GB2090415A

GB2090415A - Liquid level meter

Info

Publication number: GB2090415A
Application number: GB8041476A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-12-30
Filing date: 1980-12-30
Publication date: 1982-07-07

Abstract

A liquid level meter for measuring the level 3 of fuel 2 in a tank includes a pair of identical elongate annular capacitors 12, 14 with inner and outer tubular plates 18 and 16. Liquid is at a level 3 to be measured in one capacitor 12 while the other capacitor 14 does not contain liquid but is open to atmosphere at the top 26 and is interconnected with the top of the one capacitor at 25. Shielded twisted conductors 34 from the two inner electrodes 18 are used to connect the two capacitors and in respective square wave one-shot oscillators and controlled from a common square wave oscillator. The difference between the charging times of the two one-shot oscillator circuits and is a measure of the liquid level and during that difference time high-frequency pulses from a further oscillator can be counted in a counter. <IMAGE>

Description

SPECIFICATION Liquid level meter This invention relates to a liquid level meter for example for continuously giving an indication of the liquid level in a vehicle fuel tank.

According to the present invention a liquid level meter comprises a pair of capacitors each of which consists of two plates defining between them a dielectric space, the space of one capacitor, but not the space of the other, being arranged to contain liquid up to a level to be measured, and means for comparing the two capacitances.

By having two similar - preferably identical capacitors, one of which contains liquid up to a desired level while the other can contain ambient air which is also in the part of the one capacitor not containing liquid, compensation can be obtained for variations in ambient pressure and temperature and soon, which will affect both capacitors equally. If the capacitors each consist of concentric plates, for example concentric tubes defining an annular dielectric space, then the outer tube of each capacitor can be of ferrous material to provide shielding from radio frequency or electric induction interference.

Signals from the two capacitors can be taken from the respective inner electrodes by respective components of a twisted twin cable which can itself be shielded by an earthed conductor. Any interference will tend to be received by both components of the twin cable and will tend to counter-balance each other.

Thus the invention does not measure the absolute value of capacitance but merely the relationship of the capacitances of the two capacitors.

In a preferred comparing circuit there are two charging circuits which are similar except that each has one of the two capacitors connected in it. The charging circuits are controlled from a common square wave oscillator and the difference in the charging times of the two circuits is a measure of the liquid level. It can conveniently be measured by counting high-frequency pulses during the time interval between the completion of the charging time of one charging circuit and the completion of the charging time of the other where they start charging together.

Such a circuit can be easily calibrated by adjusting resistors so that when a fuel tank is empty for example, the charging times of the two circuits are the same while when it is full, a desired maximum reading can be obtained.

Since the two charging circuits are controlled from a common square wave oscillator, that can be at a frequency low enough for cable losses to be very small; moreover the cable capacitance merely affects the duration of the pulses in both oscillators equally rather than attenuating the signal as would be the case with a sinusoidal wave.

Thus the meter is expected to be fairly simple to manufacture but yet to be capable of giving a measure of the liquid level which tends to be accurate in spite of variations in ambient conditions and to be free from external interference.

The invention may be carried into practice in various ways and one embodiment will now be described by way of example with reference to the accompanying drawings of which Figure 1 is a diagrammatic sectional elevation of a double capacitor; Figure 2 is a diagrammatic plan view of the capacitor of Figure 1; Figure 3 is a sketch showing how the capacitor of Figures 1 and 2 can be positioned outside a fuel tank for giving an indication of the fuel level; Figure 4 is a block circuit diagram showing how the capacitor of Figures 1 and 2 can be arranged to give a direct indication of the fuel level; and Figure 5 is a set of voltage characteristics appearing in the circuit of Figure 4.

The double capacitor of Figures 1 and 2 is made up of two co-axial cylindrical capacitors which are very nearly identical. Each consists of an outer ferrous cylindrical tube 16 forming the earthed capacitor plate and a central conducting tube 18 forming the other plate with an insulating annular spacer 20 near each end for locating the tubes 16 and 18 in relation to each other. There is an epoxy resin closure plug at each end of each capacitor one of which 28 is continuous while the other 30 has an aperture 26 communicating with the space within the inner tube 18. The two outer tubes 16 are interconnected by a transverse ferrous tube 25.

The right hand capacitor in Figure 1 is a liquid level capacitor and has a number of radial holes 22 in the bottom of the inner plate 18 just above the spacer 20 for putting the space within the plate 18 into communication with the space between the two plates 16 and 18. There are similar radial holes 24 at the top of the inner plate 18. The bottom aperture 26 allows liquid 2 in which the capacitor is immersed to rise up the space between the capacitor plates as indicated at 3. The capacitance of the capacitor will vary with the amount of liquid acting as a dielectric between the plates and thus will depend upon the level of fuel in the tank.

As liquid rises in the right hand capacitor gaseous vapour is driven through the transverse tube 25 into the annular space between the plates 16 and 18 of the left hand or reference capacitor. That capacitor also has radial holes 22 nearthe lower end of the inner tubular plate 18. The lower end of the inner tubular plate 18 is closed buy a closure 28 but the upper end is open to atmosphere through the aperture 26 in the upper closure 30. Thus vapour can be displaced down the annular space between the plates 16 and 18 and then up the space within the plate 18 to atmosphere if the liquid level rises. If the liquid level falls then atmospheric vapour enters the space within the plate 18.

The dielectric space between the plates 16 and 18 of the left hand reference capacitor will always contain atmospheric vapour whatever the liquid level in the right hand capacitor so that the reference capacitance will remain more or less constant even though the capacitance of the level measuring capacitor varies with the fuel level.

The central tubular plates 18 of the two capacitors are electrically connected to respective conductors 34 of a twisted pair surrounded by a shield 36 earthed at 38 and connected to the earthed outer tubular plates 16.

If there are changes in the atmospheric pressure or temperature or other ambient conditions they will be experienced by both capacitors equally so that any consequent changes in the capacitance of one capacitor will tend to be reflected in the other and a measure of the relationship between the two capacitances can give an accurate measure of the fuel level even if the capacitance of the level measuring capacitor varies due to such variations in ambient conditions.

The twisting together of the two external conductors 34 and their shielding in the earthed shield 36 in combination with the shielding of the external earthed capacitor plates 16 tends to protect the complete device from radio frequency interference and electro-magnetic interference. Noise picked up by one conductor tends to cancel with noise picked up by the other; similarly capacitance changes of the conductor due to variations in cable length with temperature tend to compensate each other.

The device of Figures 1 and 2 can be positioned within a fuel tank with the lower opening 26 below the fuel level and the upper opening 26 above the fuel level, or if the device is to be mounted outside the fuel tank the arrangement can be as shown in Figure 3 where the tank is shown at 42 containing fuel 52 and the capacitor device is shown at 40. The upper opening 26 is connected by a traverse connection 48 with the top of the fuel tank and the lower opening 26 is connected at the bottom of the fuel tank through a transverse connection 44 containing a variable flow-restrictor 54 which can be set to enable the user to achieve any desired degree of damping. The liquid level in the level measuring component of the double capacitor device will be, as shown at 52, the same as in the fuel tank.

A circuit for producing an output at O/P dependent upon the relationship between the two capacitances is shown in Figure 4.

A one-shot liquid-level oscillator 68 comprises a Schmitt trigger 70, a diode 72, a resistor 74, and the liquid-level capacitance 76 from the right hand component of the double capacitor of Figures 1 and 2 in parallel with the capacitance 78 of the connecting cable 34.

Similarly a one-shot reference oscillator 80 comprises a Schmitt trigger 82, a diode 84, a resistor 86, the reference capacitor 88 corresponding to the left hand component in Figures 1 and 2, and the connecting cable capacitance 90.

The two one-shot oscillators 68 and 80 are controlled by a synchronising oscillator 60 consisting of an amplifier 62, a resistor 64, and a capacitor 66. The oscillator 60 is a free-running, square-wave oscillator producing a square wave as shown at A in Figure 5, the lengths of whose marks and spaces depend upon the values of the resistor 64 and the capacitor 66. As long as these values are the same on both one-shot oscillators 68 and 80, the precise values are not important so that the components do not have to be expensive, precision components. In the reference time scale shown at the top of Figure 5 times t1 and t5 represent the beginnings of consecutive marks and t represents the beginning of the intervening space.

Considering the one-shot oscillator 68 at time to during a space in the square wave A, the output, as shown at B in Figures 4 and 5, is high or logical 1-since its input is held at 0 by the square wave A through the diode 92. At the beginning of a mark at time tr, the capacitors 76 and 78 start to charge through the diode 72 and the resistor 74 until the trigger level of the Schmitt trigger 70 is reached at time t3 when the output at B drops to0 as shown in Figure 5. The output remains at 0 until the square wave A returns to Oat time 4 and that returns the output Bofthe Schmitt trigger 70 to 1.

The reference one-shot oscillator 80 is controlled from the synchronising oscillator 60 through the diode 94 in the same way. The value of the capacitors 88 and 90 determines the time t2 when the output C of that oscillator drops to 0 when the capacitors have charged.

A higher frequency, reference square-wave oscillator 96 comprises a Schmitt trigger 98, a resistor 100, and a capacitor 102. The oscillator 96 is held off while the output from the one-shot reference oscillator 80 is at logical 1 and so only runs at its frequency-determined by the resistor 100 and the capacitor 102-between the times t2 and t4 when the square wave C is at logical 0. The output of the oscillator 96 is shown at D and that is fed through the resistor 108 to the input of a gate 106 which is open as long as the output C from the one-shot liquid-level oscillator 68 is at logical 1. Thus oscillations from the oscillator 96 are fed through the gate 106 to a counter 107 from the moment t2 when the square wave C drops to logical 0 until the momentt3when the square wave B drops to logical 0.Those bursts of oscillations are shown at E in Figures 4 and 5. Three bursts are shown and they correspond to progressively lower liquid levels in the tank 42, varying the value of the capacitance 76 and changing the lengths of the marks in the square wave B. As the liquid level increases the number of high frequency pulses counted in the counter 107 in each cycle of the reference square wave A will increase. If resistors 74 and 86 are adjusted so that when there is no liquid in the tank the capacitances of the two one-shot oscillators 68 and 80 are the same so that the times t2 and t3 coincide, the output will consist of no high frequency pulses and will be represented as a 0 at O/P.

The counter 107 is reset toO at the beginning of each mark in the reference square wave A, that is at the time t-as indicated at F of Figures 4 and 5-by means of a reset circuit 114 consisting of a capacitor 118 and a resistor 120 connected to the output from the oscillator 60.

A signal to stop the counter is derived from a similar circuit 116 consisting of a capacitor 122 and a resistor 124 connected to the output C of the oscillator 80 to apply to the counter a stop signal as shown in G in Figures 4 and Sat the beginning of each mark of the square wave C. The square wave C also drives the high frequency oscillator 96 through a diode 112. The circuits 114 and 116 are simple differentiating circuits producing a pulse corres ponding to the rise of the driving square wave form at the beginning of a mark.

The output can be used to drive a light-emitting diode digital readout or any other kind of meter, graph or display.

The resistor 100 in the high frequency oscillator 96 can be set to give a desired maximum count when the tank is full. The circuit must be designed so that when the tank is full the trigger 70 can reach its trigger level before the time t4 at the end of a mark of the reference square wave A.

The advantage of the circuit of Figure 4 is that there are similar oscillators 68 and 80 containing the respective capacitors 76 and 88 merely for compar ing these capacitors, the one-shot oscillators being controlled from a common reference oscillator 60 which can be designed to operate at a frequency low enough for cable losses to be quite small. The circuit can be easily calibrated in a manner described above. Since the operation is determined by the respective charging times of the two capacitors, it can be quite sensitive even while measuring low capacitances.

i CLAIMS (filed 11.12.81) 1. A liquid level a meter comprising a pair of capacitors each of which consists of two plates defining between them a dielectric space, the space of one capacitor, but not the space of the other, being arranged to contain liquid up to a level to be measured, and means for comparing the two capaci tances.

2. A meter as claimed in Claim 1 in which each capacitor consists of concentric plates of different diameter defining an annular dielectric space be tween them.

3. A meter as claimed in either of the preceding claims in which the two capacitors are elongate and side by side with the two dielectric spaces being in communication at one end.

4. A meter as claimed in Claim 3 in which the dielectric space of one capacitor is open at one end and the dielectric space of the other capacitor is open at the other end.

5. A meter as claimed in any of the preceding claims in which one plate of each capacitor is an outer plate of ferrous material.

6. A meter as claimed in any of the preceding claims in which the comparing means comprises a twisted twin cable, one component of which is connected to one plate of one capacitor and the other component of which is connected to one plate of the other capacitor.

7. A meter as claimed in Claim 6 in which the cable is shielded by an earthed ferrous shield.

8. A liquid level meter constructed and arranged substantially as herein specifically described with reference to Figures 1 and 2 of the accompanying drawings.

9. A liquid level meter as claimed in any of the preceding claims positioned in or connected outside a tank for fuel or other liquid.

10. A meter as claimed in any of the preceding claims in which the comparing means comprises a pair of similar charging circuits one of which includes one of the capacitors and the other of which includes the other capacitor.

11. A meter as claimed in Claim 10 including a square wave generator arranged to control both charging circuits.

12. A meter as claimed in Claim 10 or Claim 11 including a source of high-frequency pulses and a gate arranged to pass pulses to a counter during a time interval representing the difference between the charging times of the two charging circuits.

13. A meter as claimed in any of Claims 10 to 12 in which the charging circuits are identical one-shot square wave oscillators.

14. A meter as claimed in any of the preceding claims including an electrical circuit arranged substantially as herein specifically described with reference to Figure 4 of the accompanying drawings.

15. A meter as claimed in any of the preceding claims arranged to give a visual indication of the liquid level measured.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. differentiating circuits producing a pulse corres ponding to the rise of the driving square wave form at the beginning of a mark. The output can be used to drive a light-emitting diode digital readout or any other kind of meter, graph or display. The resistor 100 in the high frequency oscillator 96 can be set to give a desired maximum count when the tank is full. The circuit must be designed so that when the tank is full the trigger 70 can reach its trigger level before the time t4 at the end of a mark of the reference square wave A. The advantage of the circuit of Figure 4 is that there are similar oscillators 68 and 80 containing the respective capacitors 76 and 88 merely for compar ing these capacitors, the one-shot oscillators being controlled from a common reference oscillator 60 which can be designed to operate at a frequency low enough for cable losses to be quite small. The circuit can be easily calibrated in a manner described above. Since the operation is determined by the respective charging times of the two capacitors, it can be quite sensitive even while measuring low capacitances. i CLAIMS (filed 11.12.81)

1. A liquid level a meter comprising a pair of capacitors each of which consists of two plates defining between them a dielectric space, the space of one capacitor, but not the space of the other, being arranged to contain liquid up to a level to be measured, and means for comparing the two capaci tances.