WO1990008304A1

WO1990008304A1 - Level sensor

Info

Publication number: WO1990008304A1
Application number: PCT/AU1990/000017
Authority: WO
Inventors: Kjell Hegge
Original assignee: Europa International Manufacturing Pty Ltd.
Priority date: 1989-01-19
Filing date: 1990-01-19
Publication date: 1990-07-26

Abstract

A level sensor (80) for detection of fluid levels or interfaces between fluids including flowable particulate solids comprises a hollow probe body (81) having located therein a plurality of spaced sensors (85) responsive to changes in heat capacity or dielectric constant of a surrounding fluid medium due to changes in fluid level. In an alternative form an oscillator (98) is moved within the hollow probe to detect a fluid level by a change in dielectric constant of a surrounding medium to activate or deactivate the oscillator.

Description

LEVEL SENSOR

This invention is concerned with the detection of levels in fluid media and in particular detection of a fluid level within a container, an interface between gas and liquid within a container or the level of one or more interface between two or more liquids within a container.

As used herein the expression "fluid" means any gas or liquid or any flowable solids such as powdered or particulate solids. The invention, whilst being applicable to the situations mentioned above and others, will be described by^¬ way of example only with reference to the detection of the level of an interface between gas and liquid within a container and also with reference to an interface between a flowable solids material and a gas or liquid. In particular, the invention will be described by way of example with reference to the detection of the level of liquid in a container where the liquid is liquified carbon dioxide gas CO2 and where the container has CO2 gas above the level of the liquid. The invention will also be described with reference to the detection of the level of cement powder in a silo.

Carbon dioxide in gaseous form is used in large quantities in many industries. One of the major consumers of CO2 is the hotel trade in the provision of draught carbonated beer and other carbonated beverages through a reticulated supply system. Other consumers also often require CO2 to be supplied in cylinders or tanks.

It has been the practice to provide some means to identify or detect the level of the liquid C0₂ within the cylinder and hence the quantity of liquid remaining in the cylinder. Since, in such situations the cylinder is under high pressure it is not convenient or appropriate to employ sight gauges or the like to provide an indication of liquid level. With such cylinders containing CO2 in liquid form it has been the practice to provide an electro-mechanical level detecting mechanism employing a probe positioned within the cylinder and which probe had a plurality of sensors such as reed relays along the length of the probe. A float was 5 associated with the probe and that float included a magnetic element for actuation of the reed relays. As it is difficult to 'completely remove moisture from bulk liquified industrial gases such as propane, butane, natural gas, CO2, nitrogen and the like, there is a tendency for ice particles to accumulate ^* on the surface of the liquid gas particularly when the liquid is cooled to below freezing by adiabatic expansion. In some cases, bulk liquid gases are stored at temperatures less-than 0°C. With mechanical and electromechanical liquid level detectors, it has been found that accumulation of ice 5 particles on the surface of the liquid gas frequently jams mechanical devices such as floats leading to quite unreliable operation of the level detector. In addition, where the storage cylinder was made of steel, fine rust particles have been found to accumulate at the liquid level and adversely CΣ affect the level sensing probe.

Liquid level sensing arrangements of a mechanical or moving nature were undesirable for reasons expressed above, were unreliable and therefore not accurate. Difficulties have also been encountered with prior art liquid 5 level detectors for use in determining the position of an interface between fluids of differing specific gravities. Fermentation and distillation vessels typify situations wherein the positions of one or more liquid/liquid interfaces may be required to be determined for the purpose of G⁾ controlling continuous processes.

A typical cylinder arrangement for CO2 is illustrated as part of the schematic view shown in FIG 4 of Australian Patent Application 67304/87. Such cylinders are provided with a heating element within the tank and adjacent 5 the bottom of the tank, a refrigerant evaporator within the tank and at an upper end thereof, a liquid level detection device extending into the cylinder and having an elongate rod and a captive float and take off and inlet lines.

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 Number 493224 and 407006.

United States Patent 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 eleσtral 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. International 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. While the above prior art references typify electrodes for fluid level measurement and are generally satisfactory for use in specific environments they all suffer from one or more deficiencies and are not adaptable to reliable use in a wide variety of environments. It is an obje-ct of the present invention to provide a fluid level sensor which at least alleviates or minimises some of the disadvantages referred to above. It is a further object of the invention to provide, in at least one aspect, a means for accurate measurement or monitoring of fluid levels throughout a wide range of environmental conditions.

According to one aspect of the invention there is provided a level sensing apparatus for sensing the level of a fluid, said apparatus including: a plurality of sensors positionable at spaced- intervals relative to a fluctuating fluid level, said sensors providing an output indicative of the close proximity of the fluid, output circuitry for receiving the outputs from the sensors and providing a signal indicative of the level of the material within the container.

According to another aspect of the invention there is provided a level sensing apparatus for sensing the level of a material, said apparatus including a plurality of- temperature responsive level sensors positionable within the ^" container at spaced intervals therealong, said sensors providing a temperature dependant output, respective logic circuitry coupled to each said sensor; a temperature responsive reference sensor normally located within the- container and below a level for providing a reference output, each said logic circuit including a comparator for comparing the reference output with the temperature dependant outputs; and, respective indicating means coupled to each said logic circuitry for providing an indication in response to the comparison, said indication being representative of the level of the material within the container. The fluid level sensors are preferably located along a probe positionable within the container. The probe may be elongate and constructed to enable at least a part of each level sensor to be contacted by a fluid within the container when the container is full. Preferably, the probe is tubular and the level sensors, are positioned within the tubular probe. Apertures may be present along the length of the probe through which the level sensors may be contacted by the liquid. It is preferred that the spacing of the level sensors be equal along the length of the probe. Each level sensor may be responsive to detect whether the fluid is at a desired level. For example, the level sensors may be responsive to fluid within the container being at a full level, a three-quarter full level, a half full level, a quarter full level and an empty level. Clearly, with such an arrangement five level sensors are present although there may be a greater or larger number than this. Where the level sensors provide an indication of full, three-quarter full, half full, quarter full and empty liquid levels they are equally spaced along the length of the probe. Clearly, where equal fluid level gradations are not required, the level sensors may be located along the probe at unequal spacing.

The reference sensor is preferably located adjacent the level sensor indicative of the container being empty. Where such a location is employed for the reference sensor it should be appreciated that level sensor is located adjacent a lower end of the container and below the empty indicative level sensor. The reference sensor may also be located within the probe although this is not essential. The reference sensor is temperature responsive whereby a particular current may be caused to flow through it and the magnitude of that current is temperature dependant. Thus, where the reference sensor is covered by a fluid within the container, the magnitude of the current flowing through it varies proportionately in relation to the temperature of the fluid or its specific heat. The reference sensor may consist or comprise a semi-conductor junction. Alternatively, the reference sensor may be provided by a thermistor. It is preferred however that the reference 5 sensor be a semi-conductor junction. In a particular preferred form the reference sensor may be a bipolar semi¬ conductor device such as a transistor. It is particularly preferred that the reference sensor be a diode. A zener diode has been found suitable for this purpose.

10. _ Amplifying means may be associated with the reference sensor to provide a reference output of a desired magnitude. The amplifying means may include one or more amplifiers. Preferably, amplification is carried out in two stages and thus two amplifiers may be present for providing a

15 reference output of the desired magnitude. Th reference output is conveniently a voltage level. Whilst any suitable level may be employed it has been found that a reference of about 14 volts dσ is suitable.

The level sensors may include a temperature

20 dependant or responsive semi-conductor device. Alternatively, each level sensor may include a thermistor through which a current is caused to flow and depending upon whether the thermistor is within a liquid or within gas current of a particular magnitude is caused to flow through

25 the thermistor. In a particular preferred embodiment a bipolar semi conducted device is employed. Preferably, each level sensor includes a transistor. Preferably, the sensor includes biassing components to ensure that the transistor is biassed on and into the linear portion of its conduction

30 curve.

The biassing components may include level setting means to confine the operation of the transistor within a particular range of its conducting curve. The level setting means may extend between base current setting components and

35 either the collector or emitter of the transistor. Preferably, the level setting means is a zener-diode coupled between the base current setting components and the collector of the transistor.

The logic circuitry associated with the full level sensor is preferably coupled to the logic circuitry associated with the three-quarter full level sensor such that when the liquid level falls below the full level sensor the indicating means associated with the full level sensor is not de-energized and still provides a full level indication. The logic circuitry associated with the three-quarter full level sensor is preferably operable, once the liquid level falls below the three-quarter full level sensor to ensure that the indicating means associated with the full level sensor is de- energized. Similarly, the logic circuitry associated with the three-quarter full level sensor and the half full level sensor are also associated so that the three-quarter full indicating means is not de-energized until the level of the liquid falls below the half full level sensor. Thus, as the liquid level falls within the container there is a lag in the way in which the indicating means are de-energized. This ensures that the user is not caused to be alarmed by an apparently sudden drop in liquid level from one detected level to the other. The logic circuitry for the full and three-quarter full level sensors is preferably identical to the logic circuitry employed for the half full and quarter full liquid level sensors.

The indicating means preferably provides a visual indication. The indicating means may for example consist of a lamp which is energized to identify a particular level and de-energized when the liquid level drops. Preferably, a light emitting diode is employed. In a particular form of the invention the indicating means for all but the empty level sensor are green LED's whereas the indicating means for the empty level sensor is preferably a red LED. Where the logic circuitry is unable to provide driving current of a desired magnitude to energize the indicating means it is preferred that a driver be coupled between the logic circuitry and the associated indicating means. The logic circuitry associated with the empty level indicating means functions to either illuminate the empty level indicating means when the liquid level is just above the empty level sensor and functions to cause the empty level indicating means to flash when the liquid level falls below the empty level sensor.

The logic circuit associated with the empty level indicating means preferably includes an oscillator to enable the indicating means to flash in this way. The oscillator may be coupled to the driver which provides the proper current for operation of the empty level indicating means. A Schmidt trigger oscillator may be employed although oscillators of other types may also be used.

The logic circuitry associated with the indicating means may include circuitry for enabling the heater within the container to be energized if desired. This circuitry may be coupled to the comparator associated with the empty level sensor. This circuitry may control a controllable switch to enable the heating element to be energized if desired. The controllable switch may be a relay. A driver may^"be coupled between the coil of the relay and the circuitry which receives the output from the comparator.

As mentioned above, when the liquid level falls below the empty level sensor the empty level indicating means is caused to flash. In addition to this the apparatus of the invention may provide for an audible indication of this occurrence. It is preferred that the audible indication be provided by a beeper. The beeper may be driven by an oscillator and that oscillator may be interlocked with the empty level indicating means whereby flashing of that means alternates with the sound provided by the beeper. The oscillator may be a Schmidt trigger oscillator provided with a latching circuit. Preferably, a mute circuit is provided. The mute circuit may include a mute switch connectable to the oscillator whereby when the switch is operated the oscillator associated with the beeper is rendered inoperable.

According to another aspect of the invention there is provided a level sensing apparatus for sensing^*the level of a material within a container, said apparatus including: a plurality of sensors positionable within the container at spaced intervals therealong, each said sensor including an oscillator normally providing a first output when material is not proximate the sensor and a second output when the material is proximate the sensor output circuitry for summing the outputs from the sensors to provide an indication of the level of the material within the container.

The sensors, as mentioned, include an oscillator. The sensors may be positioned within a tube and thus need not come into contact with the material whose level is being detected. The sensors need not physically be connected to the tube and thus, if the material subjects the tube to tension or other forces the relative position and spacing of the sensors is not disturbed.

The apparatus may include any desired number of sensors. Thus, where indicators of full, three quarter full, half full, quarter full and empty level indications only are required five sensors need be present. Where less level or greater level indications are required a commensurate number of sensors may be employed.

The oscillators may include an active component. Preferably the active component is a bipolar device. A transistor may be used. The transistor is preferably part of an oscillator circuit having a feed back component and bias resistors. The feed back component may comprise a capacitor. The oscillator includes a frequency control device. Preferably the frequency control device may be a reactive component.

Preferably two reactive components are present. The reactive components may comprise series connected capacitor and indicator. The reactive elements together provide energy to sustain oscillation. When the dielectric constant of the medium surrounding the reactive components is altered the amount of energy available for sustaining oscillation alters and can result in the oscillation ceasing. To enhance this effect it is preferred that an antenna or radiating element be associated with the reactive components. : _ Preferably the element increases the surface area from which energy can be radiated to the medium surrounding the reactive- components. Preferably the radiating elements is coupled to - the junction between the capacitor and the indicator. The element may be a cylinder for example.

The output circuitry may include a summing amplifier for summing the outputs from the sensors. An operational amplifier may be employed for this purpose.

The apparatus may include a power supply. The construction of the power supply is not crucial although it is preferred that a regulated supply be established for the sensors. Typically regulated 24VDC may be supplied.

The invention will now be described by way of example with reference to the accompanying drawings in which: 5 Figure 1 is a detailed circuit diagram of a fluid level sensor and associated components in accordance with a first embodiment of the present invention; and

Figures 2a and 2b are a detailed circuit diagram of a level sensing apparatus suitable for the first embodiment 0 of the invention.

Figure 3 shows a circuit diagram of a fluid level sensory element according to a second embodiment of the invention.

Figure 4 shows a circuit diagram of a fluid level 5 sensing or monitoring apparatus according to the second embodiment of the invention.

Figure 5 shows schematically a fluid level probe according to the first embodiment of the invention.

Figure 6 shows a fluid level probe according to the 5 second embodiment of the invention.

Figure 7 shows a modification to the second embodiment of the invention.

As previously mentioned, one aspect of the invention is particularly suitable for detecting the level of

10 liquid gas in a container such as a cylinder or tank containing liquified carbon dioxide gas (CO2) and may be employed to detect the interface between gaseous CO2 and liquid gas to thereby enable the level of liquified CO2 in the cylinder to be monitored or detected.

3.5 The level sensors employed in the circuit of the invention comprise a plurality of temperature responsive devices arranged at spaced intervals along the length of a probe permanently positioned within the cylinder. Whilst this cylinder probe arrangement is not illustrated it should

20 be appreciated that the sensors consist of temperature responsive devices such as transistors, thermistors or diodes. The level of current flowing through these devices is temperature dependent and from this dependence a sensed signal may be obta-ined to determine whether a particular 5 sensor is within the liquid C0₂ or within the gaseous C0₂.

In the embodiment described below there are five temperature dependent level sensors and one reference sensor for establishing a reference signal. The five temperature dependent sensors are all transistors arranged or embedded 0 within the probe and having a portion of the casing or housing of the transistor capable of coming into contact with the liquid gas within the cylinder. Thus, the probe may consist of a longitudinal extending tube having apertures along its length through which the transistors project or are 5 visible. The reference sensor, also temperature responsive, is positioned within the probe at a location adjacent a lower end of the probe in use. The transistors and reference sensor may be connected to the remainder of the circuitry by wiring extending through the probe and exiting at one end thereof. The reference sensor may conveniently be a zener diode. All of the transistors are associated with a respective light emitting diode such that, as a consequence of the circuitry, these diodes may be actuated to provide an indication of the level of the liquid gas within the cylinder.

Figure 2 of the drawings shows a detailed circuit diagram of the level sensing apparatus according to an embodiment of the invention. In this figure, regulated 12 and 14 volts DC is obtained from a 240 volts AC supply. This supply is applied to the primary winding of a transformer Tl and stepped down through that transformer. The secondary winding of the transformer Tl is coupled across a rectifier bridge 10 to provide a stepped down full wave rectified voltage for connection to a filter consisting of filter capacitors Cl and C2 and resistor Rl. A regulating zener diode Z4 is connected across the bridge output. A voltage regulating integrated circuit IC1 functions to provide a 24 volt regulated DC voltage supply across output filter capacitor C3. Voltage regulating integrated circuit IC2 receives the regulated 24 Volt DC signal and functions to provide a regulated 12 volt DC output. Filter capacitors C4 and C5 are associated with the input and output of integrated circuit voltage regulator IC2.

The apparatus of the invention includes a plurality of level sensors which in this case consist of transistors each coupled to an associated comparator. Fo the sake of simplicity the circuit elements associated with one transistor sensor only are shown. It should be appreciated that identical circuit elements are associated with each transistor sensor. In the embodiment described an LED is associated with each transistor sensor. In the preferred embodiment five LED's are present and because each of them is associated with a respective transistor sensor and these sensors are located at spaced intervals along the probe, the LED's are indicative of liquid level within the cylinder representing a full cylinder, a three-quarter full cylinder, a half full cylinder, a quarter full cylinder and an empty cylinder respectively.

The circuitry associated with each transistor sensor will now be described in relation to figure 2 of the drawings. The transistor associated with LEDl has its collector, base and emitter connected to the terminals identified by the letters C, B, and E respectively. The output available from the collector of that transistor sensor is made available as an input to the non inverting input of comparator 11. The inverting input of comparator 11 is coupled to a supply of reference potential. The reference potential is typically 14 Volts DC. The output from the comparator 11 is connected to an OR gate 12 and that OR gate has a second input derived from AND gate 13. The output from OR gate 12 is made available as one input to AND gate 14. The AND gate 14 derives a second input from the output of comparator 15. Comparator 15 is associated with that transistor sensor used to provide an indication of three- quarter liquid level in the cylinder and is associated with LED2. The output from AND gate 14 is applied as one input to OR gate 16 and OR gate 16 derives its second input from the output of comparator 11. The output available from OR gate 16 is applied to driver 17. Resistor 18 provides a pull-up function for OR gate

12, resistor 19 is a current limiting resistor required for proper operation of LEDl and resistor 20 provides a latching or memory function for the circuit.

Returning now to figure 1 of the drawings, the operation of each transistor level sensor will now be described. Transistor Ql is biased by resistors 21, 22, 23, 24 and 25 into the linear portion of its conduction curve. Resistors 25 and 26 located in the collector emitter path of transistor Ql ensure that, subject to the temperature to which transistor Ql is subjected, a preset collector emitter current flows. This collector emitter current is subject to variation depending upon the temperature to which the transistor Ql is subjected. The nominal current in the collector emitter path is chosen to provide a desired heating effect in the transistor and also to ensure that the transistor Ql when exposed to liquid CO2 is not deactivated when otherwise subject to icing. Typically, the collector emitter current ensures that the output available at the collector is about 15 volts DC when the transistor is immersed in liquid C0₂. Once the level of the liquid- uncovers the transistor Ql, the difference between the specific heat of the liquid and that of the gas enable the temperature of transistor Ql to rise whereby more current is caused to flow in the collector emitter path and the voltage at the collector available for an input to the associated comparator drops to a level of about 10.5 volts DC. That is, less than the reference voltage of 14 volts. Zener diode Zl, in this case an 8.2 volt zener diode, functions to ensure that as the temperature of the transistor Ql rises and the voltage at the collector drops, the voltage at the collector is stabilised at a minimum level. Similarly, the zener diode Zl functions to stabilise the collector voltage at a maximum level of about 19 volts subject to temperature variations. In this way, the circuit ensures that transistor Ql is biased ON at switch on of the circuit to ensure that it is maintained in a heated state which prevents icing and the zener diode ensures that the available collector voltage does not vary beyond a predetermined range typically between 10 and 18 volts DC. Thus, returning to figure 2 of the drawings, when the transistor level sensor coupled to terminal C, B and E is below the level of liquid gas in the cylinder, the current flowing through the transistor sensor produces a collector voltage of about 15 volts. As this is higher than the reference voltage of 14 volts, the output of- comparator 11 is high. Conversely, when the transistor sensor is above the level of the liquid gas, that is within the gas zone of the cylinder, the voltage provided by the collector of the transistor sensor is lower than the reference voltage and the output at comparator 11^' is low.

The output from comparator 15 is made available to OR gate 30 and that OR gate derives a second output from comparator 31. The output from OR gate 30 is applied to driver 32 and driver 32 functions to enable LED2 to be energised. Resistor 33 is a current limiting resistor and provides for proper operation of LED2.

Zener diode Z2 is located within the probe which carries the various transistor sensors. Zener diode Z2 is employed to enable the voltage reference of 14 volts to be produced. Zener diode Z2 is located within the probe and is not directly exposed to the liquid gas. Zener diode Z2, in use, is a temperature sensor and provides a current signal representative of the temperature of the liquid gas within the cylinder. In use, sensor diode Z2 is located adjacent one end of the probe and is lowermost along the length of the probe below the liquid level or the location of the transistor sensor which enables the indication of the empty level of the cylinder to be secured. Zener diode 3 provides a protection function for sensing zener diode Z2. This is to ensure that diode Z2 is not readily damaged as this diode is located within the probe and difficult to replace. Amplifier 34 together with gain adjusting resistors 35 and 36 amplify the voltage input at the non inverting input of amplifier 34. That input voltage signal is representative of the current flowing through zener diode Z2. The output from amplifier 34 is further amplified by amplifier 37. The gain of amplifier 37 is controlled by feedback resistor 38 and resistors 39, 40 and 41. Resistor 40 is adjustable to enable the gain to be varied. Diode Dl and variable resistor 42 function to swamp any initially low input signal applied to the inverting input of amplifier 37 during filling of the cylinder from empty and once steady state conditions have been achieved, the signal available at the output of amplifier 34 reverse biases diode Dl and then predominates. Zener diode Z2 and the associated circuitry for producing the 14 volt reference signal by its nature, adjusts for temperature drift. Thus, as the temperature within the liquid gas varies, so does the voltage reference level.

The output from comparator 31 is applied as an input to invertor 43. The output from invertor 43 functions as one input to Schmidt trigger 44. Resistor 45 is coupled as a feedback element between the output of Schmidt trigger 44 and a second input for that Schmidt trigger. Resistor 45 together with capacitor 46 enable Schmidt trigger 44 to _ unction as an oscillator subject to the desired input. The output of the Schmidt trigger is coupled to driver 47. Amplifier 48 is AC coupled to one input of Schmidt trigger 49 via capacitor 50. The non-inverting'^{^}input for amplifier 48 is coupled to a voltage dividing circuit comprising resistors 51 and 52. Pull-up resistors 53 and 54 are coupled to invertor 43 and Schmidt trigger 49 respectively. Schmidt trigger 49 functions as a NAND gate. The output from gate 49 is coupled to an invertor 55 and the output from invertor 55 is applied via diode 56 to one input of NAND gate 49. The output from invertor 55 is applied via resistor 57 and diode 58 to driver 59. The input of the driver 59 has a pull down resistor _ 60 coupled to it. The output from driver 59 is coupled to a beeper 61. The beeper 61 is in series with current limiting resistor 62. Diode 63 is coupled between resistor 57 and the output of driver 47. Amplifier 48 is coupled to driver 64 which in turn is coupled to a relay coil RL. Relay coil RL functions to control or switch the contacts as shown. When relay RL is energised, the associated contacts enable a heater associated with those contacts, (not shown) , to be turned on if necessary. This heater is located within the cylinder. LED 3 is that LED which indicates that the cylinder is empty. LED3 is in series with current limiting resistor 65 and is coupled to driver 47.

NAND gate 49 derives one of its inputs from pull-up resistor 54. Resistor 54 is in series with capacitor 66. Switch SW provides a mute function and is coupled in parallel with capacitor 66. The arrangement of switch SW resistor 54 and capacitor 66 is such that when the switch is closed a momentary pulse is provided as an input to NAND gate 49. Pull-down resistor 67 is coupled to the other input of NAND gate 49.

As mentioned above, for the sake of simplicity, the circuitry associated with the half full level of the cylinder and quarter full level of the cylinder have not been shown and is identical with the circuitry associated with comparators 11 and 15 and includes like comparators.

The circuitry in figure 2 functions as follows. When the cylinder is empty of liquid gas and is about to be filled, LED's 1 and 2 are off as are those LED's normally indicative of half and quarter full within the cylinder. Those LED's may typically be green LED's. LED 3 which normally indicates that the cylinder is empty is flashing. This is because the transistor sensor associated with it and coupled to comparator 31 and the circuit elements between the comparator 31 and LED 3 cause it to flash. This occurs because the current flowing through the transistor sensor is relatively high and thus the voltage at the collector is at about 10.5 volts. This ensures that the output at comparator 31 when the 10.5 collector voltage is compared to the reference voltage, is low. That low output is inverted by NAND gate 43 and applied as an input to Schmidt trigger 44. This causes Schmidt trigger 44 to provide an oscillating output to the driver 47 and thus LED3 flashes. Once the transistor sensor associated with LED 3 and coupled to the comparator 31 is covered with liquid gas the temperature of that transistor drops and the current flowing through the collector emitter path changes to provide a collector voltage of about 15 volts, that is, higher than the reference voltage. This causes the output of the comparator 31 to go high and enables invertor 43 to provide a low input to the Schmidt trigger 44. This low input ensures that the schmidt trigger does not oscillate and provides and high output only causing the LED 3 to .remain on. In addition, amplifier 48 is then able to provide an output to driver 64 to enable the coil RL to be energised to allow the heater to be turned on if necessary.

The next phase in the filling operation causes the transistor sensor associated with the LED indicative of quarter full to be energised when that transistor is below the liquid gas level. Following this when the transistor sensor associated with the LED indicative of half full is below the liquid level, that LED is then energised and so on until LED 1 and LED 2 are also energised. Thus, when the cylinder is full all 4 green LED's are energised and LED 3, the red LED is permanently on.

Once gas is withdrawn from the cylinder, the liquid level within the cylinder commences to drop until the transistor sensor associated with LED 1 is above the liquid gas level. The sensors associated with LED 3, the quarter full LED, the half full LED and the three quarter full LED 2 are still covered by liquid gas. Because- the sensor associated with LED 2 is still covered by liquid gas, comparator 15 still provides a high output. This high output is provided as an input to AND gate 13 and AND gate 14 provides a high output and this high output is latched to the second input of AND gate 13 by resistor 20. Thus, because of this, the output of AND gate 13 is high as is the output of OR gate 12. AND gate 14 thus has two high inputs and a high output as does OR gate 16. Thus, while the sensor associated with the full indicative LED 1 is above the liquid gas level, LED 1 remains on because the three quarter full sensor is still covered with liquid.

Once the liquid level within the cylinder drops to uncover the transistor sensor associated with the three- quarter full indicative LED 2, comparator 15 provides a low output and this enables LED 1 to be turned off. Thus, a degree of hysteresis or lag is provided when the level drops and while a transistor sensor may be above the liquid level, it does not cause the associated LED to be turned off until the liquid level falls below the next lowest transistor sensor.

This operation continues until the liquid level is between the transistor sensor associated with the quarter- full indicative LED and the transistor sensor associated with the empty indicative LED 3 but with the latter mentioned transistor sensor still below the liquid level. In this state, the quarter full indicative LED and LED 3 are both on. Once the liquid level falls below the transistor sensor associated with LED 3 three things happen. Firstly, the quarter full indicative LED is turned off, LED 3 is caused to flash and relay RL is de-energised. On the positive going edge of the high output at invertor 43 a pulse input is supplied to Schmidt trigger 49. As both inputs in Schmidt trigger 49 are high (when switch SW is open) the low output from Schmidt trigger 49 is inverted by invertor 55 and made available to the junction between diode 63 and 58. The effect of this is that the beeper is caused to sound alternatively with the flashing of LED 3. Should it be desired to cause the beeper to turn off, switch SW is closed and this causes Schmidt trigger 49 to provide a high output with the invertor 55 providing a low output to de-energise the beeper.

It can be seen therefore that the circuit of the invention is capable of detecting an interface between fluids of different specific heats or even between fluids at different temperatures within a cylinder and enables, via the various LED's to provide a remote indication of a liquid gas level or the like. In addition, by the very nature of the inbuilt hysteresis, the circuit has the psychological effect of not immediately showing after a small amount of gas has been withdrawn from the cylinder, that the cylinder has quickly changed from its full to its say three-quarter full condition or from two other adjacent liquid level positions. When the cylinder is empty the circuit provides for a visual flashing indication as well as an audible indication of this condition if desired.

The circuit could equally be used to detect the interface between different liquids provided of course that those liquids had different thermal co-efficients. For example, such a condition may occur in a fermenter. The invention is able to provide its level indication in a closed high pressure system at low temperature within the system without the need for sight gauges or mechanical or moving parts. In a further application the invention may be applicable in a container of heated liquid such as a storage hot water system to provide an indication of the quantity of hot water accumulated in an upper region of the container.

Figure 3 shows an oscillating sensor 40 according to a further embodiment of the invention. The active component of the oscillator is transistor Q2. Bias resistors R40, R41 are connected as shown. Feedback capacitor CIO ensures that the circuit functions as an oscillator while reactive components capacitor Cll and indicator L govern the frequency of oscillation. The connection between components Cll and L is coupled to a radiating element RE which ensures that electromagnetic energy provided by Cll and L is not only feed to the base electrode of transistor but also to one spaced surrounding the oscillator. It is believed that the amount of energy distributed to the surrounding space and to the transistor is a function of the dielectric constant of the space.

The element RE is preferably tubular to provide a large effective surface area from which the radiation may be radiated into the surrounding space. Preferably, the element is cylindrical. When th *e. element is brought under the influence or within the presence of the material whose level is being detected more of the energy provided by the capacitor/indicator combination Cll/L is absorbed by the material and the oscillator ceases to oscillate. Thus the sensor has detected the presence of the material.

Bypass capacitor C12 is coupled between the emitter electrode of the transistor Q2 and the reference rail. The output from the emitter of Q2 is supplied to base resistor R42 of transistor Q3. The transistor Q3 is biased on when an output is provided by transistor Q2 and a current is caused to flow through collector resistor R43.

All of-the—sensors S^ — S_N are coupled to the common line and thus the current caused to flow on that line is indicative of the number of oscillator sensors oscillating. By monitoring this current an indication of the level of the material can be determined.

Figure 4 shows a block diagram of a complete level detector incorporating a plurality of sensors as described with reference to figure 3.

A power supply PS which typically provides +24VDC from an AC input. A plurality of sensors S __ S-^ are shown coupled in parallel across the output of the power supply. Each of these sensors supplies an oscillating output whenever the sensor is not near the material whose level is to be detected. These signal^'s are fed to a common line as indicated and a resistor R44 is coupled between the +24V supply and the common line. Thus the potential difference developed across the resistor R44 is directly proportional to the sum of all the currents caused.to flow on the common line by the sensors. The voltage or potential diff rence developed in this way is applied'as an input to amplifier 70. The output from amplifier 70 is indicative of the level of the material whose level is being sensed by the sensors. Material such as cement for example may be present in a silo and the sensors S _N may be arranged at equi- spaσed distances within a tube which is in turn imbedded in or standing upright in the cement. The tube may have its lower end closed off to prevent the cement from entering the tube. The sensors of the invention need not physically contact the material in order to enable level determination. As is evident from figure 4, only three wires need extend down the tube.

With material such as cement it does not matter that the tube may well be placed under considerable tension (and hence stretched) when surrounded by cement - the relative position and spacing of the sensors can readily remain constant. The sensors do not physically contact the material and thus no contamination need occur by the use of the sensors of the invention.

Figure 5 shows a level sensing or monitoring probe 80 according to the first embodiment of the invention.

Probe 80 comprises a hollow plastics tube 81 sealed by a plug 82 at its lower end. The upper end of probe 80 is sealed by a gland unit 83 through which multi-cored cable 84 passes.

Transistors 85 are sealed in apertures in the wall of tube 81 with a suitable sealant such as epoxy resin. Alternatively the interior of tube 81 may be filled with a suitable insulating potting compound such as a silicone compound polyester compound or the like.

Figure 6 shows a level sensing or monitoring probe 90 according to the second aspect of the invention.

Probe 90 comprises a plastics tube 91 sealed at its lower end by a plug 92. The upper ^nd of the tube 91 is sealed by a gland nut 93 through which multi-cored cable 94 passes.

Conductors 95, 96, 97 of cable 94 are connected in parallel to oscillators 98 spaced along the interior of tube 91.

For use in detecting or monitoring the level of a flowable particulate material such as cement powder or the like, a plastics tube is preferred as the probe is subjected to considerable tension due to friσtional engagement with the body of particulate solids as it moves downwardly inside a storage silo. As the oscillators 98 are freely suspended within tube 91 their relative positions are unaffected by stretching of the tube 91 and accurate monitoring of levels relative to a datum is ensured. Figure 7 shows a variation of the second embodiment of the invention.

The apparatus comprises a probe tube 100 sealed at its lower end by a plug 101. A cap 102 at the upper end of tube 100 has an aperture 103 through which a multi-cored cable 104 may freely pass.

Cable 104 is wound around a drum or capstan 105 attached to stepping motor 106. The conductors of cable 104 are connected by any suitable means such as slip rings 106 to suitable counting circuit 107. A display unit 108 such as a liquid crystal or LED display is provided to display a reading calibrated to appropriate units of volume, height, weight etc. Counter unit 107 is operatively connected to stepping motor 106 to selectively actuate or deactivate motor 106. In use stepping motor 106 is actuated to lower energized oscillator 109 down tube 100. Whilst above the fluid level 110, oscillator 109 returns a signal (via an appropriate conductor of multi-cored cable 104) to the counter circuit 107 which in turn maintains stepping motor 106 in an energized state and at the same time circuit 107 counts the rotational steps proportional to the position of oscillator 109.

As oscillator 109 reaches a position relative to the surface of fluid 110, the changed dielectric constant causes oscillator 109 to cease sending back a signal to counter 107. Counter 107 then deactivates stepping motor 106 and display 108 then displays a value calibrated in units relative say to the height of fluid level 110 or a volume of fluid in a container corresponding to the height of the fluid level 110 in the container.

It can be seen that with this variation, highly accurate measurements of fluid levels may be determined.

The apparatus may be calibrated such that any adherent materials such, as say a layer of sewerage sludge are insufficient to prevent oscillation thereby avoiding the necessity for constant cleaning of the level probe as required with other prior art devices of this type.

The apparatus according to the second aspect of the invention is suitable for both harsh and sensitive environments alike.

Claims

1. A level sensing apparatus for sensing the level of a fluid, said apparatus including: a plurality of sensors positionable at spaced intervals relative to a fluctuating fluid level, said sensors providing an output indicative of the close proximity of the fluid, output circuitry for receiving the outputs from the sensors and providing a signal indicative of the level of the material within the container.

2. A level sensing apparatus for sensing the level of a material within a container, said apparatus including: a plurality of sensors positionable within the container at spaced intervals therealong, each said sensor including an oscillator normally providing a first output when the material is not proximate the sensor and a second output when the material is proximate the sensor; output circuitry for summing the outputs from the sensors to provide an indication of the level of material within the container.

3. The apparatus of. claim 2 -wherein each said oscillator is a transistor oscillator.

4. The apparatus of claim 2 or 3 including a capacitive feedback element.

5. The apparatus of any one of claims 2 to 4 including reactive components coupled to the oscillator for controlling the operation of the oscillator.

6. The apparatus of claim 5 including a radiating element coupled to the reactive components.

7. The apparatus of claim 6 wherein said radiating element is an antenna.

8. The apparatus of claim 7 wherein the antenna is cylindrical in shape and said reactive components comprise a series connected capacitor and indicator with the antenna being connected to a junction between the reactive components.

9. The apparatus of any one of claims 2 to 8 wherein said sensors are connected in parallel and the outputs form the sensors are connected to a common conductor.

10. The apparatus of claim 9 wherein said output circuitry includes a summing amplifier which receives as its input the outputs from the sensors.

11. A level sensing apparatus for sensing the level of a material, said apparatus including a plurality of temperature responsive level sensors positionable within the container at spaced intervals therealong, said sensors providing a temperature dependant output, respective logic circuitry coupled to each said sensor; a temperature responsive reference sensor normally located within the- container and below a level for providing a reference output, each said logic circuit including a comparator for comparing the reference output with the temperature dependant outputs; and, respective indicating means coupled to each said logic circuitry for providing an indication in response to the comparison, said indication being representative of the level of the material within the container.

12. The apparatus of claim 11 wherein said sensors are- located along a probe positionable within the container with the probe enabling at least a part of each said sensor to be contacted by the material.

13. The apparatus of claim 12 wherein said probe is tubular with apertures along its length in which said apertures said sensors are positioned.

14. The apparatus of any one of claims 11 to 13 wherein said sensors each comprise a semi-conductor junction.

15. The apparatus of any one of claims 11 to 14 wherein each said level sensor includes a bipolar transistor and said reference sensor is a zener diode.

16. The apparatus of any one of claims 11 to 15 including an amplifier responsive to the reference sensor for providing a reference output of a desired magnitude.

17. The apparatus of claim 16 wherein said amplifier is a two stage amplifier and said reference output is a reference voltage.

18. The apparatus of claim 15 wherein each said level sensor includes biassing components associated with the transistor to bias the transistor into its conducting state and level setting means to confine the construction of the transistor within a particular range of its conduction curve.

19. The apparatus of claim 18 wherein said level setting means comprises a zener diode coupled between a bare resistor and the collector electrode of the transistor.

20. The apparatus of any one of claims 11 to 19 wherein the logic circuitry associated with each said level sensor is associated with the logic circuitry of another said logic circuitry.

21. The apparatus of any one of claims 11 to 20 wherein each said indication means is a light emitting diode.

22. The apparatus of any one of claims 11 to 21 wherein said level sensors include a full level sensor, three quarter full level sensor, half full level sensor, quarter full level sensor and empty level sensor.

23. The apparatus of claim 22 wherein said logic circuitry associated with the empty level sensor includes an oscillator to enable the indicating means to flash.

24. The apparatus of claim 22 including a heater associated with the logic circuitry for the empty level sensor.

25. The apparatus of claim 22 including an audible indicator associated with the empty level sensor for providing an audible indication of when the level falls below the empty level sensor.