GB2188152A

GB2188152A - Calibration in liquid level sensing

Info

Publication number: GB2188152A
Application number: GB08706649A
Authority: GB
Inventors: David Warburton
Original assignee: PROCONTECH Ltd
Current assignee: PROCONTECH Ltd
Priority date: 1986-03-21
Filing date: 1987-05-20
Publication date: 1987-09-23
Also published as: AU7044987A; GB8607111D0; EP0246738A1; GB8706649D0

Abstract

A liquid (e.g. petrol) level sensor 112 emits a signal from a sonic emitter 114. When the signal reaches the end of an outwardly diverging metal cone 130 it disturbs the air located over the end of a dividing wall 132 of the cone, and a receiver 116 recognises that disturbance. The time taken between the signal being omitted and received over the fixed distance is used to give an indication of the speed of sound in the environment. The emitter 114 then emits a further signal which is reflected back off the surface of a liquid to the receiver 116, and the time taken for that further signal to be emitted and received is adjusted in accordance with the time taken for the previous signal to travel the fixed distance in order to give an accurate indication of the liquid level. A further embodiment uses a reflector located between the emitter and the liquid surface, Fig 1. (not shown). <IMAGE>

Description

SPECIFICATION Liquid level sensing The present invention relates to a liquid level sensor and to a method of sensing the level of a liquid and is particularly, although not exclusively applicable to the sensing ofthe level of liquid such as petrol in a storage tank.

A previous proposal for sensing the level of petrol in a storage tank has included a sensor comprising a sonic emitter and receiver located adjacent to each other above the level of petrol. The emitter sends a sonic signal towards the surface ofthe petrol which is reflected from the surface backto the emitter. The time delay between the signal being emitted and received gives an indication of the distance ofthe surface ofthe liquid from the top of the tank, and from that reading an estimate of the amount of petrol in the tank can be arrived at. However, the prior devices are very inaccurate, particularly when attempting to read the level of an almostfulltank.

That inaccuracy derives at least in part from the variation ofthe speed of sound due to temperature changes and the composition of the gas which the signal has to pass through between the emitter and the receiver. That inaccuracy may amount to a reading being 15% out from the correct one.

According to one aspect of the present invention in a liquid level sensor including an emitter and a receiver, the receiver is arranged to recognise a signal from the emitter afterthe signal has been recognised from a fixed distance or from the surface of a liquid. With such a sensor, as the fixed distance is known, the time taken forthe signal to pass that distance can be used in the calculation or obtaining of a fairly precise reading of howfarthe level ofthe liquid is from the sensor, and accordingly how much liquid is present, that reading being compensated for despite possible variations in the temperature and environment which the signal hastopassthrough which may alterthe speed ofthe signal.The accuracy ofthe sensing may be within 0.01% ofthe actual reading or in the region of, or less than 2%ofthe actual reading.

The fixed distance from which the signal may be recognised may extend between the emitter and the surface ofthe liquid. The sensor may include means for determining when a signal has been recognised from the fixed distance orthe surface of the liquid, and said means may comprise being able to determine when the signal has first been recognised from the fixed distance or the surface of the liquid, and said means may comprise being able to determine when the signal has first been recognised from the fixed distance or has subsequently been recognised from the surface ofthe liquid.

The fixed distance over which the signal may be recognised may be arranged to be provided by a reflector from which the signal may be reflected. The reflector may be afforded by a nylon or metal member. Alternatively, the fixed distance may be comprised by the emitter receiving a signal resulting from gas being disturbed, or i interference being induced in the gas at a fixed location.

The liquid level sensor may be included in a system arranged to increase the level of liquid when the sensed level reaches, orfalls below a predetermined amount.

According to a further aspect of the present invention, a method of sensing the level of a liquid comprises emitting a signal from an emitter and recognising that signal from a fixed distance by a receiver, and receiving a signal by the receiver from the emitter which has been reflected from the surface ofthe liquid, thetime taken forthe signal to be recognised as having passed the fixed distance being used to derive an indication ofthe level ofthe surface ofthe liquid as a result ofthe time taken for the signal to be reflected back from the surface ofthe liquid.

The method maycomprisefirst emitting asignal from the emitter which is recognised from a fixed distance by the receiver, and subsequently emitting a signal from the emitterwhich is received by the receiver having been reflected from the surface of the liquid.

The method may comprise continuously or repeatedly sensing the level of the liquid. The level may be sensed at regular or irregular intervals, oron demand.

The invention may be put into practice in various ways, but two embodiments will now be described byway of example and with reference to the accompanying drawings, in which Figure 7 is a schematic sectional view of part of a petrol storage tank 10 including a first embodiment of a level sensor 12, and Figure2 is a sectional view of a second embodimentofa level sensor 112.

The sensor includes a sonic emitter 14, a receiver 16 and a reflector 18 located beneath and between the emitter and sensor. The liquid 20 is located in the tank and the surface is indicated at 24. The reflector may comprise or include a nylon or metal member surface from which the sound is reflected.

In use, on each occasion that the level isto be read, theemittersendsa sonicsignal outof an outwardly diverging passage 26 which is reflected by the reflector 18 through an inwardly converging passage 28 to the emitter. As the distance which that signal passes is fixed, the time taken for that signal to pass that distance is used to give an indication of the speedofsound inthetankfortheambient temperature and gas properties. Afurther signal is then emitted and reflected to the receiver from the surfaceoftheliquid,andthetimetakenforthat signal to leave the emitter and reach the receiver is used to give an indication ofthe level ofthe liquid in the tank. The receipt ofthatfurther signal from the reflector prior to receipt of the reflected signal from the surface is ignored.

In Figure 2,the level sensor 112 includes a sonic emitter 114, a receiver 116, and a downwardly and outwardly diverging metal cone 130 of 200mm length including a downwardly tapering dividing wall 132 of 5mm width separating the emitter from the receiverthroughoutthe extent of the cone. The level sensor 112 is arranged to be mounted in the top wall of a tank with the cone being directed towards the liquid contained therein.

In use, a signal is given from the emitterwhich travels down the cone on one side of the dividing wall 132. When the signal reaches the open end of the cone it disturbs the normally still air in thetank, and that disturbance of the air, or interference, disturbs the airoverthe open end ofthe cone leading to the receiver 116. That disturbance is picked up by the receiver. Given that the distance from the emitter to the receiver is fixed and known, the time taken between a signal being emitted and one being received can be used to give a calibration reading for the speed of sound in the tank under the conditions in the tankatthattime.

The emitterthen gives a subsequent signal which is reflected off the surface ofthe liquid and returned to the receiver (the initial signal returned to the emitter as a result of the disturbance or interference atthe end of the cone is ignored). The time taken for the reflected signal to reach the receiver from the emitter can then be corrected in accordance with the time taken for the previous signal to travel the fixed distance in order to give an accurate indication of the level ofthe liquid.

As a result ofthe "test" signal to the reflector occurring prior to a signal being reflected from the surface, extremely accurate readings relevant to the level of the liquid are able to be given, by ofthe then known calibration of the probe or cone reflected upon the distance indicated from the surface reflector even where the tank is full and the distance through which the reflected signal from the surface has to travel is extremely small. In fact, accuracy of 0.01 % or in the region of 2% or less is possible.

Eitherofthe sensors may be connected to a control unitwhich may be some distancefromthetank. In Figure 2, part of the control unit is shown, and includes a printed circuit board 134 located within a housing 136. The control unitmaybeableto calculate the amount offuel in thetank once it knows the type of tank, such as rectangular, square, cylindrical (horizontal orvertical), spherical or graphic. Graphic tanks are described herein as being those whose shapes are not symmetrical and whose volume may have to be calculated in, for instance, 100 graduations of level, with approximate readings being given for level readings coming between the graduations.

The reading forthe level of liquid in the tank may be in a volume, level orweightoutput.

The control unit may include timing means which can read the level at regular or predetermined times, this feature being particularly useful where the sensor is mounted in the storage tank of a petrol station where it may be used to record the level at the end of each day. Alternatively the level can be read on demand.

Reading means may be provided to record any information referred to herein.

The control unit may be arranged to record or indicate when the level reaches a predetermined high or low level, in which case means may be provided to shut off the supply of liquid to the tank or to add further liquid. The control unit may also be arranged to note and record when the level falls such that a predetermined volume of liquid, for instance 2 gallons, has leftthetank, and to notewhen subsequent falls in the level are occurring. This feature is particularly useful to sense when the tank is leaking (in which case regularfalls in the level will be recorded), or when an unattended tank is being pilfered, (in which case the falls will be irregular).

The control unit may be arranged to cooperate with a plurality of level sensors located in different tanks, and may be ableto perform anyofthe functions therein described in relation to all ofthe levels in those tanks automatically or sequentially or on demand.

Claims

1. A liquid level sensor including an emitter and a receiver, the receiver being arranged to recognise a signal from the emitter from a fixed distance orfrom the surface of a liquid.

2. Asensorasclaimed in Claim 1 inwhichthe fixed distance from which the signal may be recognised extends between the emitterandthe surfaceofthe liquid.

3. A sensor as claimed in Claim 1 or Claim 2 in which the sensor includes means for determining when a recognised signal has been received from the fixed distance orthe surface ofthe liquid.

4. A sensor as claimed in any of Claims 1 to 3 in which the signal is arranged to be recognised from the fixed distance by being reflected from a reflector.

5. A sensor as claimed in Claim 4 in which the reflector includes a nylon surface.

6. A sensor as claimed in Claim 4 in which the reflector includes a metal surface.

7. Asensorasclaimed inanyoflaimsl to3in which the fixed distance is arranged to be comprised by the emitter receiving a signal from gas being disturbed, or interference being induced in gas, at a fixed location.

8. A sensor as claimed in any preceding claim incorporated in a system arranged to increase the level of liquid when the sensed level reaches, orfalls below a predetermined amount.

9. A liquid level sensor substantially as herein described with reference to, and as shown in Figures 1 or 2 of the accompanying drawing.

10. A method of sensing a level of a liquid comprising emitting a signal from an emitter, and recognising that signal from a fixed distance by a receiver, and receiving a signal by the receiver from the emitterwhich has been reflected from the surface of the liquid, the time taken forthe signalto be recognised from the fixed distance being used to derive an indication of the level of the surface ofthe liquid as a result ofthetime taken forthe signal to be reflected back from the surface of the liquid.

11. A method as claimed in Claim 10 comprising first emitting a signal from the emitter which is recognised by the receiver from the fixed distance, and subsequently emitting a signal from the receiver which is received by the receiver having been reflected from the surface of the liquid.

12. A method as claimed in Claim 10 or 11 comprising recognising a signal from the fixed distance by causing disturbance of or inducing interference in gas at a fixed location.

13. A method as claimed in any of Claims 10to 12 comprising continuously sensing the level of the liquid.

14. A method as claimed in any of Claims 10to 12 comprising repeatedly sensing the level of the liquid.

15. A method as claimed in Claim 14comprising sensing the level ofthe liquid at regularintervals.

16. A method as claimed in any of Claims 10to 12 comprising sensing the level at irregular intervals.

17. A method as claimed in any of Ciaims 10to 12 comprising sensing the level on demand.

18. A method of sensing the level of a liquid substantially as herein described with reference to, and as shown in Figure 1 or 2 ofthe accompanying drawings.

19. A method of sensing the level of a liquid as claimed in any of Claims 10to 18when using a liquid level sensoras claimed in anyof Claims 1 to 9.