GB2329021A - Fluid level monitor - Google Patents

Abstract

A water level monitor 1 for monitoring the level of drinking water in a caravan is disclosed. The monitor comprises a pair of electrically conductive plates 2 mounted to the external sides of an electrically non-conducting water tank containing drinking water 4 so as to lie in planes generally parallel to each other. The plates 2 form a parallel plate capacitor, the capacitance of which depends upon the ratio of air to water in the space between the plates 2. The capacitor formed by the plates 2 forms one limb of a bridge circuit 5, the other limbs of which are constituted by a pair of diodes 6, 7 and a variable capacitor 8. A digital voltmeter 22 displays an output corresponding to the level of water in the tank between the plates 2.

Description

FLUID LEVEL MONITOR The present invention relates to a fluid level monitor, and relates particularly, but not exclusively, to a fluid level monitor for monitoring the level of drinking water in a water tank of a vehicle such as a caravan.

Fluid level monitors are known which operate by means of the introduction of an electrical transducer into a liquid in a fluid tank, the level of which is to be monitored, and the fluid level is determined by passing an electrical current through the liquid. Such transducers generally comprise two or more stainless steel rods which operate by passing current through the fluid to be monitored and obtaining a reading from a meter by processing the current.

Prior art monitors of this type suffer from the disadvantage that they cause electrolysis in the fluid, which eventually corrodes the rods, and can cause hazardous pollution of the fluid. This is especially disadvantageous when the fluid to be monitored is drinking water.

Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.

According to the present invention, there is provided a fluid level monitor, the monitor comprising a pair of electrically conductive plates adapted to at least partially accommodate a fluid container therebetween such that fluid contained in the container is located between the plates; detector means electrically connected to said plates for providing an electrical output signal dependent upon the electrical capacitance between said plates; and display means for receiving the output signal from said detector means and displaying an output dependent upon the electrical capacitance between said plates.

The electrical capacitance between the plates will vary depending upon the extent to which the space is filled with air or the fluid to be monitored, and will therefore also depend upon the level of fluid between the plates. Accordingly, by displaying an output at the display means dependent upon the electrical capacitance between the plates, the output at the display means is dependent upon the fluid level in the container. This gives the advantage of providing a noninvasive fluid level monitor, i.e. a monitor which does not require the introduction of components into the fluid to be monitored, thus avoiding contamination of the fluid or corrosion of the metal plates.

The plates may be substantially flat and may extend in planes substantially parallel to each other.

The plates may be adapted to be mounted to the exterior of an electrically insulating fluid container.

This allows the plates to be directly mounted to the container, while preventing transmission of direct current between the plates.

In a preferred embodiment, the electrically conductive portion of each said plate has a width in use which increases with height above the bottom of said plate.

By suitable shaping of the plates, this provides the advantage of compensating for non linear dependence of the capacitance between the plates upon fluid level in the container, such that the monitor may have a substantially linear output with fluid depth.

In a preferred embodiment, each said plate is substantially triangular.

The fluid level monitor may further comprise signal generating means for applying an AC signal to said plates.

The signal generating means preferably comprises a crystal oscillator.

The monitor may further comprise a feedback capacitor connected between an output of the detector means and an input of the oscillator.

This provides the advantage of enabling stabler operation of the monitor.

In a preferred embodiment, the detector means comprises rectifier means for providing a rectified AC output signal.

The rectifier means may comprise a bridge circuit to which said plates are connected.

The bridge circuit preferably further comprises a variable capacitor for enabling a zero reading to be set at the display means for a predetermined minimum fluid level in the container.

This provides the advantage of enabling the fluid level monitor to be calibrated, i.e. enabling the zero of the monitor to be set to correspond to a predetermined minimum fluid level in the container.

The detector means may further comprise a peak detector connected to an output of the rectifier means.

In a preferred embodiment, the detector means further comprises a low pass filter connected to an output of the rectifier means.

The display means may further comprise a damping capacitor connector across the terminals thereof.

This provides the advantage of reducing rapid fluctuation of the display reading, for example as a result of rapid fluid movement when the fluid level monitor is incorporated in a vehicle.

The display means may further comprise a voltage divider having a variable resistor.

This provides the advantage of enabling the output voltages of the detector means to be matched to display meters of varying types.

As an aid to understanding the invention, a preferred embodiment thereof will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram of a fluid level monitor embodying the present invention; Figure 2 is an elevational view of a conductive plate of Figure 1; Figure 3 is an equivalent electrical circuit to the circuit shown in Figure 1 for positive input voltage pulses; Figure 4 is a circuit diagram corresponding to Figure 3 for negative input voltage pulses; and Figure 5 is a graph of meter output verses water quantity between the plates for the apparatus of Figure 1.

Referring in detail to Figure 1, a water level monitor 1 for monitoring the level of drinking water in a caravan comprises a pair of generally triangular, spaced apart metal plates 2 mounted to the external sides of an electrically non-conducting water tank 3 containing drinking water 4 (see Figure 2) so as to lie in planes generally parallel to each other. The plates 2 are mounted to the tank 3 by adhesive, or are moulded to the tank when it is formed, and are generally of similar dimensions to the sides of the tank to which they are attached.

The plates 2 form a parallel-plate capacitor, and the nature of the dielectric in the space between the plates 2 therefore varies, depending upon the ratio of air to water in the space between the plates. This in turn varies the electrical capacitance of the capacitor formed by the plates 2. The capacitor formed by the plates 2 forms one limb of a bridge circuit 5, the other limbs of which are constituted respectively by a pair of diodes 6, 7, and a variable capacitor 8, the function of which will be described below.

The apparatus 1 also comprises an electrical power supply 9 such as a vehicle battery, which supplies a 9 to 24 volt DC voltage to a voltage regulator 10. The voltage regulator 10 supplies a regulated 5 volt DC output voltage to a 14.3 MHZ crystal oscillator 11, which in turn supplies a 14.3 MHZ AC output voltage of approximately 4 volt peak amplitude to the bridge circuit 5.

The output voltage of the oscillator 11 is applied to input terminals 12, 13 of the bridge circuit 5, and terminal 14 of the bridge circuit 5 is connected via negative supply rail 15 and feedback capacitor 16 to an input terminal of the oscillator 11. The feedback capacitor 16 provides a feedback path from the output terminals of the apparatus 1 to the input terminal of the oscillator 11 and thereby serves to stabilise the operation of the oscillator 11.

Output terminal 17 of the bridge circuit 5 is connected to a freewheel diode 18 and to a peak detector comprising a resistor 19 and a capacitor 20. The output of the peak detector is connected to a low pass filter comprising a choke 21 which serves to filter out 14.3 MHZ components of the output signal, and the output of the choke 21 is connected to a digital voltmeter 22, across the terminals of which a damping capacitor 23 is connected in order to smooth out rapid fluctuations in the meter reading. A voltage divider 24 including a variable resistor enables the output voltage of the low pass filter 21 to suit a variety of meters, such as a digital voltmeter 22 as described above, light emitting diodes, audible warning devices, or like devices which will be known to persons skilled in the art.

The operation of the bridge circuit 5 of Figure 1 will now be described with reference to Figures 3 and 4, which show equivalent circuits to Figure 1.

The crystal oscillator 11 outputs a generally sinusoidal AC output voltage at 14.3 MHZ, which is supplied to input terminals 12, 13 of bridge circuit 5. When the positive half cycles of the output of oscillator 11 are applied to terminals 12, 13, because terminal 14 of bridge circuit 5 is connected to the negative supply rail 15, diode 6 is reversed biased and will therefore not conduct. As a result, AC signals can be transmitted from input terminal 12 to output terminal 17 of the bridge circuit 5 via the capacitor formed by plates 2 with water 4 therebetween. As a result of the voltage drop across the capacitor formed by plates 2, terminal 17 is at a lower voltage than terminal 13 and diode 7 is therefore effectively reverse biased and will not conduct.

In addition, freewheel diode 18 is reverse biased and therefore will not conduct, with the result that AC current signals from input terminal 12 to output terminal 17 flow as current I1 via peak detector 19, 20, filter, 21, meter 22, and capacitor C1 to return to the oscillator 11. At the same time, AC current signals from input terminal 13 travel as current I2 via capacitor 8 and output terminal 14 to capacitor 16.

It can therefore be seen that positive half cycles of the input signal to the bridge circuit 5 result in rectified AC output signals to the peak detector 19,20 and filter 21, the amplitude of which is dependent upon the electrical capacitance between the plates 2, which is in turn dependent upon the depth of water in the tank 3.

Referring now to Figure 4, when the negative half cycles of the signal from oscillator 11 are supplied to input terminals 12, 13, diode 6 is now forward biased and therefore conducts.

This has the effect of shunting the limb of the bridge circuit 5 formed by the plates 2, with the result that little AC current passes via the capacitor formed by the plates 2.

Since little voltage signal passes from input terminal 12 to output terminal 17, diode 7 is effectively forward biased because of the negative signal applied to input terminal 13 and therefore conducts. As a result, the voltage at output terminal 17 drops, which in turn causes freewheel diode 18 to be forward biased and thus to conduct. Current from input terminal 12 of the bridge circuit 5 consequently follows a path via diode 6 and capacitor 16 to the oscillator 11, and current from input terminal 13 follows a path via diodes 7 and 18 and capacitor 16. The effect of freewheel diode 18 is to prevent a negative voltage appearing across the meter 22, which may under certain circumstances cause damage to the meter 22.

The operation of the water level monitor shown in Figures 1 and 2 will now be described.

The monitor 1 is calibrated by adjustment of variable capacitor 8 such that the meter 22 displays zero when the water level in the tank 3 is at its minimum value. When water is subsequently added to the tank 3 between plates 2, the electrical capacitance of the capacitor formed by the plates 2 varies, which results in variation of the amplitude of the rectified AC signal appearing at output terminal 17 as a result of the positive half cycles of the input signal. The peak amplitude of the output signal appearing at output terminal 17 is therefore dependent upon the level of water in the tank 3 and is detected by peak detector 19, 20 and input to choke 21 which filters out the rapidly fluctuating 14.3 MHZ component of the output signal. The signal is then input to meter 22, which displays an output corresponding to the level of water in the tank 3.

Damping capacitor 23 slows down rapid fluctuations in the output of meter 22, which in turn reduces rapid fluctuations in the meter output as a result of movement of the water 4 in the tank 3 due to movement of the vehicle.

By making the plates 2 generally triangular as shown in Figure 2, this compensates non-linearities in the output of the monitor 1, with the result that the output at meter 22 is generally proportional to the amount of water in the tank 3 as shown in Figure 5. The plates 2 shown in Figure 2 may be constructed as triangular conductive plates, or as nonconductive plates of some other shape and having triangular conductive inserts.

It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended Claims. For example, it will be appreciated that the use of the monitor is not limited to monitoring to level of drinking water, and the level of any fluid in a non-conductive tank may be monitored, such as fuel. Furthermore, while the embodiment described above utilises passive components, the invention can also be implemented by means of active electronic components.

Claims (16)

1. A fluid level monitor, the monitor comprising a pair of electrically conductive plates adapted to at least partially accommodate a fluid container therebetween such that fluid contained in the container is located between the plates; detector means electrically connected to said plates for providing an electrical output signal dependent upon the electrical capacitance between said plates; and display means for receiving the output signal from said detector means and displaying an output dependent upon the electrical capacitance between said plates.

2. A monitor according to claim 1, wherein the plates are substantially flat and extend in planes substantially parallel to each other.

3. A monitor according to claim 1 or 2, wherein the plates are adapted to be mounted to the exterior of an electrically insulating fluid container.

4. A monitor according to any one of the preceding claims, wherein each said plate includes an electrically conductive portion having a width which in use increases with height above the bottom of said plate.

5. A monitor according to claim 4, wherein each said plate is substantially triangular.

6. A monitor according to any one of the preceding claims, further comprising signal generating means for applying an AC signal to said plates.

7. A monitor according to claim 6, wherein the signal generating means comprises a crystal oscillator.

8. A monitor according to claim 7, further comprising a feedback capacitor connected between an output of the detector means and an input of the oscillator.

9. A monitor according to any one of the preceding claims, wherein the detector means comprises rectifier means for providing a rectified AC output signal.

10. A monitor according to claim 9, wherein the rectifier means comprises a bridge circuit to which said plates are connected.

11. A monitor according to claim 10, wherein the bridge circuit further comprises a variable capacitor for enabling a zero reading to be set at the display means for a predetermined minimum fluid level in the container.

12. A monitor according to any one of claims 9 to 11, wherein the detector means further comprises a peak detector connected to an output of the rectifier means.

13. A monitor according to any one of claims 9 to 12, wherein the detector means further comprises a low pass filter connected to an output of the rectifier means.

14. A monitor according to any one of the preceding claims, wherein the display means further comprises a damping capacitor connector across the terminals thereof.

15. A monitor according to any one of the preceding claims, wherein the display means further comprises a voltage divider having a variable resistor.

16. A fluid level monitor, the monitor substantially as hereinbefore described with reference to the accompanying drawings.