GB2105046A

GB2105046A - Fluid flow rate and direction sensor

Info

Publication number: GB2105046A
Application number: GB08219893A
Authority: GB
Inventors: Hubert Overall; Brian Leonard Howe
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-07-23
Filing date: 1982-07-09
Publication date: 1983-03-16

Abstract

A device for indicating flow of fluid, e.g. air, comprises a first thermistor (14) exposed to fluid flow for generating an output signal indicative of temperature. A second thermistor (32), shrouded from fluid flow, generates a further output signal indicative of ambient temperature. An indication of the cooling effect caused by fluid flow, and hence of the magnitude of flow, is obtained by utilising the two output signals. An indication of direction of flow can additionally or alternatively be determined by utilising a number of spaced, shrouded thermistors to compare temperatures in regions differently shrouded from the fluid flow, e.g. as an anemometer controlling a central heating system. <IMAGE>

Description

SPECIFICATION Fluid flow sensor Field of invention This invention concerns fluid flow sensors and is of particular application to sensors for detecting both the rate of flow of air and the direction in which air is flowing.

Background to the invention Electron devices known as thermistors are known. It is a characteristic of these devices that their resistance changes with temperature and this characteristic can be used in a calibrated mode of operation to determine the temperature of an environment in which the thermistor is located.

It is an object of the present invention to produce a device which uses this characteristic of a thermistor to determine fluid flow.

It is another object of the present invention to provide a device which uses this characteristic of a thermistor to determine the direction from which a fluid is flowing.

It is another object of the present invention to provide a fliud flow and direction indicator using thermistors.

Summary of the invention According to one aspect of the present invention a device for detecting that a fluid medium is flowing past a given point comprises: 1. a temperature sensitive device located at the said given point, 2. circuit means connected to the said device and responsive to resistance changes in the device with changes in temperature thereof to produce a first output signal indicative of the temperature, such that the lower the temperature the greater will be the output signal, 3. a second temperature sensitive device lo cated in the environs of the said given point but not sensitive to the fluid flow but only the ambient temperature at the said given point, 4. circuit means associated with the said second device for generating a second out put signal indicative of the ambient tem perature at the said given point, and 5. circuit means generating a third output signal indicative of any difference between the first and second signals.

Preferably the third output signal comprises a two value signal such that one value obtains when the first output signal magnitude exceeds the second otuput signal magnitude by a given amount and the second signal obtains when the said first output signal falls to a magnitude such that the difference between it and the magnitude of the second output signal is equal to the said given amount.

It is to be noted that the second output signal itself may vary, the third output signal may change state as a result of a rise or fall of ambient temperature during constant fluid flow (which results in a cooling of the first device) just as can an increase in fluid flow past the first device (which will introduce a greater cooling effect of the first device and therefore an increase in the magnitude of the first output signal).

The said given amount may be adjustable.

In a development of the invention a repetitive timer is included in the circuit means and electronic sampling switch means is provided by which the third output signal is sampled at regular intervals of time and an electronic memory serves to temporarily store the sampled values of the third output signal and a read-out device is provided for reading the contents of the memory to provide an indication of fluid flow.

In another development of the invention, averaging circuit means is provided for averaging the values of the sampled third output signal (whether they have been stored or not) and indicator means is provided to indicate the averaged value of the third output signal.

By using an averaging technique so spurious variations in the third output signal due to minor fluctuating conditions in the fluid flow will be effectively "ironed out" and a general fluid flow indication obtained.

The device operates on the basis that the first device will be cooled by an amount which is proportional (at least to a first approximation) to the rate of flow of the fluid over the device. The second device whilst being shielded from the fluid flow nevertheless is responsive to the ambient temperature of the fluid medium and provides a reference signal.

By increasing the flow, so the temperature of the first device drops and the first output signal rises so as to indicate a greater fluid flow.

According to a preferred feature of the invention, the device incorporates means for raising the temperature of the first device above the ambient temperature detected by the second device. This can be used to increase the sensitivity of the device overall.

The source of heating may comprise a duct through which warmed fluid is passed, the duct leading to the first mentioned device.

Warm air for passing through the duct may be obtained from a normally warmed environment such as the interior of a building of vehicle where the device is to be mounted in association with a building or vehicle. Alternatively a small heating element such as a resistive heater may be provided in association with the first device either close to the device or in association with ducting to cause warmth from the element to transfer to the said first device where a source of appropriately warmed fluid is not available.

As so far described the invention can be used to indicate the rate of fluid flow.

Where the first device is located in a duct, the direction in which the fluid is flowing can be ascertained to a first approximation by rotating the duct slowly and noting the two positions at which the third output signal is a minimum. The fluid flow is then at right angles to the diameter passing through those two positions.

It is, of course, possible to look for the maximum value of the third output signal but it will be appreciated that unless the duct through which the fluid is forced to flow is of very small diameter, the position of the maximum (when the duct is completely in line with the fluid flow) will be very difficult to determine and it is usualiy easier to look for a minimum value.

The invention can be applied to a device which not only indicates the rate of flow but also indicates the direction of flow.

According to this aspect of the invention a plurality of temperature sensitive devices are spaced apart around a circle and are shielded one from the other by partitions so that fluid flow impinging on one of the devices will be completely shielded from at least one of the others.

By providing circuit means responsive to the signals from the devices so the general direction from which the fluid is flowing can be determined from the pattern of electrical signals.

The array of temperature sensitive devices may be stationary in which event the pattern of electrical signals can be calibrated directly taking into account the relative positions of the different devices. Thus for example the device may be set up as a wind direction indicator and calibrated using a compass.

Alternatively the device may be rotatabie about an axis through the centre of the circular array of devices and a read-out display is linked to the output from the electrical circuits associated with the devices so as to produce an indication of wind direction. In this arrangement one of the devices when impinged on directly by air flow will produce a unique indication such as North, in the display.

By rotating the array so that this device points towards the North (using a compass or other aid) the device is then calibrated and is left fixed in position or the reading taken as required.

The compass may in fact be incorporated in a housing containing the array of devices so that the whole device becomes portable and it is merely necessary to hold the device up, rotate a moveable part of the housing relative to the reminder until a mark on the moveable housing lines up with the North of the compass needle after which a wind direction reading can be obtained.

As in the first aspect of the invention, a local source of warm air or fluid is provided for warming all of the devices to a temperature very slightly above ambient so that the device is rendered more sensitive.

A further temperature sensitive device may be provided for sensing the ambient temperature and this is conveniently shrouded from the fluid flow such as the wind and, of course, also from any warm air effecting the array of devices.

If it is desirable that the number of temperature sensitive devices is restricted to a minimum, then a first approximation to ambient temperature may be obtained by referencing the maximum device output signal against the minimum device output signal at any instant.

The minimum will normally correspond to the ambient temperature since it will arise from the device or devices which are shrouded from the fluid flow.

It is an advantage of the invention in both aspects, that no moving parts need to be involved in order to obtain a measure of fluid flow and direction indication.

The invention can, therefore, be compared advantageously with devices which involve rotating veins and the like.

Whilst the device is of particular application the detection and indication of wind speed and direction the invention is not, of course, limited to such applications and may be used to determine water or liquid flow, currents in rivers and at sea and the like.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of a device for detecting fluid flow, and Figure 2 is a plan view, and Figure 3 is a cross section through a device incorporating a number of thermistors for determining wind direction.

Detailed description of drawings In Fig. 1 a constant current amplifying transistor 10 having a controlled input signal by zener diode 1 2 supplies current to a bead thermistor 1 4. The standing current is set by a calibrating potentiometer 16.

The voltage at junction 1 8 varies with the temperature of the thermistor 14 and is supplied as one input to a comparator amplifier 20. The other input of the latter is derived from a potentiometer 22. The maximum value of the input signal from the potentiometer 22 is dictated by a line resistor 24.

Adjustment of the potentiometer 22 dictates the temperature at which the output signal at junction 26 changes state. The amplifier 20 is such that when the potential at 1 8 drops below the potential of the slider of potentiometer 22 the output at 26 rises from a low level to a high level and vice versa.

It will be appreciated that the amplifier 20 can be arranged to operate in the reverse mode.

A field effect transistor 28 serves as an electronic switch and the output from amplifier 20 is supplied to the base of transistor 28.

The operating conditions of the field effect transistor 28 are dictated in part by the position of an adjustable potentiometer 30 and the value of a temperature sensitive resistor 32. The latter is rendered sensitive to the ambient temperature and the switching conditions for the transistor 28 are thus varied with temperature as determined by the element 32.

An output signal may thus be obtained from junction 34.

In use the bead thermistor 14 is exposed to the fluid flow and the temperature sensitive resistor 32 is rendered sensitive to the ambient temperature of the fluid in such a manner as not to be effected by the rate of flow of the fluid. In Figs. 2 and 3 a device is shown which incorporates four bead thermistors 36, 38, 40 and 42 arranged in a circle around a central bead thermistor 44. The central one is shrouded within a cylindrical sleeve 46 and each of the others are separated by quadrant defining walls 48, 50, 52 and 54.

Referring to Fig. 3 each of the bead thermistors 36 to 42 is located in a vertical duct and as shown in Fig. 3 thermistor 36 is located in a duct 56 and thermistor 40 in a duct labelled 58. Although not shown in Fig.

3 two further ducts 60 and 62 are provided for the thermistors 38 and 42.

Communicating with each of the first mentioned ducts are radial ducts 64, 66, 68 and 70.

As shown in Figs. 2 and 3 the wind is denoted as flowing in the direction of arrow 72 and the wind not only impinges on the top of the thermistor 36 but will also travel through the duct 64 and up the duct 56 so as to further cool the thermistor.

As shown in Fig. 3 the thermistors are arranged in a housing having a generally circular base 74 with the quadrant walls 48, 50 etc., upstanding therefrom. Conveniently the whole is integrally moulded from plastics material.

The underside of the base 74 is formed with a reduced diameter screw thereaded ring 76 by which the device can be screwed into the upper end of a cylindrical duct 78. The duct may include a resistive heating element 80 and may include part of a housing on which the device can be stood or by which it can be held.

The hollow cylindrical duct 78 may be used to duct warm air from a building or other interior such as the interior of a boat or vehicle up through the ducts 56, 58 etc., past the thermistors such as 36 and 40 so as to generally warm these thermistors. This tends to give the unit a higher level of sensitivity.

Electrical connections to the therrnistors are made by wires such as 82 and 84 which may themselves support the thermistors in the ducts.

Where the device is not to accommodate an air flow from below, the duct 78 may be dispensed with and the ring 76 formed as a solid member through which the supporting wires such as 82 and 84 extend. In this event the ducts such as 64 and 66 allow water which may collect in the ducts such as 56 and 58 (in the event that the device is left out exposed to the elements) to drain away.

Where the device follows this pattern and it is felt that a degree of localised heating is required for the thermistors, heating coils may be located below or around the thermistors, concerned in the ducts 56 and 58. However, these heating coils are not shown in the drawings.

Operation to determine flow and rate of flow.

Circuit means (not shown) is provided for comparing the signals from thermistors 36 to 42 to produce a first output signal indicative of the lowest sensed temperature. (The lower the temperature the greater will be the output signal.) Further circuit means (not shown) is associated with the thermistor 44 for generating a second output signal indicative of the ambient temperature. A third circuit means (not shown) compares the 1 sot and 2nd outputs to produce a 3rd output signal, indicative of the difference between the first and second output signals. The 3rd output signal indicates the rate of fluid flow and is proportional thereto. An indication of the value of the 3rd signal will give a flow rate value.

Direction indication Each thermistor 36 to 42 will be cooled in the presence of fluid flow to a greater or lesser extent depending on the wind direction relative to the portions of the walls such as 52.

The general direction of the fluid flow can be determined from the pattern of temperatures so obtained at the thermistors indicated by the different electrical signals due to different temperatures from the thermistors 36 to 42.

The determined direction may be indicated on a meter or read-out display (not shown).

With different electrical signals from the thermistors the least cooled i.e. warmest (and therefore lowest output signal) thermistor is selected as the reference since it must be on the leaward side of the walls 52 etc. A series of comparators can be arranged to determine this one of the four thermistors.

The output for the other three thermistors can then be compared and a wind direction by interpolation.

If the first comparison reveals two equally warm thermistors then the wind direction can be taken as approximately intersecting these two, and its precise direction is found by comparing the other two thermistor outputs.

A device in accordance with the present invention finds particular application in conjunction with a central heating system, with the device mounted on the exterior of a build ing to sense the presence of a wind impinging on the building and linked to the central heating control system to compensate for the cooling effect of the wind.

For example, a wind sensor device as de scribed in connection with Fig. 1 may be located on the side of a building, e.g. a house, facing the prevailing wind. The output signal from the device, indicative of the presence or absence of a wind over a given threshold value, is fed to a microprocessor based central heating control circuit. If the signal indicates the presence of a wind over the threshold value, the central heating control circuit alters the on/off programme of the central heating system so that the system is switched on earlier than would normally be the case, so compensate for the cooling effect of the wind.

Claims

CLAIMS 1. A device for detecting that a fluid medium is flowing past a given point comprises:

1. a temperature sensitive device located at the said given point,

2. circuit means associated with said tem perature sensitive device and respon sive to resistance changes in the de vice with changes in temperature thereof to produce a first output signal indicative of the temperature, such that the lower the temperature the greater will be the output signal,

3. a second temperature sensitive device located in the environs of the said given point, not sensitive to the fluid flow but sensitive only to the ambient temperature at the said given point,

4. circuit means associated with the said second device for generating a second output signal indicative of the ambient temperature at the said given point and, 5. circuit means generating a third output signal indicative of the difference be tween the first and second output sig nals.

2. A device according to claim 1, wherein the third output signal comprises a two value signal such that one value obtains when the first output signal magnitude exceeds the second output signal magnitude by a given amount and the second signal obtains when the said first output signal falls to a magnitude such that the difference between it and the magnitude of the second output signal is equal to or less than the said given amount.

3. A device according to claim 2, wherein said given amount is adjustable.

4. A device according to claim 1, 2 or 3 further comprising a repetitive timer included in the circuit means electronic sampling switch means being provided by which the third output signal is sampled at regular intervals of time, an electronic memory serving to temporarily store the sampled values of the third output signal and a read-out device being provided for reading the contents of the memory to provide an indication of fluid flow.

5. A device according to anyone of the preceding claims, including averaging circuit means for averaging the values of the sampled third output signal, and indicator means to indicate the averaged value of the third output signal.

6. A device according to anyone of the preceding claims, further comprising heating means for raising the temperature of the first device above the ambient temperature detected by the second device.

7. A device according to claim 6, wherein the heating means comprises a duct through which warmed fluid is passed, the duct leading to the said first temperature sensitive device.

8. A device according to claim 7, including means for passing through the duct warm air from the interior of a building or vehicle where the device is mounted.

9. A device according to claim 6 or 7, wherein the heating means comprises a small heating elelent provided in association with the first device.

1 0. A device according to anyone of the preceding claims, wherein the first temperature sensitive device is located in a duct, and means is provided for rotating the duct relative to the associated devices.

11. A device according to claim 10, further comprising means for detecting the positions at which the third output signal is a minimum or a maximum on rotation of the duct.

1 2. A device for determining the direction of fluid flow, comprising a plurality of temperature sensitive devices spaced apart and shielded from one another by partitions so that fluid flow impinging on one of the devices will be completely shielded from at least one of the others, and circuit means responsive to the signals from the devices so the general direction from which the fluid is flowing can be determined from the pattern of electrical signals.

1 3. A device according to claim 12, wherein the array of temperature sensitive devices is stationary and the pattern of electrical signals is calibrated directly taking into account the relative positions of the different devices.

14. A device according to claim 12, wherein the device is rotatable about an axis through the centre of the circular array of temperature sensitive devices and a read-out display is linked to the output from the electrical circuits associated with the devices so as to produce an indication of wind direction.

15. A device according to claim 13 or 14, wherein a compass is incorporated in a housing containing the array of temperature sensitive devices.

16. A device according to anyone of claims 1 2 to 15, further comprising heating means for raising the temperature of the temperature sensitive devices to a temperature slightly above ambient temperature.

1 7. A device according to claim 16, wherein the heating means comprises a duct through which warmed fluid is passed, the duct leading to the said temperature sensitive devices.

1 8. A device according to claim 17, including means for passing through the duct warm air from the interior of a building or vehicle where the device is mounted.

19. A device according to claim 16 or 17, wherein the heating means comprises a small heating element provided in association with the temperature sensitive devices.

20. A device according to anyone of claims 1 2 to 19, further comprising circuit means associated with the temperature sensitive devices and responsive to resistance changes in the devices with changes in temperature thereof to produce a first output signal indicative of the lowest sensed temperature such that the lower the temperature the greater the output signal, a further temperature sensitive device in the vicinity of the first temperature sensitive devices, not sensitive to fluid flow but sensitive only to ambient temperature, circuit means associate with the further temperature sensitive device for generating a second output signal indicative of the ambient temperature, and circuit means generating a third output signal indiciative of the difference between the first and second output signals.

21. A device according to claim 20, wherein the third output signal comprises a two value signal such that one value obtains when the first output signal magnitude ex ceeds the second output signal magnitude by a given amount and the second signal obtains when the said first output signal falls to a magnitude such that the difference between it and the magnitude of the second output signal is equal to or less than the said given amount.

22. A device according to claim 21, wherein said given amount is adjustable.

23. A device according to claim 20, 21, or 22, further comprising a repetitive timer included in the circuit means, electronic sampling switch means being provided by which the third output signal is ampled at regular intervals of time, an electronic memory serving to temporarily store the sampled values of the third output signal and a readout device being provided for reading the contents of the memory to provide an indication of fluid flow.

24. A device according to anyone of claims 20 to 23, including averaging circuit means for averaging the values of the sampled third output signal, and indicator means to indicate the averaged value of the third output signal.

25. A device for detecting fluid flow, substantially as herein described with reference to, and as shown in, the accompanying drawings.

26. A device for determining the direction of fluid flow, substantially as herein described with reference to, and as shown in, the accompanying drawings.

27. A central heating system in association with a device in accordance with any one of the preceding claims.