WO2018138518A1

WO2018138518A1 - Electromagnetic flow sensor

Info

Publication number: WO2018138518A1
Application number: PCT/GB2018/050236
Authority: WO
Inventors: James Mark ENGLAND; Andrew Nicholas Dames
Original assignee: Sentec Ltd
Current assignee: Sentec Ltd
Priority date: 2017-01-27
Filing date: 2018-01-26
Publication date: 2018-08-02
Anticipated expiration: 2019-07-27
Also published as: GB201701376D0

Abstract

An electromagnetic flow sensor suitable for low cost manufacturing is disclosed. The sensor comprises planar or thin film electrodes (5) formed on one or both sides of an insulating substrate (4), the substrate being located between the pole pieces of a magnetic circuit in a region where fluid flows. The electrodes are patterned conductive traces on the surface(s) of the substrate, covered where necessary with an insulator except for specific electrode regions which are to be exposed to the fluid to sense the emf resulting from the fluid flow. The insulated portions of the conductive traces may extend beyond the primary sensing area, either on the surface or internally within the substrate, bringing the connections from the exposed electrode regions out from the fluid-filled part of the sensor to a portion of the sensor which is dry or potted where electrical connections to the sensing circuitry can be made.

Description

Electromagnetic Flow Sensor

FIELD OF THE INVENTION

This invention relates to the field electromagnetic flow sensing of conductive fluids such as water, particularly where cost or power consumption are important.

BACKGROUND

Battery-powered electromagnetic flow meters (which may also be referred to as "magnetic flow meters" or "mag meters") for residential water metering are known and examples include the iPERL i^'RTM) ranee of water meters available from Sens us Inc. Reference is also made to WO 00/19174 Al. Many existing types of electromagnetic flow meter, both batten/ powered and mains powered, are not particularly suited to low-cost and/or high- volume manufacture. SUMMARY

According to a first aspect of the present invention there is provided an electromagnetic flow sensor for the measuremen t of conductive fluids. The sensor comprises a flow tube for passing conductive fluid (such as water) from an inlet to an outlet through a measurement section comprising a magnetic field generator. The flow tube includes electrically insulating material inside it defining a flow cross-section. For example, the flow tube may be electrically conductive (e.g., formed of a metal), but is provided with an electrically insulating inner surface (e.g., a plastic liner or plastic centre section) in a section housing or containing the insulating substrate. Alternatively, the flow tube may be electrically insulating (e.g. take the form of a plastic flow tube). The sensor includes an insulating substrate across the flow section with at least two planar electrodes on at least one surface, arranged such that a first planar electrode on the surface of the substrate is one side of the flow section, and a second electrode is near the opposite side of the flow section. According to a second aspect of the present invention there is provided an electromagnetic flow sensor. The sensor comprises a flow tube running between first and second tube ends (which may be referred to as "inlet" and "outlet" respectively) in a longitudinal direction and having a measurement section disposed between the first and second tube ends. The flow tube includes an electrically-insulating inner surface in the measurement section. The flow tube has an internal transverse flow cross section in the measurement section. The sensor comprises a magnetic field generator tor generating a magnetic field in the measurement section. The sensor comprises an insert (tor example, a thin, planar or "fin- like" insert) disposed in the measurement section and extending in a transverse direction (tor example, vertically) to the longitudinal direction. The insert comprises a substrate having first and second opposite faces (or "surfaces") which support at least two electrodes on at least one face. The faces are preferably oriented such that they are substantially orthogonal to magnetic flux lines produced by the magnetic field generator near the centre of the measurement section. The first and second electrodes are spaced apart in the transverse direction and are disposed at or proximate to opposite edges (or "sides") of the flow cross section (for example, if the transverse direction is vertical, at a top and bottom of the flow cross section). The flow cross section may have a first length along the transverse direction and second length along a direction orthogonal to the transverse direction and the longitudinal direction, and the first length is preferably greater than the second length. The flow cross section may diverge or van/, for example be stepped, sloped or curved, at the opposite edges, such that the first length becomes greater proximate to the insert. Preferably, changes or transitions in cross section are smooth so as to discourage or minimise, or even to avoid turbulence or flow separation and excessive, non-recoverable head loss. For example, on a divergent side, the tangent may be less than 7 degrees to the axis so to discourage or minimise, or even to avoid flow separation. Optional features are defined in the dependent claims.

Embodiments of electromagnetic flow sensors may comprise planar or thin film electrodes formed on one or both sides of an insulating substrate. The electrodes are patterned conductive traces on the surf ace(s) of the substrate, covered where necessary with in insulator except for specific electrode regions which are to be exposed to the fluid to sense the emf resulting from the fluid flow. The conductive traces extend beyond the primary sensing area, either on the surface or internally within the substrate, bringing the connections from the electrode regions out from the fluid-filled part of the sensor to a portion of the sensor which is dry or potted where electrical connections to the sensing circuitry can be made. The exact layouts ol the conductive traces can be adjusted to minimise coupling to either the local or global magnetic fields. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a perspective view of a flow tube 1 with magnetic drive circuit 3 pole pieces 2 and insulating substrate 4 containing electrodes 5 (which can be seen in Figure 2) on both sides and optional internal field sense coils. This shows the general arrangement of the flow sensor, in a particular arrangement where all the components can be inserted into the flow tube from one side. Although this may be a useful method of assembly, other mounting and assembly arrangements are possible without affecting the operation or other benefits ot the sensor design. The internal surfaces ol the flow tube 1 need to be insulated at least in the central section of the tube, or alternatively the flow tube should be fabricated from insulating materials such as glass-filled PPS or polyarnide, as appropriate for the engineering requirements of strength and pressure resistance. The flow tube material should be substantially non-magnetic, at least in the region of the sensor to ensure that the magnetic flux is not shunted through the flow tube. The exact materials of the flow tube, beyond the insulating and non-magnetic properties, do not affect the function of the sensor, and any combinations of materials that meet the functional requirements may be used for this without further limitation. Figure 2 is side view of the flow sensor m Figure 1.

Figure 3 shows a section through the flow sensor of Figure 1 showing the magnetic circuit insulated from the flow path, with the substrate 4 carrying electrodes 5 at either end of the flow path which are able to contact the fluid. This embodiment has exposed electrode material 5 on both sides of the insulating substrate 4. Detail (B) and (C) show the actual exposed electrode patterns in section, at opposite ends of the substrate. The electrodes extend along the axis of the flow tube for typically 25-50% of the total axial dimension of the substrate. The electrical connections which are not exposed to the fluid are not shown sn this figure, hut can be assumed to be planar patterned material covered with insulator, either running on the surface of the substrate or internally using layers such as might be fabricated in a printed circuit board. Figure 4 is an exploded view of the flow sensor of Figure 1 showing mounting and assembly arrangements of the components through a slot 11 in the flow tube 1. The insulating material between the magnetic circuit pole pieces and the fluid is formed as part of the flow tube 1. The substrate is inserted into the flow tube through a single aperture, which is the only aperture that needs to form a fluid-tight seal around the substrate. This seal might be formed by potting, epoxy or other structural adhesive, or by insert molding the substrate when the flow tube is moulded, or by a suitably designed elastomeric seal, or by other suitable engineering means without limitation.

Figure 5 illustrates flow tube 1 with magnetic drive coil 3, pole pieces 2 and single-sided substrate 4 with planar electrode on one surface. This is the single sided variant of the flow tube in Figure 1. The substrate 4 forms one wall of the flow measurement section, as it is displaced fully to one side. Electrodes are patterned only on one side of the substrate. The same general material and assembly descriptions apply as for Figure 1. Figure 6 is a side view^T of the sensor in Figure 5.

Figure 7 shows a section through flow sensor of Figure 5 showing pole pieces separated from water path by insulator, insulating substrate along one wall of the water path and planar electrodes 5 on the surface of the substrate facing the water. This shows the positions of the electrodes 5 at either end of the substrate.

Figure 8 is an exploded view of the flow sensor assembly in Figure 5 showing the side of the planar substrate on which the electrodes are formed. The same assembly and material notes apply as for the assembly in Figure 1.

Figure 9 is an exploded view of the same flow sensor in Figure 8 showing the reverse side of the planar substrate without electrodes. Figu re 10 is a perspective view of a flow sensor assembly before sealing using a flexible collar 6 and potting with a structural adhesive, such as epoxy, showing the flow tube assembly before addition of structural adhesive, and which includes a flow tube 1, pole pieces 2, magnetic drive circuit 3, electrode substrate 4.

Figure 11 is a perspective view of the flow sensor assembly shown in Figure 10, including structural adhesive 7.

Figure 12 is an exploded view of the flow sensor assemblv shown in Figure 10, not including structural adhesive, and which comprises a tube 1, electrode substrate 4, flexible collar 6 and magnetic subassembly 8.

Figure 13 is a cross-section of the flow tube assembly sh own in Figure 0 with the structural adhesive, taken orthogonal to flow direction at measurement section, showing flexible collar 6 and structural potting compound 7.

DETAILED DESCRIPTION

Herein are disclosed a number of embodiments of electromagnetic flow sensors and flow meters suitable for volume manufacturing and/ or battery operation that make use of electrodes patterned onto insulating surfaces or substrates. Electrodes are used in electromagnetic flow sensors to connect between the "wetted" fluid part of the sensor and the dry electrical connection to the signal processing. It prevents the passage of fluid out of the flow sensor whilst providing a means to detect the emf caused by the flowing fluid. In conventional electromagnetic flow meters, the electrodes are discrete elements or components which are in contact on one face with the fluid, and have some method to form a conductive fluid-tight seal in order to connect to the signal processing. By using a planar substrate with electrodes patterned on the surface, the function of both the emf sensing and the fluid-tight electrical connection can be combined into a single assembly that can manufactured simply and sealed into a flow tube assembly. In the embodiments described herein, the assembly may be inserted into the flow tube through a single aperture greatly simplifying the processes required to seal against the fluid. Summary

The sensor is an electromagnetic flow sensor, the general principles of operation of which are well known to skilled persons. Referring to Figures 1 to 4, the sensor comprises a flow tube 1 for passing conductive fluid such as water rom an inlet to an outlet generally arranged advantageously to accelerate the flow of fluid through a measurement section in between the inlet and outlet to increase the velocity of the fluid and hence the signal from the sensor. To avoid shorting out the emfs from the sensor, the surfaces of the flow tube 1 in the vicinity of the measurement section that are exposed to the fluid need to be insulating. The measurement section itself comprises a magnetic field generator, with electrically insulating material lining the inside fluid-wetted section which defines the flow path for fluid through this section from, inlet to outlet. The flow path section is typically rectangular. Within the flow path, there is planar insulating substrate disposed across the longer dimension of the rectangular flow path section. The insulating substrate has planar electrodes on at least one surface, arranged such that a first electrode is near a first short wall of the rectangular flow path section, and a second electrode is near the second short wall of the rectangular flow section. Sections through such a flow measurement section are shown in Figure 3 and Figure 7. The at least first and second electrodes sense voltage generated by the flowing conductive fluid as it passes through the magnetic field produced by the magnetic field generator, said voltage being substantially proportional to the velocity of the fluid, and hence the flow rate of fluid from, the inlet to the outlet, following the generally well known principles of

electromagnetic flow measurement. The measurement section may pass 100% of the fluid from, inlet to outlet, or it may allow a certain fraction such as 50%, to allow a small measurement section to be used in a larger flow tube, or only a very small portion if the assembly is used as an insert flow meter in a much larger flow tube.

Magnetic circuit details

The magnetic field may be generated by a variety of means. This might include current passing through a coil, or residual magnetism from a semi-hard magnetic material. The magnetic field ma be reversed periodically to enable the sensor to generate a periodically reversing AC e f whose amplitude is proportional to the field amplitude multiplied by the flow rate, greatly improving the performance of the sensor with low flow rates. The magnetic flux is guided by soft magnetic materials from the generating means to substantially planar pole pieces on either side of the measurement section, said soft magnetic pole pieces being electrically insulated from the fluid to prevent the emf from the sensor from being shorted out. Such insulation may be locally applied to the pole pieces, or may be formed by one or more separate insulating components, or formed by the flow tube, or formed by any other suitable means without limitation.

The magnetic field lines in the design run from one pole piece to the other in a substantially parallel direction, and run in the same overall direction across the majority of the rectangular flow path section. This is in contrast to some embodiments of sensor where the flow path is divided in two in which the magnetic field lines in the two halves of the measurement sections are substantially anti-parallel. In the embodiments described here, there is no central pole piece— the substrate (4) in Figure 3 is non-magnetic.

Substrate and electrode details

The insulating substrate may be any sufficiently insulating material, or material with sufficiently insulating coating, that is compatible w th the conductive fluid being measured and any specific application requirements such as operating temperature, chemical resistance or potable water compatibility. This may include all the standard printed circuit board materials, polyimide, apton, standard polymeric materials such PPS, polycarbonate or polypropylene, glass, ceramic, etc without limitation. The applicability of the geometry is not limited by the choice of material. The substrate may divide the measurement section into two separate substantially equal cross section flow paths, or may be disposed along one wall of the measurement section.

The surface of the substrate is patterned with electrodes, with areas exposed to the fluid as shown in e.g. Figure 8, or in section in Figure 3 or Figure 7. The exposed areas of electrodes must provide a well characterised electrical interface between the electronic measurement input circuit and the output of the sensor. Electrochemically active electrodes such as Ag coated with AgCl, may be used, or fully inert noble metal electrodes such as gold may be possible. Conductive porous materials such a graphite are particularly favoured because they are both inert and have very high effective surface area, which creates a very low noise electrode. Ion-permeable coatings such as Nafion¹*⁴ may be advantageous in reducing noise or protecting the electrode. Any other conductive materials may be chosen if they have suitable electrical and electrochemical behaviour, and are compatible with the fluid, operating temperature, specific application requirements such as potable water compatibility and the long-term performance of the sensor, without limitation.

The connections to the electrodes may be routed to one end of the substrate which is outside of the region of fluid flow, where they may then be connected to suitable electronics for amplifying digitising or recovering the signals using techniques known for operating electromagnetic flow sensors.

The electrodes themselves may be patterned by any means suitable for the type of materia! — for example, screen printing, photomasking and etching continuous layers to form the patterns, milling off material from a fully coated sheet, etc. Screen printed conductive ink based on graphite is a specific example suitable for low cost high volume manufacturing.

Areas of the electrode material that are not to be exposed to the conductive fluid (for example, bringing out the connections for the electronics to measure) can be masked by a further insulating layer over the material. There are many choices of material and patterning methods, but screen printed insulating inks (like solder resist used on printed circuit boards) are an attractive low-cost option. Any insulating coating or layer which is compatible with the fluid may be used witho t limitation. The connections to the electronics may also run through internal layers in the substrate, which may avoid the need for a further patterned insulating coating on the surface of the electrode, and electronic components, including signal connection, conditioning and processing components, may be assembled onto the same substrate as the electrodes using standard electronics assembly processes such as soldering. Sealing

The substrate is inserted into the flow tube through a single aperture, which is the only aperture that needs to form a fluid-tight seal around the substrate. This seal might be formed by potting, epoxy or structural adhesive, or by insert molding the substrate when the flow tube is moulded, or by a suitably designed elastomeric seal, or by other suitable engineering means without limitation. Many different chemistries of potting, epoxies or adhesives may be applicable, including two-part acrylics, two -step acrylics, epoxies, polyurethanes, silane modified polymers, silicones and methyl methacry kites. A

combination of methods may be used to achieve the application requirements of sealing, fluid resistance and pressure tolerance. Using a 2-part structural adhesive, for example, provides reinforcement to the flow tube around the aperture, reducing deformation when the fluid is under pressure. The preferred sealing method and material depends on the specific requirements of the application, including fluid type, fluid temperature, fluid pressure, compliance requirements such as potable water regulations, lifetime and accuracy. For example, a sealing method particularly well suited to high pressure applications is displayed m Figures 10-13. This solution uses an elastomeric collar in combination with a structural adhesive. The adhesive provides reinforcement to the flow tube, reducing tube deformation which is a source of measurement error. The collar acts to block the structural adhesive from entering in or near the aperture, increasing the distance between the adhesive-tube interface and the adhesive- electrode substrate interface. When the tube expands and the aperture deforms under high water pressure, this extra separation between the interfaces reduces strain in the adhesive and stress at the interfaces.

FURTHER EMBODIMENTS

The planar substrate may have electrodes on both sides (Figure 3) or one side (Figure 7). It may be constructed with internal conductive layers, for example like a multi-layered printed circuit board. These internal layers may be used to route the electrode connections, or to form coils or other types of layout compatible with the fabrication method, as discussed further below. All standard methods of fabricating and patterning such layers may be deployed without limitation. Designs which are single sided may advantageously have the conductive electrodes and insulating coating applied using low cost screen printing techniques suitable for mass production, since there are no requirements for any connections to pass through the substrate (i.e. no drilled and plated via holes). For designs with electrodes on both sides, the connections to each set of electrodes may be made outside of the section which is immersed in fluid by e.g. using a double sided connector. The substrate may also contain additional conductive layers, which may advantageously be used to form a coil to sense the change in magnetic field when a design with periodically reversing magnetic field or AC magnetic field is employed. The signal from this field sense may be combined with the detected emf from the electrodes in order to create an output which is substantially proportional to the velocity (fluid flow rate) but independent of any variation m the magnetic field level.

On designs with electrodes on both sides, the substrate effectively divides the measurement section into two flow channels. The most obvious way to combine the signals from the two sensors is to connect the electrodes on opposite sides of the substrate together, i.e. a parallel connection. If the substrate extends axially sufficiently far, the electrodes from each of the two channels may be connected together in series, providing a means to create a larger signal from the sensor, thereby improving the signal to noise and hence repeatability of any flow measurement. Alternatively, the two sets of electrode connections may be routed independently to the electronics and measured separately, added together electronically, multiplexed, or connected to other forms of electronic circuitry without limitation.

An important design feature of electromagnetic flow sensors is the unwanted coupling between the periodically reversing or AC magnetic circuit and the loop formed by the connection to the electrodes and the fluid. When the field changes, a parasitic emf is induced in this loop if any of the flux from the magnetic circuit couples into it. This parasitic emf can be substantially eliminated in these embodiments, because the connection to the electrode opposite the electronics end of the substrate can be routed optimally across the substrate, for example directly across the middle of the substrate. The residual flux coupled because of manufacturing tolerances etc can be kept very small because the total loop area is relatively small.

In the case of the double sided electrode arrangement, the two loops formed in the two separate flow channels have opposite senses, and this also affords substantial rejection of external AC magnetic fields, which helps to make the design easier to qualify against EMC standards.

The planar substrate may be patterned with additional electrodes. For example, a third electrode may be added centrally on one or both sides of the substrate to act as a common mode reference level for the input electronics, which helps to mitigate common mode interference effects and in some cases may simplify the design of the electronics by removing the need for biasing resistors. Additional electrodes on the planar substrate may also be used to measure the flow profile. For example, 3 or more electrodes may be equally distributed from one edge of the measurement section to the other, along a line perpendicular to the flow. By sensing the emf differences between adjacent pairs of these multiple electrodes a measure of the fluid velocit profile may be ascertained. This may allow corrections to be made to the flow rate when severe upstream or downstream flow disturbers are present. This is particularly relevant when the measurement section does not itself pass all of the flow volume, but instead is disposed as a form of insertion meter in a larger pipe. Under these

circumstances, the velocity profile locally to the sensor can be used to determine the overall velocity profile across the entire pipe, allowing a means for the insertion meter to be accurate even when installed near flow disturbers.

The invention may be used as part of a volumetric water meter, i.e. one which totals up the volume of water which passes through it as its primary function. It may be used as a flow sensor, i.e. providing a signal proportional to the flow rate for purposes such as controlling another process, detecting leaks, monitoring or logging, combining with pressure data, etc. It may be used tor both these purposes. For example, when used inside an appliance such as a washing machine or dishwasher, the sensor may provide information on the volume of water that has entered for the particular stage of the cycle, and/or the rate at which water is entering to confirm normal operation. In a continuous flow water heating device, the flow rate may be used to assist in the control of the electrical heating input power to control the output temperature of the heated water. These are examples, and many other applications of this type of low cost flow or volumetric sensing are possible without limitation. The substrate on which the electrodes are patterned may he non-planar— i.e. it may be a curved surface. The descriptions or "planar" above should be interpreted with this alternative reading in mind.

The measurement section need not have a rectangular cross-section— an oblate form of cross section is almost as good, and may have advantages tor withstanding pressurised fluid. The cross-sections shown in Figure 3 and Figure 7 display a narrowing of the flow section near the electrodes. This principle can be used to lower the fluid velocity local to the electrodes and so reduce any noise resulting from fluid surface effects. A conventional round section with electrodes patterned on the inner curved surface would also be possible. The descriptions above can be adapted with the rectangular limitation removed.

The assembly may be considered as a complete meter or sensor, rather than a tube with an insert.

A key feature of the design to maximise the signal level is to make sure the electrodes are spaced as far apart as reasonably possible, as this increases the signal level and improves the overall efficiency of the design.

Claims

1. An electromagnetic flow sensor for the measurement of conductive fluids, comprising a flow tube for passing conductive fluid such as water from an inlet to an outlet through a measurement section comprising a magnetic field generator, with electrically insulating material inside it which defines the flow cross-section, including an insulating substrate across the flow section with at least two planar electrodes on at least one surface, arranged such that a first planar electrode on the surface of the substrate is near one side of the flow section, and a second electrode is near the opposite side of the flow section.

2. A flow sensor of claim 1 in which the separation between the first and second

electrodes is substantially the maximum that can be achieved within the flow section.

3. A flow sensor of claim 1 or 2 in which the cross-section of the flow section is oblate.

4. A flow sensor of claim 1 or 2 in which the cross-section of the flow section is

rectangular.

5. A flow sensor of claims 1-4 in which the substrate is substantially planar.

6. A flow sensor of claim 1-5 in which the electrodes are on the same surface of the

substrate.

7. A flow sensor of claims 1-6 in which third and fourth electrodes are formed on the opposite side of the substrate to first and second electrodes as a substantially mirror image.

8. A flow sensor of claims 1-7 in which the electrodes are made of screen printed graphite.

9. A flow sensor of claims 1-8 in which the insulating substrate is polycarbonate.

10. A flow sensor of claims 1-8 in which the insulating substrate is PPS

11. A flow sensor of claims 1-10 in which the magnetic field is generated using semi-hard or remanent material.

12. A flow sensor of any preceding claim in which the connections to the electrodes are routed to minimise parasitic signals coupled from the magnetic circuit.

13. A flow sensor of any preceding claim in which the connections to the electrodes are routed on internal layers in the substrate.

14. A flow sensor of any preceding claim in which additional electrodes are used to bias the sensing circuitry.

15. A flow sensor of any preceding claim in which pairs of electrodes are connected in parallel.

16. A flow sensor of any preceding claim in which pairs of electrodes are connected in series.

17. A flow sensor of any preceding claim in which pairs of electrodes are connected to separate electronic sensing inputs.

18. A flow sensor of any preceding claim in which additional circuit paths such as coils are also formed on or within the substrate.

19. A flow sensor of any preceding claim in which the electrodes are non-galvanic inert construction.

20. A flow sensor of any preceding claim in which the electrodes are formed from

conductive ink.

21. A flow sensor of any preceding claim in which the electrodes or insulating layers are screen printed.

22. A flow sensor of any preceding claim in which the substrate with the electrodes passes through an aperture in the wall of the flow tube.

23. A flow sensor of claim 22 in which a fluid-tight seal is formed between the substrate and the aperture in the wall of the flow tube by cured epoxy resin.

24. A flow sensor of claim 22 in which a fluid-tight seal is formed between the substrate and the aperture in the wall of the flow tube by two-part structural adhesive.

25. A flow sensor of claim 22 in which a fluid-tight seal is formed between the substrate and the aperture in the wall of the flow tube by a potting compound.

26. A flow sensor of claim 22 in which a fluid-tight seal is formed between the substrate and the flow tube by an elastomeric seal, gasket or o-ring.

27. A flow sensor of claim 22 in which a fluid-tight seal is formed between the substrate and the flow tube by an elastomeric ring, collar or gasket in combination with a two- part structural adhesive.

28. An insertable electromagnetic flow sensor for the measurement of conductive fluids, comprising a measurement section comprising a magnetic field generator, with electrically insulating material inside it which defines a flow path, including a planar insulating substrate across the longer dimension of the flow path section with at least two planar electrodes on at least one surface, arranged such that a first planar electrode on the surface of the substrate is near one end of the parallel-sided flow path, and a second electrode on the same surface of the substrate is near the opposite end of the parallel-sided flow path.

29. A flow sensor of claim 28 in which third and fourth electrodes are formed on the opposite side of the substrate to first and second electrodes as a substantially mirror image.

30. A flow sensor of any preceding claim which is used to control the power input of an electrical heating element.

31. A flow sensor of any preceding claim where one or more electronic components are mounted onto the same substrate as the electrodes.