GB2188158A

GB2188158A - Flowmeter

Info

Publication number: GB2188158A
Application number: GB08705782A
Authority: GB
Inventors: Harvey N Smith; John R Potts; Geoffrey Mcintyre
Original assignee: Cambridge Instruments Inc
Current assignee: Leica Microsystems Inc
Priority date: 1986-03-20
Filing date: 1987-03-11
Publication date: 1987-09-23
Also published as: GB8705782D0; JPS62266416A

Abstract

A flowmeter includes a pipe section 2, which has a portion of its wall defined by flexible diaphragm 4 which carries a bridge configuration of piezoresistive devices (18, 19, 20, 21) (Figure 2). An output signal is taken from across one pair of junctions of the bridge and a fixed potential difference is applied across the other opposite pair of junctions of the bridge. The output signal from the bridge is processed to compensate for temperature and to derive the flow rate in accordance with: Flow rate = K1@ (output signal) K2. Flow rates in a range 25-250 ml. per minute are measured of liquids with pH values in the range 5 to 10. <IMAGE>

Description

SPECIFICATION Flowmeters The present invention relates to flowmeters and methods of measuring flow which are particularly relevant to measuring low flow rates in the range of 25 to 250 ml/min.

Various types of flowmeters are known for measuring fluid flows in pipes. Typically such flowmeters include moving parts such as turbines which measure the velocity of the fluid from which the flow rate can readily be derived. However for very low liquid flow rates it is impossible to use such turbines since the low flow will not drive the turbine in rotation to produce the required output signal.

Other prior art flowmeters use orifices or pitot tubes across which a differential pressure is measured. Once again, such devices are unsuitable for very low flow rates of the order of 25 to 250 ml/min since obstructions in the flow path will cause excessive turbulence at these flow rates.

The present invention accordingly provides a flowmeter comprising a pipe section having a portion of its wail defined by a diaphragm including an array of piezoresistive devices connected in a bridge configuration with two pairs of opposite junctions, conduit means for applying a reference pressure to the side of the diaphragm remote from the interior of the pipe section, means for establishing a potential difference between the junctions of one said pair, output means for carrying a first electrical signal representing the potential difference between the junctions of the other said pair, and processing means for receiving said first signal, and deriving therefrom an output signal representing the flow rate of fluid in said pipe section.

The use of such a fiowmeter does not require the presence of any components which will disturb the flow pattern within the pipe section. The pressure of the fluid within the pipe causes the diaphragm to flex changing the resistance of the piezoresistive devices which are arranged in a quadrilateral bridge configuration. Effectively this results in a differential pressure measurement being produced from which the processing means can derive the flow rate from the following equation: Flow rate=K1x (first electrical signal)K2.

K1 and K2 are found to be constants for a particularflowmeter and a specific hydraulic geometry. K2 is typically close to 0.05 and K1 is dependent on the density of the fluid flowing through the pipe.

A flowmeter embodying the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure lisa diagrammatic representation of an invention; and Figure 2 is a plan view of the diaphragm used in the flowmeter of Figure 1.

As shown in Figure 1, a flowmeter 2 consists of a pipe section 13 which is connected into the main fluid flow system by means of connectors 11 and 12 at the inlet and outlet ends of the pipe section 13. It will be appreciated that the connectors 11, 12 have been shown diagrammatically and may be of any known design.

In particular they will generally include seals to prevent escape of the fluid at the connections. A portion 14 of the internal wall of the pipe section 13 is defined by a flexible diaphragm 15. The flexible diaphragm 15 may be protected by an outer wall portion to prevent damage to it from the exterior. Between the diaphragm 15 and the outer wall portion there is a cavity which is subject to atmospheric or a reference pressure. A plug and socket connector 16 provides electrical connections to the diaphragm 15 and also communicates the atmospheric reference pressure via a conduit in a connector cable 17.

The diaphragm 15 includes four piezoresistors 18,19,20,21 which may be produced by ion implantation in the material of the diaphragm. The piezoresistors 1821 are arranged in a four element, classical bridge configuration as illustrated in Figure 2. A power supply 24 is connected to produce a potential difference between a first pair of opposite junctions 22 and 24 of the bridge by means of connecting wires 26, 28 which pass via the plug and socket connector 16. The output signal from the bridge is taken from the other pair of opposite junctions 30,32 by means of wires 34, 36 which are fed via the plug and socket connector 16 to a processing circuit housed with the power supply 10. The processing circuit converts the output signal into digital form via an A two D converter before processing it.

The arrangement of the diaphragm in a pipe section 13 together with connections for a power supply and reference pressure via a plug and socket connector 16 is sold as a solid state flow-thru pressure sensor by MICROSWITCH, a division of Honeywell Inc. Such pressure sensors are sold as the 150PC series which have been found suitable for use in the flowmeter of the present invention. Desirably, the sensor includes a temperature sensor in the wall of the pipe section 13 which is connected to the processing circuit 10 via the plug and socket connector 16 by means of a further line in the connecting cable 17. The temperature sensor may be a thermosensitive resistor which can be formed in the diaphragm 15 alongside the bridge configuration. Alternative forms of temperature sensor may also be used.

The processing circuit 10 takes the input from lines 34 and 36 and produces a first temperature compensated electrical signal representative of the pressure in the pipe section 13. This output can then be further processed with calibration data from the calibration unit 42 on line 38 in accordance with the following equation: Flow rate= K1x (pressure sensor output)K2 in order to produce an output signal representing the flow rate. The flow rate signal is passed on line 40 to a display 44 which may, for example, be a printer which prints out the current flow rate at regular time intervals, or an LCD display which shows a numerical representation of the flow rate in suitable units, and which is continuously updated.Alternatively, electrochemical sensor information can be passed to the calibration unit 42 on line 46 and calibrated electrochemical sensor data can be displayed on the display unit 44.

The values of K1 and K2 are supplied to the processing circuit 10 by means of a calibration input. K1 will vary with the density of the fluid passing through the pipe section 13. Therefore, if the flowmeter of the present invention is used for a different fluid, the value of K1 must be changed. K1 also includes the factor which is related to the type of sensor being used and the specific hydraulic geometry. The factor K2 stays constant for a particular type of sensor and hydraulic geometry. Its value is close to a half. This is controlled by a constriction 57 and a vent 58 in exhaust line 56 of sample flow chamber 53 which creates a smooth, unperturbed liquid flow rate downstream of the pressure sensor.

The typical range of values of K1 and K2 for water are: K1, 50 to 60 K2, 0.48 to 0.58 The pressure sensor output is a measure of the change in relative resistance of the piezoresistive devices in the bridge. Such a change in resistance takes place as the diaphragm flexes because of the arrangement of adjacent piezoresistive devices at an angie to one another. Thus as the degree of deflection of the diaphragm increases as the pressure increases, the relative resistance of the resistors in the opposite arms of the bridge will change and this will be picked up as a potential difference developed between junctions 30 and 32. This potential difference is the output signal which goes to the processing circuit 10.

In order to allowtheflow meter to be used with liquids of high pH, such as boiler or waste waters, a silicone elastomer coating may be placed over the diaphragm and other parts of the thru-flow pressure sensor which are exposed to the liquid. This does not impair the operation of the flowmeter.

It has been found thatflowmeters as described above produce reproducable results which accurately measure flow rates in the range 25 to 250 ml/min and are capable of measuring flow rates of liquids with a pH range from 5 to 10.

Claims

1. Aflowmetercomprising a pipe section having a portion of its wall defined buy a diaphragm including an array of piezoresistive devices connected in a bridge configuration with two pairs of opposite junctions, conduit means for applying a reference pressure to the side of the diaphragm remote from the interior of the pipe section, means for establishing a potential difference between the junctions of one said pair, output means for carrying a first electrical signal representing the potential difference between the junctions of the other said pair, and processing means for receiving said first signal, and deriving therefrom an output signal representing the flow rate in said pipe section.

2. A flowmeter according to claim 1, in which the processing means derives said output signal representing flow rate in accordance with the following expression: Flow rate=K1x (first electrical signal)K2.

3. A flowmeter according to claim 1 or 2, in which the side of the diaphragm which is exposed to the fluid is coated with a silicone elastomer.

4. A flowmeter according to any one of the preceding claims, further comprising a temperature sensor in the pipe section, the sensor having an output connected to said processing means, said processing means compensating said first electrical signal for the effects of temperature changes.

5. A method of measuring low flow rates in the range of 25 to 250 ml/min by means of passing the fluid through a pipe section which includes in a portion of its wall a pressure sensor including a diaphragm carrying an arrangement of piezoresistive devices in a bridge configuration, measuring the variation of relative resistance of the resistive devices as a result of the deflection of the diaphragm due to the pressure of the fluid, compensating said measured output signal for temperature variation, and processing said output signal in accordance with the following equation: Flow rate=K1x (pressure sensor output signal)K2 in order to derive a signal representing the flow rate.

6. Aflowmeter substantially as herein described with reference to the accompanying drawings.

7. A method of measuring low flow rates substantially as herein described.