WO1991002949A1

WO1991002949A1 - Level sensing device

Info

Publication number: WO1991002949A1
Application number: PCT/AU1990/000375
Authority: WO
Inventors: Kjell Hegge
Original assignee: Europa International Manufacturing Pty. Ltd.
Priority date: 1989-08-25
Filing date: 1990-08-27
Publication date: 1991-03-07

Abstract

A level sensing device for measuring the level of an electrically conducting liquid comprises a stationary electrode (17) and a movable electrode (20) suitably comprising a bob attached to a wire. The movable electrode is raised or lowered within a liquid container by a stepping motor (24). Upon contacting the liquid a closed circuit is established between stationary electrode (17) and movable electrode (20) which is detected by a microprocessor (28). Microprocessor computes the position of the movable electrode (20) within the container and determines the level and/or volume of liquid in the container. The device can provide accurate measurements of liquid levels, volumes or changes in volume in a liquid container.

Description

LEVEL SENSING DEVICE FIELD OF THE INVENTION THIS INVENTION relates to a level sensing device which may be used to measure the level of a liquid within a container and which may also be used in conjunction with a suitable microprocessor to accurately measure the volume of a liquid within a container_. or the change in volume of liquid within a container.

BACKGROUND OF THE INVENTION Devices for measuring the level of a liquid within containers have been known for a very long time. Some devices comprise a dipstick or like member which is inserted into the tank and the level of liquid within the tank can be measured by sighting the wetness line. An alternative method includes the use of sight glasses or gauge glasses which normally extend externally of the tank and between an upper and lower position whereby a sample of the liquid can be visually sighted and thus the level determined. These devices are deficient in that they provide only a relatively crude measurement of the level of the liquid within the container. For instance, the devices cannot compensate or adjust for froth on top of the liquid or liquid volume changes caused by changes in temperature or pressure.

Hence, these devices have been required to be used in combination with other measuring devices if an accurate measurement of liquid level or volume is required.

Other known devices include ultrasonic devices which are unsuitable for liquids which froth as the device provides erroneous readings.

Devices for measuring the change in volume of liquid within a container are also well known. The usual device used for this type of measurement is a flow meter which is usually installed within the inlet or outlet pipe of the container.

Most of these flow meters are electrode-mechanical devices which give a readout of the volume of fluid which is leaving or entering the container within a given time frame.

A disadvantage with flow meters is that they generally provide an indication of fluid flow and cannot distinguish liquid flow from gas flow. Thus when the fluid comprises a mixture of liquid and gas such as in the case with effervescent liquids or liquids which by nature tend to froth, the flow meter measures the total fluid flow which will thus not provide an accurate indication of the liquid component.

A second method for detecting volume change is by using pressure or weight sensing devices located on a bottom wall of the fluid container. These devices are not sufficiently accurate.

In situations where accurate measurements of liquid flow are required ie. in the petrol, liquor or milk industries which may attract a government excise duty in various countries, known conventional flow meters do not possess the required degree of accuracy and must be combined with other measuring devices to produce accurate measurements when ambient conditions are changing.

Other fluid level sensing and monitoring means are described in International Patent Application Number PCT/AU85/00265 , United States Patents 3911744, 2963908, 4203325, and Australian Patent Numbers 493224 and 407006.

United States Patents 3911744 and 4203325 are concerned with vertically spaced electrodes which when immersed in a conducting liquid, close an electrical circuit with a laterally spaced reference electrode. United States Patent Number 2963908 is concerned with the measurement of levels in fluids including flowable particulate solids and utilizes capacitance measurement to detect changes in dielectric constant of a surrounding medium. Australian Patent Number 493224 described a rod¬ like electrode structure comprising alternating conducting and insulating sections for liquid level measurement or pump control. The electrode requires for its operation to be immersed in a conducting liquid. Australian Patent Application Number 407006 describes a plurality of spaced thermistors operating at a normal temperature of about 300°C. Immersion of a heated thermistor in a liquid changes its electral resistivity and consequently the operating characteristics of a transistor operatively connected to the thermistor to switch the transistor between a non-conducting state and a conducting state.

Int rnation l Patent Application Number

PCT/AU85/00265 describes an electrode assembly similar to that described in Australian Patent Number 493224 except that the spaced conducting electrodes are discontinuous about the peripheral surface of the electrode body.

DISCLOSURE OF THE INVENTION It is an object of the invention to provide a level sensing device which may substantially alleviate the disadvantages of the prior art.

According to one aspect of the invention, there is provided a level sensing device for measuring the level of an electrically conducting liquid, said device comprising a stationary electrode, a movable electrode movable between an initial position above the level of the liquid to be measured and a measuring position in electrical contact with the liquid, a motive means to move said movable electrode between said initial and measuring positions, computational means to compute the position of said movable electrode relative to a reference position, and, detection means to detect a change in electrical conductivity between said stationary and said movable electrodes, whereby in use, said stationary electrode is in electrical contact with the liquid and said motive means moves said movable electrode from the initial position to the measuring position whereby there is a change in electrical conductivity between the stationary and movable electrodes to enable computation of the position of a liquid level with respect to the reference level.

The level sensing device is capable of accurately measuring the level of a liquid. In the context of the specification, the phrase "accurately measuring" is taken to mean a measurement that is within 1% of the true measurement when corrected for variations in ambient conditions which may effect the measurement (eg. variations in temperature, pressure and the like) .

The energy source may comprise alternating or direct electrical current. The electric current is preferably non-ionising so that liquid contained within the container does not experience any chemical reactions or degradation when the current flows through it.

The stationary electrode is suitably positioned such that at least part of the electrode, in use, is always in contact with liquid present within the liquid container.

For measurement of liquid levels in electrically conducting containers such as stainless steel tanks, the stationary electrode suitably comprises the conductive tank wall.

For measuring liquid levels in containers which are not electrically conducting, the stationary electrode suitably extends within the liquid container to a position closely spaced from the bottom of the container. The stationary electrode may extend vertically from an upper portion of the liquid container to a position closely spaced from the bottom of the liquid container thereby extending substantially through the interior of the container.

The stationary electrode may comprise an elongate body formed from a conductive material which is preferably chemically inert such as stainless steel. The movable electrode is suitably positioned relative to the stationary electrode such that when in the measuring position, non-ionising electric current can flow through the liquid between the electrodes.

Suitably, the movable electrode comprises a weight or bob attached to a flexible member such as a chain or wire. The free length of the flexible member can be varied so as to move the bob vertically within the container. Preferably, the flexible member and bob are formed from electrically conductive materials which may be chemically inert such as stainless steel.

The motive means may comprise a drive to raise or lower the movable member relative to the liquid level.

Suitably, the motive means comprises a spool about which is wound the flexible material attached to the bob, with the spool rotated by a suitable motor. Preferably, the motor comprises a stepping motor or a motor incorporating a shaft encoder or like device to allow the free length of the flexible member to be accurately determined.

Alternatively, the motive means may comprise a fixed pulley to support the flexible member, a linear actuator with a stationary portion secured to the pulley and a moving pulley secured to the flexible member and free to move in a linear fashion either towards the pulley or away from the pulley such that the free length of the flexible member between the pulley and the bob may vary. Preferably, the linear actuator includes a ball screw or power screw which rotatably engages the moving portion such that the moving portion is free to move along the length of the power screw in response to rotation of the screw. Advantageously, the power screw is rotated by a stepper motor or a motor incorporating a shaft encoder or like device.

The computational means may comprise a microprocessor. The microprocessor may include a digital read-out for presenting the liquid level measurement. Suitably, the microprocessor further includes an indicator means to indicate when the source of electrical energy is operative. Preferably, the indicator means is a light emitting diode (led) .

Alternatively, the microprocessor may be adapted to provide its output at a location remote from the container. Advantageously, the microprocessor is adapted to control the operation of the motive means to move said movable electrode.

The detection means may be associated with the electrodes to detect a change in electrical conductivity between the electrodes. The detection means may be operatively associated with the motive means, suitably through the microprocessor, to stop the motive means upon detection of a change in electrical conductivity between the electrodes.

The level sensing device may further include a signal generating means which generates a signal proportional to the distance moved by the movable electrode, whereby the signal may be detected by the computational means and used to compute the position of the liquid level.

The signal generating means may be associated with the motive means and preferably is associated with the motor which actuates the spool or linear actuator as described above. Advantageously, the signal generating means is a component of the motor as is provided by a stepping motor.

According to a second aspect of the invention, there is provided a volume measurement device for measuring a volume of an electrically conducting liquid within a container said volume measuring device comprising a level sensing device as described above wherein the computational means may be calibrated with calibration data relating to the container to compute the volume of liquid within the container from the position of the level of the liquid within the container.

The calibration data relating to the container may include volumetric and/or dimensional data.

The volume measuring device may further comprise a temperature sensor located within the container and adapted to provide a signal calibrated for a reference temperature within the container; wherein in use, the temperature sensor signal may be processed by the computational means to provide a measurement of volume which has been corrected to allow for variations in volume caused by variations in temperature.

Suitably, the temperature sensor is formed from chemically inert material and may be separate from or form part of the stationary electrode. Desirably, the computational means is adapted to display the temperature reading when required.

According to a third aspect of the invention, there is provided a method for measuring the volume of an electrically conducting liquid in a container comprising the following steps: (i) displacing a movable electrode from an initial position to a measuring position where the electrical conductivity between a stationary electrode in electrical contact with the liquid and the movable electrode changes;

(ii) measuring the displacement of said movable electrode; (iii) computing the volume of liquid within the container with a computational means using the displacement measurement and calibration data relating to the container.

The volume measuring method may further include correcting the volume measurement obtained in step (iii) above to allow for variations in liquid volume caused by variations in temperatur .

According to a fourth aspect of the invention there is provided a method for measuring a change in volume of an electrically conductive liquid in a container using a method for volume measurement as described above, wherein the method comprises the following steps:

(i) measuring an initial volume of liquid in a container with a volume measuring device described above;

(ii) changing the volume of liquid within a container;

(iii)measuring the displacement of the movable electrode between the initial liquid level and the changed liquid level; and

(iv) computing the change in volume of liquid within the container using the displacement measurement and calibrated data obtained from the internal dimensions of the container.

The volume of liquid may be changed with a suitable liquid control means such as a valve or pump.

Advantageously, the liquid control means comprises an electro-mechanical device which may be controlled by the computational means. The electro-mechanical device may comprise an electric motor and pump. The pump may be a variable displacement pump which may be electrically controlled such that the pump is always rotating but only produces fluid displacement in response to appropriate signals from the computational means (ie. a demand pump).

Alternatively, the electro-mechanical device may comprise a servo-valve. Preferably the servo-valve opens and closes in response to appropriate signals from the computational means thus allowing liquid to flow into or out of the container.

The method for measuring a change in the volume of a liquid in a container is suitable for use in batching operations where one or more containers contain master batches which are required to be accurately fed into a main liquid or solid. The output from the batch containers can be accurately monitored by measurement of the change in volume in the containers using the level sensing device of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better described with the following description of preferred embodiments thereof as depicted in the accompanying drawings in which

Figure 1 is a schematic view of a level sensing device according to a first embodiment for use in a container having a non-conducting container wall.

Figure 2 is a schematic view of a level sensing device according to a second embodiment and which is suitable for use in a container having an electrically conducting container wall. Figure 3 is an elevation view of a motive means according to an embodiment located within a housing.

Figure 4 is a plan view of the motive means of figure 3.

Figure 5 is a view of a movable electrode according to an embodiment of the invention.

Figure 6 is a view of an insulation plug and a stop for the movable electrode according to an embodiment of the invention.

Figure 7 is a schematic view of a number of level sensing devices associated with containers and controlled by a single control station.

Figure 8 is a block diagram of a preferred circuit arrangement of the level sensing device.

BEST MODE FOR CARRYING OUT THE INVENTION Figure 1 discloses a level sensing device associated with a liquid container and which is suitable for liquid containers having a non-electrically conducting container wall.

The level sensing device 10 is fitted to container

11 having a known internal volume and containing a quantity of liquid 12 above which is located an airspace 13. The top wall 14 of container 11 is provided with a collar 15 to secure mounting plate 16 in place.

In this embodiment, stationary electrode 17 comprises an elongate conductive body attached to the underside of mounting plate 16 and extending vertically downwards into container 11 to a position closely spaced from the bottom wall 18 of the container. In this manner, a portion of the stationary electrode will always be in contact with liquid 12. Electrode 17 is formed from stainless steel suitably about 10mm thick and includes a temperature sensor 19 fitted thereto.

Movable electrode 20 is located closely adjacent stationary electrode 17 and comprises a stainless steel bob 21 of generally conical shape the upper end of which is attached to a stainless steel wire 22. The bob is more clearly illustrated with reference to figure 5.

The upper end of wire 22 is wound around a spool 23 attached to a stepping motor 24 mounted to mounting plate 16. Stepping motor 24 is more clearly described with reference to figures 3 and 4. Stepping motor 24 is used to move bob 21 vertically between an initial or reference position 26 where the bob is located directly below mounting plate 16 and a measuring position 27 where bob 21 contacts the surface of liquid 12. The operation of a stepping motor 24 is controlled by a computational means in the form of a microprocessor 28 which is fixed to mounting plate 16. Stepping motor 24 produces a varying electrical signal which is proportional to the distance in which bob 21 is moved from its initial or reference position 26 and the signal is fed into microprocessor 28.

Microprocessor 28 controls the supply of electrical current to stationary electrode 17 and also to stepping motor

24. Microprocessor 28 includes an indicating means 29 in the form of a light emitting diode (LED) to indicate when the system is energised.

To measure the level of liquid 12 within container 11, microprocessor 28 supplies electrical current to stationary electrode 17. Thereafter, microprocessor 28 energises stepping motor 24 to move bob 21 from its initial or reference position 26 to a measuring position 27 where bob 21 contacts the surface of liquid 12.

As the liquid is electrically conductive, the contact of bob 21 with liquid 12 causes a flow of current between the stationary electrode and the bob.

The current flow is detected by a detection means (not shown - see fig 8) which suitably comprises a resistor to measure a voltage potential and which provide a signal to microprocessor 28. Microprocessor 28 stops stepping motor 24 and senses the electrical signal emitted by stepping motor 24 and processes the signal using a suitable computer program to produce a reading of the liquid level at display window 30.

The volume of liquid 12 within container 11 can be obtained by depressing switch 31 which changes the computer program within microprocessor 28 to include calibration data relating to container 11 and properties of liquid 12 and also measures the temperature of liquid 12 by temperature sensor 19 and processes the various readings to obtain a corrected or "true" volume which can be displayed at display window 30. If desired, the liquid temperature can also be displayed. The microprocessor, and stepping motor and other operative parts which are external to container 11 can be protected by a suitable weather proof cover 32.

This embodiment is particularly suitable for use with beverage containers since all components that are in contact with the beverage are made from chemically inert or terilizable material such as stainless steel which are easy to clean.

Figure 2 discloses a second embodiment of a level sensing device which is suitable for use in liquid containers having a electrically conductive container wall.

In this embodiment, container 35 is formed from conductive material such as stainless steel. Such tanks are widely used in the milk industry.

The container wall 35 being electrically conductive functions as the stationary electrode. An advantage of such a system is that a portion of the stationary electrode is always in contact with any liquid present in the container.

Movable electrode 36 comprises a bob as described in figure 1 and as more clearly described in figure 5 and which is attached to a stainless steel wire 37. Bob 36 is movable vertically within the container by a stepping motor (not shown) which is housed within the bob control unit 39. The stepping motor is more clearly disclosed with reference to figures 1, 3 and 4.

Stepping motor functions to move bob 36 between an initial reference position adjacent a top wall 40 of container 35 and a measuring position where bob 36 is in contact with the surface of liquid 41.

A computational means in the form of a microprocessor 38 is located remote from the bob control unit and is electrically connected thereto through suitable electric conduit. Microprocessor 38 as described in figure 1 and figure 8 operates the stepping motor and thus the movement of bob 36 within the liquid container 35.

The level of liquid 41 in container 35 is measured in a manner similar to that described with reference to figure 1. As the stationary electrode comprises the container wall, a separate temperature probe 42 can be located within container 35 and suitably extends from top wall 40 to a position closely spaced from the bottom wall 43 of the container, thereby ensuring that the temperature probe is always at least partially in contact with liquid 41.

In this manner, the volume of the liquid can be corrected in regard to the temperature as described with reference to figure 1.

Figures 3 and 4 disclose a preferred embodiment of the stepping motor within a housing (also called the bob control unit) .

The bob control unit comprises housing 45 spaced above the top wall 46 of a container by a collar portion 47. The lower end of collar portion 47 is mounted to the top wall 46 of the container through mounting plate 48. Bolts 49 secure mounting plate 48 to top wall 46.

An insulating plate 50 is positioned between mounting plate 48 and top wall 46 to electrically isolate the bob control unit from the container. If the container is not electrically conductive and is not used as the stationary electrode, the insulating plate need not be required.

The upper end of collar portion 47 is mounted to housing 45 through bolts 51. A stepping motor 52 is mounted to a mounting plate

53 which is mounted to a lower floor of housing 45 optionally through insulating plate 53A by bolts 51. Stepping motor 52 includes a rotatable drive shaft 58 about which is mounted a spool 54. The free end of rotatable shaft 58 is supported by bearing 56. Spool 54 stores the stainless steel wire to which the bob is attached.

An elongate roller 55 is positioned longitudinally along the outer face of spool 54 and is biassed against the spool. Roller 55 functions to maintain the wire at a predetermined tension on spool 54 by forcing the wire to pass between the roller 55 and spool 54.

Stepping motor 52 is electrically connected to a power source through connectors 57a,57b and upon energising stepping motor 52, spool 54 rotates which in turn winds the stainless steel wire onto or from the spool. The .stainless steel wire passes from the spool through a suitable opening in housing 45 (not shown), through collar portion 9, through a suitable opening in the top wall of the container and into the interior of the container. For measurement of liquids containing an upper layer of foam or froth, the bob passes through the froth or foam to contact the liquid. This results in froth or foam adhering to a portion of the stainless steel wire immediately behind the bob the amount of wire so contaminated depending on the depth of the froth or foam. Collar portion 47 functions to prevent this part of the wire being wound onto spool 54 thereby contaminating the spool. Thus, upon movement of the bob to its upper position, which is adjacent the top wall of the tank, any contaminated wire will locate within collar portion 9 and will not extend to spool 55. Obviously, the length of collar portion 47 can be varied depending on the type of liquid to be measured and the amount of frothing or foaming present on the surface of the liquid.

Figure 5 discloses a preferred form of a movable electrode according to an embodiment of the invention. The movable electrode is in the form of a bob 60 having a cylindrical body 61 formed from stainless steel. The normally lower end of cylindrical body 61 is formed with a tapered surface 62 which tapers to a point 63. The normally upper end 64 of main body 61 has a frusto conical configuration which includes a recess 65 to threadingly accommodate a plug 66 to which the wire is secured.

Plug 66 is provided with a central passageway 67 and an offset recess 68. The bob supporting wire is threaded through passageway 67 and returned into recess 68 and thereafter plug 66 is fitted to recess 65 within bob 60 thereby securing the wire to bob 60.

It should be appreciated that the operative or lower end of the bob need not be tapered in the form shown in figure 5 and advantageously may include a needle or similar structure located at the apex to further reduce the effects of surface tension in the liquid.

Figure 6 discloses a plug and bob stop assembly according to a preferred embodiment of the invention and which is fitted to a top wall of a liquid container.

The plug 70 is suitably in the form of an insulation plug and comprises a hollow internally threaded cylindrical body 71 and an upper collar 72. Body 71 is fitted through a suitable opening in an aperture on a top wall of a liquid container with collar 72 abutting adjacent the aperture. The plug is suitably formed from plastics material.

The bob stop 73 comprises an elongate body 73A having a central passageway 74 extending therethrough. The normally upper part of body 73a is of reduced diameter and is threaded to threadingly engage within the passageway of plug

70.

The lower portion of passageway 74 is flared to accept the end of plug 66 disclosed in figure 5. A compression spring 75 is located within the flared passageway to "soft stop" the bob upon the bob being moved to its upper position.

The wire extends through the passageway in the bob stop 73 and the insulation plug 70 and to the stepper motor described above.

Thus upon movement of the bob to its upper position, the upper portion 66 of the bob as disclosed in figure 5 locates within the flared opening of passageway 74 thereby avoiding contact between the bob and a wall of the container. Figure 7 discloses a schematic representation of a number of liquid containers each containing a level sensing device according to an embodiment of the invention and controlled by a single remote control station.

In this figure, there are disclosed containers 81a- 8Id which have electrically conductive walls which form the stationary electrode as described above with reference to figure 2. Each tank is provided with a temperature probe 82a-d as described above and a movable electrode 83a-d in the form of a bob fitted to a wire as described above. The movable electrode in each container is controlled by a bob control unit 84a-d as described previously and each bob control unit is controlled by a unit control box 85a-d housing a microprocessor as described previously.

In this embodiment, each unit control box can be controlled and programmed by a remote control station 86 which can be operated either directly or through a telemetric link through a suitable modem. In this arrangement, a remote operator can program an operate the various level sensing devices from a position remote from the liquid containers and is also in a position to obtain readings, at a position remote from the liquid containers. Figure 8 discloses a block circuit diagram according to a preferred embodiment of the invention.

In this figure there is disclosed a stationary electrode 90 and a movable electrode 91. The electrodes have a voltage applied across them. Movable electrode 91 is mounted to a stainless steel wire 91A which is wound around a spool 92. Spool 92 is rotated by shaft 93 of a stepping motor 94. Stepping motor 94 is powered through a power input 95 which is connected to a power source (not shown) . Stepping motor is energised upon receiving a signal from microprocessor 96 via interface 97. Upon receipt of a signal, stepping motor operates to raise or lower the movable electrode within a container.

Rotation of stepping motor 94, results in a signal being generated which varies according to the amount of rotation. This signal is detected by interface 98 and is fed into microprocessor 96, to allow the microprocessor to compute the position of the movable electrode 91 within the container. A detection means 99 comprises a resistor 100 associated with movable electrode 91, and which detects a potential difference when current flows through the resistor a potential difference is established across the resistor.

Any such potential difference is applied to interface 101 which provides an input signal to microprocessor 96. Microprocessor 96 is clocked by clock 102, and includes a memory.

A display 103 displays information outputted by microprocessor 96 through interface 104.

The operation of the circuit shall now be described.

Movable electrode 91 is initially in an upper or reference position within the container and in this position, electrodes 91 and 90 are separated by an airspace which essentially results in an open circuit between the electrodes. Therefore, no current flows between electrodes

90 and 91 or through resistor.

At a predetermined time, or upon a signal being given to microprocessor 96 the microprocessor provides a signal to stepping motor 94 through interface 97 to rotate stepping motor 94 which in turn lowers the movable electrode

91 towards the liquid in the container.

Upon rotation of stepping motor 94, a signal is sent to microprocessor 96 through interface 98, the signal indicating the amount of rotation of motor 94 and therefore the position of movable electrode within the container. When movable electrode 91 contacts the electrically conducting liquid in the container, a closed circuit is formed between movable electrode 91 and stationary electrode 90 thereby allowing current to flow between the electrodes and through the resistor 100. The current flow immediately results in a potential difference being established across resistor 100 which is applied to interface 101 which in turn provides an input signal to microprocessor 96.

Immediately upon receiving the signal from interface 101, microprocessor 96 signals the stepping motor to stop rotation and computes the position of movable electrode 91 from the signal from interface 98 and together with pre-programmed characteristics of the volume of the tank and any input from a temperature sensor (not shown) computes the volume of liquid within the tank and outputs this information to display 103.

The level sensing device provided by the present invention provides accurate measurements of liquids that froth or foam since the microprocessor can be programmed to recognise the minimum current which exceeds the current that can be conducted by the froth. That is, the level sensing device looks only at the interface between the liquid and the medium above it. The substantial difference electrically conductive properties between the two mediums allows the level sensing device to accurately determine the level of the liquid irrespective of whether the surface of the liquid contains froth or foam. This is an advantage over prior art devices in which the level readings were effected by such froth or foam.

The features described above allow the microprocessor to be adapted or programmed to control the complete measuring process, independent of human intervention. It is further preferred that the microprocessor is adapted to allow multiple or repeated readings to be performed to each measurement. Suitably, three readings are performed for each measurement. Such multiple readings allows erroneous readings or measurements to be eliminated.

Increased measuring accuracy is also obtained by having the movable electrode always moving to a reference point prior to moving to contact the surface of the liquid. However, in order to reduce motor wear and wear on the wire, it is also possible to have the movable electrode moving to a reference point at specified times and not every time a measurement is made. For instance, the movable electrode may only move approximately 30cm above the surface of the liquid to be measured for a certain number of measurements after which the microprocessor is programmed to move the movable electrode to its reference point for recalibration.

In this manner, the level sensing device provided by the present invention is basically tamper-proof since the only human input is to turn on or energise the electrical energy source of the microprocessor and thereafter the microprocessor is programmed or can be programmed to take over the entire control of the liquid level measurement processes according to predetermined programmed instructions.

The microprocessor can be programmed to provide readings upon pressing of a switch or can be programmed to give continuous or semi-continuous readings.

Furthermore, it should be appreciated that the volume measuring device provided by the present invention will produce a true and correct reading for the volume of a liquid within a container irrespective of the temperature of the liquid.

The only requirement that the microprocessor is correctly programmed to the respective characteristics of the container (ie. volume dimensions and of the liquid being measured) .

A further advantage of the level sensing device according to the invention is its ability to be periodically cleansed. Such a cleansing ability is important if the level sensing device is used in the beverage industry where strict hygiene controls are required by law.

For instance, the microprocessor can be programmed such that upon input of a appropriate signal, a cleansing operation program is initiated. In the cleansing operation, the stepping motor lowers the movable electrode and wire to its initial or upper position.

Thereafter, the liquid container can be initially cleansed by a water rinse. As the movable electrode is held against the upper portion of the container, the water jets from the spray ball which is used to cleanse the tank does not result in the bob swinging which provide stress and thus fatigue on the wire.

After the water rinse cycle, the second step in cleansing the liquid tank is a caustic rinse typically at elevated temperatures of about 82°C. As the caustic rinse operates, the microprocessor operates the stepping motor to lower the movable electrode to the base portion of the tank in a slow and even manner thereby allowing the full length of the wire and the movable electrode to pass through the cleaning spray.

The liquid tank is thereafter finally cleansed with a sterilent rinse. During this rinse cycle, the microprocessor activates the stepping motor to slowly and evenly wind the movable electrode back up to its initial or upper position where it is again tensioned against the spring of the bob stop (see for instance figure 6).

At the end of the final cleaning cycle, the microprocessor activates the stepping motor to move the movable electrode to a lower position in the fluid container and to await the passage of incoming liquid to be measured.

The level sensing device of the invention can be associated with alarms which operate when the liquid level rises above or falls below set limits. Thus, the alarms can function as high/medium/low indicators. It should be appreciated that the level sensing device according to the invention can be readily modified by persons skilled in the art without departing from the spirit and scope of the invention. The microprocessor for example may include a facility for the plug in of a telemetric device to allow interrogation of volume and temperature data over a telephone line. The device may also be modified to allow the microprocessor to control the temperature of the liquid via a suitable refrigeration or heating unit. Also the device can be readily retrofitted into existing containers or fitted to new containers. Various other changes and modifications may be made to the embodiment described without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

CLAIMS :

1. A level sensing device for measuring the level of an electrically conducting liquid, said device comprising a stationary electrode, a movable electrode movable between an initial position above the level of the liquid to be measured and a measuring position in electrical contact with the liquid, a motive means to move said movable electrode between said initial and measuring positions, computational means to compute the position of said movable electrode relative to a reference position, and detection means to detect a change in electrical conductivity between said stationary and said movable electrodes, whereby in use, said stationary electrode is in electrical contact with the liquid and said motive means moves said movable electrode from the initial position to the measuring position whereby there is a change in the electrical conductivity between the stationary and movable electrodes to enable computation of the position of the liquid level with respect to the reference level.

2. The device as claimed in claim 1, wherein said stationary electrode comprises a wall of an electrically conducting liquid container.

3. The device as claimed in claim 1, wherein said stationary electrode comprises an elongate body extending from adjacent a top wall of a liquid container to adjacent a bottom wall of the liquid container.

4. The device as claimed in any one of claims 1 to 3, wherein said movable electrode comprises a bob attached to an electrically conductive flexible member.

5. The device as claimed in claim 4, wherein said motive means comprises a drive to raise or lower said bob relative to the liquid level.

6. The device as claimed in claim 5, wherein said drive comprises a spool about which is wound said flexible member said spool being rotated by a motor.

7. The device as claimed in claim 1, including a signal generating means which generates a signal proportional to the distance moved by said movable electrode, whereby said signal may be detected by said computational means _and used to compute the position of the liquid level.

8. The device as claimed in claim 7, wherein said signal generating means is generated by a stepping motor or shaft encoder comprising said motive means.

9. The device as claimed in claim 1, wherein said computational means comprises a microprocessor.

10. The device as claimed in claim 9, wherein said microprocessor controls the operation of said motive means.

11. A volume measurement device for measuring the volume of an electrically conducting liquid within a container, said volume measuring device comprising a level sensing device as defined in any one of the preceding claims, wherein said computational means may be calibrated with calibration data relating to said container to compute the volume of liquid within said container from the position of the level of the liquid within said container.

12. The device as claimed in claim 11, further comprising a temperature sensor located within said container and adapted to provide a signal calibrated for a reference temperature with the container; wherein in use, the temperature sensor signal may be processed by said computational means to provide a measurement of volume which has been corrected to allow for variations in volume caused by variations in temperature.

13. The device as claimed in claim 12, wherein said temperature sensor forms part of said stationary electrode.

14. A method for measuring the volume of an electrically conducting liquid in a container comprising the steps of

(i) displacing a movable electrode from an initial position to a measuring position where the electrical conductivity between a stationary electrode in electrical contact with the liquid and the movable electrode changes;

(ii) measuring the displacement of said movable electrode;

(iii) computing the volume of liquid within the container with a computational means using the displacement measurement and calibration data relating to said container.

15. The method as claimed in claim 14, further including correcting the volume measurements obtained in step (iii) to allow for variations in liquid volume caused by variations in temperature.

16. A method for measuring a change in volume of an electrically conducting liquid in a container using the method as claimed in any one of claims 14 or 15 comprising the following steps:

(i) measuring an initial volume of liquid in a container with a volume measuring device as defined in any one of claims 11 to 13;

(ii) changing the volume of liquid within the container;

(iii)measuring the displacement of said movable electrode between the initial liquid level and the changed liquid level; and,

(iv) computing the change in volume of liquid within said container using the displacement measurement and calibrated data obtained from the internal dimensions of the container.

17. The method as claimed in claim 16, wherein the volume of liquid may be changed with a suitable liquid control means.