WO2009037501A1

WO2009037501A1 - Measurement of flow in a channel

Info

Publication number: WO2009037501A1
Application number: PCT/GB2008/050829
Authority: WO
Inventors: Richard Warren Jones
Original assignee: Hymetrics Limited
Priority date: 2007-09-17
Filing date: 2008-09-17
Publication date: 2009-03-26
Also published as: GB0718089D0

Abstract

An apparatus for measurement of the rate of flow of water along an open channel consists of a carrier (11) within which is a hydrometric structure (20) in the form of a weir or flume, and also sensors (3, 4) to measure a depth of water D1 upstream of the hydrometric structure, and a depth of water D2 immediately downstream of the hydrometric structure. These are connected to a computer (25) that selects an appropriate calibration, in accordance with the inclination of the apparatus, and deduces the flow rate. The two different depth measurements D1 and D2 enable the computer (25) to check for consistent operation of the apparatus.

Description

MEASUREMENT OF FLOW IN A CHANNEL

The present invention relates to an apparatus and a method for measuring flow in a channel, particularly but not exclusively in an open channel.

The most common way of measuring flow in open channels is the critical-depth method which requires the use of hydrometric weirs and flumes. By this method, a flow measurement systems comprises: a hydrometric structure to form a contraction of cross-section of the channel; a sensor for measuring head of water approaching the hydrometric structure; a head/flow calibration of the hydrometric structure; and a computing device to determine a value for the rate of flow passing through the hydrometric structure from a measurement of the head and the characteristic of the calibration. Head/flow calibrations are published in ISO Standards for types and sizes of standard hydrometric structures. Non-Standard hydrometric structures can be calibrated against a reference flow meter. To comply with metrology standards, particularly the ISO 17025 calibration standard, the reference flow-meter must have a calibration certificate that can be traced to international reference measurements.

As a general rule, flume and weir technology is perceived as a different field of technology to that of head measurement, and consequently these two components are supplied by different manufacturers to be integrated and set-up at the measurement site. To assemble and integrate a working system the computing module must be programmed with the correct calibration, and the head measurement device must be set with its zero-head measurement coincident with the zero-flow datum of the hydrometric structure. The 'Portable flow-measuring device' of US 4 195 520 (Shaver) for use in circular drains, seals the drain with an inflatable collar to divert the flow into a chamber which houses a weir plate. As the water backs-up in the drain the chamber fills causing the flow to over-top the weir and return to the drain downstream of the drain- seal. The flow rate is indicated by the water level in the chamber upstream of the weir reaching graduations on a plate. This device thus embodies a hydrometric structure, a weir, with calibrated graduations to indicate the flow rate over the weir. US 3 427 878 (Gerlitz et al) describes a portable flume that can be inserted into an open drain, imposing a restriction on the flow; an adjustable rod provided with a vernier screw enables the liquid level upstream of the restriction to be measured. Again, this liquid level can be calibrated against the flow rate along the drain. In contrast to the complications involved in measurements in open drains, turbine or electromagnetic flow meters which are used for measurement in pipes are relatively simple measurement systems. Pipe flow meters are pre-calibrated with computing device already integrated at the factory so that they can be fitted on the pipeline ready to measure the flow.

It is an objective of this invention to provide a means of integrating the components of an open channel flow measurement system so that, as an integrated system, it can be inserted into a channel to provide a pre- calibrated measurement of the flow in the channel. Such systems are ready to be switched-on for measurement, in the same way as pipe flow meters. SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided an apparatus for the measurement of a flow of liquid along an open channel comprising: i) a carrier with a cross-section shaped to fit the channel and adapted for installation in the channel such that the liquid flowing along the channel passes through the carrier;

ii) a hydrometric structure fixed within the carrier to cause the depth of the liquid approaching the hydrometric structure to vary with the rate of flow of liquid passing through the hydrometric structure;

iii) a first sensor in the carrier upstream of the hydrometric structure to measure a first depth of liquid;

iv) a second sensor in the carrier downstream of the hydrometric structure to measure a second depth of liquid;

v) means for the computation of the flow of liquid along the channel through an installed carrier from the measured first depth;

vi) and checking means reponsive to the first and the second depths;

The checking means enables correct functioning of the apparatus to be confirmed. For example, under normal circumstances, if the flow rate increases then there is an increase in both the first depth the second depth; however if the hydrometric structure is partially or completely blocked, then the first depth increases but the second depth does not correspondingly do so. As a second potential advantage, if the carrier is provided with shutter means to retain a volume of still liquid within the carrier, then the corresponding measurements of the first depth and the second depth, combined with their known separation along the carrier, enables the inclination of the carrier to the horizontal to be determined. Thirdly, if there is a blockage or flow restriction downstream of the apparatus in the channel, then the downstream liquid level may back up, potentially leading to a so-called drowned condition of the hydrometric structure; this would be detected from the second depth being significantly greater than that expected from the value of the first depth; and under these conditions the flow rate can be computed from the values of both the first and the second depths.

The computation of the flow of liquid may utilise a table of calibrations of flow passing through the carrier in relation to the measured first depth, each calibration being at a fixed and known inclination of the carrier, the table comprising calibrations at a number of fixed inclinations. Alternatively the computation of the flow of liquid may utilise an algorithm or equation, based on the measured value of the first depth. In any event the apparatus preferably incorporates means to check if the carrier is horizontal, or to measure its inclination from the horizontal. This may comprise a spirit level, or a clinometer, or as indicated above the inclination may be measured from the measurements of the first depth and the second depth when there is non-flowing liquid within the carrier. Where the computation of flow uses a table of calibrations at a number of different inclinations, the appropriate calibration may be deduced by interpolation, on the basis of the measured inclination, between calibrations that are in the table. The hydrometric structure preferably defines a tapered gap for through flow of the liquid, the gap being narrower at the bottom than the top, and preferably the gap is V-shaped as viewed in elevation along a horizontal axis. Preferably the hydrometric structure comprises opposed restricting elements on each side of the carrier. In the preferred embodiment each restricting element is defined by surfaces inclined to the longitudinal axis of the apparatus and meeting along the edge of the tapered gap; and in the preferred embodiment the inclined surfaces are plane surfaces, and the inclination to the longitudinal axis of the upstream surface is less than that of the downstream surface. In any event both the carrier and the hydrometric structure are desirably of a rigid material to ensure consistent operation.

Preferably the first sensor and the second sensor are ultrasonic transducers arranged to transmit ultrasonic pulses through the liquid. The ultrasonic transducers may be embedded within the structure of the carrier, particularly where the carrier is of a material such as glass fibre reinforced resin. Under some circumstances it may be advantageous if the second sensor is arranged to transmit pulses in a direction inclined from the vertical, preferably by an angle of no greater than 20°, more preferably between 10° and 15°. This is preferably such that the ultrasonic pulses propagate in a direction at least approximately orthogonal to the surface of the liquid in that vicinity. In this context it should be appreciated that the term 'depth' refers to the distance from the sensor to the liquid surface, and at least for the second depth is not necessarily measured in a vertical direction. The second sensor is preferably arranged sufficiently close to the hydrometric structure that the liquid surface to which the measurement is made is sloping smoothly. Further downstream the liquid would be in a disturbed state with surface waves and considerable turbulence, so that ultrasonic depth measurements from below the surface are more difficult.

In a second aspect the present invention provides an apparatus for the measurement of a flow of liquid along an open channel comprising a carrier with a cross-section shaped to fit the channel and adapted for installation in the channel such that the liquid flowing along the channel passes through the carrier, and a hydrometric structure fixed within the carrier to cause the depth of the liquid approaching the hydrometric structure to vary with the rate of flow of liquid passing through the hydrometric structure; also comprising a first sensor fixed to the carrier upstream of the hydrometric structure to measure a first depth of liquid, and means for the computation of the flow of liquid along the channel through an installed carrier from the measured first depth; wherein the apparatus is provided with means for measuring the inclination of the apparatus from the horizontal, and the computation means selects an appropriate flow/depth calibration in accordance with that measured inclination.

The invention also provides a method for measuring flow of liquid along an open channel by using an apparatus of the invention. The method may include a calibration step, after installing the carrier in the channel. This may only necessitate measurement of the inclination of the carrier from the horizontal, for example by use of a spirit level, or by trapping still liquid within the carrier and measuring the first depth and the second depth. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an apparatus of the invention installed in a drain, and shown in longitudinal section;

Figure 2 shows a cross section view of the apparatus, on the line 2-2 of figure 1 ;

Figure 3 shows a perspective view of the apparatus of figure 1 and 2 ;

Figure 4 shows typical calibration characteristics (head plotted against flow rate) for an apparatus of the invention, with a set of characteristics for sloping channels; and

Figure 5 shows the variation of first and second depths with flow rates under different flow conditions.

A PREFFERED EMBODIMENT OF THE INVENTION

Referring to Figures 1, 2 and 3, the components of the apparatus 10 are mounted on a carrier 11, which in the example shown here is a curved sheet shaped to fit a semicircular drain 50, the top edges of the sheet being connected by two rigid bars 13. The carrier 11 seals to the drain, so liquid flowing along the drain 50 then flows through a channel 12 defined by the carrier 11.

Segments 14 are mounted to the walls of the channel 12 at an intermediate position along its length, and define the geometric form of a hydrometric structure 20 which forms a contraction of the channel cross-section area. The segments 14 each consist of two plane surfaces 15 and 16 which are inclined oppositely with respect to a horizontal line along the centre of the channel 12, and which meet along an edge 17 that is inclined to the vertical, so there is a tapered gap 18 between the opposed edges 17. The gap 18 is narrowest at the bottom. The downstream plane surfaces 16 are inclined to the said horizontal line more steeply than are the upstream plane surfaces 15.

When installed in the drain 50, if water flows along the drain it accelerates through the gap 18 under gravity to a critical state in which the upstream water level 30 is higher than the downstream level, and the upstream level 30 is uniquely related to the flow rate of water along the channel 12 of the carrier 11. As the liquid approaches and passes through the gap 18 the water velocity increases and the water level 31 slopes down. Downstream of the sloping water surface 31 the water reaches a region 32 with a mean level lower than the upstream level 30, but the water is turbulent and there is usually a standing wave. The water surface in this region 32 is usually disturbed, but the water waves cannot propagate up the sloping surface 31 because the water there is flowing faster than the wave velocity. For hydrometric structures the relationship between upstream water level and flow is either found within a published ISO Standard calibration for Standard geometric forms of hydrometric structure, or may be determined by calibration, for example in a laboratory, before use of the apparatus .

In this example, the upstream water level is measured using a first sensor 3 mounted close to the upstream edge of the carrier 11. This typically emits a short burst of four cycles at a frequency of 1 MHz to form a pulse capable of resolving distances to an accuracy of about 0.001 m. The pulse travels within a narrow conical region, indicated by broken lines, with its axis along the bold dashed line to the surface at 22. The distance Dl from the base of the channel 12 to the water surface 22 is:

Dl = 0.5 Sv tl + dl

where: Sv is the speed of the pulse through water, tl the time interval between triggering the pulse of the first sensor 3 and the detection of its echo from the water surface at 22, and dl is a constant distance offset related to the construction and vertical position of the first sensor 3. The speed of a pulse of ultrasound is typically 1450 m/s but this value varies with the temperature of the water and with its composition (the speed through sea water being typically 1500 m/s) . To determine Sv, the water temperature T is measured using a thermometer 24 and this value is processed by a computing module 25 using a stored lookup table relating sonic velocity Sv to the water temperature T.

A second sensor 4 a short distance downstream of the hydrometric structure 20 measures a distance D2 to the sloping water surface at 23 of water leaving the hydrometric structure 20. The water surface slopes as it is accelerated through the contraction of the hydrometric structure 20. To ensure that a strong echo is received from the sloping water surface, the sensor 4 is arranged with its beam axis (shown by the bold dashed line) inclined from the vertical at an angle 5. The angular width of the ultrasonic beam from the sensor 4 - and the angular width over which the sensor 4 is sensitive to incident ultrasound - should be large enough for the sensor 4 to be able to receive echoes from the sloping surface 23 but also to be able to detect echoes from a horizontal surface. For example, if the beam is of angular width 25° the sensor 4 might be arranged so its axis is inclined at 10° from the vertical. When the water is flowing, and therefore sloping as shown in figure 1, the echoes will be from the closest region of the surface (at 23) , so the corresponding ultrasonic waves propagate orthogonally to the surface. Again, the distance D2 to the water surface at 23 is:

D2 = 0.5 Sv t2 + d2 where: t2 the time interval between triggering the pulse of the second sensor 4 and the detection of its echo from the water surface at 23, and d2 is a constant distance offset related to the construction and position of the second sensor 4. It will be appreciated that this downstream depth distance D2 is not in a vertical direction, but is orthogonal to the local surface 23. The computing module 25 receives measurements of time interval, tl and t2, and measurements of the temperature of the water, T, through cables 7. Cables 7 are embedded within the form of the carrier 11 beneath a surface ridge 8. External communication with the computing module is via a cable 28.

The constants dl and d2 are determined by calibration and are stored for subsequent use within the computing module 25. These offset values dl and d2 are such that, if the apparatus 10 is horizontal and there is a uniform depth of water (not flowing) in the channel 12, the values Dl and D2 are equal and are equal to the distance from the base of the channel 12 to the water surface. Where the carrier 11 is of glass fibre reinforced plastic material, the sensors 3 and 4 may be embedded within the thickness of the material so that their operative surfaces are protected from the liquid by a thin layer of resin. As indicated above, under normal operating conditions the flow rate can be deduced from the upstream depth, i.e. distance Dl, which can be obtained from the signals from the first sensor 3. As the flow rate increases, so does the upstream depth, and so also does the downstream depth D2 as measured by the second sensor 4. The downstream depth D2 increases at least approximately linearly with the upstream depth Dl. The ratio of D2 to Dl is therefore approximately constant under normal operation, and for any measured value of Dl there is an expected value of D2. By monitoring the values of Dl and D2, or monitoring their ratio, the computing module 25 can therefore check if the apparatus 10 is operating correctly. If D2 is greater than the expected value, that indicates a blockage or flow restriction in the drain 50 downstream of the apparatus 10, causing the downstream liquid to back up. Under these circumstances, as long as there is a drop of at least about 20 mm through the hydrometric structure 20, the flow rate can continue to be measured, by using an appropriate and different calibration, taking into account the values of both Dl and D2. If D2 is less than the expected value, that indicates a blockage or flow restriction in the hydrometric structure 20. If this reduction is slight, this may be due merely to contamination of the surfaces of the hydrometric structure 20, for example by growth of a biofilm or by fat deposits; and this would indicate that the hydrometric structure 20 should be cleaned. If this reduction is large, that would suggest the presence of debris of some type blocking the hydrometric structure 20 and therefore blocking the drain 50, so that urgent action should be taken to remove the blockage.

It will be appreciated that the section of drain 50 in which the apparatus 10 is installed may be horizontal, or may be inclined in the direction of flow. Drains are typically inclined at approximately 2%, that is to say an angle of about 1.14° below the horizontal. The apparatus 10 may incorporate a sensitive spirit level 26 so that the apparatus 10 may be installed horizontally, or alternatively the spirit level 26 may be used to measure the inclination of the apparatus 10 when it has been installed. It will be appreciated that the apparatus 10 may be calibrated before use at a range of different angles of slope (e.g. being calibrated at 0°, 0.5°, 1.0°, 1.5° and 2.0°), and the appropriate calibration to be used by the computing module 25 may be obtained from example by interpolation between the closest angles for which calibration values are available. The spirit level 26 is only required on installation, or subsequently if the inclination is to be checked, and need not be integral with the apparatus 10.

As an alternative to using a spirit level, flow of liquid through the apparatus 10 may be stopped, during installation, using barriers (not shown) sealed to the ends of the channel 12; with still liquid in the channel 12 so the water level is horizontal throughout the length of the channel, measurement of the depths Dl and D2 enables the inclination of the apparatus 10 to be calculated. So if the carrier 11 is inclined on a gradient, for example the gradient of a drain, then the inclination, θ, of that gradient will be: θ = (Dl'-D2' ) /L where L is the horizontal distance between the first and second sensors 3 and 4, and Dl' and D2' are measured distances to the horizontal water surface. The angle of inclination so calculated can be stored in the computing unit 25, so as indicated above an appropriate calibration can be used that takes account of the slope of the apparatus 10.

Referring now to figure 4, this shows graphically on the left-hand graph the relationship between the measured time interval t and the calculated value of the depth Dl above the base of the apparatus 10; and on the right-hand graph shows the relationship between the variation of the first depth, Dl, with the flow rate Q of water through the apparatus 10. As indicated in the left-hand graph, the depth Dl is made up of two parts, the calculated range R which is half of the product of the speed Sv of ultrasound with the time tl, and the offset constant dl . It will be appreciated that the slope of the graph is equal to half the speed Sv. As indicated in the right- hand graph, the first depth Dl - which corresponds approximately to the hydraulic head - increases as the flow rate Q increases. The exact relationship depends upon the slope of the channel 12, and graphs are shown for a horizontal apparatus 10 (marked 0%), and for slopes of 0.5%, 1.0% and 1.5%. Graphs such as these can be obtained by means of a laboratory calibration experiment carried out at a range of different inclinations. If, in a particular example, the slope of the apparatus 10 when installed in the drain 50, is measured as being 0.3%, then the appropriate calibration line, marked by the arrow, may be deduced by interpolation between the closest calibration slopes above and below (0 and 0.5% in this case) . Hence, after installation, from the known value of the inclination, any measured value of the first depth Dl may be related to the corresponding flow rate Q.

Referring now to figure 5, the graph 'a' represents the variation of Dl with Q, as discussed in relation to figure 4. The graphs 'b' and 'c' represent the variation of the second depth D2 with flow rate Q in two different circumstances. The graph 'b' is the drowning threshold, and is inherent to the design of the hydrometric structure 20, and so this graph 'b' is determined during calibration. It corresponds to a situation in which the flow rate downstream of the apparatus 10 is restricted, so that the downstream water level rises up. The drowning threshold indicates that there is insufficient difference of height between the upstream and downstream water levels so that the water does not reach critical flow as it passes over the hydrometric structure 20. The graph ^Λc' represents the variation of the second depth D2 with the flow rate Q, at the normal operating conditions in the drain 50; the second depth D2 is at least approximately linearly related to the first depth Dl; and this graph ^Λc' can be determined immediately after the apparatus 10 has been installed in the drain 50. For example measurements of D2 may be made and recorded over the first 24 hours of operation, when it may be assumed that there is no blockage or fouling of the surfaces. It will be appreciated that the exact values for this graph ^Λc' depend on the exit conditions, that is to say the conditions under which the water flows after it has left the apparatus 10.

As explained above, for any measured value of the first depth Dl the corresponding flow rate Q can be deduced. If the corresponding value of the second depth D2 lies on the graph ^Λc' then the apparatus 10 is in its normal operational state. If the value of the second depth D2 lines between graph ^Λc' and graph ^Λb' , that is to say in zone Zl, for example at D2a, this indicates some blockage of the downstream drain 50 as compared to the initial state, but insufficient to lead to drowning; and under these circumstances the measurements of flow rate Q deduced from Dl will be correct. There are two different fault conditions: values of second depth D2 below the graph ^Λc' , or above the graph ^Λb' , and these are as follows. If the second depth D2 is below the graph ^Λc' , that is to say in zone Z3, for example at D2c, this indicates partial blockage of the hydrometric structure 20, so the deduced values of flow rate Q will not be correct, and the apparatus 10 needs to be serviced to remove the blockage. This may only necessitate cleaning of the surfaces, or it may require removal of debris. If the second depth D2 is above the drowning threshold graph ^Λb' , that is to say in zone Z2, for example at D2b, this indicates that the apparatus 10 is in a flooded state. This indicates a blockage in the drain 50 downstream of the apparatus 10, and it may be necessary to request servicing of the drain 50 to remove this blockage. Under these circumstances the measurements of flow rate Q as deduced from the first depth Dl would be inaccurate, although the apparatus 10 may nevertheless be calibrated to deduce the flow rate from measurements of both the first depth Dl and the second depth D2 ; such measurements are likely to be less accurate than those under normal operating conditions.

It will be appreciated that the apparatus 10 described above may be modified in various ways while remaining within the scope of the invention. For example the hydrometric structure may have a different shape to that described, for example, formed to fit a rectangular channel. As another example, a hydrometric structure might form a weir occupying the lower portion of the carrier with a horizontal crest elevated at a distance 0.3 to 0.4 times the carrier width above the base of the channel. The form of such a weir might be defined, for example by an upstream ramp approaching the crest on a 2:1 gradient followed by a downstream downward ramp leaving the crest on a 5:1 gradient. In this arrangement, the second sensor may be located in or on the downstream ramp. This provides a hydrometric structure that has higher capacity, but has the potential disadvantage that dead water is impounded by the weir. Indeed the hydrometric structure might consist of a weir combined with a side member (like the segment 14) projecting from at least one wall of the channel 12.

In a further variation, at least part of the base of the carrier upstream of the hydrometric structure may be defined by rigid sheet material that is spaced clear of the bottom of the drain, so that the sensors 3 and 4 may be installed below or with their front faces flush with the base of the flow channel; this is particularly suited to an apparatus made from metal sheet, particularly to form a carrier of rectangular cross- section. For example the base of the channel, at the upstream end of the apparatus, may slope up from an end portion shaped to contact the channel, to a raised portion that may be for example 20 mm higher and which extends at least as far as the downstream side of the hydrometric structure. Where the structure, that is to say the carrier and the hydrometric structure, are formed of metal, the preferred material is 316 stainless-steel.

The second sensor 4 for measurement of the downstream depth where the surface is sloping may be inclined so that the axis of its ultrasonic beam is orthogonal to the sloping surface; if in that case the beam width is insufficiently wide to be able to measure the depth when the water surface is horizontal (in the no-flow situation) , then the sensor 4 might be rotatable so that its beam is vertical, or alternatively there might be two side-by-side sensors 4, one producing a vertical ultrasonic beam (for use when the water surface is horizontal) and the other producing an inclined ultrasonic beam (for use when the water is flowing through the apparatus) .

It will be appreciated that if there is a fault with the apparatus 10, for example a fault with one of the sensors 3 or 4, the apparatus 10 may be removed from the channel 50 for repair, and can then be replaced.

Claims

1. An apparatus for the measurement of a flow of liquid along an open channel (50) comprising :

i) a carrier (11) with a cross-section shaped to fit the channel and adapted for installation in the channel such that the liquid flowing along the channel passes through the courier;

ii) a hydrometric structure fixed (20) within the carrier (11) to cause the depth of the liquid approaching the hydrometric structure to vary with the rate of flow of liquid passing through the hydrometric structure;

iii) a first sensor (3) in the carrier (11) upstream of the hydrometric structure (20) to measure a first depth (Dl) of liquid;

iv) a second sensor (4) in the carrier (11) downstream of the hydrometric structure (20) to measure a second (D2) of liquid;

v) means (25) for the computation of the flow of liquid along the channel through an installed carrier (11) from the measured first depth (Dl);

vi) and checking means (25) reponsive to the first (Dl) and the second depths (D2) .

2. An apparatus as claimed in claim 1 wherein the sensors (3, 4) use ultrasonic pulses directed from the base of the carrier (11) through the liquid to the liquid surface to determine the depth of liquid.

3. An apparatus as claimed in claim 2 wherein the second sensor (4) directs ultrasonic pulses at an inclination (5) from the vertical.

4. An apparatus as claimed in claim 3 wherein the second sensor (4) is sufficiently close to the hydrometric structure (20) that, when liquid is flowing through the apparatus, the liquid surface to which the measurement is made is sloping smoothly.

5. An apparatus as claimed in any one of the preceding claims wherein the carrier (11) comprises a resin/glassfibre composite material.

6. An apparatus as claimed in claim 5 wherein the sensors (3, 4) are ultrasonic transducers and are embedded within the carrier.

7. An apparatus as claimed in any one of claims 1 to 4 wherein the carrier comprises metal sheet material.

8. An apparatus as claimed in claim 7 wherein the sensors are ultrasonic transducers and are either affixed to the underside of the metal sheet material, or are mounted in apertures through the sheet metal material.

9. An apparatus as claimed in any one of the preceding claims wherein the hydrometric structure (20) defines a tapered gap (18) for through flow of the liquid, the gap (18) being narrower at the bottom than the top and tapering linearly with height.

10. An apparatus as claimed in claim 9 wherein the hydrometric structure (20) comprises opposed restricting elements (14) on each side of the carrier, wherein each restricting element (14) is defined by surfaces (15, 16) inclined at an angle of inclination to the longitudinal axis of the apparatus and meeting along the edge (17) of the tapered gap (18) .

11. An apparatus as claimed in any one of claims 1 to 8 wherein the hydrometric structure comprises a weir with a horizontal upper surface, comprising an upstream inclined surface and a downstream inclined surface, the upstream inclined surface and the downstream inclined surface being inclined at an angle of inclination to the horizontal .

12. An apparatus as claimed in claim 10 or claim 11 wherein the inclined surfaces (15, 16) are plane surfaces, and the inclination angle for the upstream surface is less than that of the downstream surface.

13. A system for the measurement of a flow of liquid along an open channel comprising an apparatus as claimed in any one of the preceding claims, and also comprising shutter means to temporarily retain a volume of still liquid within the carrier.

14. A method of measurement of a flow of liquid along an open channel by use of an apparatus as claimed in any one of claims 1 to 12.