WO1997003394A1

WO1997003394A1 - Liquid level control system

Info

Publication number: WO1997003394A1
Application number: PCT/GB1996/001677
Authority: WO
Inventors: William Harry Kemp; Victor Joseph Shideler
Original assignee: Itt Industries Limited
Priority date: 1995-07-13
Filing date: 1996-07-10
Publication date: 1997-01-30
Also published as: GB9514338D0; GB2303226A

Abstract

A liquid level control system comprises a container (10) for receiving a flow of liquid. Level sensing means (14) is arranged to provide a control signal which is variable in dependence upon the depth of liquid in the container. A pump (18) is coupled with an outlet (20) from the container and is responsive to the control signal to expel liquid from the container at a rate which increases as a function of increase of liquid depth.

Description

LIQUID LEVEL CONTROL SYSTEM

This invention relates to a liquid level control syεtem and more particularly but not solely to such a system for controlling the level of milk in a tank of a milking machine.

Currently, milk is extracted from cows utilising a vacuum pressure system and receiver tank. When milk reaches a given level in the receiver tank, a pump is activated which pumps the warm milk from the receiver through a heat exchanger into a bulk refrigerated storage tank. The stored milk is then extracted by a pump located in a milk tanker truck for delivery to the processing plant.

Milk is a fragile material, containing a high fat/solids content. If the milk is pumped from the receiver tank too quickly by a standard centrifugal pump, the fat in the milk is separated and overall quality/value of the milk is decreased.

Further, if the milk is pumped too quickly through the heat exchanger, proper cooling of the milk is inhibited and refrigeration costs in the bulk storage tank are increased unnecessarily.

There is for this application a need to ensure that the receiver tank is not overfilled, during receipt of variable flows of milk, and to transfer the milk as slowly as possible through the heat exchanger. This invention has been arrived at by __consideration of these xequirements and has resulted in a liquid level control system which can be used in a milking machine but which is suitable for use in any application where the transfer of liquid from a receiving tank container is required.

According to the invention there is provided a liquid level control system, comprising a container for receiving a flow of liquid, level sensing means for providing a control signal variable in dependence upon the depth of liquid in the container and a pump coupled with an outlet from the container and responsive to the control signal for expelling liquid from the container at a rate which varies as a function of the liquid depth.

The pump may be an impeller pump and the impeller may be flexible. The pump may be driven by an electric motor at a speed which is variable in dependence upon the control signal to vary the rate at which liquid is expelled. The electric motor may be an AC induction motor. The induction motor may be powered by an inverter which provides a supply voltage and frequency, dependent upon the control signal, which is a function of liquid depth.

The induction motor may be powered by a power supply having an inverter and as described and claimed in US Patent Application Number 08/291,239 of the present inventor.

The system may include means for providing a control signal at switch on to run the motor at high speed for a predetermined time interval thereby to prime the pump.

The level sensing means may comprise a pair of spaced conductive probes extending downwardly or upwardly side by side in the container and wherein the resistance between the probes-which varies as a function of liquid depth is employed to provide the control signal. A further conductive probe of length shorter than said pair of conductive probes may be provided extending downwardly alongside the pair of conductive probes to determine when a predetermined liquid depth has been achieved upon detection of a resistive path through the liquid. Means may be provided for producing a signal representative of a predetermined intermediate depth of liquid upon detection by means of the further probe and to maintain that signal when the level drops so as to maintain an intermediate operating speed for the motor. In an advantageous refinement of the invention resistance change between said pair of probes between the level at which the liquid just touches the probes and the level at which the liquid just touches the further conductive probe is utilised to relate liquid depth to liquid conductivity. In one arrangement there is provided means coupled with said pair of conductive probes for deriving an electrical signal which is a function of the resistance between the probes, means for storing the amplitude of that signal when the liquid level rises to just touch the probe, means for storing the amplitude of that signal when the liquid level rises to just touch the shorter probe and means for calibrating the control signal within defined limits in dependence upon the two stored signals to adapt the control signal to the conductivity of liquid thereby to provide a substantially full pumping control range irrespective of conductivity of the liquid. In one configuration there is provided a resistor coupled in series with the pair of conductive probes, a pair of input lines for connection to a source of alternating current for feeding the series arrangement, an amplifier having an input fed by the voltage developed across the resistor, and a resistor to provide a d.c. analogue voltage output which is related to liquid depth. In a further refinement the analogue voltage output is fed to an analogue to digital converter, the two signal values are stored in digital form and are compared by a computer which provideε a control signal which is adapted to the conductivity of the liquid.

Alternatively, the level sensing means may comprise several liquid detectors arranged to detect the presence of liquid each at a different level in the container. The liquid detectors may be formed by float switches. Means may be provided for producing a signal representative of a predetermined intermediate depth of liquid upon detection by an intermediate one of the liquid detectors, and to maintain that signal when the level drops so as to maintain an intermediate operating speed of the motor.

The system may comprise a liquid falling level detector responsive to the control signal and operable to shut down the pump if a fall in liquid level does not occur within a predetermined time interval from switch on. The invention also includes a milking machine comprising a liquid level control system, as previously defined, wherein the container is a milk receiving tank, a vacuum pump is coupled to the tank and is operative to induce milk into the tank from milk cups for attachment to cows and the pump coupled with the tank outlet is coupled with a bulk milk storage tank. The coupling between the tank outlet and the bulk storage tank may be provided with a heat exchanger to effect cooling of the milk. The bulk milk storage tank may be refrigerated.

In order that the invention and its various other preferred features may be understood more easily, some embodiments thereof will now be described, by way of example only, with reference to the drawings, in which:-

Figure 1 is a simplified schematic illustration of a liquid level control system constructed in accordance with the invention,

Figure 2 is a schematic illustration of a receiving container provided with a three probe liquid level sensing arrangement usable in the construction of Figure 1,

Figure 3 is a schematic illustration of a receiver container having a range of liquid detectors arranged at different container depths to provide an alternative liquid level sensing arrangement useable in the construction of

Figure 1,

Figure 4 is a block schematic illustration showing a possible configuration of control signal generator coupled with the liquid level sensing arrangement of Figure 2, Figure 5 is a block schematic illustration showing an alternative configuration of control signal generator coupled with the liquid level sensing arrangement of Figure 3,

Figure 6 is a schematic circuit diagram of a refinement of part of the liquid level sensing system of the invention,

Figure 7 is a schematic illustration of a milking machine incorporating the principles of the invention and Figure 8 illustrates schematically the operation of the arrangement of Figure 7 in the automatic milk pumping mode of operation.

In the drawings similar components are given the same reference numerals.

Referring now to Figure 1 a container formed by a tank 10 is supplied with a liquid via an input pipe 12. A level sensor arrangement 14 extends through the top of the tank towards the bottom and is connected to a control signal generator 16 which is coupled to a variable speed pump 18. An outlet 20 from the bottom of the tank is connected to the input of the pump 18 which serves to pump liquid out of the tank at a rate which increases as a function of liquid depth as controlled by the control signal generator in dependence upon the level sensor arrangement, as will be described in further detail in connection with Figures 4 and 5.

Figure 2 illustrates one form of the sensor arrangement 14 which is influenced by the depth of liquid in the tank 10. This comprises a pair of spaced conductive probes 22, 24 which extend downwardly into the tank in spaced apart side hy side disposition. Alternatively -the probes could be arranged to extend upwardly from the base of the tank. The electrical resistance between these probes will vary as a function of liquid depth and this resistance can be utilised to generate a control signal. An optional third probe 26 can be mounted alongside and spaced from the probes 22, 24 extending downwardly into the tank. The probe 26 is of length shorter than the probes 22, 24 and its extremity is at an intermediate depth. The third probe can be utilised to determine when the liquid reaches the predetermined depth aε there is a sudden change of electrical resistance between it and one of the probes 22, 24, once liquid reaches that level. The conductive level probe is a series of 3 stainless steel probeε of equal length. One of the three probeε is cut to equal one half of the total tank height plus 2 inches. (For example, if the tank is 24 inches deep, the probe would be cut at of 24 inches plus 2 inches or 14 inches long) . Trimming the probe to thiε level informs the system of the half way point in the tank. The extra 2 inches allows for a "buffer" zone at the top of the tank, ensuring the pump is at full speed prior to the tank overflowing and proper operation of any full tank alarms.

Figure 3 illustrates an alternative form of the sensor arrangement 14 in which the tank 10 is provided with several liquid detectors 28 mounted in the tank at different depths in the container and responsive only when liquid level rises to that depth or beyond. One suitable form of detector is a float-actuated switch. In a preferred form the float level switches are logic. Extra Low Voltage, (ELV) , signal inputs which connect to a stack of three float switches. The float switches should be dry contact, normally open switches, which close in the presence of fluid.

Figures 2 & 3 are just two of many possible sensing .systems influenced by changing liquid depth and any form of sensing system which provides such information can be utilised within the scope of this invention for example a rheostat operated by a float valve, an optical beam generator and detector the path of which beam to the detector is diverted in the presence of a liquid.

The output or outputs from the liquid level sensor are coupled to the control generator 16 of Figure 1 which generates a signal which varies in dependence thereon and which is suitable for varying the rate of delivery of the variable speed pump. The pump is preferably driven by an electric motor and the control signal generator may generate a voltage or current which varies in amplitude and/or frequency as a function of depth. In the case of the sensor illustrated in Figure 3 the amplitude and/or frequency can be varied in a step wise fashion as each float switch is actuated or the change can be integrated to provide a gradual change in amplitude and or frequency between steps or the outputs from each sensor can form one input for a digital to analogue converter. It will be appreciated that altemative ways of controlling the del-rvery from a pump are possible and can be utilised in this invention. For example a pump device driven at constant speed in which the displacement is controlled as a function of the control signal or having a throttle valve which provides a restriction which is a function of the control signal.

Referring now to Figure 4 there is illustrated in simplified schematic form a possible configuration of the control signal generator utilising the liquid level εenεor of Figure 2. A proportional level sensor 30 has two inputs each coupled to a different one of the longer probes 22, 24. A voltage is generated by the sensor which is dependent upon the resistance between the two probes by for example connecting a high impedance current source across the terminals. The voltage developed by the current source is arranged to vary between 0 & V between empty and full depth in the tank. The output from the senεor 30 iε coupled to a maximum voltage detector 32. An intermediate level senεor 34 has one input coupled to one of the long probes 24 and another input coupled to the short probe 26. The resistance between the probes is very high in the absence of a liquid depth which bridges the probes but drops significantly when the liquid level rises to the level of the short probe. This resistance is utilised to provide a voltage which steps between substantially 0 and ?£V by for example connecting a resistance connection in series with a high impendence current source across the terminals and utilising the step in voltage to trigger a switch coupled between the output and a source of voltage V. The output is coupled to a second input of the maximum voltage detector 32. The output of the proportional level sensor is coupled to a fall detector 36 which is also coupled to an on/off switch 38. The fall detector is triggered at switch on and if it detects a falling voltage level within a predetermined time interval it provides an output control signal to a latch 40 connected to the output of the maximum voltage detector 32 the purpose of which will be further described. The on off switch is connected to a timer 42. The timer provideε an output voltage of V for a predetermined time interval from εwitch on of the switch 38 which voltage V is coupled to an input of^•the maximum voltage detector and also to the latch 40. When the system is switched on the timer 42 is triggered and actuates the latch 40 which connects the output of the maximum voltage generator 32 to the output 44. The timer also provides a voltage V to the maximum voltage detector 32 such that V appears at the output 44. This voltage causes the pump 18 to be run at maximum speed. The switch 38 also triggers the fall detector which determines whether the proportional level sensor 30 is detecting a fall in liquid level in the tank which shows that the pump has primed and is operating correctly. If this is the case then the detector provides a retaining signal which holds the latch on until the system is switched off. If no such signal is provided the latch drops out when the time period of timer 42 expires and the pump is shut down until the switch 38 is εwitched off and then on again. Provided that a fall in level iε—detected the pump continues to operate and provide a flow rate which is a function of the voltage at the output 44. If the level of liquid is greater than the intermediate depth then the proportional level sensor provides the highest voltage to the maximum voltage detector and this voltage is routed to the output 44 and the pump speed iε varied aε a function of changing depth. If the level drops below the intermediate level then the intermediate sensor provides the highest voltage which is fixed at V and the pump speed is maintained at a fixed low speed.

Referring now to Figure 5 there is illustrated a control εignal generator εimilar to that of Figure 4 but utiliεing the alternative liquid level sensor of Figure 3. In this arrangement the liquid detectors 28 are connected to different inputs of a proportional level senεor 44, which provideε variable output voltage increaεing in dependence upon depth, and an intermediate one of the detectorε 28 iε connected to the intermediate level sensor 45 to trigger an output voltage of W if the liquid does not reach the level to actuate that sensor. The output signals are coupled to the remaining blocks which operate in a manner similar to that described in connection with Figure 4.

The pump in the previously described embodiments can be an impeller pump and may have a flexible impeller. Such a pump is preferably driven by an AC induction motor fed by an inverter which provides a supply voltage and frequency which is dependent upon the control signal received from the output 42. In a particularly advantageous embodiment the induction motor is powered by a power supply having an inverter and aε described in U.S. Patent Application Number 08/291,239 of the present inventor filed 16 August 1994, the whole contents of which is incorporated herein by reference.

In a particularly advantageous refinement of the -invention the "half "height" probe is utilised to determine an X/Y graph, in εoftware, of the relationεhip between the probeε immersion in the liquid and the conductivity of the liquid. As a fluid becomes more conductive, a smaller amount of fluid is required to cause a proportional signal equal to that of a lesε conductive fluid. If a control microcomputer readε the conductivity value between the two long probeε at the exact moment in time that fluid reacheε the half height probe, a reference conductivity value can be determined. This allows proportional liquid level reading to be carried out over the entire length of the probe. For example, if the reference point for half height conductivity was fixed at say 50 ohms, a fluid with conductivity greater than 50 ohmε would actually reach the half height point in the tank earlier, than the "real" half height point. Likewise, fluid with less conductivity would have the reverse problem. In an uncompensated syεtem, only fluids with a conductivity exactly equal to a specific reference value would track the proportional level in accordance with the actual physical level. It can thus be determined that if the half point is not a fixed value but rather a variable data, then the control software could be arranged to adjust the value by reading the actual data when fluid physically touched the half height probe. This allows the syεtem to be adaptive to varying conductivity of fluid entering the receiver system. Referring now to -Figure 6 one method of implementing the refinement is illustrated. A transformer, 50 generates an alternating, safety-iεolated low voltage which is connected across the two long sensing probes 22, 24. A 1 ohm resistor 52 iε inserted in one leg of the tranεformer-probe circuit. As the probeε .are immerεed in liquid a minute voltage iε developed acrosε reεiεtor 52 which is proportional to depth of immersion and liquid conductivity. An amplifier 54 amplifies the developed voltage and feeds it via a rectifier circuit 56 to an analog to digital converter 58. The analog to digital converter converts the amplified, proportional voltage to an 8 bit numeric equivalent which is read by a microcomputer device 60. The half height probe 26 iε connected to a buffer amplifier 62 which provideε a digital output change of εtate when liquid touches the probe. This signal iε also rerouted to microcomputer 60. SOFTWARE AND CONTROL OPERATION: As previously described, the voltage developed across resistor 52 is dependent on the depth of immersion of the probes in liquid and the conductivity of the liquid. Assuming the receiver tank is empty, there will be no voltage across resistor 52 and the computer 60 will read a "null" output from the A/D converter. (Likewise) the half height probe will also signal no liquid to the microcomputer 60. When liquid just touches the bottom of the two long probes, a voltage is developed acrosε resistor 52, is amplified by the amplifier 54 and is converted by the A/D converter 58 and read by the microcomputer 60. This value is stored in memory as the "lower" point on an X/Y graph which will be more fully described. As liquid level increases in the receiver tank, the developed voltage across resistor 52 increases in linear proportion. When the liquid just touches the half height probe, amplifier/buffer 62 changes state and the microcomputer iε "notified" that the liquid level is now half way up the sensor probes 22, 24. Immediately, the microcomputer reads the developed voltage acroεε reεistor 52, via the amplifier 54 and A/D converter 58 as previously described. This second reading is stored aε the upper point on an X/Y graph. It can now be εeen that a liquid with higher conductivity will produce valueε for the "lower" and "upper" data pointε, greater than those of the first liquid. Similarly, a liquid with a lesεer conductivity will produce data valueε leεε than the first liquid. Regardless of liquid conductivity, the relationship between depth of probe immersion and conductivity is a linear function. Therefore, if the "lower" data point reading is considered the "zero" point on the probe and the "upper" data point the half height of the probe then all other physical points on the probe can be calculated. For example, if the lower reading is 20 units, the upper, (50% depth) , reading is 60 then 100% immersion would be a reading of 100 units. The relationship of probe immersion to pump speed may be software "coded" into memory and the appropriate output signalε from the microcomputer, 60 are fed to the INVERTER SPEED CONTROL OUTPUT. A milking machine employing the principles of the invention and based on the arrangements previously described is illustrated schematically in Figure 7. A milk receiver tank 10 has an input pipe 12 which is connected to a series of milk cups 70 for coupling to the udder of one or more cows. The tank iε closed and a vacuum pump 72 creates in the tank a suction which induceε flow of milk from the milk cupε-70 into the tank. The tank includeε a liquid level εenεor arrangement 14 which iε of the form illustrated in Figure 2 but which can be of the form illustrated and described in connection with Figure 3. From 1he bottom of the tank an outlet 20 is coupled via a pump 74, which is preferably an ITT Jabsco Food grade flexible impeller pump, via a heat exchanger 76 to a refrigerated bulk milk storage tank 70. The heat exchanger is supplied with a through flow of cooling water via inlet 80 and outlet 82. The pump is driven by a 3 phase 4 pole AC induction motor 84, the speed of the motor is controlled by a power supply 86 having an inverter as described in US Patent Application Number 08/291,239 and provided with a mains input 88 for a source of εingle or three phase power or US 2 phase 220 volt power. The power supply is controlled in dependence upon the milk level sensed in the tank 10 in the manner previously described in relation to Figures 1 to 6. A senεor alεo activateε a high milk level alarm 92.

The operation of the system is as follows:- 1. The syεtem is started by actuation of an ON/OFF switch (not illustrated in Figure 7) . 2. The pump remains off until milk reaches the shorteεt probe at the tank 1/2 level. When milk reaches this point, the pump will start on high speed for a maximum period of 20 secondε. 3. During the 20 second priming sequence. The system monitors the milk level in the receiver tank. If the level drops during priming, the prime sequence is complete and control passeε to

5 εtep 4. If the fluid level doeε not drop during the priming εequence, the prime failε. During priming, the flexible impeller pump running on high speed εhould be able to remove milk faster than the rate at which it enters the receiver

10 tank. If this doeε not happen, a failure iε assumed and the pump is shut off. The user may repeat the priming sequence by switching off and on again. Repeated priming failures may indicate leaking

15 plumbing between the milk receiver tank and pump or a defective pump. (Also note that succeεεful priming doeε not necessarily indicate that the pump is primed. If air is displaced from the intake lines by pump suction, milk level in the

20 receiver tank will drop. Depending on pipe size and diεtance between receiver and tank, suction flooding may take additional time) .

4. The pump speed will remain at low speed when milk is between the bottom and height of the

25 receiver tank. When milk level is between height and the tank "virtual full" level, the pump will "track" the milk level detected by the probes in the receiver tank. (When the tank is full) , the pump will run at maximum εpeed of 50 hertz. Aε the tank level dropε, .pump speed will drop to a minimum speed of 20 hertz. There are 32 discrete inverter speedε divided over % the total length of the probe.

5. Should the receiver tank empty when in the auto run mode, the pump will εtop. This can be arranged by actuation of the lowest εenεor or by the εtep change in resistance which occurs when liquid drops below the level of the two longer probeε. Notes/ a. Moving the toggle switch to the STOP position will immediately stop the pump. b. If float switches are utilized, instead of conductive probes, operation remains the same as described except that the speed changeε are εynthesized as the float switcheε become covered/uncovered. c. The diagram of Figure 8 illuεtrated schematically the operation in the automatic milk pumping mode of operation.

The system described provides the following advantages:- 1. GENTLE ACTION - Manufacture of certain dairy products, e.g. Dutch-style cheeses requires milk with a low level of free fatty acids (FFA's) . High speed centrifugal pumps break open the fat globuleε allowing enzymeε to attach the fat and create these acids, so a low speed gentle action pump iε desirable.

2. HIGH LEVEL MOUNTING - A Milking parlour is a 5 very wet and dirty place. Centrifugal pumps mounted near to floor level in the pit are at risk from pressure washdown jets and are prone to mechanical damage. Milk pipelines are often run at ground level as high levels and vertical drops 10 can increases FFA's - this prevents the receiver form being raised. As a result, a pump which can be mounted above the receiver is desirable, i.e. self-priming.

3. EASILY-CLEANED-IN-PLACE - The diaphragm type 15 pumps are difficult to clean due to the presence of valves in the milk flow. Alεo effective cleaning demands high cleaning fluid flow rates which may be higher than the normal milk flow. A pump which is inherently CIP-able and which 20 generates high flows is advantageous.

4. ABLE_TO DISPLACE FROTH - Centrifugals are prone to air locking when froth enters. A positive displacement pump is desirable to keep the milking process operating without

25 interruptions for venting or even a service call out.

5. UNDAMAGED BY SOLIDS - Occasionally small εtones enter the system, picked up from the teats or off the floor when the cluster is removed. Lobe pumps have been tried in milk parlours but are easily jammed; a jam free pump is desirable.

6. SIMPLE - Diaphragm pumps have many moving parts, both within and outside the milk wetted area. Dairy farmers want a simple, easy to service pump.

7. EFFICIENT COOLING -Milk commonly passes through a water plate cooler before reaching the bulk tank. A cost εaving to the farmer results if the milk pasεes as slowly and steadily as posεible through the cooler (leεε water and less refrigeration) . A two or variable speed pump allows efficient cooling of milk and high wash- water flow.

8. RELIABLE - High speed Centrifugalε εuffer from short seal life, diaphragms are expensive to replace and can last only half or a third of a season. A pump with long component life and low cost spareε is desirable.

Whilst the embodiments described all relate to the pumping of milk it will be appreciated that the Principles of this invention are applicable to level sensing and pumping of any conductive liquid e.g. water.

Claims

CLAIMS :

1. A liquid level control system, comprising a container for receiving a flow of liquid, level sensing means for providing a control signal variable in dependence upon the depth of liquid in the container a pump coupled with an outlet from the container and responsive to the control signal for expelling liquid from the container at a rate which varies as a function of the liquid depth.

2. A sensor as claimed in claim 1, wherein the pump is an impeller pump.

3. A system as claimed in claim 2, wherein the pump has a flexible impeller.

4. A syεtem as claimed in any one of the preceding claims, wherein the pump is driven by an electric motor at a speed which is variable in dependence upon the control signal to vary the rate at which liquid is expelled.

5. A εystem as claimed ± claim 4, wherein the electric motor is an AC induction motor.

6. A system as claimed in claim 5, wherein the induction motor is powered by an inverter which provides a supply voltage and frequency, dependent upon the control signal, which iε a function of liquid depth.

7. A system as claimed in claim 6, wherein the induction motor is powered by a power supply having an inverter and as described and claimed in US Patent Application Number 08/291,239.

5 8. A εyεtem aε claimed in any one of claims 4 to

7, comprising means for providing a control signal at switch on to run the motor at high speed for a predetermined time interval thereby to prime the pump.

9. A syεtem as claimed in any one of claims 4 to "0 8, wherein the level sensing means comprises a pair of spaced conductive probes extending downwardly or upwardly side by side in the container and wherein the resistance between the probes which varies as a function of liquid depth is employed to provide the control signal.

5 10. A system as claimed in claim 9, wherein a further conductive probe of length shorter than εaid pair of conductive probes is provided extending downwardly alongside the pair of conductive probes to determine when a predetermined liquid depth has been achieved upon detection 0 of a resiεtive path through the liquid.

11. A system as claimed in claim 10, wherein means is provided for producing a signal representative of a predetermined intermediate depth of liquid upon detection 5 by meanε of the further probe and to maintain that signal when the level drops so as to maintain an intermediate operating speed for the motor.

12. A syεtem aε claimed in claim 10 or 11, wherein reεiεtance change between εaid pair of probes between the level at which the liquid just touches the probes and the level at which the liquid just touches the further conductive probe is utilised to relate liquid depth to liquid conductivity.

13. A system as claimed in claim 12, comprising means coupled with said pair of conductive probes for deriving an electrical signal which is a function of the resiεtance between the probes, means for storing the amplitude of that signal when the liquid level rises to just touch the probe, means for storing the amplitude of that signal when the liquid level rises to just touch the shorter probe and means for calibrating the control signal within defined limits in dependence upon the two stored signals to adapt the control signal to the conductivity of liquid thereby to provide., a subεtantially full pumping control range irreεpective of conductivity of the liquid.

14 A system as claimed in claim 13, wherein the means for providing an electrical signal comprises a resiεtor coupled in series with the pair of conductive probes, a pair of input lines for connection to a source of alternating current for feeding the serieε arrangement, an amplifier having an input fed by the voltage developed across the resistor, and a resiεtor to provide a d.c. analogue voltage output which iε related to liquid depth.

15. A εystem as claimed in claim 14, wherein the analogue voltage output is fed to an analogue to digital converter, the two εignal values are stored in digital form and are compared by a computer which provides a control signal which is adapted to the conductivity of the liquid.

16. A syεtem as claimed in any one of claims 4 to 8, wherein the level sensing means comprises several liquid detectors arranged to detect the presence of liquid each at a different level in the container.

17. A syεtem as claimed in claim 16, wherein the liquid detectors are formed by float actuated εwitches.

18. A system as claimed in claim 16 or 17, wherein means is_ provided for producing a signal representative of a predetermined intermediate depth of liquid upon detection by an intermediate one of the liquid detectors, and to maintain that signal when the level drops so as to maintain an intermediate operating speed of the motor.

19. A syεtem as claimed in any one of the preceding claims, comprising a liquid falling level detector responsive to the control signal and operable to shut down the pump if a fall in liquid level does not occur within a predetermined time interval from εwitch on.

20. A liquid level control εystem substantially as deεcribed herein with reference to the drawings.

21. A milking machine comprising a liquid level control system as claimed in any one of the preceding claims, wherein the container is a milk receiving tank, a vacuum pump is coupled to the tank and is operative to induce milk into the tank from milk cups for attachment to cows and the pump coupled with the tank outlet is coupled with a bulk milk storage tank.

22. A milking machine as claimed in claim 21, wherein the coupling between the tank outlet and the bulk storage tank is provided with a heat exchanger to effect cooling of the milk-_

23. A milking machine as claimed in claim 21 or 22, wherein the bulk milk storage tank is refrigerated.

24. A milking machine substantially as described herein with reference to the drawings.