CAPACITIVELY COUPLED MAGNETIC FLOWMETER

Abstract: A capacitively coupled magnetic flowmeter is disclosed for measuring flow rate of fluid in a conduit in which the fluid flows, comprising means (10) for generating a magnetic field in the fluid in a direction transverse to the direction of fluid flow, electrode means (8) capacitively coupled to the fluid for measuring the potential difference induced in the fluid by the magnetic field, and electronic means (25) connected to the electrode means (8) to produce a signal proportional to the flow rate of the fluid, in which the electrode means (3, 4) comprises a pair of measurement electrodes (15), located at diametrically-opposed positions in the conduit, each measurement electrode (15) being insulated from the fluid by at least one dielectric portion (16), and a reference electrode (17) corresponding to each measurement electrode (15), each reference electrode being capacitively coupled to the corresponding measurement electrode (15) by at least one dielectric portion (18). Changes in capacitance of the dielectric portion (16) between the measurement electrode (15) and the fluid can therefore be compensated by changes in the capacitance of the dielectric portion (18) between the measurement electrode (15) and the reference electrode (17).

Description: CAPACITIVELY COUPLED MAGNETIC FLOWMETER This invention relates to capacitively coupled magnetic flowmeters, especially, but not exclusively, for the measurement of the flow rate of a fluid in a conduit. It is well known that a magnetic field applied orthogonally across a fluid flowing in a conduit induces a potential difference across the fluid. A flowmeter consisting of the magnetic field-applying means and a pair of electrodes mounted at diametrically-opposed positions in the wall of the conduit orthogonal to both the magnetic field and the direction of fluid flow can be used to obtain a measurement of the potential difference across the fluid. It is also well known for the electrodes to be capacitively coupled to the fluid using a dielectric liner inside the conduit. Coupling the electrodes to preamplifiers and a differential amplifier provides an output which is proportional to the rate of fluid flow through the conduit. A flowmeter of this kind is disclosed in EP-A1-0 114 737. A difficulty with such a capacitively coupled flowmeter arises because the output of the preamplifier connected to each electrode is proportional to the capacitance formed by the fluid/dielectric/electrode interface. Thus, any changes in the characteristics of the dielectric, such as changes induced by variations in thickness or temperature, will affect the accuracy of the flowmeter. EP-A1-0 114 737 solves this problem by making continuous measurements of the electrode capacitance during operation of the meter, but such an approach is generally undesirable, as it requires relatively complex electronic circuits. According to the present invention, a capacitively coupled magnetic flowmeter for measuring flow rate of fluid comprises a conduit through which the fluid flows, means for generating a magnetic field in the fluid in a direction transverse to the direction of fluid flow, electrode means capacitively coupled to the fluid for measuring the potential difference induced in the fluid by the magnetic field, and electronic means connected to the electrode means, the electronic means being adapted to produce a signal proportional to the flow rate of the fluid, characterised in that the electrode means comprises first and second measurement electrodes located at diametrically-opposed positions in the conduit, each being insulated from the fluid by at least one dielectric portion, and first and second reference electrodes corresponding to the first and second measurement electrodes, each reference electrode being capacitively coupled to the respective corresponding measurement electrode by at least one reference dielectric portion. The invention therefore provides a capacitively coupled magnetic flowmeter in which each of the capacitively coupled measurement electrodes is provided with a reference capacitively coupled electrode, both electrodes forming part of an electronic means so that changes in capacitance of the dielectric portion between the measurement electrode and the fluid can be compensated by changes in the capacitance of the dielectric portion between the measurement electrode and the reference electrode. This provides a simple way of compensating for changes in capacitance. The electrodes may define a first measurement capacitor having a capacitance defined by the capacitance between the flowing fluid and the first measurement electrode, a first reference capacitor having a capacitance defined by the capacitance between the first measurement electrode and the respective first reference electrode, a second measurement capacitor defined by the capacitance between the flowing fluid and the second measurement electrode and a second reference capacitor defined by the capacitance between the second measurement electrode and the respective second reference electrode. The first measurement capacitor and first reference capacitor may respectively comprise the input and feedback impedances of a first preamplifier of the electronic means, and the second measurement capacitor and second reference capacitor may respectively define the input and feedback impendances of a second preamplifier of the electronic means. The gain of each preamplifier is thus substantially proportional to the ratio of the two capacitances. This means that the output signal obtained from each of the preamplifiers is independent of the absolute values of the capacitances of these capacitors, so that any changes in the capacitance of the reference capacitors can cancel out corresponding changes in the capacitance of the measurement electrode capacitors. Preferably, the output of each of the first and second preamplifiers provides the inputs to a differential amplifier such that the output of the differential amplifier is proportional to the potential difference induced across the fluid and is thus proportional to the flow rate of the fluid. Preferably, the material of the dielectric portion (the inner portion) that couples the measurement electrode to the fluid is the same as the material of the dielectric portion (the reference portion) used to couple the measurement and reference electrodes. Changes in the gain of the preamplifiers due to the capacitors are then only dependent on geometric factors. In a simple arrangement, the inner dielectric portion may be provided by a continuous sleeve within the conduit. A suitable material for this layer would be PVC. Alternatively , the dielectric portion may comprise discrete portions or may form an integral part of the main flow conduit itself, for example, by making the conduit out of a suitable dielectric material. The reference dielectric is preferably of sheet material, such as PVC. The electrodes may be constructed from metallic foil or from a suitable conducting paint. Because the electrodes are capacitively coupled to the flowing fluid, a time variant magnetic field is required. This preferably comprises a square wave excited magnetic field. Shielding may be provided around the magnetic field generating means to concentrate the magnetic field and/or shield the electrodes from external magnetic fields. The electrode means are preferably permanently mounted in the conduit, which can form an integral part of a pipe for the fluid. The magnetic field generating means and the electronic means may be detachably mounted, so that they can be attached for measurements to be taken, and then removed. The generating means and electronic means conveniently form a single unit. The electronic means preferably includes an AD converter, control circuits for the magnetic field generating means and an interface for a display and memory means such as a microprocessor. There now follows a description, to be read with reference to the accompanying drawings, of a capacitively coupled magnetic flowmeter which illustrates the invention by way of example. In the drawings: Figure 1 shows a cross-section of a known capacitively coupled magnetic flowmeter; Figure 2 shows a side view of a flow conduit and electrode means of the flowmeter of the invention; Figure 3 shows a cross-section on the line A-A of Figure 2; Figure 4 shows an enlarged detail of Figure 3; Figure 5 shows a suitable arrangement of the magnetic field coils; Figure 6 shows a schematic view of an electronic means incorporating the flow conduit and electrode means; and Figure 7 shows a preamplifier for the electrode means. In a known capacitively coupled magnetic flowmeter, shown in cross section in Figure 1 , a potential difference induced in a fluid flowing through a conduit 1 by a magnetic field generated by coils 2 is measured by electrode means comprising a pair of first and second electrodes 3,4. Each electrode 3, 4 is capacitively coupled to the fluid through a dielectric liner 5,6 set in the wall of the conduit. The signal generated at each electrode 3,4 is amplified through a preamplifier (not shown) and passed on to the inputs of a differential amplifier (not shown) . The differential amplifier produces an output that is proportional to the difference between the signals produced by the preamplifiers, giving an indication of the flow rate of the fluid. This arrangement has the problem that changes in the value of the capacitance at each electrode result in changes in the signal. Figures 2 and 3 show a capacitively coupled magnetic flowmeter of the present invention. The flowmeter comprises an electrode means 8 inserted in a wall 9 of the conduit body 7, magnetic field generating means 10, and electronic means (not shown) . The conduit body 7 comprises a tube 11 of PVC material, two rectangular holes 12 being cut into the tube 1 1 on opposite sides for the electrode assemblies 8' , 8" . Standard flanges 13 are provided at each end of the conduit body 7 to allow the flowmeter to be incorporated into a pipe system (not shown) . The PVC tube 1 1 is bored to the correct diameter, and then the two holes for the electrode assemblies 3, 4 are machined into the tube 1 1 at diametrically-opposed locations taking care so as not to cause the tube 11 to distort. Countersunk holes 14 are formed round the edge of each hole 12 for screws 13 which attach the electrode assemblies 8' , 8". The holes 14 do not penetrate to the inside of the conduit 7. When assembled, the electrode assemblies 8' , 8" are sealed to the tube 1 1 using waterproof sealant. Figure 4 shows the detail of a first one of the electrode assemblies 8' , 8". The other assembly is of similar construction. The first electrode assembly 8' is constructed as a laminate and comprises a first measurement electrode 15 insulated from the fluid by an inner dielectric layer 16, and a first reference electrode 17 capacitively coupled to the first measurement electrode 15 by a dielectric layer 18 (the reference dielectric layer) . The inner layer 16 is constructed from PVC sheet 0.5mm thick while the reference layer 18 is machined from solid bar of PVC. The first electrodes 15, 17 are constructed from aluminium foil. Alternatively, they may be painted on using a conducting paint. The first electrodes 15 , 17 are each connected to a wire 19,20 which forms the connection to the electronic means. The first reference electrode 17 and reference layer 18 have apertures to allow the passage of the wire 19 to measurement electrode 15. The second measurement and reference electrodes are of similar construction. It is of great importance that the electrodes are well bonded to the PVC material and therefore a suitable adhesive is required. The requirements are that the adhesive should be slow setting so that time is available for adjustment during construction; that it should have low viscosity so that it flows over the whole surface; that it should adhere to aluminium and PVC; and that it should behave as an insulator. The capacitance C, between the fluid and the first measurement electrode 15 defines a first measurement capacitor, and that C between the first measurement electrode 15 and the first reference electrode 17 defines a first reference capacitor. Similar capacitances C 3 and C 4 are defined for the second electrodes. The magnetic field generating means 10, shown in Figure 5, consist of two coils 21 ,22 and an electrical steel shield 23 in two sections. This enables the magnetic means 10 to be removably attached to an existing electrode means 8. The coils are arranged to provide a magnetic field orthogonal to the two electrodes 15, 17. The electrical steel shield 23 acts as a return path for the magnetic circuit and also supports the magnetic coils 21 ,22. It is essential that the losses in the magnetic circuit are small and that there should be good mating between the two halves of the magnetic circuit in order to ensure that the reluctance of the return path is low and that a high and repeatable magnetic field is generated in the conduit 7. It can be difficult to fulfil these two requirements with electrical steel. In order to achieve low losses the thickness of the steel has to be limited. This then gives rise to problems associated with aligning thin metal sheets and the high reluctance associated with such sheets. A laminated structure is therefore desirable so that the losses can be reduced because of the insulation of one sheet from another and the overall thickness of the structure is made up from layers of material. The shield 23 is therefore constructed from two flanged sections of PVC. Four layers of mumetal (a high permeability, low loss material) are bonded together using araldite and are then bonded to the PVC. The thickness of the individual sheets of mumetal is 0.125mm and therefore they can be easily conformed to the shape of the PVC outer structure. The PVC provides the mechanical rigidity of the system and the flanges enable the two sections to be bolted together in a highly repeatable manner. The electronic means 25 are shown in Figures 6 and 7 of the accompanying drawings. Figure 6 shows a general schematic view of the electronic means 25. The electronic means 25 of Figure 6 comprises a preamplifier 26, first and second preamplifier (not shown) connected to each electrode assembly 8' , 8" to amplify the signals from the electrodes 15, 17. The output from each preamplifier 26 is passed to a differential amplifier 27 which obtains a flow signal proportional to the potential difference between the two electrode means. The two preamplifiers 26 and the differential amplifier 27 form an analogue circuit. The flow signal output from the differential amplifier 27 is passed to an analogue-to-digital (A/D) converter 28, which digitises the flow signal and passes it to an interface 29 to a microprocessor such as a PC (not shown) . The signal may be displayed on and stored by the PC. The electronic means 25 also comprises a coil driver 30 for the magnetic field generating means 10, as the capacitive coupling of the electrodes requires the magnetic field to vary with time. Operation of the coil driver 30 to generate a square wave is controlled by a control unit 31 , which also generates timing and control signals for the analogue circuit. Figure 7 shows a circuit diagram of a first one 26 of said preamplifiers. The second preamplifier is of similar construction. The first measurement capacitance C) forms the input impedance of the preamplifier, while the first reference capacitance C 2 forms the feedback impedance. The lead from the first measurement electrode 15 is shielded by a shield 32. A feedback resistor 33 is provided which has as large a resistance as possible to reduce droop on the flat part of the signal, and may be as high as 20G ohm. This also ensures that the preamplifier works at low frequency because it is AC coupled by the electrodes 15, 17. The gain of the first preamplifier in the passband is given by: gain = C 2 where C j = first measurement capacitance and C 2 = first reference capacitance A similar arrangement exists for the second preamplifier, with a corresponding equation gives the gain of the preamplifier in terms of C 3 and C 4 . The gain of each of the preamplifiers will therefore not be affected by changes in the value of the measurement capacitances provided that the reference capacitances changes by a similar amount. This will tend to happen because the two capacitors are both located at the conduit 7, and thus experience similar environmental changes. Each of the preamplifiers is thus largely self-compensating, overcoming the disadvantages of the prior art in which no feedback reference capacitor is provided. The self- compensating affect is maximised because the measurement and reference capacitors are made from dielectric materials having identical properties. Many other circuits are possible that allow the changes in the properties of the measurement electrodes to be compensated by changes in the reference electrode capacitance, and the above embodiment represents only one example of a circuit which can perform such compensation.

Claims: CLAIMS 1. A capacitively coupled magnetic flowmeter for measuring flow rate of fluid comprising: a conduit (1) through which the fluid flows, means (10) for generating a magnetic field in the fluid in a direction transverse to the direction of fluid flow, electrode means (8) capacitively coupled to the fluid for measuring the potential difference induced in the fluid by the magnetic field, and electronic means (25) connected to the electrode means (8) , the electronic means being adapted to produce a signal proportional to the flow rate of the fluid, characterised in that the electrode means (8) comprises first and second measurement electrodes (15, 15) located at diametrically-opposed positions in the conduit each being insulated from the fluid by at least one inner dielectric portion (16) , and first and second reference electrodes (17) corresponding to the first and second measurement electrodes respectively each reference electrode (17) being capacitively coupled to the corresponding measurement electrode (15) by at least one reference dielectric portion (18). 2. A capacitively coupled flowmeter according to claim 1 in which the electrodes define a first measurement capacitor C, having a capacitance defined by the capacitance between the flowing fluid and first measurement electrodes, a first reference capacitor C 2 having a capacitance defined by the capacitance between the first measurement electrode (15) and the associated first reference electrode (17) , a second measurement capacitor C 3 defined by the capacitance between the flowing fluid and the second measurement electrode (15) and a second reference capacitor C 4 defined by the capacitance between the second measurement electrode (15) and the associated second reference electrode (17) . 3. A capacitively coupled magnetic flowmeter according to claim 2 in which the first measurement capacitor C, and the first reference capacitor C 2 respectively comprise input and feedback impedances of a first preamplifier of the electronic means, and the second measurement capacitor C 3 and second reference capacitor C 4 respectively define the input and feedback impedances of a second preamplifier of the electronic means. 4. A capacitively coupled magnetic flowmeter according to claim 3 in which the output of each preamplifier (26, 26) provides the inputs to a differential amplifier (27) , the output of the differential amplifier (27) being proportional to the potential difference induced across the fluid. 5. A capacitively coupled magnetic flowmeter according to any preceding claim in which the material of the inner dielectric portion (16) that couples the measurement electrodes (15) to the fluid is substantially the same as the material of the reference dielectric portion (18) used to couple the measurement electrodes (15) to the reference electrodes (17). 6. A capacitively coupled magnetic flowmeter according to any preceding claim in which at least one inner dielectric portion (16) is provided by a continuous sleeve within the conduit. 7. A capacitively coupled magnetic flowmeter according to any preceding claim in which at least one inner dielectric portion (16) is PVC. 8. A capacitively coupled magnetic flowmeter according to any one of claims 1 to 6 in which the inner dielectric portions (16) comprise discrete portions. 9. A capacitively coupled magnetic flowmeter according to any one of claims 1 to 7 in which the at least one inner dielectric portion is provided by making the conduit (1) out of a suitable dielectric material. 10. A capacitively coupled magnetic flowmeter according to any preceding claim in which the reference dielectric portion (18) is of sheet material, such as PVC. 11. A capacitively coupled magnetic flowmeter according to any preceding claim in which the measurement and reference electrodes (15, 17) are constructed from metallic foil. 12. A capacitively coupled magnetic flowmeter according to any one of claims 1 to 9 in which the measurement and reference electrodes (15, 17) are made from a suitable conducting paint. 13. A capacitively coupled magnetic flowmeter according to any preceding claim in which the magnetic field generating means (10) comprises a square wave excited magnetic field. 14. A capacitively coupled magnetic flowmeter according to any preceding claim in which shielding (23) is provided around the magnetic field generating means (10) to concentrate the magnetic field and/or shield the electrodes from external magnetic fields. 15. A capacitively coupled magnetic flowmeter according to any preceding claim in which the electrode means (8) are permanently mounted in a conduit forming an integral part of a pipe for the fluid. 16. A capacitively coupled magnetic flowmeter according to any one of claims 1 to 15 in which the magnetic field generating means (10) and the electronic means (25) are detachably mounted. 17. A capacitively coupled magnetic flowmeter according to any preceding claim in which the magnetic field generating means (10) and electronic means 25 form a single unit. 18. A capacitively coupled magnetic flowmeter according to any preceding claim in which the electronic means (25) includes an analogue to digital (A/D) converter (28), control circuits (31) for the magnetic field generating means and an interface (29) for a display and memory means such as a microprocessor.

Inventors: SANDERSON MICHAEL LANGLEY, SANDERSON, MICHAEL, LANGLEY
Applicants: SANDERSON MICHAEL LANGLEY, SEV TRENT WATER LTD, SANDERSON, MICHAEL, LANGLEY, SEVERN TRENT WATER LIMITED
Application year: 1997
Main classification: G01F1
IPC codes: G01F1