GB2066961A - Electronic capacitive liquid level gauge - Google Patents

Abstract

The invention consists of a method of sensing, measuring, and providing appropriate digital outputs of the height of a liquid level, either in a closed container, or in a natural environment, such as rivers or the sea. The method used is to discriminate between the electrical capacity to earth, or to a common sensor, of discrete sensors placed vertically in the liquid, when they are above the surface of the liquid or below it. The sensors are insulated from the liquid by any suitable dielectric and are spaced vertically at any desired interval. They can be simple plates of metal foil. The count of sensors indicating "below the surface" or of those indicating "above the surface" or the dividing line between the two, indicates the depth. The method used in the application involves pulsing each sensor sequentially and resolving the electronic capacity indication as being "above the surface" or "below". <IMAGE>

Description

SPECIFICATION Electronic capacitive liquid level gauge (Short titlel "ECLG" For ease of reference, the invention titled above is called the ECLG in the succeeding description.

The ECLG has particular application for the measurement and recording of tidal levels in fresh or salt water, but the principle involved has universal application in the measurement of all vertical levels of practically any liquid.

The ECLG fulfills the need for an instrument to indicate, electronically, the vertical level of a liquid at a predetermined site and produce electrical signals suitable for recording in both analogue and digital form (eg in analogue form, as a time/height graph on electrically sensitised paper, and in digital form on magnetic or punched tape or disc storage, for later processing). The ECLG has the advantage of requiring no moving parts such as the floats, wires, drums, counterbalance weights, gears, spring driven clocks etc, or even pressure heads, all of which lead to periodic malfunction of "classical liquid level recorders. The electrical signals produced are ideally suited to scanning by accurate quartz-controlled clocks at predetermined intervals for subsequent processing.

By using sensors at discrete vertical intervals for example, 2 centimeter vertical spacing, the ECLG avoids inherent inaccuracies of liquid level gauges which rely on the variation of a single parameter (eg voltage, frequency, pressure etc) to indicate the liquid level, where variation of conditions and reference level of the chosen barometer can lead to inaccurate results. The ECLG, by the principle on which it relies, avoids any contact between the sensors and the liquid being measured, thus avoid mis-function caused by wetting of electrical contacts.

PRINCIPLE The ECLG reiies for its measurement of liquid level on the principle that capacitance of a metal or conducting plate to earth, or to another similar plate insulated from it, will change as its dielectric environment changes, or in other words, the capacity of such a sensor will be different if it is in air compared to its capacity when immersed in some other fluid such as water, oil etc. The sensors can thus indicate whether they are immersed in the liquid or not by their capacitance to a specified level or part of an electrical circuit, whilst the sensors remain insulated electrically from the liquid itself.Thus, if sensors are arranged vertically at the intervals at which it is desired to measure the vertical level of the liquid, the sensors below the surface of the liquid will exhibit a different capacity to earth (or some other level) from those above the surface, and thus the actual vertical level of the liquid will be easily displayed. Depending on the electrical properties of the liquid being measured, the liquid can either be treated as a dielectric (ie non-conductive material) which on reaching the level of a pair of capacitive plates alters the capacitance between the two plates themselves, or the liquid itself can form the earthed plate of a capacitor with the plate of the sensor. In either case the sensors must be insulated from the liquid by a material suitable to act as part or all of the dielectric of the capacitive sensor.

The electronic circuitry involved in forming and reading the capacitive sensors just described has scope for almost unlimited variation in design, and the formation of suitable output signals from the sensors described has similar unlimited or wide scope. The invention of the ECLG depends upon the adaption of simple electrical principles to the particular application of liquid level measurement and relies on established and widely used electronic and digital techniques for the further processing of the signals or indications obtained. The core of the invention is therefore the use of capacitive sensors to measure the vertical level of liquids. This principle has wide application potential in the manufacture of tide gauges, water-table gauges and the level of fluids in storage tanks.

DESCRIPTION The method of operation is illustrated in its simplest form in Figure 1. An oscillatory supply voltage is connected in parallel to load resistors connected in series with each plate of a number of sensors. A sensor not immersed in the liquid will have a low capacitance to earth, and no voltage will be generated across its load resistor. A sensor immersed in the liquid (but insulated from it) will have a higher capacitance to earth, current will flow, and a voltage will be generated across the load resistor.

In the case of liquids having a very low electrical conductivity, two plates are used for each sensor, as in Figure 2. When a particular sensor is above the level of the liquid the capacitance between the two plates is very low, but when immersed in the liquid, the dielectric property of the liquid increases the capacitance between the two plates of the sensor, and the resultant current flow is detected, as in Fig. 1 by the voltage across the load resistor.

Application Figure 3 shows a much more sophisticated arrangement using a commercially produced Integrated Circuit (IC) No S9263 and adapting a circuit published by R.S. Components Ltd, 1 7 Epworth Street, London EC2 in their data sheet No R/3251, August 1978 as a test circuit for "Touch Switches" for which the IC is designed.

The circuits in the l.C. scan the 16 inputs of the I.C. in succession, on specific pulses of the Clock Scan Voltage (pin 40). When the input pin being scanned has a low capacitance to earth a 'high' voltage level is detected and the integrated transistor of the corresponding output pin remains switched off. When the input pin being scanned has an increased capacitance to earth a 'low voltage is detected, the corresponding output transistor is switched on, and the output pin is raised to the user-determined output voltage connected to pin 28. The reference level for the sensed voltages at the input pins can be adjusted by adjustment of R1. An addition to the circuit, R2, permits adjustment of the Clock Scan frequency to suit the gauge and environmental conditions.With fresh water as the liquid being measured, a frequency of about 1 00kHz is suitable.

The IC is produced for use in Touch Switches (for example. the on/off switches fitted to some television sets) but is particularly useful as a controller for the water level gauge using the principles of the ECLG described. There is, or course, no suggestion that the IC is part of the new invention. Bt is just a useful component available on the open market which can be adapted for use in the ECLG.

In the ECLG for use as a tide gauge, sheet aluminium plates, 2cm long, by 1 cm wide are stuck vertically at 2 cm intervals (ie with a 1 cm gap between each) along a plastic pipe of a suitable diameter (14 inches to 4 inches).

These plates are the specific plates of the sensors. Horizontally opposed, spaced 1 cm away round the diameter of the pipe, are the common plates of the sensors. These are the same shape as the specific plates but are commoned together on the side furthest from the specific sensors, in sets of 1 6 vertically and are connected to the output of the amplifiers of the Clock Scan output on the IC. The specific sensor plates are connected, in order, to the input pins of the l.C. It is important to keep the separate wires of these specific plates as well spaced as possible to avoid the effects of stray capacitance. 3 to 4 mm spacing is desirable, and this can either be achieved by a honeycomb spacer inside the pipe or nibbed channels on the outside of the pipe. The outside of the pipe is varnished.

Figure 4 shows how three ICs can be used to control sensors for 1 metre of pipe with 2 cm spacing of sensors. A gauge of desired length can therefore be built up in 1 metre sections, each being identical in all but the final output stage to the one before. It will be seen in this design, in order to be able to use the 48 available inputs to provide for the 50 sensors required for a one metre length of gauge the sensors at zero, 34 and 68 cms distance along the gauge are not connected to IC inputs but by a little ingenuity can be coupled to other sensors to provide a signal.

The outputs of the three ICs for each metre length of gauge can be processed by standard counter techniques to produce the centimetric readings for each metre, the one metre lengths themselves providing the indication of the whole numbers of metres. Probably the best method is to resolve the indicated level in each length of gauge into binary coded decimal outputs. For up to 9.98 metres therefore, a total of 8 BCD output lines for the whole gauge would be sufficient.

Claims (4)

1. The method of measuring liquid level by comparing the electronic capacities of vertically disposed metal plates (to earth or to a common sensor) when they are above or below the liquid level. The plates, being insulated, are true capacitor plates, and they are not wet by the liquid.

2. The particular application of this method (described in claim 1 above) to tide and water level gauges in rivers and the sea, enabling true "Non Moving Part" gauges.

3. The use, in the method described in Claim 1 above of sequential pulsing of the sensors to reduce the effects of mutual capacity between adjacent sensors and electrical wiring.

4. The use of the method described in Claim 1 above to provide digital outputs of the measurements in discrete units, free from any analogue distortions or scaling errors.