined magnitude.

2. A transducer according to Claim 1, in which each means for providing fluid communication between the internal surface of each of said diaphragms and a corresponding part of the differential-pressure sensing means is a porous plug mounted in a respective one of said wall surfaces.

3. A transducer according to Claim 2, in which each porous plug is of metal.

4. A transducer according to Claim 3, in which each porous plug is of sintered metal.

5. A transducer according to Claim 2, 3 or 4 in which a face of the plug is level with the external wall surface of its associated wall.

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

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. phragm 18, which may be of any suitable material such as stainless steel, and sensing device are conventional and further description is considered unnecessary. Each sub-chamber 20, 22 has a planar, external wall surface 20a and 22a. As shown the wall surfaces 20a 22a conveniently, but not necessarily oppose each other. Two isolating diaphragms 24, 26 are fixedly mounted around their periphery to the body 19 so that in their normal, unstressed state they lie flat against the two wall surfaces 20a and 22a respectively and their centre area can move perpendicularly to their plane. The planar faces 20a, 22a of the body 19 against which the diaphragms 24, 26 are mounted are flat and smooth so that the diaphragms may only be distended away from the body 19. Means, in the form of sintered metal porous plugs 28, 30 fitted in the centre of the wall surfaces 20a, 22a respectively and ducts 32, 34, provide fluid communication between the internal surface 24a, 26a of each diaphragm 24, 26 and a corresponding surface 18a, 18b respectively of the sensing diaphragm 18. The porous plugs 28, 30 are fitted into the centre of the wall surface 20a, 22a so that the top face of the plug is level with its associated wall surface. The whole of the volume between the isolating diaphragms 24, 26 is filled with a low compressibility fluid, for example, silicone oil. A small additional volume of fluid is then introduced under slight pressure, to distend the isolating diaphragms 24, 26 away from the body 19, by an amount such that when fluids at an excessive differential pressure are applied to inlet ports 12 and 14, the isolating diaphragm on the higher pressure side collapses to lie flat against the corresponding wall surface of the body 19 and the volume of fluid so displaced from between the isolating diaphragm and the body 19 is insufficient to damage the sensing element 18. Further increases in differential pressure simply hold the isolating diaphragm on the higher pressure side firmly against the wall surface of the body 19, without applying any further pressure to the sensing element 18. Because the isolating diaphragm 24 or 26 is returned to its original, flat condition it suffers no local straining, and because its support, the wall surface 20a or 22a of the body 19 is flat, manufacture is cheap and simple. Where the normal movement of the sensing element 18 is small, typically a few tenths of a thousandth of an inch, the amount by which the isolating diaphragms 24 and 26 must be distended is also small and the need for manufacturing tolerances may impose limitations on the achievable overpressure conditions. Further, the effects of compressibility and thermal expansion of the filling fluid must be small compared to the displacement of the isolating diaphragms 24 and 26. To increase the travel of the isolating diaphragms 24 and 26 without increasing compressibility and thermal effects, the sensing element 18 may be mounted in a rigid holder 36 which may be mounted in a flexible member 38 such as another diaphragm or bellows. The stiffness of the diaphragm 38 may be adjusted to give the required travel of the isolating diaphragms 24 and 26. Thus there has been described a transducer which eliminates the need for complex shut-off valves and avoids expensive profiling operations. It is applicable to d.p. transducers in which the isolating diaphragms are used solely to transmit pressure from the process fluid to the fill fluid in the cell and are not mechanically connected to the sensing element. WHAT WE CLAIM IS:

1. A differential pressure transducer for determining the differential pressure between first and second fluids, or first and second parts of a fluid continuum, comprising differential-pressure sensing and measuring means so arranged in a hollow chamber as to define two fluid-tight sub-chambers each having a planar external wall surface, two isolating diaphragms secured around their peripheries to the chamber so that in their unstressed state they lie flat against the two planar wall surfaces respectively, means in each of said walls for providing fluid com municataion between the internal surface of each of said diaphragms and a corresponding part of the differential-pressure sensing means, and first and second means for applying the first and second fluids to the external surfaces of respective diaphragms, the volume of said chamber, defined in part by the diaphragms, being filled with a low compressibility fluid at a pressure such that each diaphragm is distended slightly from its associated planar wall surface when the differential pressure between said first and second fluids is less than a predetermined magnitude.

2. A transducer according to Claim 1, in which each means for providing fluid communication between the internal surface of each of said diaphragms and a corresponding part of the differential-pressure sensing means is a porous plug mounted in a respective one of said wall surfaces.

3. A transducer according to Claim 2, in which each porous plug is of metal.

4. A transducer according to Claim 3, in which each porous plug is of sintered metal.

5. A transducer according to Claim 2, 3 or 4 in which a face of the plug is level with the external wall surface of its associated wall.

6. A transducer according to any one of

Claims 1 to 5, in which the differentialpressure sensing means is mounted in a rigid holder which is, itself, mounted on a flexible member such as a diaphragm or bellows.

7. A transducer according to Claim 6, in which the stiffness of the flexible member is adjustable.

8. A transducer according to any one of Claims 1 to 7, in which the differentialpressure sensing means is a diaphragm.

9. A transducer according to Claim 8 in which the differential-pressure sensing diaphragm has a strain gauge bridge thereon.

10. A transducer according to any one of the preceding claims, in which the said external wall surfaces oppose each other on either side of the differential-pressure sensing means.

11. A differential pressure transducer substantially as hereinbefore described with reference to and as illustrated in the sole figure of the accompanying drawing.