GB2262811A - System for measuring fluid flow - Google Patents

Abstract

A system for measuring fluid flow, e.g. in an air duct 10, comprises a first sensor 16 adapted to produce an output signal dependent on the temperature of the fluid, and a second sensor 18 which is disposed in the flow path of the fluid. The second sensor 18 includes a temperature responsive integrated circuit device 23 and a heater 24 located adjacent thereto, and produces an output signal indicative of the resultant temperature due to the heating effect of the heater and the cooling effect of the flow of fluid past the second sensor. The signals from the first and second sensors are processed to produce an output indicative of the rate of fluid flow. As shown the sensor 18 comprises an aerodynamically shaped pod 22 containing paste 23 to provide thermal coupling between the heater and temperature responsive device. The pod is mounted on a transverse strut 17 attached to a frame 13 intended to be bolted between two duct sections. A display 20 shows both temperature in the duct and flow rate. <IMAGE>

Description

"Svstem for Measurinq Fluid Flow" The invention provides a system for measuring fluid flow which is particularly suitable for use with air ducting systems, but which may also be applied to flows of other gases or liquids.

The invention makes use of the cooling effect of a flow of fluid over an element disposed in its flow path. In one known form of measurement system based on this principle, one or more thermistors are mounted in the flow path of air through ducting. The associated electric control circuitry maintains the thermistor at a constant temperature. The power required is related to the speed of flow and may be used as a measure of such speed. However, thermistors are fragile components and are susceptible to damage and failure.

Consequently, such devices cannot normally be left permanently in the air flow to provide constant monitoring of the flow rate, but can only be introduced into the air flow at intervals to sample the flow rate at particular times.

Other devices for measuring fluid flow make use of mechanical or pressure-operated devices which are mounted in the flow path. However, such devices generally have moving parts and/or comparatively small apertures and are therefore susceptible to failure as a result of wear, mechanical breakdown, or blockage.

The present invention sets out to provide a novel flow measuring system which may overcome these disadvantages.

According to the invention there is provided a system for measuring fluid flow comprising one or more first sensors adapted to produce an output signal dependent on the ambient temperature of the fluid, one or more second sensors which, in use, are disposed in the flow path of the fluid, each second sensor including a temperature responsive device and a heater located adjacent the temperature responsive device, the second sensor being adapted to produce an output signal indicative of the resultant temperature due to the heating effect of the heater and the cooling effect of the flow of fluid past the second sensor, and means for processing the signals from said first and second sensors to produce an output indicative of said rate of flow of fluid.

Preferably the temperature responsive device is a temperature responsive linear integrated circuit, and the heater is an electrical heater. The first sensor may also include a temperature responsive linear integrated circuit.

A system according to the invention may thus have no moving parts and may comprise robust components which may be left substantially permanently in a fluid flow path to provide constant monitoring of the flow rate.

The temperature responsive device and the heater are preferably contained within a common enclosure, for example an elongate pod which, in use, extends generally parallel to the fluid flow path. The pod may be aerodynamically shaped and may be filled, around the temperature responsive device and heater, with a material of high thermal conductivity.

In the case where there are provided a plurality of first sensors and/or second sensors, the signal processing means is preferably adapted to average the signals from the sensors of the same type.

The output from the signal processing means may control a display unit giving a visual display indicative of the rate of flow of fluid. The display may indicate the velocity of fluid flow, the volume rate of fluid flow, or both.

The visual display may comprise one or more visual display meters, and may also include a meter indicating the ambient temperature of the fluid such meter being responsive to the output signal from the first sensor.

In a specific embodiment, the system may comprise a frame, adapted to be so mounted in ducting through which the fluid flows that the fluid passes through the frame, at least one first sensor mounted on the frame so as to be disposed in the fluid flow, at least one support strut extending across the frame, and at least one second sensor mounted on the support strut.

The support strut may be formed from a material of low thermal conductivity. Alternatively, the second sensor may be mounted on the support strut by means of a carrier of low thermal conductivity, so as to provide a thermal break.

The frame may be in the form of a short length of ducting, having means for connecting it between two normal lengths of ducting.

The following is a more detailed description of a specific embodiment of the invention, by way of example, reference being made to the accompanying drawings in which: Figure 1 is a perspective view, partly cut away, of an air duct incorporating a system in accordance with the invention, Figure 2 is a perspective view on an enlarged scale, again partly cut away, of a sensor for use in the system, and Figure 3 is a block diagram showing the arrangement for processing the output signals from the system and providing a suitable display.

Referring to Figure 1: the air ducts 10 are of a common type, being formed from sheet metal and of rectangular cross section. The ends of each length of ducting are formed with outwardly extending flanges 11 so that two adjacent lengths of ducting may be secured together by bolts 12 passing through the flanges.

In the present case there is disposed between the flanges 11 on the adjacent lengths of ducting 10 a rectangular frame 13. The frame 13 comprises, in effect, a short length of ducting 14 formed with flanges 15 which are bolted to the flanges 11 on the main lengths of ducting.

An ambient temperature sensor 16, incorporating a temperature responsive linear integrated circuit, is mounted in one side wall of the frame 13 so as to project into the interior of the frame adjacent a corner thereof.

A strut 17 extends transversely of the frame 13, and across the flow path of air along the ducting, and carries at its centre a temperature responsive sensor 18. The strut 17 may be formed of material of low thermal conductivity, for example a suitable rigid plastics, or it may be formed from stronger material, such as aluminium or stainless steel, the sensor 18 then being mounted on the strut 17 by means of a carrier of low thermal conductivity, for example formed from plastics, to provide a thermal break.

The sensor 18 is shown in greater detail in Figure 2. The housing 22 of the sensor is in the form of an elongate aerodynamically shaped pod which in use, as shown in Figure 1, extends generally parallel to the direction of air flow and points towards the oncoming air.

The pod 22 contains a temperature sensitive linear integrated circuit device 23 and a small electrical resistive heater 24 located adjacent the device 23. The interior of the pod 22 is filled with a paste 27 of high thermal conductivity which surrounds the device 23 and heater 24.

Electrical connectors 25 from the device 23 and heater 24 pass out through the end of the pod 22 through a silicone seal 26. The support strut 17 for the pod is hollow and the connectors 25 pass along the hollow strut to the side of the frame 13 carrying the sensor 16.

Electric cables 19 from the sensors 16 and 18 lead to a visual display unit 20 mounted on the exterior of the ducting, or in any other suitable location. Output control cables 21 also lead from the display unit 20. The cables 19 include cables supplying power to the sensors from an electrical power source in the display unit 20.

In use, the sensor 16 detects the ambient temperature of the air flowing through the ducting and outputs a signal indicative of such ambient temperature.

The integrated circuit device 23 is both heated by the heater 24 and cooled by the flow of air over the pod 22, and the device 23 outputs a signal indicative of the resultant temperature. Since the cooling effect of the flow of air over the pod 22 is dependent on the velocity of flow, the output signal from the IC device 23 is indicative of such velocity.

Referring to Figure 3: the analogue output signal 28 from the ambient temperature sensor 16, and the analogue output signal 29 from the linear IC device 23, are fed to an analogue-to-digital converter 30 and the respective digital signals are fed to a digital processing device 31 in the form of a ROM or EPROM.

The signal from the ambient temperature sensor 16 serves to provide a datum against which the signal from the sensor 18 can be measured and the two signals are compared and processed accordingly by the device 31.

Digital output signals 32, 33 from the device 31 are fed to a digital visual display meter 34 in the display unit 20.

The device 31 is pre-programmed with the results of extensive testing and calibration to act as a "look-up table" and to produce, from the sensor signals, output signals which are indicative of the velocity of air flow over the sensor 18. Alternatively, by suitable programming using data regarding the dimensions of the ducting, the output signals from the device 31 may be indicative of the volume flow rate through the ducting.

The meter 34 is therefore arranged to display, for example by means of an LCD, a figure indicating the velocity or volume flow rate of the air.

The display unit 20 may also include a meter 35 indicating the ambient temperature. Such meter is controlled directly by the analogue signal 28 from the sensor 16, as indicated by the line 36 in Figure 3.

As previously mentioned, the display unit 20 also incorporates a power supply, indicated diagrammatically at 37 in Figure 3, which provides electric power to the heater along the line indicated at 38 and to the linear IC device along the line indicated at 39.

In addition to providing a visual display indicating the fluid flow and ambient temperature, it may be desired to use the signals indicative of such values for further purposes, for example for operating an alarm unit should the flow rate move outside a desired range, or for controlling other apparatus the operation of which requires to be modified in accordance with the air flow rate. Output signals may also be used as part of a feedback circuit to maintain the flow rate within a desired range.

For this purpose, the digital output signals 32, 33 from the device 31 are also fed, along lines 40, 41 to a digital-to-analogue converter 42 from which the corresponding analogue signals 43, 44 are fed to the appropriate output circuits 45.

For simplicity, the arrangement described in relation to Figure 1 has been shown as incorporating only a single ambient temperature sensor 16 and only a single flow sensor 18. Such arrangement may be suitable for comparatively small ducts.

However, where large volumes of air are being moved, using ducting of large cross-sectional area, a plurality of ambient sensors 16, and/or flow sensors 18, may be distributed over the internal area of the frame 13. Thus, there may be provided two or more struts 17 extending across the frame, one above the other, and two or more sensors 18 spaced apart along each strut.

Similarly, ambient temperature sensors 16 may be mounted at different locations around the frame 13 so as to detect the ambient temperature in different parts of the duct.

In each case, the signals from the various sensors of the same type are averaged, as necessary, to produce the two signals which are fed to the device 30 and then the device 31. These two signals are then processed as before to produce the digital output signals for the display unit.

Claims (17)

1. A system for measuring fluid flow comprising one or more first sensors adapted to produce an output signal dependent on the ambient temperature of the fluid, one or more second sensors which, in use, are disposed in the flow path of the fluid, each second sensor including a temperature responsive device and a heater located adjacent the temperature responsive device, the second sensor being adapted to produce an output signal indicative of the resultant temperature due to the heating effect of the heater and the cooling effect of the flow of fluid past the second sensor, and means for processing the signals from said first and second sensors to produce an output indicative of said rate of flow of fluid.

2. A system according to Claim 1, wherein the temperature responsive device is a temperature responsive linear integrated circuit, and the heater is an electrical heater.

3. A system according to Claim 1 or Claim 2, wherein the first sensor includes a temperature responsive linear integrated circuit.

4. A system according to any of Claims 1 to 3, wherein the temperature responsive device and the heater are contained within a common enclosure.

5. A system according to Claim 5, wherein the common enclosure comprises an elongate pod which, in use, extends generally parallel to the fluid flow path.

6. A system according to Claim 5, wherein the elongate pod is aerodynamically shaped.

7. A system according to Claim 5 or Claim 6, wherein the elongate pod is filled, around the temperature responsive device and heater, with a material of high thermal conductivity.

8. A system according to any of Claims 1 to 7, and in which there are provided a plurality of first sensors and/or second sensors, wherein the signal processing means is adapted to average the signals from the sensors of the same type.

9. A system according to any of Claims 1 to 8, wherein the output from the signal processing means controls a display unit giving a visual display indicative of the rate of flow of fluid.

10. A system according to Claim 9, wherein the visual display indicates the velocity of fluid flow, the volume rate of fluid flow, or both.

11. A system according to Claim 9 or Claim 10, wherein the display unit includes one or more visual display meters.

12. A system according to any of Claims 9 to 11, wherein the display unit includes a meter indicating the ambient temperature of the fluid such meter being responsive to the output signal from the first sensor.

13. A system according to any of Claims 1 to 12, and including a frame, adapted to be so mounted in ducting through which the fluid flows that the fluid passes through the frame, at least one first sensor mounted on the frame so as to be disposed in the fluid flow, at least one support strut extending across the frame, and at least one second sensor mounted on the support strut.

14. A system according to Claim 13, wherein the support strut is formed from a material of low thermal conductivity.

15. A system according to Claim 13, wherein the second sensor is mounted on the support strut by means of a carrier of low thermal conductivity, so as to provide a thermal break.

16. A system according to any of Claims 13 to 15, wherein the frame is in the form of a short length of ducting, having means for connecting it between two normal lengths of ducting.

17. A system for measuring fluid flow substantially as hereinbefore described with reference to the accompanying drawings.