GB2218812A - A capacitive apparatus for measuring liquid volume and flow rate - Google Patents

Abstract

In a capacitive measurement of the volume of a conductive liquid, unwanted effects due capacitance Co caused by an oxide layer on an uninsulated electrode (16, figure 5) in contact with the liquid are avoided by feeding two other electrodes (12, figure 5), insulated from one another and from the liquid, with respective alternating voltages to give rise to equal and opposite currents i(1), i(2) in the electrodes (12, Figure 5). The electrodes (12) form a pair of capacitors Ci(1), Ci(2) with the liquid acting as the co-operating electrode in each case, and the capacitance Ci(1) is measured. In this arrangement, there is no current through capacitance Co and hence it does not disturb the measurement. Flow rate may be derived from volume by differentiating with respect to time. A planar dipstick (Figure 4) used for the measurement is provided with a conductive coating (22, figure 4) to eliminate the large initial change in capacitance usually seen. <IMAGE>

Description

A CAPACITIVE TRANSDUCER This invention is concerned with the measurement of volumes and flow rates of liquids. It is particularly related to the measurement of these parameters in clinical diagnostic practice, for example the measurement of volume and flow rate of urine carried out as part of the diagnosis of bladder and urinary tract function.

Several different methods are commercially available specifically for the measurement of urine volume and urine flow rate, and this invention is essentially an improvement to one of these methods. It is, however equally applicable to these measurements applied to any conductive liquid which is being accumulated in a vessel.

The method improved by this invention is that known as the capacitive dipstick" method. In this, the level of liquid accumulated in a collection vessel is continuously measured by means of a 'dipstick" transducer. The collection vessel is usually of known cross sectional area, and hence a measurement of liquid level can be accurately and continuously converted to a measurement of liquid volume. In order to obtain flow rate, the electronic signal derived from the "dipstick" is differentiated.

The method operates by forming a long strip capacitor on the "dipstick" transducer. This capacitor is mounted in the collection vessel, and its total capacitance varies depending on the level of liquid surrounding it. The capacitor is formed by two plates, one of which is insulated from the fluid while the other is uninsulated, except by any naturally formed oxide layer.

Several different physical arrangements are in use to implement this technique. Some designs are constructed in a 'planar" fashion, such as shown in Figures 1 and 2, in which the strip electrodes 2 are arranged on opposite faces of a flat substrate 4, one of the electrodes 2 being covered with an insulating layer 6. Another design places the insulated and uninsulated plates concentrically.

When the transducer is placed in liquid, an equivalent electrical circuit is formed which consists of two capacitors in series, the connection between them being made by the conductive liquid, as shown in Figure 3, where Ci is the capacitance due to the insulating layer 6; CO is the capacitance due to the naturally formed oxide layer; Cs is stray capacitance; Rj is the resistance of the junction between the liquid and the uninsulated electrode and Rb is the bulk resistance of the liquid.

This equivalent electronic circuit is essentially the same for any implementation of the current method.

Several different techniques are used to detect the change in capacitance as liquid level changes, but a further common feature is that the liquid is connected to the system common or ground (usually via a large series capacitance). This reduces artificial contributions (artefact) due to interference or electrostatic effects.

With reference to Figures 1, 2 and 3, a more detailed description of the function of prior art systems is as follows: As the height of liquid surrounding the transducer varies, so too do the capacitances Ci and CO. Normally, efforts are made to ensure that CO, the capacitance due to the natural oxide layer of the uninsulated electrode, is very large compared to the insulated layer capacitance and to ensure that the bulk resistance of the liquid is low compared to the reactive impedance of Ci. The measuring circuit therefore effectively monitors the value of Ci.

This value is proportional to the height of the surrounding liquid, which in turn is proportional to the volume in the vessel. Since the cross sectional area of the vessel is known, the volume can be accurately computed.

The oxide layer on the uninsulated electrode forms a complex electrochemical junction with the liquid, which although predominantly capacitive, also has resistive component(s) as well as electrochemically formed potential(s). Its presence is necessary, however, in order to provide a low impedance connection between the liquid and the 0 volts or ground of the measuring system, in order to provide interference rejection. The unwanted effects of the presence of this junction in series with the measured variable capacitance must, however, be minimised in order to provide satisfactory performance.

This can be partly achieved by careful design of the electronic measuring system as well as by the physical design of the transducer itself.

This effect constitutes one of the major drawbacks of the prior art systems, since minimisation of the effects of CO are sometimes not totally effective, and, when they are, are frequently expensive to implement.

A further drawback of conventional transducers relates to the fact that there is a large change in measured capacitance between the condition where fluid touches the transducer, and when it is in free air. This step change in capacitance can produce a large transient when the output is differentiated. Electronic means can be used to mask this transient, or alternatively the system is "primed" by having a small quantity of liquid in the collection vessel prior to the commencement of the flow to be measured.

The invention to be described hereinafter overcomes both of these disadvantages.

The capacitive dipstick of the present invention provides the function of artefact rejection by actively "deriving' the liquid potential to eliminate interference while removing the need for the measuring circuit to measure the capacitance Ci via the capacitance CO. Thus the complex electrochemical junction does not form part of the measured capacitance, but (where required) merely appears in the interference rejection earthing circuit. In many situations an uninsulated junction is not required at all when this design is used.

Additionally, since the need for a direct contact with the liquid is obviated, the design of the transducer can be exploited to provide an inbuilt "starting volume" which reduces the initial transient to very low levels.

According to the present invention there is provided an apparatus for measuring the volume and/or flow rate of a conductive liquid comprising (i) a capacitive dipstick having first and second electrodes supported on a substrate, said electrodes being insulated from one another and, in use, from the liquid being measured; (ii) means for applying a first alternating voltage to drive a current into the first electrode; (iii) means for applying a second alternating voltage to drive a substantially equal and opposite current into the second electrode; and (iv) means for measuring the capacitance between the first electrode and the liquid.

According to another aspect of the present invention there is provided a method of measuring the volume and/or flow rate of a conductive liquid comprising the steps of (i) entering into a liquid container a capacitive dipstick having first and second electrodes supported on a substrate, said electrodes being insulated from one another and from the liquid being measured; (ii) applying a first alternating voltage to drive a current into the first electrode; (iii) applying a second alternating voltage to drive a substantially equal and opposite current into the second electrode; and (iv) measuring the capacitance between the first electrode and the liquid.

According to a further aspect of the present invention there is provided a capacitive dipstick for use in measuring the volume and/or flow rate of a conductive liquid comprising first and second planar electrodes supported on a planar substrate, said electrodes being insulated from one another and, in use, from the liquid being measured, and a conductive coating over the bottom of the dipstick which forms a capacitor with a portion of each of the first and second electrodes.

A specific embodiment of the invention is now described by way of example only with reference to the accompanying drawings in which: Figure 1 is a perspective view of part of a prior art capacitive dipstick; Figure 2 is a sectional view along the line A-A of Figure 1; Figure 3 shows an electrical circuit equivalent to that produced by the dipstick of Figures 1 and 2; Figure 4 is a sectional side view of a capacitive dipstick according to the present invention; Figure 5 is a sectional view along the line B-B of Figure 4; Figure 6 shows an electrical circuit equivalent to that produced by the dipstick of Figures 4 and 5; and Figure 7 shows a diagrammatic electrical circuit based on the circuit of Figure 6 and incorporating capacitance sensing equipment and an inverter.

The construction of the improved dipstick (in planar form) is shown in Figure 4.

Referring to Figures 4 and 5, a capacitive dipstick according to the invention has two insulated electrodes 12 formed on opposite sides of a flat substrate 14. A third, uninsulated electrode 16 is formed on one or both of these sides.

The two insulated electrodes 12 together with their insulating layers 18 are conveniently made as near as possible identical to each other, in order that equal and opposite drive voltages may be used. The resulting equivalent circuit is shown in Figure 6.

The equivalent circuit components are now Ci(l), which is the same as the Ci of the prior art circuit; Ci(2), which is the new capacitance formed by the second insulated electrode and the liquid; CO which is the capacitance due to the oxide layer as before; and Rx which is the resistance between the measuring circuit 0 volts common and the uninsulated plate (- this can be very large).

In principle, the measuring circuit still measures the value of one of the capacitances due to the insulated electrodes 12 in series with the capacitance of the oxide layer. The voltage (referred to 0 volts or ground) applied to the insulated electrode 12 is continuously measured, however, and is electronically inverted by inverter 20 as shown in Figure 7. The resulting voltage V2 is applied to the second insulated electrode 12 and this drives a current into this capacitor which is equal and opposite to that in the measured capacitor, since the two capacitors are designed to be as near as possible identical in value. A "null" voltage is therefore created in the liquid, relative to measuring system ground V3.

It should be noted that the voltage measured on the first capacitor wCi(l)" is in fact the sum of the voltage due to the measuring circuit, plus any extraneous interference signal(s). Since this is inverted and a current proportional to it is driven into the liquid in antiphase, any interference signal present in the liquid is also reduced to zero at the 'null1 point.

Since the liquid is driven to be at a bnull potential relative to measurement 0 volts, none of the measurement current flows through the variable component r 'C0, and this effectively removes le,l from the measurement. Thus variations due to electrochemical effects, aging or tarnishing do not affect the measurement. The uninsulated connection is still available, however, and since it is connected to measurement system ground or 0 volts, may still be used to provide a degree of interference rejection, for example where interference potentials exceed that of the linear range of the inverted output.

Owing to the fact that no measurement currents flow through this junction, and to the fact that interference potentials are also "nuller', the junction can be connected to system 0 volts via an indefinitely large resistance.

Under normal circumstances, in fact, the connection may be dispensed with entirely, and the uninsulated electrode 16 is not required when this is done.

This has the incidental advantage (in medical systems) of providing total electrical isolation between the measuring circuit and the liquid, even in the presence of high voltage a.c., and between the liquid and/or its container and the measuring equipment.

Since the transducer may be configured with no direct connection between the liquid and an electrode, a small initial 'starting volume" may be designed into the transducer as shown in Figure 4. When the transducer is mounted within the collection vessel, an equivalent small volume due to the conductive coating 22 over the insulation 18 is measured, prior to any liquid being introduced. As liquid flows into the vessel, it surrounds the conductive tip and eventually its level exceeds that of the conductive coating, at which point the capacitance begins to vary as the level increases. No physical conditions are changed when this occurs, however, and hence no step changes are introduced. This obviates the need for any "priming volume" or for any electronic masking of the first part of the flow measurement.

The oscillator or other capacitance sensing equipment 24 is well known and requires no further description in this specification.

It will of course be understood that the present invention has been described above purely by way of example and modifications in detail can be made within the scope of the invention.

Claims (19)

1. An apparatus for measuring the volume and/or flow rate of a conductive liquid comprising (i) a capacitive dipstick having first and second electrodes supported on a substrate, said electrodes being insulated from one another and, in use, from the liquid being measured; (ii) means for applying a first alternating voltage to drive a current into the first electrode; (iii) means for applying a second alternating voltage to drive a substantially equal and opposite current into the second electrode; and (iv) means for measuring the capacitance between the first electrode and the liquid.

2. An apparatus according to claim 1 wherein the first and second electrodes are substantially identical, allowing the first and second drive voltages to be substantially equal and opposite.

3. An apparatus according to claim 1 or claim 2 wherein the first and second electrodes are planar and are mounted on either side of a planar substrate.

4. An apparatus according to any preceding claim wherein the capacitive dipstick supports one or more uninsulated electrodes which, in use, are at ground potential.

5. An apparatus according to claim 4 wherein a conductive coating covers the bottom of the capacitive dipstick to act, in use, as a small initial starting volume.

6. An apparatus according to any preceding claim wherein the applying means includes an inverter for inverting the alternating voltage applied to the first electrode and for applying a proportion of the inverted voltage to the second electrode.

7. An apparatus according to any preceding claim which provides, in use, a substantially "null" voltage in the conductive liquid.

8. A method of measuring the volume and/or flow rate of a conductive liquid comprising the steps of (i) entering into a liquid container a capacitive dipstick having first and second electrodes supported on a substrate, said electrodes being insulated from one another and from the liquid being measured; (ii) applying a first alternating voltage to drive a current into the first electrode; (iii) applying a second alternating voltage to drive a substantially equal and opposite current into the second electrode; and (iv) measuring the capacitance between the first electrode and the liquid.

9. A method according to claim 8 wherein the first and second electrodes are substantially identical,allowing the first and second drive voltages to be substantially equal and opposite.

10. A method according to claim 8 or claim 9 wherein the first and second electrodes are planar and are mounted on either side of a planar substrate.

11. A method according to any one of claims 8-10 wherein the dipstick supports one or more electrodes, which are at ground potential, in contact with the liquid.

12. A method according to claim 11 wherein a conductive coating covers the bottom of the dipstick and acts as a small initial starting volume.

13. A method according to any one of claims 8-12 wherein the alternating voltage supplied to the first electrode is inverted by an inverter and a proportion of the inverted voltage is supplied to the second electrode.

14. A method according to any one of claims 8-13 wherein the capacitance of the first electrode is measured by an oscillator.

15. A capacitive dipstick for use in measuring the volume and/or flow rate of a conductive liquid comprising first and second planar electrodes supported on a planar substrate, said electrodes being insulated from one another and, in use, from the liquid being measured, and a conductive coating over the bottom of the dipstick which forms a capacitor with a portion of each of the first and second electrodes.

16. A capacitive dipstick according to claim 15 wherein the substrate supports one or more uninsulated electrodes.

17. An apparatus for measuring the volume and/or flow rate of a conductive liquid substantially as hereinbefore described with reference to or as shown in Figures 4-7 of the accompanying drawings.

18. A method of measuring the volume and/or flow rate of a conductive liquid substantially as hereinbefore described with reference to or as shown in Figures 4-7 of the accompanying drawings.

19. A capacitive dipstick substantially as hereinbefore described with reference to or as shown in Figures 4-7 of the accompanying drawings.