GB2354585A

GB2354585A - Apparatus for sensing the level of a fluid in a container

Info

Publication number: GB2354585A
Application number: GB9922866A
Authority: GB
Inventors: Stephen Francis Gallagher
Original assignee: CHARIS TECHNOLOGY Ltd
Current assignee: CHARIS TECHNOLOGY Ltd
Priority date: 1999-09-27
Filing date: 1999-09-27
Publication date: 2001-03-28
Anticipated expiration: 2019-09-27
Also published as: GB9922866D0; GB2354585B

Abstract

Apparatus (2) for sensing the level of a fluid (6) comprises a plurality of electrodes (12), and control means (14) for causing the electrodes (12) to operate such that they perform a first function or a second function, the first function being a measuring function for measuring current flowing through the fluid (6), and the second function being an electrical guard function, the control means (14) being such that the electrodes (12) are each set in sequence to perform the first function whilst adjacent electrodes (12) are set to the second function, whereby the electrical current from the electrodes (12) when they are performing the first function is able to be steered in terms of the direction in which the electrical current travels.

Description

2354585 APPARATUS FOR SENSING THELEVEL OF A FLUID This invention relates

to apparatus for sensing the level of a fluid, for example the level of a fluid in a container, pond or channel.

There are numerous different types of known apparatus for sensing the level of a fluid. The cost of these different types of apparatus varies widely in dependence upon their construction and their intended use. The known different types of apparatus may be.mechanical, electrical, hydrostatic, ultrasonic, optical, radar, nucleonic or thermal. The mechanical apparatus may utilise float switches or tuning forks. The electrical apparatus may utilise conductivity, capacitance or admittance. The hydrostatic apparatus may be a pressure transducer hydrostatic apparatus. The optical apparatus may be infrared or fibreoptic apparatus. The various known types of apparatus all have their advantages and their disadvantages. They may be used. for measuring various types of fluid including liquids, foams and powders. Very few of the known different types of apparatus are suitable for use with foam, and the foam is a nuisance factor for many different types of known apparatus. Many liquids have foam floating on their surface. Sometimes the foam level is required and sometimes it is not. However the foam nearly always causes interference or complications for the liquid level measurement.Where the known apparatus for 2 sensing the level of a fluid is electrical apparatus, then the apparatus may use electrically operated level sensors which utilise an electrical parameter of the fluid in order to sense the amount or level of fluid. As indicated above, the known electrical apparatus may utilise the electrical conductance or the dielectric constant of the fluid. The electrical apparatus has the disadvantage that the measurement is effected by foam on the liquid surface, the presence of a build up of sticky material on the surface of the apparatus, and changes in the electrical characteristics of the fluid being measured. These complications frequently occur in process control applications and they limit the use of the electrical apparatus.

It is an aim of the present invention to provide apparatus for sensing the level of a fluid, which apparatus operates electrically and in which the above mentioned disadvantages are obviated or reduced.

Accordingly, in one non-limiting embodiment of the present invention there is provided apparatus for sensing the level of a fluid, which apparatus comprises a plurality of electrodes, and control means for causing the electrodes to operate such that they perform a first function or a second function, the first function being a measuring function for measuring current flowing through the fluid, and the second function being an electrical guard function, and the control means being such that the electrodes are 3 each set in sequence to perform the first function whilst adjacent electrodes are set to the second function, whereby the electrical current from the electrodes when they are performing the first function is able to be steered in terms of the direction in which the electrical current travels.

The apparatus of the present invention is advantageous when compared with the above mentioned known apparatus in that the apparatus of the present invention is immune to fouling by the build up of sticky material on the surface of the apparatus. The apparatus is able automatically to compensate for changes in fluid characteristics. The apparatus is able to measure fluids in the form of foams and/or liquids. The apparatus is also able to measure fluids in the form of fluid powders and other fluid particulate materials. The apparatus is able to measure foam profiles. The apparatus is also able to measure fluids where layers of different materials occur, one above another, e.g. oil and water. The apparatus can be designed in a hygienic manner for use in the food industry. The apparatus of the present invention is also able to withstand high temperatures and pressures, and it can be produced to be easily cleanable. The apparatus is suitable for clean in place schemes, and it is able to have a low maintenance requirement.

Generally, the apparatus of the present invention may be useful for the measurement of fluid levels in a wide 4 variety of process applications. The apparatus of the invention can be used to measure the level of a liquid where there is a layer of foam on top of the liquid. The apparatus can also be used to measure how much foam is present on top of the liquid. The apparatus may be especially useful for use in situations where foam is generated in a process as a result of agitation, or the mixing of gas into the process. Thus the apparatus of the present invention may be useful in a wide variety of process applications including biotech process applications, chemical manufacture, food manufacture and gas scrubbing. The apparatus of the present invention may also be useful for detecting interfaces between different liquids.

The control means will normally operate such that the electrodes are set in turn. Other sequences could however be employed if desired.

The control means may be a switching circuit. Alternatively, the control means may be a multiplexer.

The apparatus may be one in which the electrodes are arranged to extend in a line, and in which the electrodes are insulated from each other by electrically insulating members.

The electrodes are preferably made of stainless steel. Other materials for the electrodes may be employed. The electrically insulating members are preferably made of poly ether ether ketone. other materials may be employed if desired.

The electrodes are preferably constructed to be in the form of an elongate sensor. The electrodes are then easily located in a vertical position, for example in a container for the fluid. The electrodes may however be in a different construction if desired.

The apparatus may be one in which there is a terminal head at the end of the elongate member that is uppermost during use of the apparatus. The terminal head may contain a terminal block for making electrical connections.

The apparatus may include compensation means for compensating automatically for changes in characteristics of the fluid. The compensation means may be such that it uses one or more submerged electrodes to measure the electrical characteristics of the fluid. A check that the relevant electrode is submerged may be obtained from the measurement by the next electrode above. Submerged electrodes will provide a constant measurement unless the electrical characteristics of the fluid change.

Preferably, there are eight of the electrodes. The apparatus may however include more or less than eight of the electrodes.

The present invention also extends to the apparatus of the invention when in combination with a container.

The container may be made of an electrically conducting material, whereby the electrical current from 6 the electrodes when they are performing the first function is constrained to travel in a horizontal plane to an edge of the container, and whereby the container is able to act as a return electrode for the flow of the electrical current.

In an alternative embodiment, the container may be made of a nonelectrically conducting material, in which case the container will include an additional electrode, whereby the electrical current from the electrodes when they are performing the first function is constrained to travel in a horizontal plane to the additional electrode, and whereby the additional electrode is able to act as a return electrode for the flow of the electrical current. The additional electrode will normally be positioned adjacent a wall of the container but it may be positioned elsewhere if desired.

An embodiment of the invention will now be described solely by way of example and with reference to the accompanying drawings; Figure I shows apparatus in use for sensing the level of a fluid in a container; Figure 2 is a circuit block diagram showing the electrical circuit employed in the apparatus shown in Figure 1; and Figures 3, 4 and 5 illustrate how the apparatus shown in Figure 1 is able to be steered in terms of the direction in which the electrical current travels.

7 Referring to Figure 1, there is shown apparatus 2 for sensing the level 4 of a fluid 6 in a container 8. Foam 10 is on top of the fluid 6 as shown.

The apparatus 2 comprises eight electrodes 12, and control means 14 for the electrodes 12. The control means 14 is for causing the electrodes 12 to operate such that they perform a first function or a second function. The first function is a measuring function for measuring current flowing through the fluid 6. The second function is an electrical guard function. The control means 14 is such that the electrodes 12 are each set in turn to perform the first function whilst adjacent electrodes are set to the second function. Thus the electrical current from the electrodes 12 when they are performing the first function is able to be steered in terms of the direction in which the electrical current travels.

The control means 14 is in the form of a switching circuit as shown in detail in Figure 2.

As can be seen from Figure 1, the electrodes 12 are arranged to extend in a line. The electrodes 12 are insulated from each other by electrically insulating members 16. The electrodes 12 are made from stainless steel tube, and the electrically insulating members 16 are made of poly ether ether ketone. The apparatus 2 is such that the electrodes 12 are in the form of an elongate sensor which is tube-like and which is easily vertically positioned in the container 8 as shown in Figure 1. In 8 Figure 1, it can be seen that the apparatus 2 has a clamp 18 for holding the electrodes 12 in place in a top part 20 of the container 8.

The electrodes 12 are associat.ed with a terminal head 22 at the end of the elongate sensor that is uppermost during use. The terminal head 22 contains a terminal block (not shown) for making electrical connections.

The container 8 is made of an electrically conducting material. Thus the electrical current from the electrodes 12 when they are performing the first function is constrained to travel in a horizontal plane to an edge of the container 8. The container 8 is able to act as a return electrode for the flow of electrical current.

The apparatus 2 shown in Figure 1 can be regarded as having three main parts, mainly the electrodes 12 in the form of the elongate sensor, separate control means in the form of the electronic circuit shown in Figure 3, and a cable (not shown) which connects the sensor shown in Figure 1 to the control means 14 shown in Figure 2.

The electrodes in the elongate sensor measure the impedance of the fluid which surrounds the electrodes. The fluid may be the liquid 6 and/or the foam 10. The impedance is a combination of the conductance and the capacitance of the fluid. The measurement is done by using an alternating voltage of fixed amplitude which is connected to each electrode 12. The resulting current is measured. The current flow is dependent on the electrical 9 characteristics of the fluid being measured, ie its conductance and capacitance. In an alternative embodiment of the invention, the same measurement can be carried out by using a constant current circuit and measuring the voltage instead.

As can be seen f rom Figure 1, the elongate sensor formed of the electrodes 12 is in the form of a cylinder. The terminal block in the terminal head 22 is used to make appropriate connections to the electrodes 12. Each electrode 12 has an internal connection which connects to a wire (not shown) inside the sensor and is connected to the terminal block. The electrode 8 at the bottom of the sensor is made from stainless steel bar so that the lower end of the sensor is closed. The external surfaces of the sensor are free from ridges and crevices and are polished. This makes the sensor suitable for a wide range of applications such for example in the pharmaceutical and food industries.

The electrodes 12 and the electrically insulating members 16 are sealed so that no material can gain ingress into the inside of the sensor, even if a high pressure is present.

The control means 14 is an electronic circuit as shown in Figure 2. As can be seen from Figure 2, the electronic circuit comprises an oscillator 24, a current measuring circuit 26, a guard circuit 28, a sense multiplexer 30, a guard multiplexer 32 and a control circuit 34. The oscillator 24 generates an alternating voltage which is used to carry out the measurements. The. current measuring circuit 26 is used to measure the current flowing from each electrode 12. The guard circuit 28 dynamically reproduces the voltage at the output of the current measuring circuit 26, but increases the available current. The sense multiplexer 30 is used to connect the current measuring circuit to any one of the electrodes 12 as shown. The circuit controller 34 co-ordinates the operation of the oscillator 24, the current measuring circuit 26, the guard circuit 28, the sense multiplexer 30 and the guard multiplexer 32. The circuit controller 34 is a microprocessor. It may however alternatively be another type of control means such for example as a dedicated logic circuit, or a field programable gate array.

The control means 14 as shown in Figure 2 is such that the circuit is constructed on two printed circuit boards. One printed circuit board contains the current measuring circuit 26, the circuit controller 34 and the power supply. The other printed circuit board contains the sense multiplexer 30 and the guard multiplexer 32. The multiplexers 30, 32 plug into the other printed circuit board as a piggy-back.

The oscillator 24 generates an a.c. signal of approximately 1 volt and a frequency which may be set between 1KHz and 100KHz, but may be higher or lower for some unusual applications. This is used to energise the 11 elongate sensor. The current measuring circuit 26 is used to apply the 1 volt ac signal to the electrode 12 under test, and the current flowing is measured. If there is no liquid 6 or foam 10, then no current flows. If there is foam 10, then current flows through the foam and/or the liquid 6 to the wall of the container 8. The more the electrode 8 is submerged in liquid 6 or foam 10, the more current flows. If the foam has more liquid in it, ie it is denser, then more current flows. Once an electrode 8 responds, then by definition the electrode 8 below must be completely submerged. The current is then used to tell how much the electrode 8 is submerged, and if the electrode 8 is submerged in liquid or foam. The liquid 6 gives a much larger current flow than.the foam 10.

The guard circuit 28 is an amplifier which measures the voltage at the current measuring circuit 26, and reproduces this voltage. The guard circuit 28 can drive more current than the current measuring circuit 26, but this current is not measured. The current measuring circuit 26 and the guard circuit 28 are each connected to the multiplexers 30, 32 as shown. The multiplexers 30, 32 just act as a switch to connect the current measuring circuit 26 or the guard circuit 28 to each electrode 12 in a chosen sequence, which sequence is each electrode in turn. Thus each electrode 12 can be connected to either the current measuring circuit 26, the guard circuit 28, or nothing. In order to operate the sensor, it is possible to 12 start at the uppermost electrode 12 or the lowermost electrode 12. Each electrode can be connected in turn to the current measuring circuit 26 and the current flowing can be measured. All other electrodes 12 are connected to the guard circuit 28. In this way it is possible to measure what is happening at every point along the length of the sensor as shown in Figure 1, in order to f ind out if there is liquid present, or foam, and how much.

The circuit controller has a microprocessor which processes the information on the current measurements. The current measuring circuit 26 is connected to the microprocessor via an internal analogue to digital converter (not shown) The guard circuit 28 operates to prevent stray or leakage currents affecting any one of the electrodes 12. The guard circuit 28 acts as a guard conductor used in proximity to a measuring conductor, for example a wire or a printed circuit board track. The guard conductor is driven at the same voltage as the measuring conductor, but at low impedance. There is no voltage drop between the conductors. No voltage difference means that no current can f low. This thus guards or isolates the measurement conductor from stray currents caused by humidity or contamination, f or example on a printed circuit board surface.

The ability of the electrical current to be steered in terms of the direction in which the electrical current 13 travels will now be explained further with reference to Figures 3, 4 and 5. Consider the following three conditions at electrode n which is operating as a sensing electrode by reason of being switched to the measuring circuit. The electrode 12 above and below will be n-1 and n+1. Depending on the function switched to these electrodes, the current from electrode n will change in direction as described below.

(a) No guard is connected to n and n+1. In this case, the electrodes above and below the sense electrode have no effect, and the current from electrode n will flow in all directions. This flow of the current is shown in Figure 3.

(b) A guard is connected to n+1 above the sense electrode only. This forces the current at the top of the electrode n to be horizontal. This is because there is no potential difference between the electrodes n and n+1 so the current cannot flow sideways. This is shown in Figure 4.

(c) The guard electrode is connected to electrodes n+1 and n-1. In this case, the current. from electrode n can only flow horizontally. This will usually be the normal mode of operation. The pattern of the three electrodes will be moved from bottom to top so that each electrode in turn becomes a sense electrode. In this way, it is possible to find out what material is at each 14 electrode since the electrode is measuring a flat slice of material.

In addition to the above, if the phase difference between the guard and the sense function is changed, the current "beam" can be moved between the shapes shown in Figures 3, 4 and 5 as intermediate patterns.

During operation of the apparatus 2, the switching between the first and the second functions gives substantial immunity to the apparatus 2 from fouling, because it prevents the current flowing between adjacent electrodes.

It will be appreciated that the switching also enables changes in fluid characteristics to be measured as the lower electrodes can be used to measure the fluid parameters once the electrodes are fully submerged as the electrode dimensions are fully known, and the volume of fluid being measured is fixed by means of the horizontal current f low. The lower electrodes can be found to be submerged when the upper electrodes start to detect fluid. For example, the second electrode 8 may be submerged, when the third electrode detects fluid because the third electrode is vertically above the second electrode.

The apparatus 2 operates with good overall accuracy. once each electrode 8 is fully covered, as indicated by the electrode above, then the level of fluid at that electrode is entirely accurate. The only uncertainty then is the amount of submersion of the electrode at the top of the is fluid. This amount of submersion can be found by measuring the current f low and comparing it to the current from the electrode below. This gives a very highly accurate result.

Generally, foam can be discriminated from liquid as the density of the foam is at most only 10% of the liquid density. This results in a current flowing foam which is one tenth of that in a liquid. This can be used to identify if a completely submerged electrode is measuring foam or liquid or a combination of both. If foam is identified, then the electrode at the top of the sensor column will be surrounded by foam only, since the foam is always at the surface. The only uncertainty is if a small amount of foam were present within the height of an electrode. This can be discriminated from a smaller amount of liquid by changing the current flow pattern as described below.

The information obtained can be improved by directing the current flow away from the horizontal by switching off the guard signals to one side or both sides of the electrode used to measure at the top of the liquid. If the guard signal is switched off on top of the sensing electrode, the current flow will be directed upwardly as well as horizontally. This can be used to give more information about the interface of the foam and the liquid. Alternatively, the guard means can be switched off below 16 the electrode which is measuring, and then the current can be directed downwardly as well as horizontally.

It is to be appreciated that the embodiment of the invention described above with reference to the accompanying drawings has been given by way of example only and that modifications may be effected. Thus, for example, the sensor column may be designed to be of a different size and shape to that shown in Figure 1. Also, the circuit shown in Figure 2 may be developed further by using phase shifting circuits which can be used to control the guard circuit signals, in order to steer the current beam in a more precise way. This will allow the current beam to be steered from the electrodes 12 more precisely, and small adjustments in the angle to be carried out. Still further, although Figure 1 shows the use of eight electrodes 12, more or less than eight electrodes 12 may be employed. Instead of being in a container, the fluid may be elsewhere, for example in a pond or a channel.

17

Claims

1. Apparatus for sensing the level of a fluid, which apparatus comprises a plurality of electrodes, and control means for causing the electrodes to operate such that they perform a first function or a second function, the first function being a measuring function for measuring current flowing through the fluid, and the second function being an electrical guard function, and the control means being such that the electrodes are each set in sequence to perform the first function whilst adjacent electrodes are set to the second function, whereby the electrical current from the electrodes when they are performing the first function is able to be steered in terms of the direction in which the electrical current travels.

2. Apparatus according to claim I in which the control means operates such that the electrodes are set in turn.

3. Apparatus according to claim 1-or claim 2 in which the control means is a switching circuit.

4. Apparatus according to claim 3 in which the control means is a multiplexer.

5. Apparatus according to any one of the preceding claims in which the electrodes are arranged to extend in a line, 18 and in which the electrodes are insulated from each other by electrically insulating members.

6. Apparatus according to claim 5 in which the electrodes are made from stainless steel.

7. Apparatus according to claim 5 or claim 6 in which the electrically insulating members are made from poly ether ether ketone.

8. Apparatus according to any one of claims 5 - 7 in which the electrodes is in the form of an elongate sensor.

9. Apparatus according to claim 8 in which there is a terminal head at the end of the elongate sensor that is uppermost during use of the apparatus.

10. Apparatus according to claim 9 in which the terminal head contains a terminal block for making electrical connections.

11. Apparatus according to any one of the preceding claims and including compensation means for compensating automatically for changes in characteristics of the fluid.

12. Apparatus according to any one of the preceding claims in which there are eight of the electrodes.

19

13. Apparatus for sensing the level of a fluid, substantially as herein described with reference to the accompanying drawings.

14. The combination of apparatus according to any one of the preceding claims and a container.

15. The combination according to claim 14 in which the container is made of an electrically conducting material, whereby the electrical current from the electrodes when they are performing the first function is constrained to travel in a horizontal plane to an edge of the container and whereby the container is able to act as a return electrode for flow of the electrical current.

16. The combination according to claim 14 in which the container is made of a non-electrically conducting material, and in which the container includes an additional electrode, whereby the electrical current from the electrodes when they are performing the first function is constrained to travel in a horizontal plane to the additional electrode, and whereby the additional electrode is able to act as a return electrode for the flow of the electrical current.

17. The combination according to claim 14 in which the additional electrode is positioned adjacent a wall of the container.