FLUID VOLUME MEASUREMENT DEVICE

Abstract: A fluid volume measuring device includes a rigid support (3) of annular configuration at one end of which a resilient membrane (1) is secured. Membrane (1) includes an electrically conductive portion (2) and, in use, is arranged to support the fluid whose volume is to be measured. The membrane resiliently deforms causing deformation of the electrically conductive portion (2) giving rise to a change of resistance in the electrically conductive portion (2) which is measured by a measuring device (18). The membrane (1) and the electrically conductive portion (2) are each silicone elastomers.

Description: ^ Fluid Volume Measurement Device This invention relates to a fluid measuring device, especially but not exclusively, for measuring a quantity of liquid in a tank. Conventional fluid volume measuring devices such as, for example, devices for measuring the quantity of fuel contained in a vehicle fuel tank, are generally in the form of mechanical float devices. Such devices comprise a float mounted on the end of a pivoting arm which is in turn connected to a variable resistance. A rise or fall in the level of liquid in the tank causes a corresponding movement of tne float and in turn a variation in resist¬ ance which is generally represented by a change in position of a needle on a gauge. There are, however, a number of disadvantages associated with this type of device. The float and operating arm are generally lightweight in construction to ensure that they do not occupy too great a space in the tank and to ensure accuracy in response to changes of liquid level and thus these items are easily susceptible to damage. Further¬ more, as the float relies on its buoyancy in liquid to function the device is not suitable for use in measuring quantities of other fluids such as particulate solids. It is an object of the present invention to provide a fluid volume measuring device of robust construction and capable of operation with different types of fluid. According to the present invention there is provided a fluid volume measuring device comprising a resilient membrane peripherally. secured to a support structure, means imposing a reference pressure on one surface of the membrane, the other membrane surface being presented to the pressure of the fluid volume whereby, in use, the shape of the membrane changes in response to the fluid pressure acting on the membrane, and wherein the membrane comprises an electrically conductive portion the conductivity of which is determined by the membrane shape, electrical conductivity measuring means being connected to said portion. Preferably, the elastic modulus of the resilient membrane is the same as that of the electrically conductive portion. Preferably the resilient membrane is a circular sheet of elastomeric material fixedly secured to an annular support structure. Preferably, the electrically conductive portion is made of a material having an electrical resistivity related to stress loading on the material. Examples of such mater¬ ials are described in European Published Patent Application Number 0089 843A1. Preferably the electrically conductive portion defines a continuous elongate conductive path the ends of which extend from the measuring means and which is configured to traverse the surface area of the membrane with at least part of the path leading from the membrane periphery to the membrane centre. Alternatively, the electrically conductive portion may define a region contiguous with the greater part of the surface area of the membrane, the measuring means being connected to said region at contacts one of which extends along the membrane perip ery and the other of which is at the membrane centre. i_mbodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of one embodiment of a resilient membrane of a fluid volume measuring device in accordance with the present invention; Fig. 2 is a perspective view of a support structure for the resilient membrane of Fig. 1; Fig. 3 is a plan view of the resilient membrane of Fig. 1; . Fig. 4 is a sectional side view showing a fluid volume measuring device according to the present invention in use; Fig. 5 is a perspective view of a second embodiment of a resilient membrane of a fluid volume measuring device; and Fig. 6 is a perspective view of a support member for the resilient member of Fig.5. Referring to the drawings, a fluid volume measuring device comprises a circular resilient membrane 1 formed from an elastomeric material having an electrically conductive portion 2 which defines a continuous elongate conductive path extending from a first contact point 5 at the periphery of the membrane 1 to a second adjacent contact point 5 with parts of the conductive path extending generally radially of the membrane. The membrane 1 is about 0.25 mm thicK and made of non-conductive elast¬ omeric material and conductive portion 2, which is bonded to the surface of the membrane 1, is made of an electrically conductive elastomeric material about 0.3 mm thick. The membrane 1 is mounted on and secured around its periphery by bonding to the end of a tubular housing 3 which forms the membrane support structure with the contact points 5 aligned with and making electrical connection with leads 4 secured to the housing 3 and extending therefrom in a plastic-clad earth shielded cable 7 for connection to a conductivity measuring circuit (not shown) ,the cable 7 extending through a cond¬ uit 6 projectin from the sidewall of the housing -3. The membrane l has an effective or operational diameter of 38 mm and, as is shown in Fig. 3, the conductive path formed by the portion 2 has four limbs each with a pair of pathlengths leading from the membrane centre towards the membrane periphery and about 20 mm in length. The conductive path is about 1 mm in width throughout its length with the pair of path lengths of each limb separated by about 1mm. Each pair of path lengths of three of the limbs are interconnected at their ends adjacent the periphery of the membrane 1 and these three limbs are angularly separated by about 60 . The other limb extends from the contact point 5 in a direction which bisects the angle between the adjacent two limbs. The housing 3 is made of plastic and has a diameter of about 38 mm. Referring now to Fig. 4 one example of the fluid volume measuring device in use is illustrated. The device is arranged so that the pressure of a fluid reservoir 9 acts on the upper surface of the membrane l and the space 10 below the membrane 1 is open to atmosphere via the cable conduit 6. The membrane 1 and the associated conductive portion 2 are deformed in proportion to the pressure of fluid acting upon them Thus as the pressure of the fluid acting on the membrane 1 is proportional to the depth of fluid above the membrane 1 the membrane 1 deforms and the conductivity of the portion 2 varies accordingly. A suitably calibrated device can thus be used to provide a readout of the fluid volume. Calibration is effected using a depth gauging rod 8 set at a known height H above the membrane 1, and filling the reservior y with a Jnown fluid (e.g. water) to tne level H set by the rod 8. The particular shape of the conductive portion 2 shown in Fig. 3 means that the stress acting on the portion 2 is integrated over its length and compensates for variations in stress across the membrane 2 caused by tilting of the device. The second embodiment, illustrated in Figs. 5 and 6 operates in a similar fashion to that of the first embodiment but the conductive portion is in the form of a sheet li bonded to the membrane 1, the sheet 11 extend¬ ing over almost the whole area of the membrane 1 and having a central contact point 12. The sheet 11 is locat¬ ed on the undersurface of the membrane 1 and when mounted on its drum like housing 13 (Fig. 6) an annular peripheral zone of the sheet 11 makes electrical contact with a ring contact 14 mounted on the. end wall of the housing 13 whilst central contact point lϋ is connected to a flex¬ ible conductor 15 within the housing 13, contact 14 and conductor 15 being connected to a plastic-clad earth snielded cable 17 extending from the housing 13 via con¬ duit 16 to the conductivity monitoring device which, as shown in Fig. 5 forms part of a computer system 18. The membrane 1 is about 38 mm in diameter and about 0.1 mm thick whilst the sheet 11 is about 0.3 mm thick. The conductive sheet 11 does not produce discrete conductive paths, but provides an integrated measure of stress over the whole area of the sheet 11. In a modification of the second embodiment the sheet 11 is enlarged and secured directly to the housing 13 and functions itself as the membrane so that a separate membrane is not required. The membrane 1 is preferably comprised of a cured and crosslinked combination of non-conductive silicone elastomers. These are Polymer A, Alpha - Omega - dihydroxypolydimethysiloxane having a viscosity of 80,000 centipoises and polymer C2501, a similar vinyl-ended polysiioxane filled witn "fumed" silica, of 300- 500 centiposes viscosity. Silester OS is the preferred curing agent. The proportions of tne polymers are controlled to provide the membrane 1 with a known pre-cured viscosity. The conductive portion 2 is preferably made from a layer of the same mixture as membrane 1 plus 70 per cent (by weight) of micron sized graphite particles and 5 per cent by weight of aracnis oil, and is cut from the cured layer to the Fig. 3 configuation and affixed to the non- conductive membrane i using the same mix as above. Table 1 below illustrates the resistivity of the conduct¬ ive portion 2 measured with different depths of water above the membrane 1 in the example shown in Fig. 4 thus demonstrating the basic relationship between fluid depth and conductivity of the device. TABLE I Depth (mm) Resistance (Kohms) 700 1280 650 1150 600 1050 550 950 500 890 450 810 400 710 350 675 300 614 250 535 200 470 150 383 100 320 50 290 0 272 A specific formulation of the non-conductive polymer which forms the membrane 1 is as follows: Polymer gym C2501 50 gms Polymer σum C 50 gms Curing agent (DBTL) 2 gms Crosslinker (ailester 0.S) 5 gms A specific formulation of the conductive polymer which forms the portion 2 is as follows: Polymer gujn C 2501 50 gms Polymer gum C 50 gms Silester O.S. (cross- linker) 5 gms Lonza Graphite T.15 40 gms Aracnis υil (BDH) 14 gms Solvent (SBP3) 10O ml DBTL ( curing agent) Z gm In manufacture of each polymer the two gums are mixed together with the crosslinker and the viscosity of the mixture adjusted by addition of small quantities of only one of the gums until the vicosity measure equals a predetermined value (150 + 10 poises) . In the case of the conductive polymer the remaining constitutents, other than the curing-agent, are added to the mixture and mixed in therewith and the vicosity adjusted by addition of small quantities of the solvent until the vicosity measure equals a predetermined value (15.8 + 0.2 poises) . The final constituent to be added in each case is the curing agent which is thoroughly mixed in and the mixture is thereafter left to set (or cure) with room temperature vulcanisation.

Claims: CLAIMS 1. A fluid volume measuring device comprising a resilient membrane (1) peripherally secured to a support structure (3), means (6,10) imposing a reference pressure on one surface of the membrane (1), the other membrane surface being presented to the pressure of the fluid volume (9) whereby, in use, the shape of the membrane (1) changes in response to the fluid pressure acting on the membrane (1) , and wherein the membrane (1) comprises an electrically conductive portion (2) the conductivity of which is determined by the membrane shape, electrical conductivity measuring means (18) being connected to said portion (2) . 2. A fluid volume measuring device according to claim 1, wherein the elastic modulus of the resilient membrane is the same as that of the electrically conductive portion (2). 3. , fluid volume measuring device according to claim 1 or claim 2 , wherein the electrically conductive portion (2) defines a continuous elongate conductive path the ends of which extend from the measuring means (18) and which is configured to traverse the surface area of the membrane (1) with at least part of the path leading from the membrane periphery to the membrane centre. 4. A fluid volume measuring device according to claim 1 or claim 2 , wherein the electrically conductive portion (2) defines a region contiguous with the greater part of the surface area of the membrane (1) , the measuring means (18) being connected to said region at contacts one of which extends along the membrane periphery and the other of which is at the membrane centre. 5. A fluid volume measuring device as claimed in any preceding claim, wherein both the membrane (1) and the conductive portion (2) are made of silicone elastomeric material.

Inventors: JACKSON JOHN, JACKSON, JOHN
Applicants: FLEXIGAGE LTD, FLEXIGAGE LTD.
Application year: 1986
Main classification: G01F23
IPC codes: G01F23, G01L9, H01C10