GB2102954A

GB2102954A - Liquid depth measurement

Info

Publication number: GB2102954A
Application number: GB08219996A
Authority: GB
Inventors: Ian Richard Pullen; John Harvey Strange
Original assignee: Emco Wheaton UK Ltd
Current assignee: Emco Wheaton UK Ltd
Priority date: 1981-07-10
Filing date: 1982-07-09
Publication date: 1983-02-09
Also published as: GB2102954B

Abstract

There is provided an apparatus for measuring the contents of a container of a liquid. Open ended probes (3, 4) are positioned near the bottom of and in the ullage space above tank (1) of liquid (2). The probes are provided with a small flow of gas from a gas supply (6) via constant flow devices (7, 12) and are connected to two sides of a differential pressure transducer (9) which generates an output signal representative of the hydrostatic pressure resulting from the column of liquid above the open end of probe (3). To avoid errors resulting from excess or stopped air flow through probe (3), the rate of bubbling of the gas therefrom is monitored by bubble detector (8). <IMAGE>

Description

SPECIFICATION Tank contents measurement This invention relates to a method and apparatus for measuring the contents of a container of liquid and may be used, for example, for measuring the contents of a fuel storage tank or fuel delivery tanker.

The quantity of liquid in a fuel tank has been measured by the use of a graduated dipstick.

However, this method is inaccurate because of the difficulty of reading the scale; the method is also open to abuse if the dipstick is not properly used. Further, where the dipstick is to be used in a road tanker, there is a danger to personnel who have to climb onto the vehicle.

It has also been proposed to measure the contents of a tank by use of a probe positioned near the bottom of the tank and arranged to measure the hydrostatic head of liquid. The volume or mass contents of the tank can then be calculated from a knowledge of the tank dimensions and the density of the liquid. In one form of such apparatus, a small flow of air is caused to pass out of the end of the probe against the hydrostatic pressure and the pressure of air required to cause this flow is monitored and assumed to be equal to the hydrostatic pressure.

However, we have found that if the air flow ceases or if the rate of air flow is too high, erroneous readings of the hydrostatic pressure can result.

According to the invention there is provided apparatus for measuring the contents of a container of liquid comprising a probe arranged to be positioned in the liquid for monitoring the hydrostatic pressure of the liquid, means for supplying a flow of gas to the probe, means for measuring the pressure of gas supplied to the probe, and means for monitoring the rate of flow of the gas supplied to the probe. The rate monitoring means may be arranged to prevent the pressure measuring means from giving an erroneous indication when the gas flow ceases or the flow rate is too high.

The gas supplied to the probe may be caused to bubble through a liquid, which may be the liquid in the container or a liquid in another container remote from the probe, and the gas flow rate may then be monitored by monitoring the rate at which bubbles are formed. The bubbles may be observed directly, or alternatively or in addition the bubbles may be detected optically or electrically, or by monitoring the small variations in pressure as the bubbles are formed.

For example, a twin optical fibre may be passed down a probe which is positioned in a storage tank and from which a small flow of gas bubbles. The twin fibre may comprise a sender and receiver and the amount of reflected or transmitted light changes when a bubble is formed. The reflected or transmitted light may be detected by an opto-electronic detector remote from the storage tank.

Alternatively, the air may be passed through a detector unit remote from the tank on its way to the probe and the bubbles may be detected optically or electronically in the detector unit.

Preferably the bubbles are formed in a conductive liquid and a pair of electrodes are disposed within the liquid such that the impedance therebetween varies as bubbles are formed. An electronic circuit may then produce a signal representative of the bubbling rate in response to the impedance changes.

The circuit preferably comprises an oscillator arranged to supply an alternating signal to the electrodes at a frequency higher than the bubbling rate. This higher frequency signal is then modulated by the varying impedance caused by the bubbles and a detector circuit may be arranged to detect the modulation. The detected output may be converted to a signal representative of the frequency of the bubbles whereby too high or too low a gas flow rate may be indicated. Alternatively, the circuit may be arranged to provide an output when a flow above a predetermined minimum exists but not otherwise.

In an alternative embodiment a pulse signal is derived on the occurrence of each bubble and this may be supplied to digital circuitry arranged repeatedly to count the pulses over a predetermined time whereby an indication of the flow rate may be obtained.

Alternatively, a satisfactory flow of gas can be detected from the small oscillations in pressure of the gas that occur as bubbles are formed and then become detached from the probe near the bottom of the tank. The frequency of these oscillations can be measured by the apparatus monitoring the hydrostatic pressure and since the frequency is dependent on the bubble rate, the gas flow rate can be measured.

The apparatus of the invention may be used with advantage for measuring the contents of a fuel storage tank. Preferably two probes are provided in the tank, one near the bottom of the tank and one in the ullage space. The depth of the liquid may then be determined by subtracting the pressure above the liquid from the hydrostatic pressure near the bottom of the tank. A third probe may be provided at a known intermediate level in the liquid and the density of the liquid may then be determined by subtracting the hydrostatic pressure at the third probe from that near the bottom of the tank. The calculation of the density and of the contents of the tank is preferably performed by a suitably programmed microprocessor and the output from the flow rate monitor is supplied to the microprocessor for generating a warning indication when the gas flow rate is too high or ceases.

Some embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a fuel storage tank provided with a contents measurement apparatus in accordance with the invention; Figure 2 shows a bubble monitoring device of the apparatus of Figure 1; Figure 3 is a circuit diagram of the bubble rate indicating circuit associated with the device of Figure 2; and Figure 4 is a sectional view of an optical bubble monitoring device.

Referring first to Figure 1, there is shown a fuel storage tank 1 containing fuel 2. A probe 3 is positioned in the tank with its end near the bottom of the tank. A probe 4 is positioned to monitor the pressure in the ullage space 5. The probe 3 is supplied with a small flow of air from an air supply 6 via a constant flow device 7 and a bubble detector 8. The probe 3 has an open end so that the pressure within the probe is equal to the hydrostatic pressure at the lower end of the probe 3. This pressure is communicated to one side of a differential pressure transducer 9 and the pressure in the ullage space 5 is communicated to the other side of the pressure transducer 9. The output of the pressure transducer 9 is representative of the hydrostatic pressure due to the column of liquid above the lower end of the probe 3.The differential pressure transducer 9 may be of a commercially available type which employs a flexible diaphragm coupled to four strain gauges connected in a bridge arrangement which is unbalanced when unequal pressures exist on the two sides of the diaphragm whereby an electrical output dependent on the pressure difference is produced. The electrical output of the transducer 9 is connected to a control unit 10 which may incorporate a microcomputer.

A further probe 11 is disposed in the tank with its open end at a level intermediate the ends of the probes 3 and 4. The probe 11 is similarly supplied with air from the source 6 via a constant flow device 12 and a bubble detector 13. The pressure sensed by the probe 11 is communicated to a differential pressure transducer 14 and the pressure of the probe 4 is communicated to the other side of the pressure transducer 14. The electrical output of the transducer 14 represents the hydrostatic pressure due to the liquid above the open lower end of the probe 11 and is similarly coupled to the control unit 10.

The control unit 10 is arranged to display numerically the volumetric contents of the tank 1.

The vertical distance between the open ends of the probes 3 and 11 is accurately known and so by subtracting the outputs of transducers 9 and 14 the density of the liquid 2 may be calculated.

Given the density and the pressure signal produced by detector 9, the control unit can calculate the depth of the liquid and thus the volumetric contents may be calculated from the known dimensions of the tank 1. The numerical indication is prevented and an error warning is given whenever the bubble detector 8 indicates that an insufficient flow of gas is occurring. The detector 1 3 operates similarly to prevent erroneous density calculations from being made.

In an alternative arrangement, the pressure in the probe 3 may be communicated to the right hand side of the differential transducer 14 as shown in Figure 1 instead of the pressure in the probe 4. The transducer 14 then gives directly an output representative of the difference in the pressures at the ends of the probes 3 and 11 for use in the density calculation.

The bubble detectors will now be described with reference to Figures 2 and 3. Referring to Figure 2, the bubble detector 8 comprises a transparent bowl 21 with a sealing closure 22 and containing a conducting liquid 23, e.g. water with a suitable additive. A small flow of gas from the constant flow device 7 enters the detector via tube 24, the gas bubbles through the liquid 23 and exits via tube 25 to be supplied to the probe.

A pair of electrodes 26 and 27 are positioned in the liquid 23 such that the impedance therebetween varies as bubbles are formed.

Figure 3 shows the circuit for monitoring the impedance variations. It comprises an oscillator 31 connected to the electrodes 26 and 27, a detector 32, an amplifier 33, and a further detector 34. The oscillator 31 comprises an amplifier IC1 provided with positive feedback via resistors R1 and R2. The inverting input is connected to a capacitor C1 which alternately charges and discharges via resistor R3 to produce an output signal at a frequency of about 50 kiloHertz. The output signal is coupled to the electrodes via a capacitor C2. The detector 32 comprises diodes D1 and D2, capacitor C3 and resistor R4.The circuit time constant is such that the voltage appearing at the junction of diode D2 and capacitor C3 is an alternating low frequency signal corresponding to the impedance changes between the electrodes 26 and 27 caused by the bubbles. The output of the detector 32 is coupled by a filter consisting of capacitor C4 and resistor R5 to amplifier 33 comprising integrated circuit IC2 and resistors R6 and R7 and the amplified signal is applied to capacitor C5 via diode D3 and resistor R8. The voltage on capacitor C5 is applied to the base of transistor Trl via resistor R9. TransistorTrl is connected as an emitter follower and the output signal at the emitter of the transistor which is coupled to the control unit 10 follows the voltage on the capacitor C5. Capacitor C5 charges when the output of amplifier 33 is positive and discharges via resistors R9 and R10 and transistorTrl between positive transitions of the amplifier output signal. Thus if there is a long delay between such positive signals, indicating a long delay between bubbles, capacitor C5 discharges and a low output signal results. Thus the circuit indicates when the bubbling rate is below a predetermined minimum. If the time constant of the detector 34 is suitably chosen, the output voltage may vary in accordance with the frequency of the bubbles and a subsequent level detecting circuit may indicate when the bubbling rate and hence the gas flow is too high.

In an alternative embodiment, the output of the detector 32 may be connected to a pulse-forming circuit, e.g. a Schmitt trigger, to generate pulses corresponding to the formation of the bubbles.

These pulses may be processed in digital circuitry e.g. including a counter, whereby a digital indication of the flow rate may be obtained.

In an alternative embodiment, a twin optical fibre may be positioned in place of the electrode 26, one fibre being arranged to supply light and the other to receive light reflected from the bubbles. An opto-electronic detector may then produce output pulses each representing the formation of a bubble. The fibres could be positioned at the end of the probes 3 and 11 and such an arrangement would be acceptable even where inflammable liquids are involved because no electrical power need be supplied to the probes. All of the electrical circuitry would be remote from the storage tank in a flameproof enclosure.

Referring to Figure 4, one optical method for monitoring the bubble rate, which has proved satisfactory in operation, is to use the difference in the refractive index of air and the liquid contents of the tank. Two optical fibres 41 and 42 (the sender and receiver) run parallel beside or inside the probe and terminate where the bubbles form at the end of the probe. A small flat section at the side of the end of each fibre is cleaned and the two fibres joined by optically clear cement 43 to provide an optical path between sender and receiver. The ends of the fibres are cut at an angle at 44 and 45 so that, in air or gas, the light is internally reflected from sender to receiver. One cut face, e.g. 45, may be silvered with advantage.

When immersed in liquid the other angled face (or both if unsilvered) will no longer internally reflect the light, causing a reduction in light intensity transmitted along the receiving fibre. Hence the formation of bubbles in the liquid can be monitored.

In operation of the apparatus during filling of the tank 1, a pressure signal representative of the depth of the liquid is supplied to the control unit 10 by the transducer 9. The control unit 10 assumes a value for the density of the liquid 2 so as to be able to give an indication of the volume and contents of the tank 1. When the level of the liquid rises above the end of the probe 11, a pressure signal is supplied to the control unit 10 by the transducer 1 4. The control unit is preprogrammed with the vertical distance between the ends of the probes 3 and 11 as well as the dimensions of the tank, and so it is then enabled to calculate accurately the density of the liquid 2. The accurate density value is then used in producing the numerical indication of the tank contents and the filling operation may be stopped when the desired quantity of liquid has been put in the tank. During subsequent discharging operations, the same density value may be used or it may be measured again from time to time, and would be measured again when the tank is refilled.

Claims

1. Apparatus for measuring the contents of a container of a liquid comprising a probe arranged to be positioned in the liquid for monitoring the hydrostatic pressure of the liquid, means for supplying a flow of gas to the probe, and means for monitoring the rate of flow of the gas supplied to the probe.

2. Apparatus as claimed in claim 1 wherein the said means monitoring the rate of flow of the gas supplied to the probe comprises means for monitoring the rate of formation of bubbles of the gas in the said liquid.

3. Apparatus as claimed in claim 1 wherein the said means for monitoring the rate of flow of the gas supplied to the probe comprises means for passing the said gas through a liquid in a detector unit on its way to the probe and means for monitoring the rate of formation of bubbles of the gas in the liquid in the said detector unit.

4. Apparatus as claimed in claim 2 or claim 3 wherein the said means for monitoring the rate of formation of bubbles operates by detection of variations in optical, electrical or pressure characteristics which result from bubble formation.

5. Apparatus as claimed in claim 4 wherein the said means for monitoring the rate of formation of bubbles operates by detection of variations in transmitted or reflected light in the region of bubble formation.

6. Apparatus as claimed in claim 5 wherein the said means for monitoring the rate of formation of bubbles comprises a light source optically connected to a light detection means by two optical fibres, the said fibres being optically connected by a partially reflecting means positioned in the region of bubble formation, whereby light from the said source is passed down one of the said fibres onto the partially reflecting means by which it is reflected down the other said fibre to the light detection means or through which it passes to enter the liquid in the region of bubble formation.

7. Apparatus as claimed in claim 6 wherein the said partially reflecting means comprises two mutually angled reflecting surfaces of an optically clear material one of which surfaces is silvered.

8. Apparatus as claimed in claim 4 wherein the said means for monitoring the formation of bubbles comprises two electrodes disposed in the liquid about the region of bubble formation and means for detecting changes in impedance between the said electrodes.

9. Apparatus as claimed in claim 8 wherein the said means for monitoring the formation of bubbles further comprises an oscillator arranged to supply an alternating signal to the said electrodes at a frequency greater than the rate of bubble formation and a detector circuit arranged to detect the modulation in the signal resulting from impedance variation on bubble formation.

10. Apparatus as claimed in claim 4 wherein the said means for monitoring the formation of bubbles comprises means for detecting oscillations in the pressure of the gas which occur on bubble formation.

11. Apparatus as claimed in any one of claims 1 to 10 wherein a second probe is arranged at a position above the surface of the liquid in the said container by which the pressure above the liquid may be monitored.

12. Apparatus as claimed in any one of claims 1 to 11 wherein a further probe is arranged at an intermediate level within the said container by which the hydrostatic pressure at that intermediate level may be determined, whereby the density of the liquid in the said container may be calculated.

1 3. Apparatus for measuring the contents of a container of a liquid substantially as herein described with reference to the accompanying drawings.