GB2119927A - Liquid flow meter - Google Patents

Abstract

The flow rate of liquid 14, in the direction indicated by arrow 52, through bore 12 of a capillary tube 10 (which flow rate may be very small, e.g. of the order of 1ml. per hour) is determined by sensing the arrival of a gas (air) bubble 46 at a second sensor 38 which stops a timer 44 and actuates a piston 24 to introduce into the bore 12 a subsequent bubble 40 whose arrival is sensed by a first sensor 36 which starts the timer 44. Thus a succession of bubbles are introduced into the liquid 14 and the timer 44 senses the time they take to move between the sensors 36, 38. These times, as well as fixed or constant data, such as the capillary tube geometry, the spacing between the sensors 36, 38 and ambient temperature, supplied by data store 48, enable a microprocessor 46 to compute the precise liquid flow rate. Alternatively, just one sensor is present which is adapted to sense the leading and trailing edges of the bubble. <IMAGE>

Description

SPECIFICATION Liquid flow meter This invention relates to a liquid flow meter which is particularly suitable for the measurement of very low flow rates, for example flow rates as low as 1 ml per hour.

Existing methods of measurement of low flow rates usually involve the use of automaticallysequenced valves which control liquid flow alternately into one and then the other of two measuring vessels. Whilst the first vessel is filling, the second vessel is draining, and vice-versa. A datum liquid level in each vessel is detected, for example, by photo detection of the liquid/air meniscus, to provide a signal which triggers the change-over of valves for controlling the filling and emptying sequences. The liquid flow rate is determined by the frequency of the filling/emptying cycles. These cycles are counted and related to time to evalulate volumetric flow. This method is complicated, requiring typically four valves to be switched in sequence and it is of limited accuracy, which is determined largely by the size of the vessels.

Various other indirect methods exist for measuring very low liquid flow rates, involving, for instance ultrasonic, thermal and/or microwave sensors, turbine arrangements and the like, but none of these gives very accurate results at very low flow rates.

An object of this invention is to provide a liquid flow meter which can be used for measuring very low flow rates, down to the order of 1 ml per hour, and which will provide a very accurate determination of the flow rates.

With this object in view, the present invention provides a liquid flow meter for measuring very low flow rates comprising a capillary tube adapted for flow of the liquid therethrough, gas introduction means for introducing a volume of gas into liquid flowing through the capillary tube to form a discrete bubble therein, and sensing means for sensing the rate of progress of the bubble along the capillary tube.

Various possibilities exist for the arrangement of the sensing means. For instance, the sensing means may comprise a sensor adapted to sense the arrival of the bubble from the gas introduction means, or it may comprise a sensor adapted to sense the pas sagetherepast, successively, of the leading end and the trailing end of the bubble.

Preferably, however, the sensing means comprises a sensor operative, upon sensing a bubble at a predetermined location, to trigger the gas introduction means to introduce a further volume of gas, thereby to introduce a succession of bubbles into the liquid flowing through the capillary tube, the sensing means serving also to sense the time taken for each bubble to move along a predetermined length of the capillary tube.

In the latter instance, the sensing means may comprise a first sensor operative to sense each bubble as it is introduced and a second sensor spaced from the first sensor and operative both to sense the arrival of each bubble and to trigger the gas introduction means preferably after a short delay.

The sensing means conveniently further comprises a microprocessor or computer, supplied with fixed data from a store, and operative to compute the liquid flow rate from the timing of the signals received from the sensors and the volume of the tube between the sensors.

In practical embodiments of the invention, the or each sensor is an optical or infra-red sensor associated with the capillary tube so as to be affected by change in light and/or infra-red energy transmission at the liquid/air interfaces between the bubbles and the liquid, due to change in refractive index and/or capacity.

The gas introduction means conveniently comprises a cylinder and piston device in a branch connection to the capillary tube, the piston of the cylinder and piston device being, for example, solenoid operated, and being fitted with non return valves.

As will be understood from the foregoing, the sensing means may comprise a single sensor which, for example, senses the passage firstly of the leading and meniscus of a bubble, and starts a timer as it does so, the sensor then sensing the trailing end meniscus of the bubble and stopping the timer. If the volume of the bubble is known, the flow rate can be calculated by a microprocessor to which a signal from the timer is fed.

Preferably, however, the arrangement comprises two sensors spaced apart along the capillary tube by a known distance, one after the other, in the direction of flow. Then the arrangement is preferably such that the first sensor senses the passage of the leading end meniscus of a bubble and starts a timer, preferably within the computer, which runs until the second sensor senses the passage of the leading end meniscus of the same bubble and acts to stop the timer by means of an input to said computer. If the distance between the sensors, and the crosssectional area of the conduit are known, a microprocessor can calculate the flow rate from the time determined by the timer.

The invention will be descrjbed further, by way of example, with reference to the accompanying drawing in which the single figure is a diagrammatic part-sectional view of a preferred embodiment of the liquid flow meter of the invention.

As illustrated, the illustrated flow preferred embodiment of the flow meter of the invention comprises a capillary tube 10 having a bore 12 of sufficiently small diameter that liquid 14 flowing therethrough (and whose flow rate is required to be measured) will be bound by surface tension to form a meniscus with any gas (such as air) introduced therein and the liquid is subjected to capillary action. A branch 16 joins the capillary tube 10 at a T-connection 18 and leads to a chamber 20 of a piston and cylinder air injection device comprising a cylinder 22 and a piston 24 which is slidable in the chamber 20. The piston 24 is connected by a rod 26 to the armature (not visible) of a solenoid 28 and has an air inlet passage 30 closed at the chamber end of the piston 24 by a one way valve member 32.The piston is biased into the illustrated position by a coil spring 34 which bears on the valve member 32 which effec tivelyforms the bottom end face of the piston 24.

Sensing means of the flow meter comprises a first optical sensor 36 mounted across the capillary tube 10 downstream of the T-connection 18, and a second similar optical sensor 38 mounted across the capillarytube 10 at a predetermined and accurately known distance downstream from the first sensor 36.

The liquid 14 is flowing very slowly along the bore 12 of the capillary tube 10 in the direction indicated by the arrow 52 in the figure.

Operation of the solenoid 28 is effective to displace the piston 24 downwards thereby compressing a small volume of air into the bore 12 via a non-return valve 35 as a bubble 40 at the connection 18. On return of the solenoid 28 to the illustrated position the piston 24 is correspondingly retracted and the pressure in the chamber 20 is reduced to below atmospheric pressure, causing the one way valve member 32 to open and air to be admitted into the chamber 20 until atmospheric pressure is reached. The non-return valve 35 prevents the liquid 14 moving into the chamber 20, due to the pressure in the capillarytube 10 until an equilibrium air pressure is reached in the chamber 20. Further flow of air into the chamber 20 is then pevented by the action of the spring 34 on the valve member 32.

The flow member operates as follows: When liquid flow is taking place, and the chamber 20 is already charged with air, the solenoid 28 is operated by manual actuation of a starting switch (not shown) or by an output signal from the computer, thereby displacing the piston 24 as just described and injecting a predetermined and exactly known volume of air into the capillary tube 10 at the junction 18. The first sensor 36 is located in such a position that as soon as the volume of air has been completely injected, the leading end meniscus 42 of the bubble 40 found thereby is detected, the sensor then giving an electrical signal which is applied to start a timer 44.

When the bubble has moved along the capillary tube 10 to the position shown at 46 its leading end meniscus is detected by the second sensor 38 which provides an electrical signal to stop the timer 44 and actuate the solenoid 2 to inject another volume of air into the capillary tube 10 thereby to form a next successive bubbie. Accordingly, the operation of the device is such as to introduce into the liquid 14 flowing within the capillary tube into a series of long liquid slugs of exact and consistent length separated by air bubbles which are equally spaced and are each of exact and consistent length.

Fixed or constant data, such as the diameter of the capillary bore 12, the distance between the sensors 36 and 38, and ambient temperature are supplied to a microprocessor 46 from a data store 48 together with the output from the timer 44 and this enables the flow rate of the liquid 14 to be determined by the microprocessor 46.

At a given temperature, flow rate is inversely proportional to the period between sensor inputs.

Using a conventional timer, the flow can be expressed as Viz, where V is a constant determined by the geometry of the capillary tube 10 and the sensors 36 and 38, and T is the variable time between sensor inputs.

The microprocessor 46 (or a small computer) may be used to divide V by Tto express the results as volume per unit time. In this case the microprocessor 46 would be provided with a digital output switch to operate the solenoid 28 and two digital inputs to sense the proportions of the air bubbles.

The invention is not confined to the precise details of the foregoing example and variations may be made thereto. Thus, for instance, in simpler forms of the invention, only one sensor is used and the timing period runs from the actuation of the solenoid to the detection of the resultant bubble by the sensor. This method is not as accurate as with the preferred embodiment described above because of the possibility of slight inconsistences in the bubbles delivered by the piston 24.

In another possible embodiment, a simple sensor senses the time it takes for each bubble to pass, by successively sensing the leading end and the trailing end of each bubble. Again this is less accurate than the preferred embodiment of the invention described above in detail.

Claims (10)

1. A liquid flow meter for measuring very low flow rates comprising a capillary tube adapted for flow of the liquid therethrough, gas introduction means for introducing a volume of gas into liquid flowing through the capillary tube to form a discrete bubble therein, and sensing means, for sensing the rate of progress of the bubble along the capillary tube.

2. A liquid flow meter as claimed in claim 1 in which the sensing means comprises a sensor adapted to sense the arrival of the bubble from the gas introduction means.

3. A liquid flow meter as claimed in claim 1 in which the sensing means comprises a sensor adapted to sense the passage therepast, successively, of the leading end and the trailing end of the bubble.

4. A liquid flow meter as claimed in claim 1 in which the sensing means comprises a sensor operative, upon sensing a bubble at a predetermined location, to trigger the gas introduction means to introduce a further volume of gas, thereby to introduce a succession of bubbles into the liquid flowing through the capillary tube, the sensing means serving also to sense the time taken for each bubble to move along a predetermined length of the capillary tube.

5. A liquid flow meter as claimed in claim 4 wherein the sensing means comprises a first sensor operative to sense each bubble as it is introduced and a second sensor spaced from the first sensor and operative both to sense the arrival of each bubble and to trigger the gas introduction means.

6. A liquid flow meter as claimed in claims 5 or 6 wherein the sensing means further comprises a microprocessor or computer, supplied with fixed data from a store, and operative to compute the liquid flow rate from the timing of the signals received from the sensors and the volume of the tube between the sensors.

7. A liquid flow meter as claimed in claim 4,5 or 6 wherein the or each sensor is an optical or infra red sensor associated with the capillary tube so as to be affected by change in light transmission at the liquid/air interfaces between the bubbles and the liquid, due to change in refractive indes and/or opacity, or infra red absorption, or optical infra red reflection.

8. A liquid flow meter as claimed in any preceding claim wherein the gas introduction means comprises a cylinder and piston device in a branch connection to the capillary tube.

9. A liquid flow meter as claimed in claim 8 wherein the piston of the cylinder and piston device is solenoid operated.

10. A liquid flow meter substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.