GB2180939A - Equipment for detecting when the level of a liquid in a container drops below a predetermined level - Google Patents

Abstract

Equipment for detecting when the level of a liquid (e.g. "Real Ale") in a container (C) drops below a predetermined level is disclosed. The equipment comprises a probe (A) for insertion into the container, the probe defining first and second electrodes which are remote from the top of the container; means (C) for monitoring the electrical resistance across the electrodes; and means (D) for disabling the supply of liquid from the container in response to the monitoring means indicating that the resistance monitored across the electrodes is no longer characteristic of the presence of liquid forming a connection between the electrodes. The control means (C) may include a 6kHz square-wave oscillator for energising the probe. The probe may be provided with means for adjusting the depth of the electrodes, for adaptation to a variety of containers. <IMAGE>

Description

SPECIFICATION Equipment for detecting when the level of a liquid in a container drops below a predetermined level According to the present invention, there is provided equipment for detecting when the level of a liquid in acontainerdropsbelowa predetermined level,com- prising: a)a probeforinsertion into such a container, the probe defining first and second electrodes which are remote from the top of such a container in use of the equipmentwhenthe probe is inserted intothecontainer; b) means for monitoring the electrical resistance across the electrodes; and c) means for disabling the supply of a liquid from such a container in response to the monitoring means indicating that the resistance monitored across the electrodes is no longer characteristic of the presence of such liquid forming a connection between the electrodes.

The present invention will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a diagram ofafirstform of equipment for detecting the level of beer in a cask; Figure2isaschematicofacontrol unitoftheequi- pment; Figure 3 is a diagram of a modification of the equipment of Figure 1; Figure4 is a schematic of a control unit of the equipment of Figure 3; Figure 5is a diagram ofadevelopmentofthefirst form of equipment; Figure 6is a schematic of a control unitoftheequi- pment of Figure 5; and Figure7isa diagram of a probeassemblyofthe equipments of Figures 1,3 and 5.

The purpose ofthe two forms offirst equipment to be described isto detect when the level of beer in a cask hays fallen to a pre-determined level and then to operate a device that will prevent any more beer being drawn from the cask. Such an equipment would prove valuable in the dispensing of "real ale".

Real ale casks contain a proportion of sediment, and by setting the detector to operate above the level of this sediment, it is possible to automatically prevent beer containing sediment from entering into the pipeworkand being dispensed.

Adevelopmentoftheequipmentwill also be des- cribed which will, in addition to preventing the dispense of beer from a depleted cask, connect automatically a second, full cask to the system. Such equipment would allow the continuous, uninterrupted dispensing of clear beer.

Referring to Figurel,thefirstequipment,usinga solenoid valve, operates as follows: 1. A probe A is placed inside the cask B and detects whether the level of the liquid is above or below a predetermined level. The probe A, in conjunction with a control unitC,doesthis bydetectingthe change in electrical conductivity when the probe is uncovered by the beer, i.e. when the base of the probe is immersed in the beer an electrical circuit between the two electrodes in the probe is completed by the beer, and when the beer is below the probe level a high resistance circuit results.

2. This high resistance condition is recognised by an electronic circu it in the control u nit C. The control unitthen energises a normally-open solenoid valve D situated in the outlet pipework E of the cask. Energising the solenoid causes it to close, thereby cutting off theflow of beer.

3. The height of the probe within the cask determines the level at which the equipment operates to stop the beerflow.

4. Figure 2 is a schematic of the electronic circuit of the control unit. Thisfunctions asfollows: a) High voltage from a mains power supply connected to terminals 5 and 6 is converted by trans formerTI to a low a.c. voltage. This is rectified by a diode D5 and smoothed by a capacitor C5 to provide a d.c. supply which its further reduced and regulated bya network comprising a resistorRII and diodes D1, D2 and D3to provide supplies of approximately + 5V and -5V with respect to the reference line, terminal 2. These supplies are used to power operational amplifiers Al, A2, A3 and A4 (the connections are not shown on Figure 2 to aid clarity).

b) Am pl ifier Al,tog ether with resistors R2, R3 and R4, and a capacitorCl,form a free-running, square waveoscilatorrunningatabout6 KHZ. This a.c.

signal isappliedtotheprobethrougha resistor R5 and a capacitor C2. Capacitor C2 ensures that no d.c.

component is applied to the probe connected toterminals 1 and 2. A pure a.c. signal is used to overcome plating problems incurred by using a d.c. source.

c) The probe may be considered as a variable resist ancedevicehavinga a low value of resistance when immersed in the beer and a high value of resistance when not immersed. The amplitude ofthe a.c. signal across the probe is larger when the probe resistance is high than when it is low. AmplifierA2 operates as a differential comparator. When the probe resistance is low the positive voltage excursions on the inverting input of amplifierA2 do not exceed thed.c.

threshold level on the non-inverting input. The output of the amplifier therefore remains at a constant high positive level. When the probe resistance is high,the positive voltage peaks on the inverting input of A2 exceed the threshold level on the non- inverting input. The output of the amplifiertherefore switches to a negative voltage, i.e. when a high resistance is present at the probe terminals, the output of amplifierA2 is an a.c. signal with afrequency equal tothat of the signal from amplifierA1. Diode D2 sets the approximate level of the threshold voltage.

Resistors R5 and R6 set the sensitivity, i.e. the probe resistance at which the circuit differentiates between beer and no beer.

d) Resistors R8 and R9, a diode D4 and a capacitor C3 form a filter network with the time constant for negative going excursions significantly less than for positive going excursions. Thus when the output of amplifierA2 is a continuous high positive voltage, the voltage on the inverting input of amplifierA3 is a similar high positive voltage. When the output of A2 becomes an alternating signal, however, the voltage on the inverting input of amplifier A3 drops to a substantially constant low negative voltage.

This change is detected by amplifierA3 and the signal polarity is inverted at the output of amplifier A3. Resistors R1 and R7 are a feedback network which slightly modify the threshold voltage atthe positive input of amplifierA2. This ensures rapid switching at the transition point.

e) A resistor R10 and a capacitor C4 provide further filtering and help guard against electrical interference causing spurious operation. The output of amplifierA4changes from a low negative to a high positive voltage when the probe resistance goes high. This action through a resistor R12 and a diode D6 causes a TRIAC 1 to change from the nonconducting state to the conducting state, thereby energising the solenoid valve connected to terminals 3 and 4. A diode D6 is of the light emitting typeand provides an indication in the "no beer" state.

The principle of operation of a form ofthefirst equipment when used with an electrically operated beer pump or meter is very similar to the operation as described previously for a solenoid valve. The es sential difference is that with a beerpump or meter the electrical supply to the pump or meter is simply removed when the beer fall below the predetermined level. This action is sufficientto prevent thefurther dispense of beer making the use of a solenoid valve to shut off the beerflow unnecessary.

Figure 3 shows the arrangement with an electric motor F driving a pump G . The electrical supply line to the motor F passes through a relay contact in the control unit C and this relay contact opens when the beer level drops to the set point.

Figure 4 shows a schematic of the control unitfor this form of equipment. It operates in a very similar mannerto the circuitof Figure 2, exceptthatthe output of amplifierA4 switches a transistor TR1 rather than a triac. This transistor energises a relay coil RL whose normally closed contacts RU1 are used to in terruptthe motor or beer meter supply.

A development ofthe equipment is shown in Figure 5. In this arrangement a second, full cask H is connected through a second solenoid valve J to the primary pipework E via a tee junction K. Whilst the level ofthe beer in the primary cask B is above the preset level of the probe A, the control unit C maintains solenoid valve J energised, i.e. closed, and sol enoid valve de-energised, i.e. open. the beerdis pense equipment is thus supplied from the primary cask B. When the beer level drops below the preset level ofthe probe A, the control unit C energises sol enoid valve D, so shutting off the supply from the primary cask B. This action is identical to the princ iple described previously.In addition, solenoid valve J is de-energised so thatthe beer dispense equip ment is now supplied from the secondary cask H. The supply of beer to the dispense equipment is ef fectivelycontinuous and uninterrupted.

The exhausted cask can be removed and replaced at anytime before the secondary cask is also exhaus ted. This is done byfirst closing casktap L. The probe A is then transferred to the second cask H, and a switch on the control unit operated to interchange the operation ofthe solenoid valves D and J. If cask B is now exchanged for a fresh, full cask, dispensing can continue and the system will automaticaliy change casks when cask H is exhausted. The operation is identical to that described above exceptthat cask H is nowthe primary cask with Bthesecondary cask.

Aschematicforthe control unit of Figure 5is shown is shown in Figure 6. It differs from Figure 2 by the addition of an extra amplifier AS, TRIAC 2 and changeover switch SW 1. Amplifier A5 inverts the output of amplifierA4, and its output is fed to the gate of TRIAC 2. Therefore, when TRIAC 1 is in the conducting state, TRIAC 2 is in the non-conducting condition and vice-versa. The changeover switch SW 1 selects which output terminal 9 or loins connected to TRIAC 1 and which is connected to TRIAC 2. If normally-open solenoid valves are used as previously described, then switch SW 1 should be positioned so thatTRIAC 1 is connected to the solenoid valve in the outlet of the cask in which the probe is placed.

Aconstructionforthe probeAisshown in Figure7, and comprises an assembly of an electrically conductive body 11 forming one electrode; anotherelectrode 12 at one end of the probe body, insulated by insulators 13 and 14; a removable electrical connector 15 atthe other end ofthe body 11; a collarcomprising a portion 17 slidable along the body 11 and having a portion 18 screwed on to itto be adjustable in position relative to portion 17, the latter having radial grooves 19to allowventing ofthecask; and scales 20 each with a zero point marked on the body 11, only one being shown.

The probe assembly is designed for use with various sizes of casks. This is important since in the brewery industry for example, several different capacitycasks are used, i.e. 9gallons, 18 gallons, 36 gallons and 54gallons.

Afeature of the assembly is that the base of the cask is used as a reference point with the probe electrode being positioned a known distance (hence cap acity) from the cask base. In orderto achieve this, the probe assembly is placed into the cask with the portion 17 seated on the casktop. The probe body 11 is free to slide through the collar and is pushed through the collardown until the bottom insulator 13 touches the cask base.

Each of scales 20 reiates to a particular cask size.

Using the appropriate scale, the portion 18 is unscrewed until thetop side ofthe collar lines upwith zero on the correct scale. The body 11 is now carefully withdrawn upwards through the collar. The scales indicate volumes of liquid which will be left in the caskwhen the electrode of the probe assembly is uncovered. When the desired volume of liquid which is to be left in the cask is shown, the assembly has been correctly set.

Although the probe body isfreeto slidethrough thecollarthere is sufficient friction to preventaccidental movement afterthe assembly has been set.

The collar can be completely removed and the body can be easily cleaned with detergent without adverse effect.

Claims (11)

1. Equipmentfordetecting when the level ofa liquid in a container drops below a predetermined level, comprising :- a) a probe for insertion intosuchacontainer,the probe defining first and second electrodes which are remote from the top of such a container in useofthe equipment when the probe is inserted into the container; b) means for monitoring the electrical resistance across the electrodes; and c) meansfordisabling the supply of a liquid from such a container in response to the monitoring indicating that the resistance monitored across the electrodes is no longer characteristic of the presence of such liquid forming a connection between the electrodes.

2. Equipmentaccordingtoclaim 1,adaptedfor use with a container wherein such liquid is beer.

3. Equipmentaccording to claim 1 or2,wherein the monitoring means comprises: a) meansforapplying an alternating signal across the electrodes; and b) means for detecting a change in the alternating signal due to a change in the resistance across the electrodes.

4. Equipment according to claim 3,wherein the alternating signal isa square wave signal.

5. Equipmentaccordingtoclaim3or4,wherein the detector means comprises means for detecting a change in the amplitude of the alternating signal due to a change in the resistance across the electrodes.

6. Equipmentaccording to any preceding claim, wherein the disabling means comprises a solenoid valve.

7. Equipment according to any of claims 1 to 5, whereinthedisabling means comprises meansfor disabling a liquid pump or meter.

8. Equipmentaccordingtoanyofclaims 1 to5, wherein the disabling meansfurthercomprises means for connecting a further container of liquid to an outlet in response to the monitoring means indicating that the resistance across the electrodes is no longer characteristic ofthe presence of such liquid forming a connection between the electrodes.

9. Equipment according to any preceding claim, wherein the probe comprises: a)aprobebody b) collarthrough which the probe body is slidable, the collar having a first portion which can bear on the top of such a container and a second portion longitudinally adjustable in position relative to the first portion; and c) a scale marked on the probe body and against which the second portion is adjustable.

10. Equipment according to claim 9, wherein the probe body comprises one of the electrodes, the other of the electrodes being electrically insulated therefrom.

11. Equipment for detecting when the level of a liquid in a container drops below a predetermined level, substantially in accordance with any example herein described with reference to the accompanying drawings.