WO2001050094A2

WO2001050094A2 - Flow meter structure

Info

Publication number: WO2001050094A2
Application number: PCT/GB2001/000063
Authority: WO
Inventors: Bryan Franklin; Roger Turner
Original assignee: Abb Automation Limited
Priority date: 2000-01-06
Filing date: 2001-01-05
Publication date: 2001-07-12
Also published as: AU2532001A; GB0218247D0; GB2377030A; AU1005101A; WO2001050094A3

Abstract

Structures and methods are disclosed for measuring fluid flow in an open channel, such as an irrigation channel. By making use of the measures disclosed, a non-intrusive meter, such as an electromagnetic flow meter may be reliably deployed where conventionally a mechanical meter was required, without requiring excess flow conditioning apparatus. The meter may even be made to work reliably under conditions of partial filling.

Description

FLOW METER STRUCTURE

The present invention relates to a flow meter.

Flow meters of a variety of types are known and can generally be classified as intrusive or non-intrusive. An electromagnetic flow meter is an example of a non- intrusive meter and has advantages in that it does not impede the flow or have moving parts to wear out. Normally, however, use of electromagnetic flow meters (or other non-intrusive meters such as ultrasonic meters) is confined to applications where the flow is stable and conditioned and such meters normally require generally non-turbulent flow in a filled conduit to operate reliably. Electromagnetic flow meters are therefore normally installed in a length of pipe which is arranged to be filled and in which the upstream length of pipe is sufficient to stabilise the flow. Intrusive meters include vanes and paddle wheels and are generally less sensitive to flow conditions, but are more bulky and have moving parts which tend to wear out. The flow conditions and fluid types that may need to be measured may vary considerably and, in view of this, flow meters are often designed for particular applications and are not readily interchangeable.

The present invention relates in particular to flow measurement in a channel or measuring conduit in which the liquid level may vary. In most practical situations the channel will be an open channel (but could also be a closed channel) such as an irrigation channel, or one having at least one open portion along its length. In the following description such a channel will simply be referred to as an "open channel".

Various flow meter arrangements for measuring the flow of a liquid through an open channel have been tried with varying degrees of success. A successful conventional example of such an arrangement is illustrated in Figure 1 . This figure shows a sluice 100 provided with a sluice gate 102 which can be moved up and down in a groove 104 so as to limit the flow of the liquid 106 (in this case water) through the open channel 108 to varying degrees. Situated downstream of the sluice 100 is a flow meter 1 10 commonly known as a "Deathridge wheel". This mechanical flow meter comprises an undershot wheel 1 12 with vanes 1 14 which are moved by water 106 flowing past the flow meter 1 10, thereby causing the wheel 1 12 to rotate. The speed of rotation is indicative of the flow rate. A major drawback associated with this type of flow meter is the fact that in particular at low flow rates the relationship between flow and rotation is non-linear. Further, this mechanical flow meter is vulnerable to vandalism and, having exposed moving parts, may also be a danger to children or animals.

Another type of flow meter employs ultrasonic techniques. The speed of the flow of the liquid and also the depth of the liquid are measured to determine the volume flow rate of the liquid. This type of flow meter requires relatively complicated apparatus, in particular because two physical properties have to be measured and processed.

US 3,929,016 discloses a flow meter structure for an open aqueduct, in which an electromagnetic flow meter measures the flow through a metering conduit of circular cross section. Electromagnetic flow meters are convenient metering devices but, due to the widely accepted limitations of such flow meters, this document teaches that steps must be taken to condition the flow and this document discloses mounting a relatively conventional circular conduit flow meter in a sump with barriers around it so that the conduit remains filled and behaves essentially as a conventional flow meter installed in a filled pipeline. The sump must be provided in the aqueduct and the flow meter structure inserted therein so that the metering conduit was below the floor level of sections of the aqueduct adjacent the sump. Hence, even if no liquid was present in the aqueduct the sump (and hence the metering conduit) would always be filled with liquid.

Pursuant to the invention it has been appreciated that this prior art technique has several disadvantages. Firstly, providing a sump in a channel / aqueduct may be expensive, especially when such a sump is to be built into an existing channel. Secondly, forming a metering conduit with a circular cross section makes sub- optimal use of the available space in a channel having a flat bottom, and hence the metering conduit, having a relatively small cross section, unduly restricts the liquid flow.

The present invention has been made in view of the above problems associated with conventional flow meters and in a general aim seeks to provide an improved meter for use in an open channel, particularly, but not exclusively, an irrigation channel.

In a first aspect the present invention provides apparatus for measuring liquid flow in a channel comprising baffle means for directing liquid flowing through the channel through a metering conduit of predetermined, substantially rectangular cross sectional area, the baffle means being arranged above the metering conduit so that, when the apparatus is installed in the channel, the baffle means serves to reduce variation of the liquid level in the metering conduit, and an electromagnetic flow meter for measuring liquid flow in said metering conduit.

The combination of an upwardly extending baffle with a metering conduit with a rectangular cross section can condition the flow, even without a sump or upstream conduit, so that an electromagnetic flow meter can be used reliably. Whilst usually an electromagnetic flow meter only provides accurate readings of flow through a conduit of circular cross section if the flow has been conditioned (e.g. by means of a relatively lengthy tube upstream of the conduit), no such conditioning is necessary with a conduit of rectangular cross section.

With the baffle means being located above the metering conduit, the liquid would be expected always to fill the metering conduit, provided the liquid level is above a pre-determined minimum liquid level such as the upper internal edge of the (upstream) inlet side of the conduit (only in exceptional conditions such as extreme drought the liquid level might drop below this minimum level).

Preferably, when the apparatus is installed, the metering conduit is located towards the bottom of the structure, preferably within the lower 20, 1 5, 10 or 5 % of the apparatus.

Preferably, the baffle means serves to reduce variation of the liquid level in the metering conduit over a range of liquid heights within the channel from above the upper internal edge of the metering conduit to the top edge of the baffle means.

The height of the metering conduit may be substantially smaller than the height of the baffle means, or small compared to half the height of the baffle. It may be as small as one-third of the height. The height will usually be at least one-tenth of the height of the baffle. This will further alleviate problems caused by partial filling of the conduit.

Preferably, the height of the metering conduit is small compared to half its width, preferably less than one-third or one quarter of its width, and/or the height of the metering conduit is preferably small when compared with the combined height of the baffle means and the metering conduit, preferably less than 50, 20, 10 or 5 % of the combined height.

However, in the case of a tall or very tall baffle or a dam, a minimum height of one-tenth of the baffle or dam may be inappropriate. In such cases a smaller minimum height may be more desirable.

The height of the baffle is preferably greater than 0.5 metres, 0.8 metres, 1 .0 metre, 1 .2 metres, 1 .5 metres or 2.0 metres for most flow situations. In some cases, however, the height of the baffle may be less than 0.5 metres.

The width of the metering conduit is preferably more than 50% of the width of the baffle. It may ideally be more than 65% or 70% of the width of the baffle. It may be less than the width of the baffle, often less than 95% or 85% of the width of the baffle. There may be a gap between the outer vertical edge of the metering conduit and the outer vertical edge of the baffle in the range of 50 to 400mm. Preferably this is in the range of 100 to 300mm or 1 50 to 250mm.

As the flow meter is a non-intrusive flow meter, in this case an inductive or electromagnetic flow meter, the flow area is not obstructed and is less susceptible to silting-up because the speed of the flow of the liquid is increased due to the smaller cross sectional area of the metering conduit when compared with that of the channel.

Preferably, the apparatus has a generally flat bottom. This is convenient for inserting the apparatus in an irrigation channel that has a flat bottom.

By way of background, a typical electromagnetic flow meter comprises two field coils of the same size and strength situated, typically, on opposing faces of the vertical axis of the metering conduit of the flow meter and which therein induces a substantially uniform magnetic field within the metering conduit. The magnetic field induces an electrical potential in the fluid, flowing at velocity through the measuring conduit and a potential difference (voltage) across the fluid, in the direction perpendicular to the direction of fluid flow. This potential difference across the width of the measuring conduit is detected by measuring electrodes positioned, typically, opposite one another on the horizontal axis of the measuring conduit. The potential difference of the two electrodes is measured and is proportional to the flow velocity and the magnetic field strength. The volumetric flow rate is then calculated from this measured voltage.

According to the first aspect of the present invention, the metering conduit is of generally rectangular cross section. This is preferred to, for example, a circular cross section, because the measurement and processing of the measured values is simpler with the rectangular cross section when compared with, for example, a circular cross section. In particular, inductive measurement using a rectangular shaped flow-section is independent of flow profile effects and thus partial blockage, for example, is irrelevant. Further, when an inductive flow meter is used, arranging the means for generating the magnetic field and the means for detecting the electric voltage transverse to the magnetic field and the direction of flow is relatively simple. In addition, since many open channels such as irrigation channels are of generally rectangular cross section the structure may be manufactured to suit the dimensions of the channel.

Preferably the potential sensing electrodes are positioned a distance downstream of the mouth of the metering conduit (where the baffle is located), preferably a distance of at least 5cm, preferably at least 10cm, preferably at least A the length of the metering conduit, typically about midway along the metering conduit, preferably at least about half the height of the metering conduit. These measures contribute to providing a more reliable measure less susceptible to turbulence at the mouth of the metering conduit.

In a most preferred arrangement, the meter has field generating means which produce a substantially vertical field and potential sensing electrodes are provided on the (preferably substantially vertical) side walls of the metering conduit, preferably extending over substantially the whole height, preferably (but not necessarily) being substantially continuous over a major portion of the side walls. This arrangement ensures that the potential generated across the fluid is averaged (βvertfjin the event that the metering conduit is not completely filled and can lead to more accurate results.

In a further aspect there is provided apparatus for measuring liquid flow in a channel in which the liquid level may be subject to variation, the apparatus having a generally flat bottom and comprising a metering conduit of predetermined, substantially rectangular cross sectional area located towards the bottom of the apparatus, and baffle means, located above the metering conduit, for directing liquid flowing through the channel through the metering conduit, and a non- intrusive flow meter for measuring liquid flow in said metering conduit. Forming the apparatus with a flat bottom is preferred, as mentioned above, as it can lead to greater filling of the conduit under low flow conditions, without requiring a sump.

Preferably, the flat bottom extends over a substantial portion of the length of the metering conduit, preferably the entire length thereof.

In a further aspect the present invention provides a flow meter structure installed in an irrigation channel and for measuring a liquid flow through the channel, the flow meter structure comprising baffle means for directing liquid flowing through the channel through a metering conduit of predetermined cross sectional area, the baffle means being arranged so as to reduce variation of the liquid level in the metering conduit over a first range of liquid levels in the channel, and a flow meter for measuring liquid flow in said metering conduit, wherein the flow meter structure is installed at a position in the channel where the liquid level in the metering conduit varies over a second range of liquid levels in the channel.

According to this aspect there is no need to build a sump, which is required in US 3,929,016. It is accepted that there may be conditions in which the metering conduit is not filled with liquid. Surprisingly, this can be advantageous, in particular if the liquid is allowed to flow in the channel only at certain times. The absence of liquid in the metering conduit can then be detected, preferably based on a measure of flow or measure of conductivity or impedance between potential sensing electrodes of the flow meter and the flow meter can then be switched to a power saving or sleep mode. Preferably in the sleep mode, the meter does not provide full metering functionality and consumes less power than in a normal mode but monitors conduit filling level at intervals, preferably of less than 1 minute, preferably about every second, and resumes normal metering mode when fluid is detected.

Preferably, the metering conduit is located near the bottom of the structure.

Preferably, the first range of liquid levels extends from above the upper internal edge of the metering conduit to the top edge of the baffle means. ln the case where the fluid level varies in the metering conduit, it can normally be assumed that this varies as a function of flow rate. The meter will normally return a value indicative of velocity flow rate. Preferably, the velocity flow rate is converted to a volumetric flow rate by multiplying a measure of flow velocity by an estimate of cross sectional area which estimate is a function of velocity. The estimate may comprise a constant, equal to the actual area for velocities above a predetermined minimum velocity and may be a function of velocity for velocities below the minimum. Alternatively, but similarly, at least one correction factor may be applied to a measure of volumetric flow rate for velocities below a predetermined value to compensate for partial filling of the conduit. Use of a rectangular conduit greatly simplifies correction for partial filling.

In a related aspect the present invention provides a method of replacing a Deathridge wheel (or other mechanical flow meter) in a channel in which the liquid level may be subject to variation, the method comprising removing the Deathridge wheel; and inserting, into the channel, apparatus for measuring liquid flow in the channel, the apparatus comprising baffle means for directing liquid flowing through the channel through a metering conduit of predetermined cross sectional area, the baffle means being arranged so as to reduce variation of the liquid level in the metering conduit, and means for measuring liquid flow in said metering conduit.

Preferably, the apparatus is inserted into the channel at a position at which there is no sump, i.e. no existing sump. Further preferably, no sump is built at the installation site of the apparatus prior to, or during installation of the apparatus.

In the most important application the liquid will be water, e.g. for irrigating farm land.

The metering conduit is preferably located towards the bottom of the structure when the structure is deployed.

Preferably, the structure further comprises fixing means for securing the structure to a retaining formation of a sluice provided in or at the channel. This makes it particularly easy to install the structure in the channel. When the flow meter is intended to replace an existing flow meter situated near a sluice, the existing flow meter may simply be removed, and the structure accommodating the new flow meter may be fixed to the retaining formation of the sluice. The term sluice is used herein broadly to mean any form of moveable gate used to control flow in an open channel.

This concept is a further aspect of the invention. Accordingly, the present invention also provides a structure for insertion into a channel and for measuring a flow of a liquid through the channel, the structure comprising a housing body accommodating a flow meter; and fixing means for securing the structure to a retaining formation of a sluice provided in or at the channel.

Preferably, the fixing means is adapted to be secured to a groove of the sluice, such as found in many types of sluices. The groove is normally intended to receive a sluice gate for controlling the flow of liquid through the channel. If the structure is to be secured to a groove of the sluice, the existing sluice gate is simply taken out of the groove, and the fixing means is secured to the groove.

Preferably, the fixing means comprises at least one flange projecting laterally from the housing body.

Preferably, the flange is adapted to be slid into the groove.

Preferably, the structure further comprises filter means, such as a coarse screen for filtering the liquid. This may alleviate problems of obstruction of the metering conduit. Preferably, the structure further comprises a sluice gate. This is advantageous if the structure is mounted to a groove of an existing sluice because in this case the sluice gate of the existing sluice would have been removed.

Preferably, the sluice gate is moveable along one or more rail(s).

Preferably, the structure comprises moving means for moving the sluice gate in dependence upon the output of the flow meter. This enables the amount of liquid flowing through the channel to be regulated by closed loop control.

Preferably, the flow meter is powered by a power supply, the apparatus or structure further comprising detection means for detecting whether the flow of liquid in the channel is less than a predetermined amount, and means for interrupting power supply or reducing power consumption to the flow meter in dependence upon an output of the detection means. This may be used to reduce energy consumption. Preferably, the interrupting means is arranged to interrupt said power supply when the detected liquid flow is below a predetermined value. Normally the meter will be controlled by a processor and the processor will switch to a sleep or standby mode to reduce power when low flow is detected. Reducing energy consumption is particularly important when, as is preferred, the power supply means is provided by means of a battery or array of battery cells.

In a preferred embodiment the apparatus comprises detection means for detecting whether or not any liquid is present in the channel within a predetermined range of the flow meter; and means for interrupting power supply to the flow meter in dependence upon an output of the detection means. This feature can result in reduced power consumption and cost savings. It is of particular advantage if the flow meter is battery powered since the life of the battery can be extended by up to several times. In a closely related aspect the present invention provides apparatus for measuring liquid flow in a channel in which the liquid level may be subject to variation comprising baffle means for directing liquid flowing through the channel through a metering conduit of predetermined cross sectional area and of a lesser vertical extent than the maximum liquid height in the channel, the baffle being arranged so that, over a range of heights of liquid in the channel, substantially all of the liquid flowing through the channel is directed through the metering conduit and the metering conduit remains filled with liquid, and means for measuring liquid flow in said metering conduit.

The baffle may be inserted into the open channel such that the bottom edge of the baffle is resting on the bottom of the open channel. Alternatively, the baffle may be inserted to a depth at which it is found that more optimal conditions for measuring fluid flow are obtained.

A further advantage of the invention is that it may be possible to insert the structure directly into an open channel and obtain accurate measurements, without the need for any conditioning of the fluid flow upstream from the structure.

In some cases, rather than directing substantially all of the liquid, only a predetermined portion may be arranged to flow through the metering conduit. By metering conduit is preferably meant a closed conduit such as a pipe (preferably of substantially rectangular section) and the means for measuring liquid flow may comprise an electromagnetic flow meter integrated with the conduit, the conduit preferably defining the metering bore of the flow meter. The length of the conduit need only be sufficient for the purposes of metering and, in the limiting case, may have negligible length in the dimension of the liquid flow. The liquid need not be a pure liquid, but may contain entrapped solids and may be, for example, a slurry or suspension. It has been found that, at lower flow rates, when a conduit is less than completely filled, the fluid in the partially filled conduit tends to flow at a higher velocity and the meter, which primarily measures fluid velocity and determines volumetric flow rate on the assumption that the conduit is filled, tends to produce an overestimate of volumetric flow based on the higher measured velocity. Whilst it would be possible to measure the amount of fluid in the partially filled channel and to calculate the flow based on this measurement, this may require a complex arrangement. For example, it may require multiple sensing electrodes and substantial signal processing to process the measured data. Surprisingly, we have found that it is possible to compensate in our measurements for partial filling without directly measuring the depth of filling and thus it may be possible to obtain reasonable metering even when the metering conduit is not completely filled.

Preferably, the apparatus or structure further comprises means for adjusting the measured value of the liquid flow by multiplication with a filling factor when the measured value is below a predetermined value. Appropriate filling factors corresponding to various measured values of flow may be determined empirically and stored in a look-up table.

Although arrived at following consideration of the problems of metering in an open channel, the inventors have appreciated that the invention is more broadly applicable to other situations in which conduits may be partially filled.

In a further aspect the present invention comprises apparatus for obtaining a measure of volumetric fluid flow in a metering conduit, comprising an electromagnetic flow meter, characterised by means for generating a magnetic field in the metering conduit having a strength which varies with position to compensate the measure of volumetric fluid flow for partial filling of the metering conduit. Surprisingly this can compensate for partial filling without the depth of fluid being directly measured. Preferably, the non-uniform magnetic field varies with position so that a lower voltage is generated across the fluid for a given flow velocity when the conduit is partially filled than when the conduit is filled. This reduced voltage may counteract the inaccuracies of an increase in measurements due to the increased fluid flow velocity in the partially filled conduit.

Preferably, the field varies with position so that the ratio of the cross-sectional area of the conduit occupied by the fluid to the average effective field strength in that area is substantially constant. This may give a more linear output.

In a preferred implementation the fluid flows through the metering conduit at a flow velocity, the magnetic field inducing a voltage across the fluid which is a function of the strength of the magnetic field and the flow velocity, the meter comprising potential sensing electrodes to measure the voltage across the fluid, wherein the volumetric fluid flow rate is a function of the degree of filling of the conduit and the flow velocity, the conduit having a section which is a first function of position and the magnetic field strength being a second function of position, the first and second functions being selected so that the measured voltage is substantially a third predetermined function of said volumetric flow rate through the conduit.

Preferably, the third predetermined function is substantially a linear function of volumetric flow rate. This may facilitate measurement.

Again, the conduit preferably has a substantially rectangular section.

In a preferred embodiment of the invention, the magnetic field is substantially uniform in the metering conduit. The field need not necessarily be continuous, but may have irregularities in shape or strength, but may provide an average field strength within the conduit. Alternatively, the field within the conduit may be non- uniform. Preferably, the measuring electrodes of the flow meter extend substantially across the full height of the metering conduit.

Preferably, each electrode comprises a single electrode plate which is preferably substantially rectangularly shaped. Alternatively other shapes such as circular, trapezoidal or oval can be used.

However, the electrodes need not be continuous (i.e. formed in one plate). Instead, each electrode may comprise a plurality of electrode segments such as plates or strips. The segments are preferably positioned on the same wall of the flow meter as one another and spaced apart, or positioned adjoining one another, and preferably of rectangular shape.

The conduit may be seen to be divided into first and second portions, wherein when partially filled, the first portion is filled more than the second portion and wherein the field generating means generates a magnetic field of greater strength in the second portion of the measuring conduit than in the first. This may reduce the signal when partially filled.

Usually, the primary force acting on the fluid in a partially filled conduit is gravity, so the fluid will tend to flow in the lower part of a partially filled conduit, hereinafter referred to as the "first" part. The uppermost part of the conduit shall hereinafter be referred to as the "second" part. In some cases other forces may play an important role in determining the distribution of the fluid in a partially filled conduit. For example in a curved conduit the liquid may be subjected to centrifugal forces, or in the case of a fluid which is magnetically or electrically attracted to or repelled from certain portions of a metering conduit, references to the "first part" and the "second part" should be construed accordingly so that "first part" corresponds to that portion or those portions of a partially filled conduit to which fluid "gravitates" under the influence of whatever net force acts on the fluid within the conduit and the "second part" is the area that the fluid "gravitates" away from.

A good first correction in generating a non-uniform field may be made simply by means of generating a single sided magnetic field, preferably with a field coil, only at the side of the second portion of the measuring conduit. This feature may be provided independently in a further aspect in which the invention provides an electromagnetic flow meter having a non-uniform field-generating assembly, for example a coil on one side of the meter.

Although a single field generating element is remarkably and surprisingly effective to produce a correction, the shape of the field may be more precisely tailored to generate a more accurate correction. A second field generating element, preferably a field coil, of smaller size or strength, may be positioned at another side of the measuring conduit.

Although the invention may be applied to any shape of conduit, the conduit preferably has a substantially rectangular section. This may simplify correction as varying sections of the flow conduit do not need to be taken into account.

However, the section of the conduit may be varied to ensure the correction is more accurate, preferably the conduit side walls may be profiled to improve the linearity of the measure of volumetric fluid flow.

The flow meter apparatus as described above may be arranged for insertion into an open channel having a varying fluid level.

A preferred embodiment of this arrangement comprises a baffle at an inlet to the apparatus and/or a dam at the outlet of the apparatus to reduce the fluctuation of fluid level in the metering conduit.

One embodiment of the present invention comprises an electromagnetic flow meter, characterised by a non-uniform magnetic field which is generated within the conduit, which varies with position, to compensate for inaccuracies in flow measurement. The non-uniform magnetic field is generated so that the magnetic field strength is stronger nearer the top portion of the conduit and weaker towards the bottom portion. By having a stronger magnetic field near the top of the conduit and a weaker magnetic field near the bottom of the conduit, the velocity- dependent signal from the portion of the fluid near the bottom of the conduit is reduced and so a reduced voltage will be detected at the measuring electrodes, which will tend to balance out the over-reading of the meter. With this arrangement, the field will vary with position so that the ratio of the area occupied by the fluid to the effective field strength in that area is approximately constant.

Although a single magnetic coil is remarkably and surprisingly effective at producing a correction, the shape of the field may be more precisely tailored to generate a more accurate correction. In addition or alternatively, the shape of the channel may be selected to enhance or provide a correction. For example, in place of a truly rectangular conduit, a trapezoidal conduit or one having shaped sides may be employed. The shape of the conduit or of the field or both may be determined by calculation or empirically. More than one field-generating coil may be provided so that the shape of the field pattern may be altered, optionally independent on a measure of flow rate or fluid depth in the channel.

The present invention further provides a method of measuring fluid flow in an open channel comprising positioning an electromagnetic flow metering assembly integrated into a baffle at a metering point in the channel and measuring flow by means of the electromagnetic flow meter. The meter may also have a varying field pattern in accordance with an aspect of the invention mentioned previously. In an alternative embodiment, the non-uniform field is generated by a permanent magnet.

In a preferred embodiment a dam is installed downstream of the metering point to increase filling of the metering conduit under low flow conditions. This may further reduce variation of the liquid level in the metering conduit. Further aspects and preferred features are as set out in the claims.

Features of one aspect may be applied to other aspects; apparatus features may be applied to the method aspects and vice versa.

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 shows a schematic side view of a conventional flow meter arrangement;

Figure 2 shows a perspective view of an embodiment of the present invention.

Figure 3 shows the measuring section formed by a metering conduit in an embodiment of the present invention; and

Figure 4 shows a partial view of the measuring section shown in Fig. 3.

Figure 5 illustrates variation of output with flow for different field generating arrangements.

The conventional flow meter illustrated in Figure 1 has been described above. The following description of a preferred embodiment of the present invention will be made with reference to an open channel through which water flows.

The structure 1 shown in Figure 2 has a generally cuboid shape with parallel side walls 2 for insertion into an open channel such that the side walls are oriented parallel to the longitudinal dimension of the channel. At the upstream end 3 thereof the side walls 2 are linked by a coarse screen 4 formed with parallel vertical bars 5. Alternatively, a coarse mesh extending between the side walls 2 could be used. The coarse screen 4 acts as a filter to prevent large items such as driftwood from flowing into or through the structure 1 . Towards the other (downstream) longitudinal end 6 the side walls 2 are connected by a vertical end wall 7 which extends from the top of the structure 1 (at the same height as the top of the side walls 2) to a height spaced from the bottom of the structure. Located at a level below the vertical end wall 7 and joining onto this vertical end wall 7 is an undershot measurement portion 8 incorporating an inductive flow meter described hereafter. The vertical extent of the portion 8 is smaller than the overall vertical dimensions of the structure 1 as defined by the vertical dimensions of the side walls 2. The measuring portion 8 shown in Fig. 2 is a conduit of generally rectangular cross section, having the same width as the spacing between the vertical side walls 2. As shown in Fig. 2, the sides 9 of the measurement portion 8 are formed by extensions 9 of the side walls 2. The height and also the width of the measurement portion 8 should be selected to suit the range of flow rates for which the meter 1 will be used. In prototypes good results have been achieved using heights less than 20cm.

A pair of flanges 10 are provided on the side walls 2 at the upstream end 3. These flanges 10 can be separately attached to the side walls 2, e.g. by welding, or they can be formed by outward bending of a portion of the side walls 2.

A pair of vertically extending rails or runners 1 1 (only one being shown in Fig. 2; the other is located opposite the first) is mounted to the inside of the side walls 2 downstream of the screen 4.

Replacing an existing measurement arrangement as the one shown in Fig. 1 by one according to the present invention is as follows. The existing mechanical flow meter 1 10 is removed, and the existing sluice gate 102 is taken out of the grooves 104 of the sluice 100. The flanges 10 of the structure 1 shown in Fig. 2 are slid into the grooves 104 of the existing sluice 100. The structure is hence secured to the channel 108. A new sluice gate (not shown) is then slid into the rails 1 1 .

Operation of the arrangement so far described is as follows. Water enters the structure 1 through the screen 4 at the end 3 and flows towards the end 6. The flow of water (indicated by arrow 12) can be limited by operating the new sluice gate. In the case shown in Fig. 2 this would be achieved by raising or lowering the sluice gate. It will be appreciated that other types of sluice gates such as pivotable sluice gates may be used in combination with appropriate mountings such as hinges. The water which has passed the sluice gate is constrained by the vertical end wall 7 to flow through the measurement portion 8, where measurement of the flow takes place. The water then leaves the structure 1 via the downstream end 6.

From the foregoing description it will be appreciated that the measurement portion 8 is always entirely filled with water provided the water level does not drop below the top edge of the measurement portion 8 and the flow rate is within certain limits.

The measurement portion 8 will now be described in more detail with reference to Figs. 3 and 4. A magnetic (or inductive) flow meter is shown as installed around the measurement portion 8. Reference numerals 13, 14 and 9 respectively denote top and bottom walls and the left side wall of the measurement portion 8 (only the left hand side is shown in Fig. 4; the right hand side has a corresponding configuration). An electromagnet with laminated magnetic circuit 16 and top coil 17 and bottom coil 18 as shown in Figs. 3 and 4 is situated at the measurement portion 8 near the top and bottom walls 13 and 14 and extends over a substantial part thereof. The electromagnet powered together with a controller/processor 20 by a battery, generator, solar panel or mains connection, generates a magnetic field across the measurement portion 8, in the case shown in a vertical direction with the field being strongest in the area adjacent to the magnet. Electrodes 19 are provided at the side walls 9 of the measurement portion 8, at least one at each side wall 9.

In the embodiment detailed above, accurate measurements can be made provided the conduit remains substantially filled, for example if a dam is present downstream of the meter. However, according to a preferred embodiment, accurate measurements of volumetric flow may be made in the cases wherein the measuring conduit is not entirely filled.

As shown in Fig. 3, water enters the measurement portion 8 at the right hand part of the figure, the bottom end of the vertical end wall 7 joining onto the measurement portion at the top right hand corner. A voltage horizontally across the measurement portion 8 is generated due to the flow of the water (arrow 12) through the magnetic field, as known per se. This voltage is detected via the electrodes 19 provided at the sides 9 of the measurement portion 8. The voltage is a function of the flow velocity and the magnetic field strength and is communicated to a central controller 20 (shown in Fig. 2 only). The controller 20 includes means for processing the detected voltage and is preferably calibrated to detect and measure reduced flow rate, for example a storage device for recording the voltage (as indicative of flow rate) and the time of the measurement, and/or integrating it to record the total volume which has passed through the meter, or a smoothed value indicative of average flow rate. The value of the detected voltage and/or derived data may be stored in the controller 20 or an external storage medium connected to the controller 20. In one embodiment the controller 20 includes or is connected to a transmitter for transmitting the data to a satellite. This is particularly advantageous because it enables the flow of water, i.e. water consumption, to be monitored at a remote location, and because the installation costs of this flow meter arrangement could be lower when compared with a connection by land line.

The electrodes 1 9 are fixed to the wall of the conduit 8 by suitable means such as a bolt 21 passing through the wall. Electrical connections also pass through the wall at this point. In the preferred embodiment the joint is therefore sealed by an O-ring 22. Also in the preferred embodiment a tamper-proof cover 23 sealed by an O-ring 24 gives access for servicing the electrode 1 9. Since the inductive flow meter forms a substantially sealed unit the risk of vandalism is reduced. Further, having no moving parts the structure 1 represents a smaller risk to children and animals than the mechanical flow meter 1 10 shown in Fig. 1 . The structure 1 may include an actuator (not shown) for operating the sluice gate. Any suitable actuator can be used, for example a chain driven by a reversible electric motor and connected to a rack and pinion on the sluice gate. This actuator can be controlled to operate in dependence upon an output of the flow meter by the controller 20. Thus the flow of water can be controlled to a required value.

The controller 20 is connected to a suitable detector (not shown) provided at or near the flow meter. The detector delivers an output to the controller 20 and, if this output indicates that no water is present or the depth (and thus the flow) is less than a minimum amount the controller 20 interrupts the power supply to the flow meter and switches to a stand-by mode. The detector monitors the presence of water either continuously or at intervals. When subsequently the output from the detector indicates that water is present power is again supplied to the flow meter.

As mentioned above, the measured voltage is a function of the fluid flow velocity and magnetic field strength. Provided the metering conduit is completely filled with fluid and the flow is uniform, the volumetric flow rate is simply a product of the fluid flow velocity and the area of the conduit, which is a product of the voltage output, the magnetic field strength and a constant. However, when the conduit is not filled completely, inaccurate measurements will be made as in this instance, the fluid generally tends to flow at a higher velocity through the metering conduit, thereby generating a higher voltage for a given volumetric fluid flow.

Various methods to overcome this problem were investigated including the installation of a dam in the channel downstream of the flow meter to ensure that the metering conduit is always completely filled with fluid. The baffle described above reduces this variation.

In a preferred embodiment a correction is made in order to compensate for this over-reading of the flow meter by varying the strength of the magnetic field within the conduit in order that the field is weaker towards the bottom of the conduit. A voltage of lesser magnitude will thereby be generated across the fluid when the conduit is partially filled thus compensating for the higher fluid flow velocity, so the volumetric flow is still substantially proportional to the voltage output.

The simplest method of varying the field strength in this way is simply by removing the bottom magnetic coil 18 of the flow meter. Alternatively, it is possible to reduce the size or strength of the bottom coil 18 (for example by reducing the current through the coil) or indeed to move it to another position of the metering conduit.

A comparison of the various methods described above is shown in Figure 5. Typically the voltage signal is processed by an electronic signal processor to a frequency signal which is substantially proportional to the volumetric flow rate. In one embodiment, the signal processor is a pulse generator which generates a pulse per (chosen) unit volume of fluid, for example one pulse per litre. A typical correlation between flow rate and output of a pulse generator (frequency is proportional to flow rate) is shown in Figure 5. It was found that best results were achieved with the use of a dam downstream and both coils. This ensured the most linear characteristic. For the other cases where no dam was used, the case where both magnetic coils 17 and 18 of the flow meter are energised produced the output of greatest magnitude, but further correction is required to ensure a linear characteristic. The case where only the bottom coil 18 of the flow meter was energised was also investigated. When this alone was energised it produced an output with lesser magnitude and an even less linear characteristic. However, the case where only the top coil 17 of the meter is energised produced a signal of smaller magnitude than with both coils, as would be expected, but a surprisingly substantially linear characteristic. Thus by simply energising the top coil and altering the constant, a reasonable output can be achieved under a variety of flow conditions. In order to provide an optimal characteristic, the metering conduit profile may be modified. For example, in place of a truly rectangular conduit, a trapezoidal or conduit having shaped sides may be employed. The shape of the conduit or of the field or both may be determined by calculation or empirically to improve linearity.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

Claims

CLAIMS:

1 . Apparatus for measuring liquid flow in a channel comprising baffle means for directing liquid flowing through the channel through a metering conduit of predetermined, substantially rectangular cross sectional area, the baffle means being arranged above the metering conduit so that, when the apparatus is installed in the channel, the baffle means serves to reduce variation of the liquid level in the metering conduit, and an electromagnetic flow meter for measuring liquid flow in said metering conduit.

2. Apparatus according to Claim 1 , wherein, when the apparatus is installed, the metering conduit is located towards the bottom of the structure, preferably within the lower 20, 15, 10 or 5 % of the apparatus.

3. Apparatus according to Claim 1 or Claim 2, wherein the baffle means serves to reduce variation of the liquid level in the metering conduit over a range of liquid heights within the channel from above the upper internal edge of the metering conduit to the top edge of the baffle means.

4. Apparatus according to any preceding claim, wherein the width of the metering conduit is smaller than the width of the baffle means, and preferably less than 75 percent or 50 percent of the width of the baffle means.

5. Apparatus according to any preceding claim, wherein the height of the metering conduit is small compared to half its width, preferably less than one-third or one quarter of its width, and/or wherein the height of the metering conduit is small when compared with the combined height of the baffle means and the metering conduit, preferably less than 50, 20, 10 or 5 % of the combined height.

6. Apparatus according to any preceding claim, having a generally flat bottom.

7. Apparatus for measuring liquid flow in a channel in which the liquid level may be subject to variation, the apparatus having a generally flat bottom and comprising a metering conduit of predetermined, substantially rectangular cross sectional area located towards the bottom of the apparatus, and baffle means, located above the metering conduit, for directing liquid flowing through the channel through the metering conduit, and a non-intrusive flow meter for measuring liquid flow in said metering conduit.

8. Apparatus according to Claim 6 or Claim 7, wherein the flat bottom extends over a substantial portion of the length of the metering conduit, preferably the entire length thereof.

9. A flow meter structure installed in an irrigation channel and for measuring a liquid flow through the channel, the flow meter structure comprising baffle means for directing liquid flowing through the channel through a metering conduit of predetermined cross sectional area, the baffle means being arranged so as to reduce variation of the liquid level in the metering conduit over a first range of liquid levels in the channel, and a flow meter for measuring liquid flow in said metering conduit, wherein the flow meter structure is installed at a position in the channel where the liquid level in the metering conduit varies over a second range of liquid levels in the channel.

10. Flow meter structure according to Claim 9, wherein the metering conduit is located near the bottom of the structure.

1 1 . A flow meter structure according to Claim 9 or Claim 10, wherein the first range of liquid levels extends from above the upper internal edge of the metering conduit to the top edge of the baffle means.

12. A method of replacing a Deathridge wheel in a channel in which the liquid level may be subject to variation, the method comprising removing the Deathridge wheel; and inserting, into the channel, apparatus for measuring liquid flow in the channel, the apparatus comprising baffle means for directing liquid flowing through the channel through a metering conduit of predetermined cross sectional area, the baffle means being arranged so as to reduce variation of the liquid level in the metering conduit, and means for measuring liquid flow in said metering conduit.

13. A method according to Claim 12, wherein the apparatus is inserted into the channel at a position at which there is no sump.

14. A method according to Claim 12, wherein no sump is built at the installation site of the apparatus prior to, or during installation of the apparatus.

15. A method according to any of Claims 12 to 14, wherein the apparatus is secured in a retaining formation of a sluice.

16. A structure for insertion into a channel and for measuring a flow of a liquid through the channel, the structure comprising: a metering conduit accommodating a flow meter; and, fixing means for securing the structure to a retaining formation of a sluice provided in or at the channel.

17. A structure according to Claim 16, wherein the fixing means is adapted to be secured to a groove of the sluice.

18. A structure according to Claim 17, wherein the fixing means comprises at least one flange projecting laterally from a housing body of the structure and adapted to be received into the groove.

19. A method of installing the flow meter structure according to Claim 16 in a channel, the method comprising securing the fixing means in a groove of a sluice provided in or at the channel.

20. A method according to Claim 19, wherein securing the fixing means into the groove of the sluice comprises sliding the fixing means into the groove.

21. Apparatus or structure according to any of Claims 8 to 1 1 or 16 to 18, wherein the flow meter is an electromagnetic flow meter.

22. Apparatus or structure according to any of Claims 1 to 7 or 21 , further comprising means for adjusting the measured value of the liquid flow by multiplication with a filling factor when the measured value is below a predetermined value.

23. Apparatus or structure according to any of Claims 1 to 7 or 21 , wherein the liquid flows through the metering conduit at a flow velocity, the magnetic field of the electromagnetic flow meter inducing a voltage across the liquid which is a function of the strength of the magnetic field and the flow velocity, the flow meter comprising potential sensing electrodes to measure the voltage across the liquid, wherein the volumetric liquid flow rate is a function of the degree of filling of the conduit and the flow velocity, the conduit having a section which is a first function of position and the magnetic field strength being a second function of position, the first and second functions being selected so that the measured voltage is substantially a third predetermined function of said volumetric flow rate through the conduit.

24. Apparatus for measuring liquid flow in a channel comprising a metering conduit and an electromagnetic flow meter, wherein the liquid flows through the metering conduit at a flow velocity, the magnetic field of the electromagnetic flow meter inducing a voltage across the liquid in the conduit which is a function of the strength of the magnetic field and the flow velocity, the flow meter comprising potential sensing electrodes to measure the voltage across the liquid, wherein the volumetric liquid flow rate is a function of the degree of filling of the conduit and the flow velocity, the conduit having a section which is a first function of position and the magnetic field strength being a second function of position, the first and second functions being selected so that the measured voltage is substantially a third predetermined function of said volumetric flow rate through the conduit.

25. Apparatus or structure according to Claim 23 or 24, wherein said third predetermined function is substantially a linear function of volumetric flow rate.

26. Apparatus or structure according to any of Claims 23 to 25, wherein the conduit has a substantially rectangular section.

27. Apparatus or structure according to any of Claims 23 to 26, wherein the magnetic field is substantially uniform over the section of the metering conduit.

28. Apparatus or structure according to any of Claims 23 to 27, wherein each electrode comprises a single electrode plate.

29. Apparatus or structure according to any of Claims 23 to 28, wherein each electrode comprises a plurality of electrode plates or strips.

30. Apparatus for obtaining a measure of volumetric liquid flow in a metering conduit, comprising an electromagnetic flow meter, characterised by means for generating a magnetic field in the metering conduit having a strength which varies with position to compensate the measure of volumetric liquid flow for partial filling of the metering conduit.

31 . Apparatus according to Claim 30, wherein the magnetic field varies with position so that a lower voltage is generated across the liquid for a given flow velocity when the conduit is partially filled than when the conduit is filled.

32. Apparatus according to Claim 30 or 31 , wherein the field varies with position so that the ratio of the cross-sectional area of the conduit occupied by the liquid to the average effective field strength in that area is substantially constant.

33. Apparatus according to any of Claims 30 to 32, wherein the conduit has first and second portions, wherein when partially filled, the first portion is filled more than the second portion and wherein the field generating means generates a magnetic field of greater strength in the second portion of the metering conduit than in the first.

34. Apparatus according to any of claims 30 to 33, wherein the field generating means comprises a single field generating element, preferably a field coil, only a ^~"t the side of the second portion of the metering conduit.

35. Apparatus according to Claim 34, wherein the field generating means comprises one field generating element, preferably a field coil, adjacent to the side of the second portion of the metering conduit and a field generating means, preferably a field coil, of smaller size or strength, at another side of the metering conduit.

36. Apparatus or structure according to any of Claims 1 to 1 1 , 16 to 18 or 21 to 35, wherein the metering conduit has side walls profiled to improve the linearity of the measure of volumetric liquid flow.

37. Apparatus or structure according to any of Claims 1 to 1 1 or 21 to 36, wherein the height of the measuring electrodes extends substantially across the full height of the metering conduit.

38. Apparatus or structure according to any of Claims 1 to 1 1 , 16 to 18 or 21 to 37, wherein the field generating means of the electromagnetic flow meter is provided only at the bottom of the metering conduit of substantially rectangular cross section, and potential sensing electrodes are provided on both lateral sides of the metering conduit.

39. Apparatus or structure according to any of Claims 1 to 1 1 , 16 to 18 or 21 to 38, further comprising a sluice gate.

40. Apparatus or structure according to Claim 39, further comprising moving means for moving the sluice gate in dependence upon an output of the flow meter.

41. Apparatus or structure according to any of Claims 1 to 1 1 , 16 to 18 or 21 to 40, further comprising a debris screen as a filter.

42. Apparatus or structure according to any of Claims 1 to 1 1 , 16 to 18 or 21 to 41 , wherein the flow meter is powered by a power supply, the apparatus or structure further comprising detection means for detecting whether the flow of liquid in the channel is less than a predetermined amount, and means for interrupting power supply to the flow meter or reducing power consumption by the flow meter in dependence upon an output of the detection means.

43. Apparatus or structure according to claim 42, wherein the interrupting means is arranged to interrupt said power supply when the detected liquid flow is below a predetermined value.

44. Apparatus or structure according to claim 43, wherein said power supply means is provided by means of a battery or array of battery cells.

45. A method of obtaining a measure of volumetric liquid flow in a metering conduit which may be subject to partial filling using an electromagnetic flow meter which generates a magnetic field and measures an induced voltage representative of liquid flow rate, the method comprising generating a non-uniform magnetic field which varies with position within the conduit to compensate the measured induced voltage for the partial filling of the metering conduit.

46. A method according to Claim 45, wherein the magnetic field varies with position so that a reduced voltage is generated for a given flow velocity when the conduit is partially filled preferably so that the measured voltage is a substantially linear function of volumetric flow rate.

47. A method according to Claim 45 or Claim 46, wherein the side walls of the metering conduit are profiled to improve the linearity of the measured voltage as a function of volumetric flow rate.

48. A method of installing a flow meter for metering the flow of liquid in an open channel comprising providing apparatus or structure according to any of Claims 1 to 1 1 , 16 to 18 or 21 to 44 at a metering point in the channel.

49. A method according to any of Claims 12 to 15, 19, 20 or 45 to 48, wherein a sluice gate is provided in or at the channel and is moved in dependence on the output of the flow meter.

50. A method according to any of Claims 12 to 15, 19, 20 or 45 to 49, wherein for a defined range of liquid flow rates, the pressure of the contents of the conduit is greater than atmospheric pressure.

51 . A method according to any of Claims 12 to 15, 19, 20 or 45 to 50 for measuring liquid flow within a metering conduit which is subject to partial filling, wherein a dam is installed downstream of the metering point to increase filling of the metering conduit under low flow conditions.

52. An electromagnetic flow meter having a non-uniform field-generating assembly.

53. Apparatus or structure substantially as any one herein described with reference to or as illustrated in Figures 2 to 4, or 5 of the accompanying drawings.

54. A method substantially as any one herein described with reference to or as illustrated in Figures 2 to 4, or 5 of the accompanying drawings.

55. Use of a compensating factor with an electromagnetic flow meter positioned in an open channel susceptible to partial filling to obtain a measure of fluid flow rate in the channel.