GB2128744A - Flowmeter - Google Patents

Abstract

A flowmeter has a probe 12 which is placed in the flow to be measured and which deflects by an amount which is related to the flow rate. A strain gauge 26 mounted on the probe 12 responds to the degree of deflection, and the resultant varying voltage from the strain gauge 26 is used to produce a reading of flow rate. The probe may be stiff in one plane and able to deflect in another plane at right angles. Two of these probes 12, 14 can be combined at right angles to produce a meter which will measure flow direction (by a vector combination of the outputs from the two strain gauges) as well as flow rate. <IMAGE>

Description

SPECIFICATION Flowmeter This invention relates to a flowmeter for measuring flow rates of gases or liquids, either in a closed or an open system. For the purposes of this specification, a closed system is one where the flow is confined by surfaces, e.g.

flow through a pipe; an open system is one where there is no such constraint, e.g. the wind.

According to the invention, there is provided a flowmeter comprising a probe arranged generally normal to the flow to be metered, the probe being held at one point and being arranged so that at another point it will be deflected by the flow by an amount which depends on the flow rate, a strain gauge associated with the probe and sensitive to probe deflection, and means for processing output signals from the strain gauge to produce an indication of the flow rate.

The strain gauge may conveniently be a single or bridge type strain gauge.

The probe may be constructed so that it is stiff in one plane and capable of deflection in a second plane at right angles to the first plane. For example, it could be of rectangular cross-section, with a region, between the points where it is held and where it deflected, where the cross-section is substantially reduced in width relative to the rest of the probe. The strain gauge can then be mounted on the probe adjacent to the reduced crosssection.

Preferably the probe is kept substantially out of contact with the fluid being metered, and is maintained within a shroud on which the flowing fluid impinges. As the shroud deflects under the influence of the flowing fluid, so it passes on this deflection to the probe.

The two probes may be used together, with their planes at right angles to one another.

The outputs from the strain guages attached to the probes can be combined vectorially to produce an output representative of both flow speed and direction. The two probes may be dissimilar and arranged so that one deflects more than the other for a similar load. This may be appropriate in special applications e.g.

boat speed and leeway.

The outputs from the strain gauges will normally be passed to an amplifier. In some cases, it may be enough simply to display the outputs. In other cases, the output signals could pass to a computer which could compute desired results from this input. For example, the inside temperature in a greenhouse is very sensitive to external wind conditions. A flowmeter as set out above mounted outside a greenhouse could be connected to a microprocessor controlling the greenhouse heating system so that the heating system could react to changes in external wind veloxity and direction to maintain a constant temperature inside.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a vertical section through a flowmeter according to the invention; Figure 2 is a horizontal section through the flowmeter of Fig. 1, on the line ll-ll; Figures 3, 4, 5, 6 and 7 show schematically various sensor positions relative to a fluid flow; Figure 8 is a section through an alternative form of flowmeter according to the invention; and Figure 9 is a vertical section through a modified form of the meter shown in Figs. 1 and 2.

The flowmeter shown in Fig. 1 is capable of measuring both flow speed and direction. The meter is mounted on a fixed post 10, and two probes 1 2 and 14 are located inside a shroud 1 6. The lower end of the probe 1 2 is rigidly fixed to the post 10 by a fastening 11. The upper end of the probe 14 is rigidly fixed to the upper end of the probe 12, and the lower end of the probe 14 is rigidly fixed by a fastening 1 7 to a carrier plate 1 8 at the base of the shroud 16. The carrier plate 18 is mounted around the shaft 10, and has a central hole 20 which has a diameter larger than that of the shaft. As a result, the carrier plate, and thus the shroud, can move in a horizontal plane relative to the shaft.

Both probes 1 2 and 14 are rectangular in cross-section, as can particularly be seen in Fig. 2. At the top, they are in fact integral with one another, but they are separated by a slit 22 which extends right to their lower edges. As can most clearly be seen on probe 14 in Fig. 1, each of the probes has a region 24 where it is substantially reduced in thickness, right across its width. This formation ensures that deflection of the probes is concentrated in these regions.

A strain gauge 26 is mounted on each probe, adjacent to the reduced thickness region 24, to respond to bending of this region.

The imposed voltage on the strain gauges will change as the probes bend, and this voltage can be amplified to produce a signal.

A flexible boot 28 is provided around the lower end of the carrier plate 18, to prevent foreign matter entering the shroud and, in particular, the gap between the carrier plate and the post 10.

When a fluid flow (in a horizontal plane) impoinges on the outer surface of the shroud, it will tend to push the shroud in the direction of the flow. Since the two probes are able to deflect, the shroud is moved relative to the post 10. In most cases, both probes will deflect to allow this movement. The amount of such deflection will be related to the flow rate, and the relative deflection of the two probes will be related to the flow direction. It will thus be seen that the output signals from the two strain gauges 26 can be used to produce a reading of, say, the wind speed and direction. The total deflection will be small. For example, a movement of the carrier plate by 5/1000 of an inch could be used to produce a full scale deflection. Strain gauges are available which are capable of measuring the corresponding small bending movements of the probe.

It is possible that inaccuracies may result from the shown arrangement where the shroud extends on one side only of the carrier plate, since the carrier plate might tend to tilt.

To avoid any such difficulties, it would be possible to extend the shroud an equal distance on the other side of the carrier plate to ensure that the carrier plate moves in a horizontal plane.

If necessary, some form of spring compensation could be included to alter the sensitivity at ends of the scale.

In Figs. 3, 4, 5, 6 and 7, the wind direction is indicated by an arrow 30. Fig. 3 shows the probe 1 2 normal, and the probe 14 parallel to the wind direction. As a result of its cross-sectional form, the probe 14 is stiff in this direction, and will not deflect, whilst the probe 1 2 will. The resulting output signal will then all be produced by the strain gauge 26 on probe 12.

As the probe orientations change with respect to the wind direction, it has been found that the combined strain gauge outputs produce one cycle of a sine wave. By identifying the point on the sine wave each time a reading is produced, the direction of the wind can be identified.

Thus, the relative positions of the probes and the wind directions 30 shown in Fig. 4 produce a maximum negative reading; the positions of Fig. 5 produce a maximum positive reading and the positions of Figs. 6 and 7 produce opposite null readings.

A suitable electronic calculating circuit can be used to evaluate the output signals and to produce a desired reading.

Fig. 8 shows an alternative form of meter being used to meter the flow in a pipe 32.

The flow direction is indicated at 34. This device only measures flow rate and not direction. A single probe 36 is mounted inside a shroud 38, and the shroud projects into the flow cross-section of the pipe through a hole 40 in the pipe wall. A flange 42 supporting the shroud is sealed to the pipe wall.

Inside the shroud, thrust blocks 44 are interposed between the probe 36 and the internal wall of the shroud. Flow through the pipe will cause the shroud 38 to deflect, and this deflection will be transferred to the probe through these thrust blocks. Any differential expansion between the probe and the shroud will merely result in the blocks sliding between the shroud and the probe.

As in Fig. 1, the probe 36 has a region 46 of reduced thickness. This region is arranged normal to the flow direction 34. The lower end of the probe is then secured by a bracket 48 to a fixed point, in this case the wall of the pipe 32. A strain gauge 50 is then mounted on the probe adjacent the region 46, to measure probe deflection. Suitable circuitry (not shown) will be used to evaluate the output signal from the strain gauge to produce an indication of flow rate.

Since the deflection of the probe would be proportional to the flow of the fluid, the device could be used to measure mass flow.

In some applications, it may be necessary to completely seal the space inside the shroud, to make it fully fluid-tight. This space could be evacuated, or gas or fluid-filled, to ensure that no foreign matter gets in.

The flowmeters described have many potential applications other than those already mentioned. For example, they could be used to measure circulation in reactor vessels or tanks.

They could measure ship or boat speed (with the two probes measuring speed and leeway) or water movement in rivers or lakes.

A flowmeter as shown in Fig. 9 could be used in all the applications described. This meter is essentially the same as shown in Figs. 1 and 2, but inverted in operation. It has the advantage of making the sealing of the moving parts easy. Corresponding parts are given the same reference numerals as in Figs. 1 and 2.

The meter is mounted by means of a bracket 60 on a support. A lower shroud 62 is rigid with the bracket and contains the probes 1 2 and 14. The probe 1 2 has an upward extension 1 2a which passes through a diaphragm 64 which covers an aperture 66 through the bracket 60.

The upper end of the probe may be bare or may be surrounded by a shroud 68. The flowing fluid then acts on the probe extension 1 2a to move it in accordance with the flow gate and direction. As the probe moves, the diaphragm will distort to allow the movement to be transferred to the reduced thickness regions 24 and to the strain gauges 26.

Claims (7)

1. A flowmeter comprising a probe arranged generally normal to the flow to be metered, the probe being held at one point and being arranged so that at another point it will be deflected by the flow by an amount which depends on the flow rate, a strain gauge associated with the probe and sensitive to probe deflection, and means for processing output signals from the strain gauge to produce an indication of flow rate.

2. A flowmeter as claimed in Claim 1, wherein the probe is constructed so that it is stiff in one plane and capable of deflection in a second plane at right angles to the first plane.

3. A flowmeter as claimed in Claim 2, wherein the probe is of rectangular crosssection with a region, between the points where it is held and where it deflects, where the cross-section is substantially reduced in width relative to the rest of the probe.

4. A flowmeter as claimed in any preceding claim, wherein the probe is contained within a shroud and the flow to be metered deflects the shroud and the shroud deflects the probe.

5. A flowmeter as claimed in any one of Claims 2 to 4, wherein two probes are provided, with their stiff planes at right angles to one another.

6. A flowmeter as claimed in Claim 5, wherein means are provided to vectorially combine the output signals from the strain guages on the two probes to produce an indication of flow speed and direction.

7. A flowmeter substantially as herein described, with reference to Figs. 1 to 7 or Fig.

8 or Fig. 9 of the accompanying drawings.