Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
A SENSOR FOR MEASURING THE LEVEL OF A MEDIUM
Document Type and Number:
WIPO Patent Application WO/2003/102515
Kind Code:
A1
Abstract:
A sensor (1) for measuring the level of a medium (9) in a vessel, the sensor including: a sensor body (1) disposed in said vessel for at least partial immersion in said medium (9), said sensor body (1) having a first fluid (5) and a second fluid (11) contained therein, said first and second fluids being immiscible and said second fluid (11) having a specific gravity less than said first fluid (5) for maintaining the first fluid at the base of the sensor body, said sensor body (1) further including a flexible portion (3B) for adjusting the level of said first fluid in said sensor body in response to pressure exerted by said medium, and a means (7) for detecting the level of said first fluid in said sensor body.

Inventors:
DUNBAR NEIL JAMES (AU)
BOOTHMAN ROBERT JOHN (AU)
PARSONS MATT GOODALL (AU)
MCFARLANE SCOTT (AU)
OLBRICH WOLFGANG ERICH (AU)
Application Number:
PCT/AU2003/000678
Publication Date:
December 11, 2003
Filing Date:
May 30, 2003
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
SAMARAN INTERNAT PTY LTD (AU)
DUNBAR NEIL JAMES (AU)
BOOTHMAN ROBERT JOHN (AU)
PARSONS MATT GOODALL (AU)
MCFARLANE SCOTT (AU)
OLBRICH WOLFGANG ERICH (AU)
International Classes:
G01F23/16; H01H29/28; H01H35/18; (IPC1-7): G01F23/16; G01F23/18; G01F23/24; G01F23/292
Foreign References:
DE3203175A11983-08-18
US4238652A1980-12-09
US5167155A1992-12-01
Attorney, Agent or Firm:
Allen, Leon K. (1 Little Collins Street Melbourne, Victoria 3000, AU)
Download PDF:
Claims:
CLAMS :
1. A sensor for measuring the level of a medium in a vessel, said sensor including: a sensor body disposed in said vessel for at least partial immersion in said medium, said sensor body having a first fluid and a second fluid contained therein, said first and second fluids being immiscible and said second fluid having a specific gravity less than said first fluid for maintaining the first fluid at the base of the sensor body, said sensor body further including a flexible portion for adjusting the level of said first fluid in said sensor body in response to pressure exerted by said medium, and a means for detecting the level of said first fluid in said sensor body.
2. The sensor of claim 1, wherein said first fluid has a specific gravity less than the medium to partially immerse said sensor body in the medium.
3. The sensor of claim 2, wherein the first fluid and the second fluid have a combined effective specific gravity less than the specific gravity of the medium.
4. The sensor of any one of the preceding claims, wherein the flexible portion has a crosssectional area generally greater than said sensor body to promote adjustment of the level of the first fluid.
5. The sensor of any one of claims 1 to 3, wherein a first end of the sensor body has a crosssectional area generally greater than said sensor body to promote adjustment of the level of the first fluid.
6. A sensor of claim 5, wherein said first end is at least partially immersed in the medium.
7. The sensor of claim 6, wherein said sensor body includes a middle portion connected to and in fluid communication with said first end.
8. The sensor of claim 7, wherein the sensor body includes a second end opposed to said first end, said second end having a crosssectional area generally greater than said middle portion to further promote adjustment of the level of the first fluid.
9. The sensor of any one of claims 5 to 8, wherein said first end is flexible to form said flexible portion.
10. The sensor of claim 9, wherein said flexible first end has a general shape selected from one of a cylindrical, spherical, ovoid, or rectangular shape.
11. The sensor of any one of claims 8 to 10, wherein said second end is also flexible to increase its crosssectional area in response to pressure exerted by at least said second fluid and promote adjustment of the level of said first fluid.
12. The sensor of any one of claims 5 to 8, wherein said first end is generally rigid and said flexible portion connects a first rigid member to said first end, wherein said rigid member is moveable relative to said first end in response to pressure exerted by said medium to reduce the internal volume of said first end and promote adjustment of the level of the first fluid.
13. The sensor of claim 12, wherein said first rigid member forms part of a base of the first end and is movable inwardly of said base.
14. The sensor of claim 12 or 13, wherein said first rigid member is a disc and said flexible portion includes a bellows shaped membrane.
15. The sensor of any one of claims 8,12, and 13, wherein said second end is generally rigid and a further flexible portion connects a second rigid member to said second end, wherein said second rigid member is movable relative to said second end in response to pressure exerted by said second fluid to increase the internal volume of said second end and assist detection of the level of the first fluid.
16. The sensor of claim 15, wherein said second rigid member is initially spaced away from said second end and is movable to engage said second end.
17. The sensor of claim 15 or 16, wherein said second rigid member is a disc and said further flexible portion includes a bellows shaped membrane.
18. The sensor of any one of the preceding claims, wherein the first fluid is an electrically conductive liquid to assist detection of the level of the first fluid in the sensor body.
19. The sensor of claim 18, wherein the second fluid is electrically inert so as not to interfere with detection of the first fluid.
20. The sensor of claim 18, wherein the second fluid conducts electricity to a lesser extent than the first fluid so as to assist detection of the level of the first fluid.
21. The sensor of any one of the preceding claims, wherein at least one of the first fluid and the second fluid has a low surface tension to promote adjustment of the level of the first fluid.
22. The sensor of claim 21, wherein said one of at least the first fluid and the second fluid includes an additive to lower its surface tension.
23. The sensor of claim 22, wherein the additive includes a detergent.
24. The sensor of any one of the preceding claims, wherein said first fluid is a liquid.
25. The sensor of claim 24 when said first fluid is water.
26. The sensor of any one of the preceding claims, wherein said second fluid is a liquid or gas.
27. The sensor of claim 26, wherein said second fluid is paraffin.
28. The sensor of claim 26, wherein said second fluid is a gas generally located in a rigid, air tight end of the sensor body.
29. The sensor of any one of the preceding claims, wherein said sensor body contains therein a third fluid, immiscible with said first fluid and said second fluid, said third fluid having a specific gravity less than said second fluid so as to further promote adjustment of the level of the first fluid.
30. The sensor of claim 29, wherein said third fluid is a gas.
31. The sensor of any one of the preceding claims, wherein the detecting means includes an electronic system to detect the level of the first fluid.
32. The sensor of claim 31, wherein the detecting means includes an electrical resistance switch.
33. The sensor of any one of claims 1 to 30, wherein the detecting means includes a light detecting system to detect the level of the first fluid.
34. The sensor of claim 33, wherein part of said light detecting system is located outside of and adjacent to said sensor body.
35. The sensor of claim 33, wherein said light detecting system includes a light reflective material having a specific gravity between the first fluid and the second fluid to indicate the level of the first fluid.
36. The sensor of any one of claims 31 to 35, wherein part of the detecting means is located in the sensor body.
37. The sensor of any one of the preceding claims, wherein the sensor body is elongate and extends generally vertically in the vessel so as to be at least partially immersed in the medium over the range of medium levels to be measured.
38. The sensor of any one of the preceding claims, wherein the detecting means detects a selected level of the medium.
39. The sensor of claim 38, wherein the selected level indicates that the medium has substantially filled the vessel.
40. The sensor of any one of the preceding claims, wherein the detecting means detects multiple discrete levels of the medium.
41. The sensor of any one of the preceding claims, wherein an adjustable holding means is provided for selectively positioning the sensor body in said vessel.
42. The sensor of any one of the preceding claims, wherein other media are present in the vessel in addition to the medium being measured.
43. The sensor of claim 42, the fluids in the sensor body have a combined effective specific gravity less than the medium but greater than said other media to maintain said sensor at the approximate level of the medium.
44. A sensor, substantially as described with reference to the drawings.
Description:
A SENSOR FOR MEASURING THE LEVEL OF A MEDIUM FIELD OF THE INVENTION The invention relates to a sensor and method for measuring the level of a medium in a vessel. The invention has particular application for measuring the level of liquids, loose solids or gases in storage tank, for example a septic tank. The invention can also be used to determine the interface or boundary level between two media having different densities in such a tank.

BACKGROUND TO THE INVENTION In principle there are numerous sensors and methods for determining the level of a medium in a storage vessel, such as liquids, loose solids and gases. However, the use of a sensor in a vessel, such as a septic tank, is often restricted by difficult environmental factors. For example, the medium being measured may be sticky, contains suspended and/or abrasive debris, has varying specific gravities, contains potentially explosive and/or corrosive chemicals or other such environmental hazards. Where more than one medium is in the storage vessel, such as a combination of loose solids and liquids in a septic tank, the same types of environmental hazards may be posed by media in addition to the medium to be measured.

SUMMARY OF THE INVENTION Accordingly, the invention provides a sensor for measuring the level of a medium in a vessel, said sensor including: a sensor body disposed in said vessel for at least partial immersion in said medium, said sensor body having a first fluid and a second fluid contained therein, said first and second fluids being immiscible and said second fluid having a specific gravity less than said first fluid for maintaining the first fluid at the base of the sensor body,

said sensor body further including a flexible portion for adjusting the level of said first fluid in said sensor body in response to pressure exerted by said medium, and a means for detecting the level of said first fluid in said sensor body.

By providing a first fluid in the sensor body which responds to changes in the level of the medium but is safely contained in the sensor body, the level of the first fluid can be readily and easily detected. Thus, the level of the medium can be determined by reference to the first fluid level without the risk of damage, contamination, corrosion or other environmental hazards adversely affecting operation of the sensor in the vessel.

The first fluid can be chosen to have desired characteristics, such as being relatively inert, easily detected or having a low surface tension.

It is preferred that the first fluid has a specific gravity less than the specific gravity of the medium to partially immerse the sensor body in the medium. Preferably, the first fluid and the second fluid have a combined effective specific gravity less than the specific gravity of the medium.

The sensor body is preferably elongate and extends generally vertically in the vessel so as to be at least partially immersed in the medium over the range of medium levels to be measured. In some applications, it is desirable to sense that a selected level (usually a full level) has been reached. In these applications, the sensor body need only be disposed to be partially immersed in the medium when the medium is at or about the selected level. In other applications, multiple discrete levels may be detected or the sensor may function as a gauge giving an indication of the level of the medium in the vessel over a range of levels.

Preferably, the flexible portion has a cross-sectional area generally greater than said sensor body to promote adjustment of the level of the first fluid.

It is also preferred that a first end of the sensor body has a cross-sectional area generally greater than said sensor body to promote adjustment of the level of the first fluid.

Preferably, the first end of the sensor body is flexible to form the flexible portion. The first end is preferably at least partially immersed in the medium. In one embodiment, the flexible first end is generally spherical in shape. Other possible shapes for the first end include cylindrical, ovoid, and rectangular.

It is preferred that the sensor body includes a middle portion of generally lesser cross- sectional area than said first end, said middle portion being connected to and in fluid communication with said first end. In a preferred embodiment, the middle portion is rigid.

The sensor body preferably includes a second end opposed to said first end having a cross-sectional area generally greater than the middle portion to further promote adjustment of the level of the first fluid. The second end may also be flexible to further increase its cross-sectional area in response to pressure exerted by at least said second fluid and promote adjustment of the level of the first fluid. In one preferred embodiment, the sensor body is constructed in the shape of an hourglass with the first and second flexible ends forming the"bulbs"of the hourglass shape and the middle portion forming a"throat"section between the first and second flexible ends. The middle portion can be of any length and in this embodiment is preferably a rigid manometer tube.

A variation of this embodiment provides that the first end is generally rigid and said flexible portion connects a first rigid member to said rigid first end, wherein said rigid member is movable relative to said first end in response to pressure exerted by said medium to reduce the internal volume of said first end and promote adjustment of the level of the first fluid. The first rigid member preferably forms part of a base of the first end and is moveable inwardly of said base. In a specific form of this embodiment, the flexible portion includes a bellows shaped membrane and the first rigid member is a disc.

In another variation of this embodiment, the second end is generally rigid and a further

flexible portion connects a second rigid member to said second end, wherein said second rigid member is movable relative to said second end in response to pressure exerted by said second fluid to increase the internal volume of said second end and assist detection of the level of the first fluid. It is preferred that the second rigid member is initially spaced away from said second end and is moveable to engage said second end. The second rigid member may be a disc and the further flexible portion preferably includes a bellows shaped membrane.

In another separate embodiment, the flexible portion is formed by the side walls of the sensor body. In this embodiment, the flexible portion is formed by joining two frusto- conical sections of the side walls at their respective diameters.

The first fluid is preferably a liquid. Preferably, the first fluid is an electrically conductive liquid to assist detection of the level of the first fluid in the sensor body. In a preferred embodiment, the first fluid is in the form of water.

The second fluid is preferably a liquid or gas. In a preferred embodiment, the second fluid is electrically inert so as not to interfere with detection of the first fluid.

Alternatively, the second fluid may conduct electricity to a lesser extent than the first fluid so as to assist detection of the level of the first fluid. The second fluid in one embodiment is paraffin.

The first fluid, the second fluid, or both of the first and second fluids may include an additive to lower their respective surface tensions to assist detection of the first fluid.

By lowering the surface tension of the first and second fluids, this reduces any tendency of the respective fluids to cling or stick to the sensor body walls and hinder the accuracy of detection of the first fluid. The additive can be any chemical that lowers the surface tension of the first or second fluid. In one embodiment, the additive is a detergent.

The sensor preferably includes a third fluid, immiscible with said first fluid and said second fluid, said third fluid having a specific gravity less than said second fluid so as to

further promote adjustment of the level of the first fluid. The third fluid in one embodiment is a gas. The gas may be compressible and/or at subatmospheric pressure.

The detecting means may include a visual and/or electronic systems to detect the level of the first fluid. These systems may include colour dyes, floating level indicators, reed switches, micro-switches, light switches or sensors, induction switches, capacitance switches, piezoelectric switches, ultrasonic sensors, radio wave/electromagnetic sensors and other electronic systems. In a preferred embodiment, the detecting means includes an electrical resistance switch.

Preferably, part of the detecting means is located in the sensor body. The detecting means may be located fully within the sensor body. In a preferred embodiment, a portion of the detecting means extends into said body spaced from the first fluid.

Alternatively, the detecting means includes a float having a specific gravity between the specific gravities of the first fluid and the second fluid to indicate the level of the first fluid in the sensor body.

Preferably the sensor body is fully submerged to minimise temperature variations as well as to inhibit air bubbles (or air leaks) forming within the sensor body, which can affect the sensor's accuracy. The sensor body is preferably made from a material with a low coefficient of thermal expansion to resist temperature variations within the vessel.

The sensor body and walls are preferably resilient to resist damage, contamination and/or corrosion from the medium or other media in the vessel. The body may have a low surface tension to prevent sticky or highly viscous media from fouling the sensor.

It is preferred that an adjustable holding means adjustably holds the sensor body at a selected position in the vessel. The holding means can be attached at a generally rigid part of the sensor body, such as a rigid end or a rigid throat section. This allows the flexible portion of the sensor maximum freedom to move in any direction under the pressure exerted by the medium being sensed. In one embodiment, the holding means

includes a bracket.

The vessel may include other media in addition to the medium being measured. Where there is other media in the vessel, it is preferable that the fluids in the sensor bodyhave a combined effective specific gravity less than the medium but greater than said other media to maintain said sensor at the approximate level of the medium.

The object of preferred embodiments of the invention is to provide a relatively inexpensive and robust sensor and method for measuring the level of a medium, including liquids, loose solids and gasses. It is particularly aimed for use in media that are sticky or contain suspended and/or abrasive debris, that have varying specific gravities or contain potentially explosive and/or corrosive chemicals, etc. However, by its nature the sensor and method of the invention may also operate in less challenging environments.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will now be described, by way of example only, with reference to the drawings, of which: Figure 1 is a schematic sectional view of a sensor according to a preferred embodiment of the invention in an initial position; Figure 2 is a cross-sectional view along line A-A of the sensor of Figure 1; Figure 3 is a cross-sectional view along line B-B of the sensor of Figure 1; Figure 4 is a schematic sectional view of the sensor of Figure 1 in an operative position; Figure 5 is a cross-sectional view along line A-A of the sensor of Figure 4;

Figure 6 is a cross-sectional view along line B-B of the sensor of Figure 4; Figure 7A is a schematic sectional view of another embodiment of the invention in an initial position; Figure 7B is a cross-sectional view along line A-A of the sensor of Figure 7A; Figure 8A is a schematic sectional view of the sensor of Figure 7A in an operative position; Figure 8B is a cross-sectional view along line B-B of the sensor of Figure 8A; Figure 9A is a schematic sectional view of another embodiment of the invention in an initial position; Figures 9B and 9C are cross-sectional views along lines C-C and D-D, respectively, of the sensor of Figure 9A; Figure 10A is a schematic sectional view of the sensor of Figure 9A in an operative position; Figures 10B and 10C are cross-sectional views along lines E-E and F-F, respectively, of the sensor of Figure 10A ; Figure 11 is a schematic sectional view of a further embodiment of the invention; Figure 12 is a schematic sectional view of a further embodiment of the invention; Figure 13 is a schematic sectional view of another variation of the sensor of Figure 7A using a light sensing system for detecting the first fluid level; and

Figure 14 is a schematic sectional view of the sensor of Figure 13 with a different light sensing system.

BEST MODE FOR CARRYING OUT THE INVENTION Figure 1 illustrates a sensor according to the preferred embodiment of the invention.

The sensor has a body 1 with flexible side walls 3A, 3B, the sensor body 1 containing water 5 and an electrical resistance switch assembly 7 to detect the level of water 5.

The sensor body 1 is partially immersed in a waste liquid 9 to be measured within a septic tank (not shown). The density or specific gravity of water 5 is less than the density or specific gravity of waste liquid 9.

The sensor body 1 also contains paraffin 11, which is immiscible with water 5 and has a lower specific gravity than the specific gravity of water 5. The difference in specific gravities and the immiscibility of the water 5 and paraffin 11 ensure that water 5 and paraffin 11 do not mix and that the level of water 5 is at the base of sensor body 1 to correspond to the level of waste liquid 9 at all times, as shown in Figures 1 and 4. The paraffin 11 is also electrically inert so as not to interfere with switch assembly 7 detecting the level of water 5. A simple density calculation can be used to indicate what proportions are required for the water 5 and paraffin 11 in the sensor body 1.

Each of the side walls 3A, 3B of sensor body 1 is a frusto-conical section, the side walls 3A, 3B being joined at their respective maximum diameters. The sensor body 1 is sealed to ensure that there is no leakage of water 5 or paraffin 11 into the septic tank or leakage of waste liquid 9 into the body 1. Other configurations of the side walls 3A, 3B may be selected but generally the walls 3A, 3B are such that the sensor body 1 has a portion with an enlarged cross-sectional area, as shown in Figures 2 and 3. This allows side walls 3A, 3B to readily flex inwardly to adjust the level of the first fluid in the sensor body 1. As the side walls 3A, 3B flex inwardly, the level of water 5 rises in the sensor body 1 to be detected by resistance switch 7.

The resistance switch assembly 7 partially extends into sensor body 1 to detect the level of water 5 in the sensor body 1. The switch assembly body surrounds the switch to seal it from the tank environment, except for electrodes 8. The switch assembly 7 may be used to position the sensor body 1 in the tank by making the assembly body substantially rigid. Switch assembly 7 uses a switch, which detects when water 5 reaches a certain level in sensor body 1, which corresponds to waste liquid 9 reaching a certain level in the septic tank. That is, the switch assembly 7 only triggers when the level of the electrically conductive water 5 rises up in sensor body 1 to form a circuit with the switch assembly 7 via electrodes 8. The sensor body 1 is designed so that the level of water 5 at which the switch assembly 7 is triggered corresponds to a predetermined level of waste liquid 9 in the septic tank. This trigger level in switch assembly 7 may be varied by adjusting the amount of water 5 in the sensor body lor varying the initial shape of sensor body 1. By selecting a different fluid having a different density or viscosity to water 5, the trigger level can also be adjusted. In addition, other types of detecting devices can be used besides resistance switch assembly 7. For example, a resistance tape, micro-switch, reed, inductance or a capacitance switch can be used.

The sensor works by first placing the sensor body 1 into the septic tank containing the waste liquid 9 with other media, for example scum. The sensor body 1 is partially immersed in waste liquid 9 to stand generally vertical. As paraffin 11 has a lower specific gravity than water 5 and is immiscible with water 5, the water 5 remains at the lower part of body 1. This ensures that the level of water 5 in body 1 corresponds to the level of waste liquid 9.

The sensor body 1 is in its initial frusto-conical shape under the influence of gravity, as shown in Figure 1. As the level of waste liquid 9 increases, the pressure exerted by waste liquid 9 on side walls 3A, 3B increases. This increased pressure causes side walls 3A, 3B to flex inwardly, causing the sensor body 1 to contract to the shape shown in Figures 4 to 6 and compressing water 5 and paraffin 11. Consequently, the level of water 5 rises within the sensor body 1. As paraffin 11 has a lower density than and is

immiscible with water 5, the water 5 and paraffin 11 do not mix during contraction of the sensor body 1. When the level of water 5 rises to a level so as to form a circuit with electrodes 8, the switch assembly 7 is triggered. This indicates that the waste liquid 9 has reached a predetermined level in the septic tank, usually a"full"level.

A further embodiment of the invention is illustrated in Figures 7A-8B. According to this embodiment, sensor body 21 has a flexible end 23 joined to a rigid throat section 24 and rigid upper section 26. Flexible end 23 is generally spherical in shape and thus has a cross-sectional area greater than either throat section 24 or upper section 26, as shown in Figure 7B. Flexible end 23 need not be spherical and can adopt a cylindrical, ovoid, or even rectangular shape. Sensor body 21 contains an electrically conductive liquid 25 as the first fluid and an electrically inert, second fluid 31. There is also an amount of a compressible gas 32 (such as air) in the rigid upper section 26. The sensor body 21 is sealed to prevent leakage of the compressible gas 32. The addition of a gas 32 as a third fluid further facilitates movement of first liquid 25 and second fluid 31 in the sensor, allowing for a quicker response to changes in the level of medium 29. Gas 32 may also be a subatmospheric pressure when the sensor is operating at atmospheric pressure to enhance this effect.

A switch assembly 27, much like the switch assembly 7 of the first embodiment, is fitted to the end of the sensor body 21 opposite to flexible end 23. The electrical switch assembly 27 has two electrode probes 28a, 28b extending downwardly of the sensor body, the probes having different lengths. The length of the shorter probe 28b determines the level at which the first fluid 25 in the sensor body 21 is detected.

Additionally, the different lengths of electrodes 28a, 28b ensure that their live ends do not make inadvertent contact. When the electrically conductive liquid 25 level rises so that it reaches the shorter probe 28b, an electrical contact is formed between the probes 28a, 28b via the electrically conductive liquid 25. This triggers the assembly 27 to indicate that the selected level has been detected and thus indicate the selected level of sensed medium 29.

The sensor body 21 also includes an adjustable holding bracket 22 for selectively locating the sensor body 21 at a desired position in the tank. Thus, the sensor can be used to selectively measure differing levels of the medium in the tank by adjusting the position of the holding bracket 22 accordingly.

The sensor of the second embodiment works in much the same way as the sensor of the first embodiment. That is, the flexible spherical end 23 contracts under pressure from the rising level of medium 29 as shown in Figures 8A and 8B. This causes the level of liquid 25 to rise in sensor body 21 towards rigid throat section 24 and rigid upper section 26. Second fluid 31 is also forced into rigid upper section 26, compressing gas 32. As the gas 32 is compressed, the second fluid 31 rises rapidly, facilitating movement of first liquid 25 and hence create a quicker response from sensor body 21 to the rise in the level of medium 29. Electrode 28a is already immersed in liquid 25, so that when the liquid 25 reaches electrode 28b, the switch assembly 27 is triggered to indicate that the level of the sensed medium 29 has reached a predetermined level.

In a variation of the second embodiment, second fluid 31 and gas 32 may be replaced with a compressible gas to achieve a similar effect. That is, a rise in the level of medium 29 causes a rise in the level of first liquid 25, compressing the gas (acting as the second fluid) in the rigid, air tight upper section 26. Again, once the level of first liquid 25 reaches electrode 28b, switch assembly 27 is triggered.

A further embodiment of the present invention is shown in Figures 9A-10C. In this embodiment, the rigid throat section 24 and rigid upper section 26 of the second embodiment has been replaced in sensor body 41 with an upper flexible end 46 joined via throat section 44 to lower flexible end 43, which is partially immersed in medium 49. Throat section 44 can be slightly flexible, but it is preferably rigid. The first fluid 45 is located generally in lower flexible end 43 while the second fluid 51 is located in the upper flexible end 46.

Again, the sensing system 47 is a switch assembly the same as that of the embodiment of Figures 7A-8B. That is, sensing system 47 uses two electrodes 48a, 48b of different lengths so that a circuit is formed with electrically conductive first fluid 45 when the first fluid 45 reaches the level of shorter electrode 48b. Similar to the embodiment of Figures 7A-8B, an adjustable holding bracket 42 is used to position the sensor in the tank.

As can be seen from Figures 9B and 9C, the upper flexible end 46 has a cross-sectional area greater than throat section 44 and initially less than the cross-sectional area of lower flexible end 43. The upper flexible end 46 can expand to increase its cross- sectional area under the combined pressure of the first fluid 45 and the second fluid 51.

This allows first fluid 45 to move quite quickly in the sensor body 41 in response to the pressure exerted by medium 49, and so enhances its sensitivity.

Thus, when the level of sensed medium 49 rises, the pressure applied to sensor body 41 causes flexible end 43 to flex inward, contracting its cross-sectional area as shown in Figures 10A and lOB. The first fluid 45 then progresses through throat section 44 and compresses second fluid 51. This combined internal pressure from the second fluid 51 and the first fluid 45 results in the upper flexible end 46 flexing outward and increasing its cross-sectional area, as shown in Figures 10A and 10C. This increase in the cross- sectional area of upper flexible end 46 allows first fluid 45 to rapidly move through the sensor body 41, and thus give a quicker indication of the level of sensed medium 49.

Yet another embodiment of the invention is shown in Figure 11. Sensor body 61 is partially immersed in sensed medium 69 and has a lower end 63 and upper end 66 joined by rigid throat section 64. In this embodiment, the throat section 64 is a manometer tube section. Both the lower end 63 and upper end 66 each have an enlarged cross-sectional area relative to manometer tube 64. The sensor body 61 also contains first fluid 65 and second fluid 71, which are immiscible with each other. As second fluid 71 has a lower specific gravity than first fluid 65, first fluid 65 gravitates towards lower end 63 while second fluid 71 rests on first fluid 65 in upper end 66.

Both lower and upper ends 63,66 each have rigid walls, except where flexible bellows shaped membranes 72a, 72b are provided. The membranes 72a, 72b each connect their respective lower and upper ends 63,66 to rigid discs 75a, 75b. The rigid discs 75a, 75b are moveable relative to their respective lower and upper ends 63,66 via membranes 72b, 72b in response to pressure exerted by the sensed medium 69.

In its initial state, the sensor body 61 has disc 75a resting on lower end 63, forming part of the base of lower end 63 with its membrane 72a contracted while disc 75b extends into the internal volume of upper end 66 with its associated membrane 72b also extended.

Thus, when the sensor body 61 is partially immersed in medium 69, the disc 72a is slightly lifted from the base of lower end 63. As the level of medium 69 increases, the pressure on lower end 63 causes moveable disc 75a to extend, via membrane 72a, into the internal volume of lower end 63. This causes the level of first fluid 65 to rise and compress second fluid 71, forcing moveable disc 75b into engagement with the top of upper end 66 and contracting membrane 72b. The movement of disc 75b is then detected to determine the level of the first fluid 65 and thus the level of medium 69.

This may be done, for example, by disc 75b making an electrical contact when it reaches the top of upper end 66.

A further embodiment, similar to that of Figure 11, is illustrated in Figure 12. Again, the sensor body 81 has rigid lower and upper ends 83, 86 joined by rigid manometer tube 84. However, in this embodiment, only lower end 83 has a flexible bellows shaped membrane 92 joining rigid disc 95. An appropriate detecting system, say for example that illustrated in Figures 7A or 9A, can be provided in upper end 86 to detect the level of first fluid 85 and hence level of medium 89.

In addition, the sensor body 81 contains not only first fluid 85 and second fluid 91 (of lower specific gravity than first fluid 85) but also a compressible gas 98. In this

particular embodiment, the gas 98 is air. The provision of air 98, which naturally rises to the top of upper end 86, further assists in the first fluid 85 gravitating to the lower end 83 at a level corresponding to sensed medium 89. The air also allows the first fluid 85 to flow more rapidly in sensor body 81 due to the lower specific gravity of the air 98 and its compressibility. Consequently, the first fluid 85 is more responsive to increases and decreases in the level of medium 89.

The advantages conferred by both embodiments providing a rigid disc (75a, 75b, 92) to the either both ends (63,66) or a lower end (83) of the sensor are as follows: (a) the interface (77,97) between the first fluid (65,85) and second fluid (71,91) can move more easily than a flexible end portion having strengthened walls to withstand shock; (b) the general rigidity of the lower and upper ends (63,66, 83,86) reduces damage and resists tipping over of the sensor due to horizontally directed pressures; and (c) there is a reduced risk of the flexible membranes (72a, 7sb, 92) being broken when the internal pressure in the sensor body (61, 81) becomes excessive.

The embodiments of Figures 11 and 12 have been used to mathematically model the invention. In both cases, the mathematical models have been developed on the basis of four conditions: (1) An initial state where the disc (75a, 95) in the lower end (63, 83) has not lifted off the base of the lower end (63,83) ; (2) An intermediate state where the disc (75a, 95) has lifted off the base of the lower end (63,83) ; (3) A final state where the first fluid (65,85) has risen to the level where it is about to be detected; and (4) The sensed medium (69,89) is sludge and water is present in the vessel as another medium.

It should be noted that for all the following equations, the term"reference level"means the base of the lower end (63,83) in both sensors of Figures 11 and 12.

Mathematical Model of the Figure 11 Sensor: For Condition (1) variable values are subscripted with"0"and one obtains: Pwzo = #lhu0+(#d - #l)hi0 ...(1), where: hi = the height of the liquid interface in the manometer tube 64 above reference level; An == the height of the rigid disc in the upper end 66 above reference level; z = the height of the external clear water free surface above reference level; Pd the density of the first fluid 65 internal to the sensor; #l = the density of the second fluid 71 inside the sensor; and Pw the density of the water external to the sensor.

For this condition hd=0 because the disc 75a is still in contact with the base of lower end 63.

For Condition (2) variable values are subscripted with"1"and we obtain: Pwzl = ? Ai + (PdP,) hil + (Pw-Pd) hdl... (2), where: hd-the height of the rigid disc 75a in the lower end 63, above reference level.

Subtracting Equation (1) from Equation (2) gives: #w(z1 - z0) = #l(hu1 - hu0) + (#d - #l)(hi1 - hi0) + (PW-Pd) hdi... (3) As there is an equality of all other volumes swept out by the movable discs 75a, 75b, this gives:

AV = Am (hil-hio) = Au(hu1 - hu0) = 4A,,... (4), where: Al = the cross-sectional area of the lower end 63; A", = the cross-sectional area of the manometer tube 64; Au = the cross-sectional area of the upper end 66; and Au té increase in first fluid 65 volume in the manometer tube 84.

These reactions may be used to obtain an equation relating additional submersion of the sensor body 61 to the increase in the level of interface 77 in the manometer tube 64: This equation illustrates two points pertinent to the sensitivity of the sensor of this embodiment.

Firstly, if the cross-sectional area of the upper end 66 were equal to that of the manometer tube 64, the density of the second fluid 71 would not appear in the result.

This emphases the importance in this embodiment that these areas be significantly different.

Secondly, if the cross-sectional areas of the lower and upper ends 63,66 are indeed very large relative to that of the tube 64, then maximum sensitivity would occur, giving: hi1 - hi0 # #w(z1 - z0) ... (6) Pd - Pl This underlines the importance or the difference between the densities of the first and the second fluids 65,71 used inside the sensor. Clearly substantial further immersion of the sensor beyond the depth found at condition (1) is not desirable since the multiplier: PW I (Pd-p,) results in a value of between 6 and 7 if the first fluid 65 was water and the second fluid 71 was paraffin of specific gravity 0.85.

For Condition (3) variable values are subscripted with"2"and we begin with: <BR> <BR> <BR> #w(z1 - zsl,2)+#slzsl.2 - #sl(hd2 - hd1) - #whd1 = #l(hu2 - hi2) + #d (hi2 - hd2) ...(7), where: psi té height of the sludge/clear water interface above reference level; and Psi the density of the sludge external to the sensor.

In this case, z2 = zl. Equation (7) can be re-arranged using the similar expressions for the displaced volumes of Equation (4) for this condition: AV = Am (hi2hio) = Au = Alhd2... (8) and Equation (2) to obtain: Again it is apparent that values of Am / Au and Am / Al significantly less than unity are required to achieve a value for the best possible sensitivity of the sensor, which is: <BR> <BR> <BR> <BR> hi2 - hi1 # zsl,2(#sl - #w) ...(10)<BR> <BR> <BR> <BR> Pd - Pl Mathematical Model of the Figure 12 Sensor: For Condition (1) variable values are subscripted with"0"and since the disc 95 in the lower end 83 has not yet lifted off the lower annulus base of lower end 83, one obtains: #wz0 = #lhf0 + (#d - #l)hi0 ...(11), where: hf the height of the free surface in the upper end 86 above reference level.

This is due to the fact that at that stage the air pressure in the upper end 86 is still as it was when assembled, i. e. equal to atmospheric pressure.

For Condition (2) variable values are subscripted with"1"and the compression of the air volume in the upper end 86 must be considered to obtain: pair[Vair,0 - #V)] = pairVair,0 ...(12), where: pair the air pressure in the upper end 86 ; and Vairo = the air volume in the upper end 86 when the air pressure is atmospheric.

As before, the equalities between the volumes swept out by the moving disc 95 in the lower end 83 results in: AV, = Am(hi1 - hi0) = Au(hf1 - hf0) = Alhd1 ...(13), where: A vi the increase in second liquid 91 volume in the upper end 86, and: <BR> <BR> <BR> <BR> (pair - patm)<BR> #w(z1 - hd1) = + #l(hf1 - hi1) + #d (hi1 - hd1) ...(14), g where : patna té atmospheric pressure.

Substituting for the standard atmospheric pressure by the product of a water column height: hatm and the density of water times the acceleration due to gravity, we obtain: where: 9 the acceleration due to gravity; and hatm = the height of a column of water that exerts a pressure at its base equal to

#atm (i.e. #atm/(#wg)).

Here the AV in the denominator is also the product: A", (hi1 - hi0). For the sake of conciseness in notation we define : fV1 = hatm Am which is essentially the ratio of [Vair,0 - 0V1] the volume of a manometer column of height hatm to the air volume in the upper end 86.

Thus, the result that is obtained is: where: fvl is a constant equal to the ratio of volumes: = hatmAm/[Vair,0 - #V1] The same comments made above regarding the need for an area ratio, Åm/Au, that is different from unity equally applies to the sensitivity of the sensor of this embodiment.

That sensitivity then has a maximum limiting value of : <BR> <BR> <BR> #w(z1 - z2)<BR> hi1 - hi0 #<BR> (#d - #l + #wfV1) ...(17) If, for example, the air volume is of the order of 0.75 litre and the manometer inside diameter is 6.0 mm, then the volume ratio fvl is of the order of 0.4. This reduces the sensitivity from the corresponding value for the sensor of Figure 11 due to the effect of the extra pressure of the slightly compressed air 98.

For Condition (3) variable values are subscripted with"2"and we begin with: <BR> <BR> <BR> #W (z1 - zsl,2) + #slzsl,2 - #sl(hd2 - hd1) - #whd1 = Patm #V2 + #l(hu2 - hi2) + #d(hi2 - hd2)<BR> <BR> <BR> [Vair,0 - #V2] ... (18), where:

A V2 the decrease in first fluid 85 volume in the lower end 83.

It is noted that z2 = zl. This can be re-arranged using the similar expressions for the displaced volumes for Condition (3): #V2 = Am(hi2 - hi0) = Au(hu2 - hu0) = Alhd2 ... (19) and Equation (2) to obtain: where: fv 2 is a constant equal to the ratio of volumes: = hatmAm/[Vair,0 - #V2] Again it is apparent that values of A", l A"and A", l A1 which are significantly less than <BR> <BR> <BR> <BR> unity are required to achieve a value for the best possible sensitivity, which is:<BR> zsl,2(#sl - #w)<BR> <BR> <BR> <BR> hi2 - hi1 # ...(21)<BR> <BR> <BR> <BR> <BR> <BR> (#d - #l + #wfV2) There is a similar reduction in the sensitivity due to the extra pressure exerted by the slight compression of the air volume in the upper end 86.

On the basis of the above models, it has been determined that the sensitivity of the sensors of Figures 11 and 12 are largely influenced by the relative potential cross- sectional areas of the ends (63,66, 83,86) of the sensor, compared to the throat section (64,84) of the sensor. The greater the difference in cross-sectional area, the greater the sensitivity of the sensor. Furthermore, the volume of the flexible ends of the sensors in Figures 7A-10C can also have an effect on the range of levels of the sensed medium.

Generally, the larger the volume of the flexible end (s) of the sensors, the larger the range of levels of the medium that can be measured.

Various measuring systems can be used with the sensor, such as visual, electromagnetic, ultrasonic or electronic measuring systems. Where a visual sensing system is used, the first fluid and the second fluid can have different colours to help define the liquid boundaries and/or with transparent side walls for the sensor body. Electromagnetic measuring systems include light, radio wave or electromagnetic switches, sensors or other detectors. Electronic systems include reed switches, micro-switches, inductance switches, capacitance switches and piezoelectric switches.

Figures 7A, 8A, 9A, and 10A illustrate only one type of detecting means that may be used with the present invention. Figures 13 and 14 show two variations of the embodiment of Figures 7A-8B using different types of light detecting systems. In both Figures 13 and 14, the same reference numerals indicate the same elements as used in Figure 7A-8B.

In Figure 13, the light detecting system has a light source 100 which is connected to a bundle 102 of optic fibres 102a, 102b, 102c, 102d, and 102e. The bundle 102 extends alongside the sensor, each optic fibre 102a-102e branching off from bundle 102 at different locations corresponding to predetermined discrete levels in the sensor.

The first fluid 25 and throat section 24 are transparent to propagate light from optic fibres 102a-102e. The second fluid 31 is optically opaque to block light from optic fibres 102a-102e.

Another bundle 104 of optic fibres 104a-104e extends along the side of the sensor body 21 opposite to bundle 102. The optic fibres 104a-104e branch off from bundle 104 at the same locations as optic fibres 102a-102e and positioned so as to receive light from respective optic fibres 102a-102e. The bundle 104 is connected to a light detector unit 106 which detects the light (or lack thereof) from each optic fibre 104a-104e and indicates an appropriate level of the sensed medium 29 in the tank based on which optic fibre (s) 104a-104e receive or do not receive light from optic fibre (s) 102a-102e. An appropriate display indicating the level is also located on unit 106.

In this embodiment, as the level of first fluid 25 rises in response to a rise in sensed medium 29, light from optic fibres 102a-102e of bundle 102 is gradually received by optic fibres 104a-104e of bundle 104. In the position shown in Figure 13, the light transmitted by optic fibre 102e is received by optic fibre 104e and transmitted to unit 106, while opaque second fluid 110 blocks light from optic fibres 102a-102d. Thus, the unit 106 displays the level of the medium 29 corresponding to the level of first fluid 25 in the sensor.

A variation of the light detecting system is shown in Figure 14. In this case, the first fluid 25 is optically opaque while the second fluid 31 of lower specific gravity is optically transmissive. A floating light reflective material 115 is placed in throat section 24 and has a specific gravity less than the first fluid 25 but greater than second fluid 31. This allows light reflective material 115 to be positioned as the interface between the first and second fluids 25,31. The light reflective material 115 may take any suitable form, in this case being small reflective plastic beads. The light reflective beads 115 do not interfere with the fluid contact between the first and second fluids 25, 31.

The light detecting system has a light source 100 connected to optic fibres 117a, 117b.

Optic fibre 117a transmits light from light source 100 towards the light reflective medium 115 while optic fibre 117b receives the light reflected from beads 115. The light received by optic fibre 117b is then sent to light detection unit 119 to determine the level of first fluid 25 in the sensor and then determine the corresponding level of the medium 29. As the level of first fluid 25 rises in response to the level of the medium 29 rising in the tank, the light reflected from beads 115 is sent to light detection unit 119 to determine the level of first fluid 25 in sensor body 21 and thus the level of the medium 29. A particular application of this light detecting system is where the medium being sensed or other media in the vessel are potentially explosive or chemically reactive to an electronic detecting system.

The detection range of the sensor can be modified as required for the measured medium and vessel under consideration. For example, the sensor body can be manufactured so as to predetermine the threshold pressure at which the side walls of the sensor body will begin to flex. The amount of the first fluid in the sensor body, the proportion of first and second fluids in the sensor body and the range of flexure of the sensor body also affect the range of detection of the sensor.

The sensor body operates most effectively when it is free to flex inwardly and outwardly. Thus, the sensor body may have a rigid portion to support the sensor body.

In the first embodiment, the body of switch assembly 7 fulfils this supporting function whereas in the embodiments of Figures 7A-8B, the rigid upper section 26 provides this supporting function as does holding bracket 22. The size and shape of the sensor body can also be altered to offer more resistance to compression forces from the medium to be measured.

The physical properties of the sensor body may also be modified to suit a particular a particular medium being measured. Where the medium is formed from loose solids, the sensor body can be constructed from a thick walled material that offers high abrasive resistance. Where a highly viscous or"sticky"medium is being measured, the sensor body can be constructed from a material that has low surface tension properties. In this circumstance, the flexible sensor body may also have a shape that that does not allow the sticky media to pool on the surface; i. e. it drip-dries. Where a chemically aggressive or corrosive medium is being measured, the sensor body can be made of materials resistant to chemical attack.

Thus, it can be seen from the description of the preferred embodiments, the invention provides a simple, effective and robust sensor for measuring a level of a medium. The simple structure of the sensor allows for a relatively inexpensive manufacture. As the first fluid is fully enclosed within the sensor body, there is a reduced risk of damage, contamination or corrosion of the sensor.

The sensor of the preferred embodiments may also be used to detect the boundary level between media having different densities. For example, the sensor can be used to detect the interface or boundary between water and sludge in a septic tank.

While operation of the sensor has been described in relation to detecting an increase in the level of a medium, the sensor may also be used to detect a predetermined decrease in the level of the medium, including an empty level of the vessel. For example, the first embodiment can be modified to detect a decrease in the level of medium 9 by initially having water 5 in electrical contact with switch assembly 7. The switch assembly is then triggered to indicate the level of a medium 9 (such as a substantially empty level) when the level of water 5 drops (in correspondence with the level of medium 9) and breaks off electrical contact with electrodes 8. The sensor may also be adapted to detect progressive change (increase or decrease) in the level of the medium, including variations in the level of the medium in the vessel over time. For example, the electrical resistance switch assembly 7 of the first embodiment may be substituted with a resistance tape level detector, capacitance level detector or an ultrasonic level detector to detect a progressive increase or decrease in the level of the water 5. With these types of detectors, the level of the detecting fluid can be initially measured to obtain base values for the water 5 and medium by which the sensor can be calibrated.

While a waste liquid 9 has been selected for measurement, other media may be chosen, such as loose solids or semi-solids. In addition, water 5 and/or paraffin 11 may be substituted with a fluid suitable for the required specific gravity of medium to be measured. Thus, the choice of the first fluid and second fluid will influence how the sensor works in various ways. For example, the sensitivity of the sensor can be adjusted by choosing a first fluid with a high viscosity relative to the medium. In another example, the relationship between the densities of the first fluid and the medium will dictate whether the sensor body is fully or partially immersed in the medium.

The materials used for the sensor body can be modified to increase the longevity of the sensor. Preferably, the material of the sensor body has a low coefficient of thermal

expansion to resist temperature variations in the vessel. In the case of septic tanks, the water temperature is usually stable at 12-18°C, which may change by a few degrees due to seasonal temperature fluctuations. It has been found that suitable materials include rubber and plastics, such as polyethylene or polypropylene as well as glass reinforced membranes. Generally, the flexible portion cannot be too flexible, otherwise the flexible portion will tend to plastically deform or stretch over time, reducing its effectiveness. Thus, the flexible portion should change its shape by a flexing action rather than a plastic bending action. This may be achieved by using thicker walls with a relatively large radius of curvature.

Several trials were conducted using the sensor of Figure 9A having water and paraffin as the first and second fluids, respectively. The sensor had a length of about 1.4m with the flexible portion being made of polyethylene and having a wall thickness of 500 microns. The sensor was able to predict the sludge level in a septic tank with a 93% accuracy, the 7% error partly due to the estimate of the sludge density. However, other parameters may be chosen for the sensor, as discussed above.

The foregoing describes only several embodiments of the invention and modifications can be made without departing from the scope of the invention.