GB2571243A

GB2571243A - Level probe and sensor

Info

Publication number: GB2571243A
Application number: GB1720651.7A
Authority: GB
Inventors: Jorge Gomes Gois Paulo
Original assignee: BDR Thermea Group BV
Current assignee: BDR Thermea Group BV
Priority date: 2017-12-12
Filing date: 2017-12-12
Publication date: 2019-08-28
Also published as: WO2019115639A1; GB201720651D0

Abstract

A liquid level probe has two electrically conductive members e.g. 8a and 8b separated by an electrically insulating member comprising a downward-facing cup. The cup traps a bubble of air 14, preventing electrical bridging of adjacent conductive members in the face of scale, or retained liquid films. Both resistive and capacitive probes are described. Electrical resistance may be measured between each conductive member and a reference electrode using an AC voltage. An electrically powered water heater comprising the level sensor is also claimed. The reference electrode may comprise a submerged heating element of the water heater.

Description

LEVEL PROBE AND SENSOR

Field of the Invention

The invention relates to level probes for sensing the level of a liquid e.g. within a tank. The invention also relates to level sensors incorporating such probes, and to water heaters incorporating such sensors.

Background and Prior Art

Level sensors, for sensing the level of liquids in vessels are well-known, and take many forms. For all types of liquid, including both aqueous and non-aqueous liquids, such sensors include ultrasonic distance measuring units that reflect an ultrasonic wave from the liquid surface, and measure its time of flight. Such a sensor can give a continuous measure of the liquid height, but relies on knowledge of the acoustic properties of the liquid, and for the liquid to be able to transmit ultrasound. Other sensors include mechanically-coupled floats whose height can be measured to provide a measure of the liquid level. These, again, can give a continuous measure of the liquid level.

Other types of sensors rely on the liquid breaking a series of light beams crossing the tank as the liquid level changes. These have no moving parts, and in contrast to the sensors described above give a discrete measure of liquid level, i.e. the level in the tank is subdivided into a number of discrete ranges, and the sensor can detect which range includes the liquid interface.

Such a discrete measure of liquid level is often adequate for applications where the sensor is used to detect over- or under-filling events, or to confirm that the liquid level is within a desired range.

For electrically-conducting liquids (i.e. predominantly water-based), the electrical properties of the liquid can also be used to provide such a discrete level measurement. Such systems are often used in water heaters, especially heaters used to produce boiling or near-boiling water for beverages. Two such systems are illustrated in schematic view in Figures 1 and 2.

In Figure 1, three electrically-conductive probes la, lb, lc, of different lengths are positioned within a water tank 2. As the level of the water 3 changes, the conductivity of the water can provide an electrical connection between two or more of the probes. In the configuration illustrated in Figure 1, there will be an electrical flow path between probes lb and lc, but not between la and the other two probes. Measurement of the electrical conductivity between the probes can thus be used to determine that the water level is somewhere between the bottom end of probe la and the bottom end of probe lb. Once the level falls below the bottom of probe lb, no electrical flow path exists between the probes, and conductivity measurement can therefore be used to determine that the water level is somewhere below the bottom end of probe lc. In a slight variant of this, a submerged electrically-conducting heating element 4 can also be used as one of the electrodes. Although functional, systems such as this tend to be relatively bulky to avoid bridging of the probes by capillary action and scale build-up. Also, due to the reduced quantity of metal in contact with the water, negative effects of corrosion on this type of electrode tend to be noticeable quite prematurely. For very long electrodes, there is also a problem of mechanically stabilising them in place.

In Figure 2, three electrically-conductive studs 5a, 5b, 5c are mounted through the side wall of the tank 2. In a way analogous to that described in reference to Figure 1, measurement of the electrical conductivity between the studs 5 or between the studs 5 and a heating element 4 can allow the determination of the approximate height of the water 3. Whilst this is less bulky than the arrangement of Figure 1, the arrangement of wall-penetrating studs 5 can be a source of leakage. In addition, if any of the studs corrode, or need replacing, this can be difficult, e.g. requiring the draining of the tank and subsequent access to both the interior and exterior of the tank wall.

A further known level sensor is illustrated in Figures 3-6. In this sensing arrangement, a level probe 6 is provided having alternate regions 7a-7e of insulating material and regions 8a-8d of conducting material. Again, in a way analogous to the systems described above, the electrical resistance between each conductive member 8, or between each conductive member and a reference point (such as the element 4) can be used to determine the approximate liquid level. In this example, the electrical resistance between each conductive member 8a-8d and the element 4 is denoted Rj - R₄ respectively. For the arrangement and liquid level of Figure 3A the relative resistances are shown in Figure 3B. It can be seen that, as all electrodes are covered with water, the electrical resistance between each region 8 and the element 4 is approximately the same.

Figure 4A illustrates the same arrangement as Figure 3A, but with a lower liquid level. Like elements described in Figure 3 A are correspondingly numbered. It can be seen in Figure 4B, that the resistances R1 and R2 are significantly higher than R3 and R4, indicating that the water level is somewhere between the bottom of region 8b and the bottom of region 8c.

Such systems are compact, and work well when first installed. However, Figures 5-6 illustrate a common fault that occurs after repeated filling and emptying of such a tank. Again, like elements previously described are correspondingly numbered. In areas of hard water (i.e. water with a high dissolved mineral content, typically calcium and magnesium carbonates), and especially where such a level sensor is used in a water heater, a layer of precipitated minerals (often called “scale”) forms on the surface of the probe. This scale 9 is typically the result of boiling water having what is called “temporary hardness”, characterised by the presence dissolved bicarbonate minerals such as calcium bicarbonate and magnesium bicarbonate. When the water is boiled, carbonates are formed from the bicarbonate anions and calcium carbonate is precipitated onto the surface as scale 9. Such scale 9 is typically very porous, and can be completely wetted by the water 3. Figure 5A illustrates such a situation, with a full tank 2 of water 3. In this situation, the relative resistances R1-R4 will be as shown in Figure 5B. As all of the electrodes are covered with water, and the water permeates the layer of scale 9, the resistances will all be about the same. The system can then correctly determine that the water level must be above the lower end of region 8a.

In Figure 5B, however, the water level has dropped, but soon after dropping, the scale 9 on the surface of the probe 6 remains wet, and therefore electrically conductive. As a result, the relative resistances will be as shown in Figure 6B, with R| and R₂ (solid bars) very similar to R₃ and R₄. It is therefore very difficult to interpret this result as being different from the situation illustrated in Figure 5. Furthermore, should the scale 9 on the exposed region of the probe 6 dry out, then the measured resistances R i and R₂ might return to the expected values as indicated by the dotted bars in Figure 6B. So, not only does the presence of scale give incorrect readings, but the incorrect readings are also dependent on whether the readings are taken when the scale is wet or dry. Over time, of course, the phenomenon becomes more pronounced, but might become less pronounced if the tank and the probe are de-scaled. As a result, it is very difficult to programme a sensor to analyse the resistance measurements and provide a reliable level estimate.

In addition to the problem of scaling described above, this type of sensor is also unreliable because, as the water level drops, a film of liquid (e.g. water) can be left on the surface of the probe providing a false reading due to e.g. conductivity of the water film or (for capacitive sensors) the capacitive effect of the liquid film.

Figures 22-23 illustrate, in plan and sectional elevation view respectively a known capacitive level sensor, generally indicated by 29. The sensor comprises an insulating base 30 and a series of electrically conductive regions 31. Each conductive region 13 is operably connected to an electrical connector 32 from which a bundle of signal wires 33 may be taken to a measuring unit. The fluid contacting surface of each conductive region 31 is covered by a dielectric material 34. By this means, the assemblage of each conductive region 31, its dielectric covering 34 and the air or liquid in contact with the dielectric covering form a capacitor. The capacitance of each assemblage therefore varies with the presence or absence of adjacent liquid. This can be measured to estimate the level of a liquid in which the sensor 29 is submerged.

Figures 24 - 26 illustrate the sensor of Figures 22-23 in-situ in a tank 2 partially filled with a liquid such as water 3. It can be seen that conductive region 31A is out of the liquid, whereas regions 31B-31F are submerged. As a result, the capacitance of the assemblage comprising region 31A will be lower than that of the assemblages comprising regions 31B-31F. Measurement of each capacitance can thereby be use used to determine the approximate position of the liquid level 13.

However, for some combination of fluids 3 and dielectric coverings 34, when the liquid level 13 drops, as illustrated in Figure 25, a fluid film 35 can remain on the surface of the dielectric covering 34, leading to false readings. In addition, and analogous to the problem described above, is the sensor 29 is used to measure the level of hard water, a buildup of scale 9 can form on the surface of the sensor 29 after repeated use. This is particularly the case where the sensor is used to measure level in a water heater, in which the temperature cycling can lead to a rapid accumulation of carbonates forming a scale as described above. As illustrated in Figure 26, when the liquid level falls from 13A to 13B the layer of scale 9 remains wet, thereby causing false readings from the sensor.

It is amongst the objects of the present invention to provide a solution to these problems.

Summary of the Invention

Accordingly, the invention provides, in a first aspect: a level probe, for use in sensing the level of a liquid, said probe comprising: (a) a first electrically conductive member; (b) a second electrically conductive member; and (c) an electrically insulating member comprising a downward-facing cup; said electrically insulating member separating said first and second electrically conductive members.

Preferably, said probe comprises three or more electrically conductive members, each separated from an adjacent such member by an electrically insulating member comprising a downward facing cup.

More preferably, said electrically insulating member has a hydrophobic surface.

More preferably also, said probe further comprises wires operatively connected to said electrically conductive members to enable electrical measuring apparatus to be connected thereto.

More preferably also, said electrically conductive members are hollow and/or said electrically insulating member(s) comprise(s) a hollow lumen extending therethrough. This allows for wires to be passed through the interior of the components.

In preferred embodiments, the liquid-contacting surface of said electrically conductive members is covered by a dielectric material. This allows the probe to be used to make capacitive measurements.

In a second aspect, the invention provides a level sensor, for sensing the level of a liquid, comprising: (a) a probe of the invention; (b) electrical measuring apparatus operably connected to each electrically conductive member; said measuring apparatus being configured to measure an electrical of each electrically conductive member.

Preferably, when no dielectric covering is provided, said electrical property is electrical resistance between each conductive member and a reference electrode in contact with said liquid.

Where electrical resistance is measured, it is preferred that this is measured using an alternating voltage, preferably having a frequency of at least 1 kHz, and more preferably using a voltage of less than 5V.

Where a reference electrode is used, it is preferred that said reference electrode is forms part of said probe and is situated at or adjacent the lower end of the probe, in use.

Also included within the scope of the invention is a level sensor as first described above and using a probe having a dielectric covering wherein said electrical property is the capacitance of the electrical assemblage comprising each conductive member, its respective dielectric covering and any liquid in contact with each said dielectric covering.

Also included in the scope of the invention is an electrically powered water heater comprising a level sensor described herein.

Also included in the scope of the invention is such an electrically powered water heater in which said electrical property is resistance and wherein said reference electrode comprises part or all of a submerged heating element.

Brief Description of the Figures

The invention will be described with reference to the accompanying drawings, in which:

Figures 1-6 illustrate, schematically, prior art level probes;

Figures 7-8 illustrate in partial cross-section view, embodiments of level probes of the invention;

Figures 11-12 illustrate the operation of level probes of the invention;

Figure 13 illustrates, in schematic view, the pattern of scale formed on a level probe of the invention;

Figures 14-15 illustrate the operation of level probes of the invention;

Figure 16 illustrates a level sensor of the invention;

Figures 17 - 21 illustrate constructional details of a level probe of the invention;

Figures 22-26 illustrate prior art capacitive level sensors; and

Figures 27-28 illustrate, in cross-sectional view, embodiments of level probes of the invention.

Description of Preferred Embodiments

Figure 7 illustrates, in cross-sectional view, an embodiment of a level probe of the invention, generally indicated by 10. The probe comprises a first electrically-conductive member 8a, of elongate cylindrical form. The conductive member may be made of metal, such as copper, stainless steel, or a metal alloy, such as brass, and superalloys such as the predominantly ironnickel alloys sold under the registered trade mark INCOLOY® or from a conductive polymer. This is the case in all embodiments of the invention. Alternatively, the conductive member could just be provided with an electrically-conductive surface. The lower end of the first conductive member 8a is connected to an electrically insulating member 7 comprising an inner elongate insulating rod 11 surrounded by a downward-facing cup 12. At the lower end of the rod 11 is connected a second like electrically-conductive member 8b. A further member 16 may be connected to the top of the conductive member 8 a to allow it to be conveniently positioned within a tank. When placed in a water tank, as water is added to the tank and the water level 13 rises, the downward-facing cup 12 causes a pocket of air 14 to be trapped within the cup 12. Asa result, a region of the inner surface 15 of the cup is never exposed to liquid water. In order to improve the performance further, it is particularly preferred that the insulating member 7, or just the cup 12, or even just the inner surface 15 of the cup 12 is composed of, or covered with a hydrophobic material to help shed droplets of water that might splash onto the inner surface 15 of the cup 12. It will be appreciated that some of the water will enter the inner confines of the cup 12, due to the pressure exerted by the weight of the water above, but as the water level falls again, the cup 12 will empty. However, entry of water into the cup 12 will only be significantly apparent when submerged to a depth of several metres.

Figure 8 illustrates the level probe of Figure 7 after scale 9 has formed on the probe. It can be seen that, as no (or little) water has been able to contact the upper region of the inner surface 15 of the cup 12, there is no continuous film of scale 9 bridging the conductive members 8a and 8b. In this way, the problems described above can be avoided.

Figure 9 illustrates a similar, but alternative embodiment of a level probe of the invention, generally indicated by 10. Again, like elements previously described are correspondingly numbered. In this embodiment, the electrically-conductive members 8 are smaller, rather than being elongate. A further insulating member 17 is also attached to the bottom of conductive member 8b, extending downwardly therefrom. In any embodiment of the invention, the cup can be of any convenient shape that provides a downward-facing void in which an air pocket can form when water rises up the probe. The distance 19 between the descending inner face 18 of the cup and elements that extend within the cup’s void should be chosen to be greater than twice the thickness of scale expected to be deposited within the design life of the probe, or within the expected time between de-scaling. This ensures that scale cannot bridge this gap and thereby form an unwanted electrically conductive path between adjacent conductive members 8.

The depth of the cup 12 should be chosen such that an air pocket is retained therein as the water level rises up past the mouth of the cup 12. This may readily be determined by trial and error for the situation in which the level probe 10 is to be used. For example, in situations where the probe 10 is to be used in a hot water environment, the depth of the cup should be chosen such that an air pocket is retained within it in the face of subsequent cooling of the air and consequent volume reduction of the air within the pocket. In a boiling water environment, there may also be movement of the water due to the presence of not only convection currents but also bubbles of steam and previously dissolved gases. A typical depth for most situations would be at least 5mm, and preferably at least 10, 15, 20 or even 30mm.

Figure 10 illustrates a further embodiment of a level probe of the invention, generally indicated by 10. In this embodiment, a reference electrode 19 is provided at the lower end of the probe 10 that may be used to make the measurements described above.

Figures 11-12 illustrate, in a way analogous to Figures 3-6, the performance of a probe 10 of the invention. Like elements already described are numbered accordingly. In Figure 11 A, an embodiment of a probe 10 of the invention is immersed in water 3 in a tank 2. As the water level rises, an air pocket 14 is trapped in each of the cups 12 in the insulating regions 7 of the probe 10. Electrical resistance R is measured between each of the electrically conductive members 8 and a reference electrode, in this case the element 4 of a water heater. As shown in Figure 1 IB, as all of the conductive members 8 are immersed in the water 3, each resistance is relatively low, indicating that the water level 13 is above the lower end of the highest conductive member 8a.

In Figure 12A, the water level 13 has fallen, and the water has drained out of the cup forming part of insulating region 7b. The corresponding resistance measurements shown in Figure 12B show that the resistance between the reference electrode and conductive members 8 a and 8b is high as there is no electrical connection between them and the reference, whereas the resistance between conductive members 8c and 8d is low, due to the presence of the electrically-conducting water path.

Figure 13 illustrates the scaling pattern on a portion of a probe 10 of the invention. The left hand side of the figure illustrates the probe submerged below the liquid level 13. An air pocket 14 is retained within the cup 12 of the non-conducting member 7. As a result, a portion of the inner region 15 of the cup 12 is kept dry, and is therefore protected from scaling. Scale 9 therefore can only form in the regions indicated. The right hand side of the figure illustrates the probe when the liquid level 13 has fallen below the lower rim of the cup

12. Even though the scale 9 might be wet, there is no electrical conduction path between the conducive member 8a and the conductive member 8b, or a reference electrode below the water level. As a result, performance of the level probe is unaffected by scale.

Figures 14-15 illustrate the arrangement of Figures 11-12 after scaling. Like elements are again correspondingly numbered. As a result of the discontinuity of scaling 9, when the water level 13 drops as shown in in Figure 15A, the same resistance profile (shown in Figure 15B) as that obtained for the unsealed situation of Figure 12A is obtained.

Figure 16 illustrates in schematic cross-section view, a level sensor of the invention. A level probe 10 of the invention is provided, which can be immersed in water 3 within a tank 2. Electrical connections 20 in the form of cores of a wire link each of the conductive members 8 to electrical measuring apparatus 21. A reference electrode 19, in this embodiment the heating element 4, is also electrically connected to the measuring apparatus 21 by a wire 22. The measuring apparatus 21 is configured to measure an electrical property of each conductive member. Such an electrical property could include capacitance, but in preferred embodiments the property is electrical resistance between each conductive member 8 and a reference electrode, such as the heating element 4. It is particularly preferred that the electrical resistance is measured using an applied alternating voltage. By using an alternating voltage, electrolysis of the water 3 (and consequent degradation of the conductive members 8 and element 4) is largely avoided. The inventor has found that an alternating voltage having a frequency of at least 1 kHz is preferred. A peak voltage of less than about 5 V is also preferred, for example between 3 and 4V. The electrical connections 200 to each conductive member 8 may advantageously be made via the interior of the probe as illustrated in Figure 17.

Additionally, in order to reduce corrosion it is preferred that the conductive members 8 are formed of a corrosion-resistant material such as that sold under the registered trademark Incoloy®. In any choice of metal for the conductive members 8, it is especially preferred to use the same material as other metals in contact with the liquid (e.g. a heating element) to avoid electrolytic corrosion.

Figure 17 illustrates, in cross-sectional view, a portion of a level probe 10 of the invention. The electrically conductive members 8 are formed of a metal tube, e.g. a copper tube. The electrically insulating member 7 with its cup 12 is also provided with a channel 23 from one end to the other through which one or more wires 20 may pass to make electrical contact with the conductive members 8. In this embodiment, each end of the insulating members 7 is provided with a reduced diameter, forming a step 24 that can be pushed into the end of the conductive members 8 to join them together. The insulated wire 20 has and exposed portion of conductor 25 that can be wrapped around the step feature 24 before it is inserted into the conductive member 8 to make an electrical connection with the conductive member 8.

This arrangement also illustrates a further preferred feature of any described embodiment of the invention. It can be seen that the region where the electrical connection is made between the wire 20 and conductive member 8 is located within the cup, i.e. within the cavity that traps air as the liquid level rises. In this way, the electrical connection is less exposed to moisture, thereby increasing its durability.

Figures 18-20 illustrate in end elevation, side elevation and perspective view, the end region 26 of the insulating member 7. A series of axially extending channels 27 is also provided on the surface of the step feature 24. After assembly of the conductive 8 and insulating 7 members, and the wires 20 connecting to each conductive member 8, an insulating potting compound can be introduced into the lumen of the probe. The potting compound can then be exuded through the channels 27, to provide not only a water-tight seal to the lumen of the probe, but also to secure the various elements together and ensure that the electrical connections are secure.

Figure 21 illustrates an additional means of securing the elements together, and providing a water-tight seal. In this embodiment, water-tight sleeve 28 is secured around the junction between an insulating member 7 and a conductive member 8. Such a sleeve might be heatshrink tubing, or an elastomeric band. The sleeve can be used on its own, or in conjunction with potting compound as described above.

Figure 27 illustrates, in cross-sectional view, a level probe of the invention for use where electrical resistance is the electrical property to be measured. Eike elements previously described are numbered accordingly. In this embodiment, each insulating region 7 comprises a cup 12 in the form of a hollow cone. The inventor has found that this form, having a continually downward sloping top surface 36 increases the likelihood that any scale that does form on the surface 36 is more easily sloughed off the surface by the action of movement in the water. In addition, the probe is also provided with a temperature sensor 37 located at the bottom end of the probe. No electrical connections are shown, for reasons of clarity.

Figure 28 illustrates, in cross-sectional view, a level probe of the invention similar to that illustrated in Figure 27, but configured for use as a capacitance probe. In order to achieve this, each of the fluid-contacting surfaces of the conductive regions 8 is covered by a dielectric material 34. In this configuration, there is no possibility of a scale layer bridging between adjacent conductive regions, thereby reducing the deleterious effect of scale on the performance of the probe. Again, no electrical connections are shown, for reasons of clarity. Also, the elements are shown in a slightly spaced-apart relationship, for clarity. Before use, the cavity within the probe is preferably filled with electrical potting compound to secure the elements together and provide water resistance.

Figure 29 illustrates, in a way analogous to Figure 16, a level probe 10 of the invention configured to sense capacitance (as illustrated in Figure 28) used as part of a level sensor io comprising the probe 10 operably connected by electrical connections 20 to an electrical measuring apparatus 21 configured to measure capacitance. Eike elements previously described are correspondingly numbered.

Claims

1. A level probe, for use in sensing the level of a liquid, said probe comprising:

(a) a first electrically conductive member;

(b) a second electrically conductive member; and (c) an electrically insulating member comprising a downward-facing cup;

said electrically insulating member separating said first and second electrically conductive members.

2. A level probe according to Claim comprising three or more electrically conductive members, each separated from an adjacent such member by an electrically insulating member comprising a downward facing cup.

3. A level probe according to either of Claim 1 or Claim 2 wherein said electrically insulating member has a hydrophobic surface.

4. A level probe according to any preceding claim further comprising wires operatively connected to said electrically conductive members to enable electrical measuring apparatus to be connected thereto.

5. A level probe according to any preceding claim wherein said electrically conductive members are hollow.

6. A level probe according to any preceding claim wherein said electrically insulating member(s) comprise(s) a hollow lumen extending therethrough.

7. A level probe according to any preceding claim wherein the liquid-contacting surface of said electrically conductive members is covered by a dielectric material.

8. A level sensor, for sensing the level of a liquid, comprising:

(a) a probe according to any preceding claim;

(b) electrical measuring apparatus operably connected to each electrically conductive member;

said measuring apparatus being configured to measure an electrical of each electrically conductive member.

9. A level sensor according to Claim 8 when dependent on any of Claims 1 to 6 wherein said electrical property is electrical resistance between each conductive member and a reference electrode in contact with said liquid.

10. A level sensor according to Claim 9 wherein said electrical resistance is measured using an alternating voltage.

11. A level sensor according to Claim 10 wherein said alternating voltage has a frequency of at least 1 kHz.

12. A level sensor according to any of Claims 9 to 11 wherein said electrical resistance is measured using a voltage of less than 5V.

13. A level sensor according to any of Claims 9 to 12 wherein said reference electrode is forms part of said probe and is situated at or adjacent the lower end of the probe, in use.

14. A level sensor according to Claim 8 when dependent on Claim 7 wherein said electrical property is the capacitance of the electrical assemblage comprising each conductive member, its respective dielectric covering and any liquid in contact with each said dielectric covering.

15. An electrically powered water heater comprising a level sensor according to any of Claims 8 to 14.

16. An electrically powered water heater according to Claim 15 when dependent on any of Claims 9 to 12 wherein said reference electrode comprises part or all of a submerged heating element.