GB2379026A

GB2379026A - Pitot flow meters

Info

Publication number: GB2379026A
Application number: GB0120492A
Authority: GB
Inventors: Edward Francis Read
Original assignee: L M TECHNICAL SERVICES Ltd
Current assignee: L M TECHNICAL SERVICES Ltd
Priority date: 2001-08-23
Filing date: 2001-08-23
Publication date: 2003-02-26
Also published as: GB0120492D0

Abstract

A sensing head (12) for an anemometer or other flow meter comprises a hollow disc-like body having a peripheral wall (40) and an axis (44). Four similar chambers (C1-C4) are formed in the body, and the wall has, for each chamber, a plurality of Pitot holes (42) extending through the wall into the respective chamber. The holes are generally symmetrically arranged around the axis. Means (30) are provided to enable the pressure difference between each diametrically-opposite pair (C1,C3; C2,C4) of the chambers to be sensed. The provision of two or more Pitot holes for each chamber (i.e. eight or more Pitot holes in total) produces a substantially lobe-less characteristic for the anemometer, and the head can still be used to produce a measurement (albeit somewhat inaccurate) even if one of the holes becomes blocked. The provision of the four chambers, each serving two or more of the Pitot holes, enables wind velocity to be measured by sensing only two pressure differences.

Description

TITLE Pitot Flow Meters DESCRIPTION This invention relates to flow meters and components thereof.

The invention was originally conceived as an anemometer, i. e. for measuring wind speed and direction, but it is also applicable to the measurement of the velocity of other gases and of liquids.

Several ways of measuring wind speed and/or direction are known, including the wetted finger technique, weather vanes, wind socks, spinning-cups devices, heated wire anemometers and anemometers that rely on the Pitot effect. The invention provides a development of the Pitot anemometer.

A Pitot anemometer is described in patent document GB-A-757829. That anemometer includes a sensing head having a disc-like body with four Pitot holes formed in its edge equiangularly spaced at 90 degrees around the axis of the disc. Signals indicative of wind speed and direction past the head are produced in dependence upon a first pressure difference between a first opposite pair of the holes and a second pressure difference between the other, second, opposite pair of the holes. The component of the wind velocity in the direction of the first pair of holes is taken to be a trigonometric function of (e. g. proportional to) the first pressure difference, and the component of the wind velocity in the direction of the second pair of holes is taken to be a trigonometric function of (e. g. proportional to) the second pressure difference.

One problem with the anemometer of GB-A-757829 is that its angular characteristic is significantly lobed. In other words, if a wind of speed S in the first direction produces a first pressure difference of P and a second pressure difference of zero, and if a wind of the same speed S in the second direction produces a first pressure difference of zero and a second pressure difference of P, it should be the case that a wind of the same speed S at 45 degrees to the first and second directions produces first and second pressure differences both of 2-'. P. However, that is not the case. Another problem with the anemometer of GB-A-757829 is that, if one of the holes should become blocked, for example by debris, a bird's claw or bird droppings, the anemometer is rendered useless (except to measure wind speed in a direction at right angles to the direction of the blocked hole).

A Pitot anemometer that alleviates the above problems to some extent has been described in web page http://www. ao. coml-toby/projects/met96/pitot/intro. html. That anemometer has five double-ended differential pressure sensors equiangularly spaced by 36 degrees about an axis. It can be expected that such an anemometer would have a less lobed characteristic than the anemometer of GB-A-757829 and that it would still produce a measurement (albeit somewhat inaccurate) if one of the openings to one of the sensors were blocked. However, a disadvantage of the anemometer described in that web page is that it requires five or more, rather than two, differential pressure sensors and that calculation of wind speed and direction from the five or more differential pressures is more complicated.

The present invention (or at least specific embodiments of it) aims to alleviate these problems at least to some extent.

A first aspect of the present invention provides sensing head for an anemometer or other flow meter, comprising a hollow disc-like body having a peripheral wall and an axis, wherein: four similar chambers are formed in the body; the wall has, for each chamber, a plurality of Pitot holes (preferably at least three such holes, and more preferably at least four such holes) extending through the wall into the respective chamber; the holes are generally symmetrically arranged around the axis; and means are provided to enable the pressure difference between each diametrically-opposite pair of the chambers to be sensed.

The provision of two or more Pitot holes for each chamber (i. e. eight or more Pitot holes in total) produces a less lobed characteristic than the anemometer of GB-A-757829, and the head can still be used to produce a measurement (albeit somewhat inaccurate) if one of the holes becomes blocked. The provision of the four chambers, each serving two or more of the Pitot holes, enables wind velocity to be measured by sensing only two pressure differences, unlike the arrangement described in the web page mentioned above.

Preferably, the holes are generally equiangularly spaced around the axis. Preferably, the chambers are generally equiangularly spaced around the axis. Preferably, the size of each hole is substantially smaller than the size of each chamber.

Preferably, each hole is flared out towards the outer surface of the peripheral wall.

In typical use, the axis would be arranged generally vertically, and preferably each of the holes provides an opening extending downwardly to the level of a floor of the respective

chamber. Accordingly, if rainwater or seawater should enter the chamber it can easily drain out again.

Preferably, the chambers are separated around the axis by generally uniform-thickness dividing walls. Preferably, the peripheral wall is of generally uniform thickness.

Preferably, the peripheral wall is generally circular or has rotational symmetry of 45 degrees or less (and more preferably 30 degrees or less, and even more preferably 221/2 degrees or less) about the axis. Accordingly, the head produces an identical or somewhat similar disturbance to the fluid flow whatever the orientation of the head relative to the flow direction.

Preferably, the means to enable the pressure difference between each diametricallyopposite pair of the chambers to be sensed comprises a passageway communicating with each chamber adjacent the axis. In this way, the passageway is generally equidistant from each of the Pitot holes of the respective chamber.

A second aspect of the invention provides an anemometer or other fluid flow meter comprising: a head according to the first aspect of the invention; and means for producing an output signal indicative of fluid flow velocity past the head in dependence upon a first pressure difference between an opposite pair of the chambers and a second pressure difference between the other opposite pair of the chambers.

Preferably, the output signal producing means comprises: means for producing first and second analogue signals in dependence upon the first and second pressure differences, respectively; means for converting the first and second analogue signals to first and second digital signals, respectively; and means for processing the first and second digital signals to produce the output signal.

Preferably, the meter further comprises means for sensing the orientation of the head about its axis relative to a geographical datum direction, the processing means being responsive to the orientation sensing means so that the output signal is indicative of the fluid flow direction relative to the geographical datum direction.

A specific embodiment of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side view of an anemometer;

Figure 2 is an under plan view of a head cap of the anemometer of Figure 1, partly sectioned on the section line 2-2 shown in Figure 1; Figure 3 is a plan view of a head base of the anemometer of Figure 1; Figure 4 is a side view of the head cap and head base, sectioned on the section line 4-4 shown in Figures 2 and 3; Figure 5 is a schematic circuit diagram of circuitry used in the anemometer; and Figure 6 is a plot of raw analogue values measured using a prototype of the anemometer of Figure 1 for various wind speeds with the anemometer approximately oriented in various directions.

Referring to Figure 1, the anemometer 10 comprises a head 12 mounted by a boss 14 on the upper end of a tubular stem 16. The lower end of the stem 16 has a base 18 by which the anemometer 10 can be secured to any suitable mounting, such as a vertical pole or mast. The head 12 is disc-shaped with its axis vertical.

Referring now to Figures 1 to 4, the head 12 comprises a disc-shaped base 20 (including the boss 14) and a cap 22 secured together by screws (not shown) in holes 24. The base 20 has a rebated upper peripheral edge 26 and a rounded lower peripheral edge 28. Four sensing holes 30 are formed through the base 20 adjacent its centre for connection to pressure sensors as will be described later. The cap 22 is formed as a disc, thicker than the base, and has a flanged lower peripheral edge 32, for fitting the rebated upper edge 26 of the base 20, and a rounded upper peripheral edge 34. The underside of the cap 22 is formed with four segment-shaped recesses 36 symmetrically arranged around its centre, leaving four uniform-thickness, equiangularly-spaced, radial dividing walls 38 between the recesses 36, and a uniformthickness, circular peripheral wall 40. The peripheral wall 40 is formed with sixteen radial Pitot holes 42 equiangularly spaced around the centre of the cap 22. Each hole 42 is externally countersunk to about half of its depth. The holes 42 are symmetrically arranged with respect to the radial walls 38 so that four of the holes 42 open into each of the recesses 36. When the cap 22 and base 20 are screwed together, the recesses 36 and upper surface of the base 20 form four segment-shaped chambers C1-C4, each associated with four of the Pitot holes 42 through the peripheral wall 40 of the cap 22, and each associated with one of the sensing holes 30 through the base 20 adjacent the axis 44 of the head 12. Apart from the sensing holes 30 and Pitot holes 42, the chambers C1-C4 are sealed. To this end, a gasket and/or gasket cement (not shown) is

provided between the base 20 and the cap 22. As seen in Figure 4, the lower extremes of the non-countersunk portions of the Pitot holes 42 are level with the upper surface of the base 20 so that any water entering the chambers C 1-C4 can readily drain away.

The circuitry for the anemometer 10 is housed on a printed circuit board in the stem 16.

Referring now to Figure 5, the circuitry includes a first differential pressure sensor 44U having its two pressure inlets connected by flexible pipes 46 to the sensor holes 30 for an

opposite pair of the chambers Cl, C3 so as to sense the pressures P 1, P3 in those chambers Cl, C3. Similarly, a second differential pressure sensor 44V has its two pressure inlets connected by flexible pipes 46 to the sensor holes 30 for the other opposite pair of the chambers C2, C4 so as to sense the pressures P2, P4 in those chambers C2, C4. Suitable pressure sensors 44U, 44V are as designated the"SDX series"and produced by Sensym Inc. The analogue differential electrical outputs from the pressure sensors 44U, 44V are fed to respective op-amps 46U, 46V, each with adjustable gain and offset, to produce a pair of pressure component signals U, V that are fed to a four-channel analogue-to-digital converter 48.

The circuitry also includes a pair of magneto-resistive sensors 50X, 50Y that are arranged with their primary axes mutually-orthogonal and lying in a plane normal to the axis 44 of the head 12. Each of the magneto-resistive sensors 50X, 50Y produces a respective differential analogue electrical signal dependent upon the component of the Earth's magnetic field strength in the direction of the respective sensor's primary axis. Suitable magneto-resistive sensors 50X, 50Y are as designated the"HMC series"and produced by Honeywell Corp.. The analogue differential electrical outputs from the magneto-resistive sensors 50X, 50Y are fed to respective op-amps 52X, 52Y, each with adjustable gain and offset, to produce a pair of magnetic component signals X, Y that are fed to the other two channels of the analogue-todigital converter 48.

The analogue-to-digital converter 48 is connected to a microprocessor or microcontroller 54 with associated working RAM 56 and EEPROM 58 that stores the operating program for the microprocessor 54 and correction factors to be described later. The microprocessor 54 is connected to an RS232 I/O port 60 which is used to load the correction factors into the EEPROM 58 and to output the measurements from the anemometer 10.

In order to calibrate the anemometer 10, the following steps are performed: 1. The gains and offsets of the amplifiers 46U, 46V are adjusted to provide predetermined outputs when two reference pressures are applied to the respective sensors 44U, 44V.

v 2. The gains and offsets of the amplifiers 52X, 52Y are adjusted to provide predetermined outputs when the respective sensors 50X, 50Y are pointing to Magnetic North and Magnetic South.

3. The anemometer is then placed in a wind tunnel with a precision speed reference and is rotated several times through 360 at various wind speeds. The CPU 54 outputs the values U, V, X, Y via the I/O port 60. The pressure values U, V are plotted to produce generally elliptical plots at the various wind speeds, as shown in Figure 6. The magnetic values X, Y are also plotted to produce a generally elliptical plot.

4. From the elliptical plots of the pressure values U, V, correction factors Fu, Fv, Ou, Ov are calculated that, when multiplied by and added to the values U, V would transform the plots into generally circular plots of the required amplitude and origin.

5. From the elliptical plot of the magnetic values X, Y, correction factors Fx, FY, Ox, Ov are calculated that, when multiplied by and added to the values X, Y would transform the plot into a generally circular plot of the required amplitude and origin.

6. The calculated correction factors Fu, Fv, Ou, Ov, Fx, Fv, Ox, OY are then written to the EEPROM 58 via the I/O port 60.

7. The anemometer is then taken to the site where it is to be installed and mounted in position.

To remove variations in the local magnetic field from the place of initial calibration, the anemometer 10 is placed in a calibration mode and rotated several times through 360 . To recalibrate, the CPU54 continually measures the magnetic components for a fixed period of time during rotation and then performs a statistical calculation to normalise the measured ellipse to circular characteristics.

8. The revised correction factors Fx, FY, Ox, Ov are stored in the EEPROM 58 for use in the normal measurement cycle of the instrument.

In normal operation, the microprocessor 54 is programmed to perform the following steps repeatedly: 1. Calculate the current magnetic bearing B of the anemometer 10 by: 1.1. Reading X, Y from the analogue-to-digital converter 48; 1.2. Reading X and Y correction factors Fx, FY, Ox, Oy from the EEPROM 58 and

calculating corrected X and Y values Xc, Yc from Xc= (X. Fx)-Ox and Yc= (Y. Fv)Ov ; 1.3. Calculating B from the full quadrature arc tangent of Yc/Xc and converting from mathematical notation (in radians, with zero is East and positive rotation is counter-

clockwise) to bearing notation (in degrees, with zero is North and positive rotation is clockwise).

2. Calculate the current mean values UM, VM of U, V over a sampling period of, say, two seconds, by: 2.1. Every, say, 1/10 second, reading U, V from the analogue-to-digital converter 48 and accumulating as UA, VA ; and 2.2. At the end of the sampling period, calculating UM=UA/N and VM=VA/N, where N is the number of samples read during the sampling period.

3. Read U and V correction factors Fu, Fv, Ou, Ov from the EEPROM 58 and calculate corrected U and V values Uc, Vc from Uc= (UM. Fu)-Ou and Vc= (VM. Fv)-Ov.

4. Calculate the current wind direction A relative to the bearing B of the anemometer 10 from the full quadrature arc tangent of Vc/Uc and converting from mathematical notation (in radians, with zero is East and positive rotation is counter-clockwise) to bearing notation (in

degrees, with zero is North and positive rotation is clockwise).

5. Calculate the current wind direction D relative to magnetic North from A-B (mod 360 ).

6. Calculate the current wind speed S from S= (Uc+Vc)'.

7. Restart the sequence at"1"above.

The calculated wind speed S and direction D are output via the I/O port 60 to a logging/monitoring device via a wire or wire-less link. The wind speed S and direction D may be output in a variety of formats and units.

Periodically during normal operation, the processor 54 performs a set/reset cycle to remove residual magnetism and temperature drift effects that are known to affect the magnetoresistive sensors 50X, 50Y in accordance with the manufacturer's specifications.

It should be noted that many modifications and developments may be made to the embodiment of the invention described above.

For example, the number of Pitot holes 42 for each chamber C1-C4 may be two, three, or more than four, and the holes 42 may have different shapes.

The chambers C1-C4 need not be arranged around the axis of the head 12, but instead could, for example, be layered.

The head 12 need not be circular in plan view, but instead could, for example, be polygonal having, for example, the same number of sides as the number of Pitot holes 42.

However, it is preferred that the head is circular so as to minimise turbulence caused by the head and to produce a pressure interpolation effect around the perimeter of the head.

Although the Pitot holes 42 have been shown to be equiangularly-spaced around the axis 44 of the head, it is possible that some other spacing of the holes 42 may be effective. However, it would be preferred that the holes 42 are mirror-symmetrical about two mutually-orthogonal mirror planes and are rotationally symmetrical through 900 about the axis 44 of the head.

Rather than correcting the pressure values U, V and magnetic values X, Y and performing a series of mathematical operations on them to determine the wind speed S and direction D, the EEPROM 58 may alternatively be set up as (a) a first two-dimensional array addressable by the pressure values U, V to produce the wind speed S, (b) a second twodimensional array addressable by the pressure values U, V to produce the wind direction A and (c) a third two-dimensional array addressable by the magnetic values X, Y to produce the bearing B of the anemometer 10. It is then merely necessary to calculate the current wind direction D relative to magnetic North from A-B (mod 3600). The values written into the three arrays in the EEPROM 58 would be calculated at the time of calibration.

It should be noted that the embodiment of the invention has been described above purely by way of example and that many other modifications and developments may be made thereto within the scope of the present invention.

Claims

CLAIMS 1. A sensing head for a flow meter, comprising a hollow disc-like body having a peripheral wall and an axis, wherein: four similar chambers are formed in the body; the wall has, for each chamber, a plurality of Pitot holes extending through the wall into the respective chamber; the holes are generally symmetrically arranged around the axis; and means are provided to enable the pressure difference between each diametrically-opposite pair of the chambers to be sensed.
2. A head as claimed in claim 1, wherein the holes are generally equiangularly spaced around the axis.
3. A head as claimed in claim 1 or 2, wherein the chambers are generally equiangularly spaced around the axis.
4. A head as claimed in any preceding claim, wherein the size of each hole is substantially smaller than the size of each chamber.
5. A head as claimed in any preceding claim, wherein each chamber has at least three such holes.
6. A head as claimed in any of claims I to 4, wherein each chamber has at least four such holes.
7. A head as claimed in any preceding claim, wherein each hole is flared out towards the outer surface of the peripheral wall.
8. A head as claimed in any preceding claim, wherein: in use, the axis is arranged generally vertically; and each of the holes provides an opening extending downwardly to the level of a floor of the respective chamber.
9. A head as claimed in any preceding claim, wherein the chambers are separated around the axis by generally uniform-thickness dividing walls.
10. A head as claimed in any preceding claim, wherein the peripheral wall is of generally uniform thickness.
11. A head as claimed in any preceding claim, wherein the peripheral wall is generally circular or has rotational symmetry of 45 degrees or less about the axis.

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12. A head as claimed in any preceding claim, wherein the means to enable the pressure difference between each diametrically-opposite pair of the chambers to be sensed comprises a passageway communicating with each chamber adjacent the axis.
13. A sensing head for a flow meter, substantially as described with reference to the drawings.
14. The use of a head as claimed in any preceding claim in an anemometer.
15. A fluid flow meter comprising: a head as claimed in any of claims 1 to 13; and means for producing an output signal indicative of fluid flow velocity past the head in dependence upon a first pressure difference between an opposite pair of the chambers and a second pressure difference between the other opposite pair of the chambers.
16. A meter as claimed in claim 15, wherein the output signal producing means comprises : means for producing first and second analogue signals in dependence upon the first and second pressure differences, respectively; means for converting the first and second analogue signals to first and second digital signals, respectively; and means for processing the first and second digital signals to produce the output signal.
17. A meter as claimed in claim 15 or 16, further comprising means for sensing the orientation of the head about its axis relative to a geographical datum direction, the processing means being responsive to the orientation sensing means so that the output signal is indicative of the fluid flow direction relative to the geographical datum direction.
18. A fluid flow meter, substantially as described with reference to the drawings.
19. The use of a fluid flow meter as claimed in any of claims 15 to 18 as an anemometer.