WO2006061232A1

WO2006061232A1 - Fluid level measurement apparatus

Info

Publication number: WO2006061232A1
Application number: PCT/EP2005/013200
Authority: WO
Inventors: Timothy Clark
Original assignee: Sensor Systems Jersey Ltd
Current assignee: Sensor Systems Jersey Ltd
Priority date: 2004-12-08
Filing date: 2005-12-07
Publication date: 2006-06-15
Anticipated expiration: 2007-06-08
Also published as: GB0426876D0

Abstract

An apparatus for measuring the level of fluid in a container. The apparatus comprises a measuring chamber; a fluid level indicator comprising a float device located outside of the measuring chamber; and a float follower located inside the chamber. The float device and the float follower are each provided with at least one magnet, the or each magnet of the float device creating a first magnetic field inside the measuring chamber, and the or each magnet of the follower creating a second magnetic field inside the measuring chamber. The first and second magnetic fields are mutually opposing to create a repelling magnetic force that is capable of supporting the float follower.

Description

Fluid Level Measurement Apparatus

FIELD OF THE INVENTION

The present invention relates to fluid level measurement apparatus especially, but not exclusively, for measuring the level of liquid in a tank.

BACKGROUND TO THE INVENTION

Apparatus for measuring the level of liquid in a tank are well known and typically include an ultrasonic, or other acoustic, transceiver located at one end of a measuring tube and arranged to bounce acoustic signals off a level indicator (which may comprise the surface of the liquid itself) inside the measuring tube. The liquid level is measured by determining how long an emitted acoustic pulse takes to return to the transceiver.

Conventional apparatus are often unsuitable for use in applications where the fluid (usually liquid) is aggressive or corrosive as is the case for, for example, petroleum and similar products. A first problem is the volatility of the liquid which may lead to vapour in varying degrees of concentration existing in the measurement zone, that is to say between the liquid surface and the acoustic transceiver. The speed of sound is reduced as the concentration (density) of the vapour increases and so the measurements taken at the transceiver increase in value and become progressively inaccurate. Moreover, the concentration of the vapour varies depending on the volatility of the actual fluid and may vary as ambient temperature rises and falls. A second problem relates to the corrosive effect that an aggressive fluid, especially chemicals, can have on the apparatus itself.

The level indicator may comprise a float and a float follower coupled by magnetic attraction. If magnetic attraction is used, the follower is pulled towards the inner wall of the tube and the float towards the outer wall of the tube. As the level of liquid in the tube rises, the buoyancy of the float needs to overcome both the mass of the float and that of the follower as well as being sufficient to overcome the resistance to movement caused by friction. The physical effect of this is to cause the float and follower to lurch from position to position rather than to travel smoothly. This hampers responsive and accurate measurement. Problems also arise with downward movement of the float and follower since the mass of the float and follower have to be such that gravitational force causes them to overcome the magnetic attraction so that they fall in line with the outside fluid level - if they are not heavy enough they will hang for a time as the buoyancy of the float is progressively reduced as the liquid level falls until this overcomes the frictional effect and they can fall. Whilst it is possible to establish a balance of magnetic attraction to float buoyancy and mass this is difficult and requires a deal of fine tuning, usually resulting in a compromise.

In addition, when a float and follower are coupled together by magnetic attraction, it is possible for them to separate during use, especially since they are often loosely coupled together in an attempt to mitigate the frictional effect referred to above. When they separate, incorrect measurements are taken and re-coupling is necessary.

SUMMARY OF THE INVENTION

Accordingly, a first aspect of the invention provides an apparatus for measuring the level of fluid in a container, the apparatus comprising a measuring chamber; a fluid level indicator comprising a float device located outside of the measuring chamber and a float follower located inside the chamber, the float device and the float follower each being provided with at least one magnet, the or each magnet of the float device creating a first magnetic field inside the measuring chamber, and the or each magnet of the follower creating a second magnetic field inside the measuring chamber, wherein the first and second magnetic fields are mutually opposing to create a repelling magnetic force that is capable of supporting the float follower.

In use, the measuring chamber, which typically takes the form of a tube, extends into the container and is at least partially surrounded by the fluid to be measured.

The follower is advantageously located within the measuring chamber such that the second magnetic field is located, in use, above the first magnetic field. As a result, the follower "floats", i.e. is supported against the effects of gravity, inside the measuring chamber by means of the magnetic repulsion between the first and second magnetic fields.

The mass of the follower and the strengths of the magnetic fields are advantageously selected such that the mass of the follower is insufficient for it to force its way past the magnetic field created in the measuring chamber by the float magnets, but sufficient to fall (under gravity) inside the measuring chamber when allowed to do so by the position of the float.

It is preferred that the float device and the follower each comprise a plurality of spaced-apart magnets arranged in an annular manner. More preferably, an odd number of magnets is provided in one of the float or follower and an even number of magnets is provided in the other of the float and follower. This encourages the follower to float generally centrally inside the measuring chamber and not to favour any specific position.

A second aspect of the invention provides said fluid level indictor.

A third aspect of the invention provides an installation comprising said measuring apparatus installed in said container.

Preferred features of the invention are listed in the dependent claims. Further advantageous aspects of the invention will become apparent to those skilled in the art upon review of the following description of a specific embodiment and with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention is now described by way of example and with reference to the accompanying drawings in which:

Figure 1 is a schematic side view of a fluid level measurement apparatus embodying one aspect of the invention, the apparatus being shown in situ in a vessel, the vessel being only partly shown;

Figure 2A is a perspective view of a first part of a float device included in the apparatus of Figure 1;

Figure 2B is a side view of the float device part shown in Figure 2 A;

Figure 2C is a perspective view of a second part of the float device;

Figure 2D is a plan view of the float device part of Figure 2C;

Figure 3 A is a cross-sectional side view of a first part of a float following device included in the apparatus of Figure 1 ;

Figure 3 B is a perspective view of the part shown in Figure 3 A;

Figure 3 C is a plan view of a second part of the float following device; and

Figure 3D is a perspective view of the part shown in Figure 3C. DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to Figure 1 of the drawings, there is shown, generally indicated as 10, a fluid level measurement apparatus embodying one aspect of the invention. The apparatus 10 is shown in situ mounted in a vessel or tank 12, only a top portion of which is shown in Figure 1. The tank 12 contains a quantity of fluid 14, usually liquid, the surface of which is indicated by line 16.

The apparatus 10 comprises a transceiver 18 located at one end of a measuring chamber that typically takes the form of a tube 20 (usually but not necessarily a substantially cylindrical tube). The tube 20 is advantageously formed from a non- ferrous, but preferably conductive material, especially metal, for example copper or stainless steel. When fitted to the tank 12, the tube 20 extends into the tank 12 such that the liquid to be measured surrounds at least a part of the tube 20. The transceiver 18 is arranged to emit signals that are propagated along the interior of the tube 20 in a direction generally away from the transceiver 18, and to receive signals that are returned to the transceiver 18 along the interior of the tube 20. Typically, the transceiver 18 transmits and receives acoustic signals, for example comprising audible sound pulses or ultrasonic pulses, but may alternatively transmit/receive electromagnetic signals, or any other suitable signals. In preferred embodiments, the transceiver 18 includes an acoustic transducer (not shown) which may comprise a conventional piezo-electric transducer that vibrates upon the application of an electrical signal to produce acoustic signals, and which generates electrical signals upon application of acoustic signals. Typically, the transceiver 18 includes, or is associated with, means, usually comprising a data processor, for controlling the transmission and reception of signals and for measuring the elapsed time between an emitted pulse and a reflected pulse, the reflected pulse having been reflected by a level indicator whose position corresponds to the fluid level in the tank. However, this, and other components relating to the transceiver 18, are not illustrated or described herein as they are not essential to understanding the invention. It will be understood that the transceiver 18 may take a variety of conventional forms.

The apparatus 10 may include an acoustic coupler 22 adjacent the transceiver 18. The coupler 22 may comprise a substantially conical body as indicated in Figure 1. The coupler 22 manipulates the acoustic waves travelling to and from the transceiver 18 but is not described in detail herein as it is not essential to the invention.

Advantageously, the tube 20 is sealed in an air-tight or substantially air-tight manner around the transceiver 18. The in use top end 26 of the tube 20 is typically carried by a stopper 24 that may be fitted into an aperture in the tank 12 in any convenient manner. Advantageously, the stopper 24 is formed at least partly from an electrically conductive material in order to ground or earth the tube 20 to the tank 12. In the preferred embodiment, the stopper 24 is formed from a conductive plastics. The in use bottom end 28 of the tube 20 is also advantageously sealed in an air-tight or substantially air tight manner. The arrangement is such that the tube 20 is sealed to the external environment, i.e. the tank 12, during use, so that the ingress of vapours or liquid is substantially prevented. In the illustrated embodiment, the transceiver 18 and associated components are located at the in use top end 26 of the tube. In alternative embodiments, the transceiver 18 and associated components may be located at the bottom end 28 of the tube 20.

The apparatus 10 further includes a fluid level indicating device, generally indicated at 30. The fluid level indicating device 30 comprises a float device 32 and a float following device 34. The float device 32, which is also shown in Figures 2 A to 2D, includes one or more magnets 36, conveniently permanent magnets although electro-magnets could alternatively be used, the preferred arrangement of which is described in more detail below. In the preferred embodiment, the float device 32 includes a generally sleeve-like body 38 shaped and dimensioned to fit around the external periphery of the tube 20. Advantageously, the width of the passage defined by the sleeve-like body 38 is wider than the width of the tube 20 such that the interior surface of the body 38 does not engage with the exterior surface of the tube 20 when the tube 20 and the body 38 are coaxial with one another. Conveniently, the body 38 may be substantially cylindrical in shape.

The body 38 includes or carries a float 40 to give buoyancy to the float device 32 so that it floats, during use, in the liquid 14. The float 40 may be formed from any suitable material and may be selected depending on the density of the liquid 14. Conveniently, the float 40 surrounds the body 38 in a sleeve-like manner.

In the preferred embodiment, the float device 32 comprises a plurality of magnets 36 arranged in an annular fashion so that, in use, the magnets 36 surround the tube 20. Alternatively, however, the float device 32 may comprise a single magnet (which may or may not be annular in form) or two or more spaced-apart or contiguous magnets (which may or may not be annular in form). The arrangement of the or each magnet 36 is such that, during use, the or each magnet 36 creates a magnetic field inside the tube. As is described in further detail below, the follower 34 includes one or magnets for creating an opposing magnetic field inside the tube 20 such that the follower 34 is urged away from the magnetic field created by the magnet(s) 36 of the float 32 by a repelling magnetic force.

Figures 2C and 2D show a carrier 42 for carrying the magnets 36 in the preferred embodiment. The carrier 42 includes four spaced-apart first stations 44, each station 44 being shaped and dimensioned to receive a respective magnet 36 (in alternative embodiments, the carrier 42 may be adapted to accommodate more or fewer magnets). The carrier 42 is generally annular in shape and so, in use, the magnets 36 carried in the stations 44 are spaced-apart around the external periphery of the tube 20. The carrier 42 preferably also includes one or more second stations 46, each second station 46 being shaped and dimensioned to receive a bearing, for example a ball bearing 48 or a roller bearing (Figure 1). The bearing 48 may be made from any suitable material, for example glass. Each station 46 is shaped to define a gap at the inner side of the carrier 42 so that, when the respective bearing 48 is seated in the station 46, a portion of the bearing 48 protrudes through the gap. Preferably, at least two spaced-apart bearing stations 46 are provided, the spacing being such that at least two respective bearings are substantially oppositely disposed around the tube 20 during use. In the illustrated embodiment, the carrier 42 includes four stations 46 spaced substantially evenly around the carrier 42. The magnets 36 are aligned or orientated such that the magnetic field that they create, during use, has a predominant North or South polarity.

The carrier 42 and the body 38 are fixed together (releasably, permanently or integrally formed) in any convenient manner. For example, the carrier 42 may include the male member(s) 50 of one or more spring clips, the corresponding female member(s) 52 being provided on the body 38 (or vice versa). To facilitate the interconnection of the carrier 42 and the body 38, the body may be provided with a flange 54, for example an annular flange, adjacent one end. The flange 54 may also help to support the float 40. The body 38, the flange 54 and the carrier 42 may be made from any suitable material, for example nylon.

When the float device 32 is fitted around the tube 20, the magnets 36 are located adjacent (but typically spaced-apart from) the exterior surface of the tube 20 and the bearings 38 are in contact with the exterior surface of the tube 20. The float device 32 is slidable with respect to the tube 20 in a direction generally parallel with the longitudinal axis of the tube 20. hi the preferred embodiment, the carrier 42 grips the tube 20 by means of the bearings 48, the bearings 48 serving to facilitate said sliding movement. Advantageously, the bearings 48 also help to locate the float device 32 in a generally coaxial manner with the tube 20. It is preferred to provide an (electrical) earth connection between the float device 32 and the tube 20 primarily to prevent the build up of electrostatic charge. To this end, a bushing 56 may be provided between the float device 32 and the tube 20. The bushing 56 may take the form of an annular cap fitted over the end 58 of the body 38. The end 58 of the body 38 may be provided with a groove 60 for seating the bushing 56. Advantageously, the bushing 56 helps to maintain the float device 32 in its preferred coaxial arrangement with the tube 20. The bushing 56 may be made from any suitable electrically conductive material, for example conductive plastics.

Referring now to Figures 1 and 3A to 3D, the preferred float follower 34 is described in more detail. The follower 34 includes one or more magnets 60, conveniently but not necessarily permanent magnets, the preferred arrangement of which is described in more detail below. The follower 34 comprises a body 62 having a surface 64 for reflecting signals that are, during use, emitted by the transceiver 18. It is preferred to form the body 62 from a relatively lightweight material such as nylon or other plastics and so the reflecting surface 64 may be provided by an insert 66 formed from, or coated with, a reflecting material.

In the preferred embodiment, the width w of the follower 34 is less than the internal width of the tube 20 so that the follower 34 does not engage (or at least has minimal engagement with) the internal walls of the tube 20 when the follower 34 is coaxially located within the tube 20. However, it is also preferred that the width w of the follower 34 (at least at its widest region if the follower is of non- uniform width) is approximately the same width as the tube 20 while still being narrower than the tube 20. This limits the amount by which the follower 34 can tilt within the tube 20 . Similarly, the length / of the follower 34 may be selected to limit the amount by which the follower 34 can tilt within the tube 20. The shape, including length and/or width, of the follower 30 is preferably selected such that it prevents (by engagement with the tube walls) the follower 34 from revolving about an axis generally perpendicular with the longitudinal axis of the tube 20. Moreover, the shape and dimensions of the follower 34 are preferably selected such that, should the follower 34 tilt and engage with the sides of the tube 20, it tends to move back towards the ideal coaxial position rather than coming to rest in the tilted position. This may, for example, be achieved by arranging for the centre of gravity of the follower 34 to remain within a region bounded by the lateral dimensions of the base of the follower 34 when tilted, hi preferred embodiments, the length I of the body 62 is preferably at least approximately equal to the internal width of the tube 20 and, more preferably, is at least approximately 1 to 2 times the width of the tube 20.

In the preferred embodiment, the follower 34 includes a plurality of magnets 60 arranged such that respective like magnetic poles are aligned with one another, i.e. such that the respective North poles are facing generally in the same direction and the respective South poles are facing in generally the same direction. Alternatively, a single magnet may be provided.

Figures 3C and 3D show a carrier 68 for carrying the magnets 60 in the preferred embodiment. The carrier 68 includes five spaced-apart stations 70, each station 70 being shaped and dimensioned to receive a respective magnet 60 (in alternative embodiments, the carrier 68 may be adapted to accommodate more or fewer magnets). The stations 70 are preferably spaced-apart in an annular manner. In preferred embodiments, one of the float device 32 and follower device 34 carries an odd number of magnets, the other carrying an even number of magnets.

The carrier 68 and the body 62 are fixed together (releasably, permanently or integrally formed) in any convenient manner. For example, the body 62 may include the male member(s) 72 of one or more spring clips, the corresponding female member(s) 74 being provided on the carrier 68 (or vice versa). The carrier 68 may be formed from any suitable material, preferably lightweight material such as nylon or other plastics. It is preferred that, apart from the magnets 36, 60 themselves, the tube 12, the float device 32 and the follower 34 are all formed from non-magnetic material.

The magnets 36 of the float device 32 create a magnetic field inside the tube 20. The follower 34 is orientated within the tube 20 so that an opposing magnetic field, created by magnets 60, faces, and is in use located above, the magnetic field created by the float device 32. As a result, a repelling magnetic force acts on the follower 34 urging it away from the field created by the float device 32 which, in use, is in a generally upward direction. The respective strengths of the magnetic fields and the mass of the follower 34 are selected such that the follower 34 is unable to fall past the magnetic field created by the float device 32 under the action of gravity. Hence, the follower 34 floats within the tube 20, supported by the magnetic fields inside the tube 20.

By way of example, the magnetic field created by the float device 32 is indicated as Ml in Figure 1, while the magnetic field created by the follower 34 is indicated as M2. In the illustrated embodiment, the field M2 emanates from one end (the in use bottom end) of the follower 34 with the result that the follower 34 is located above the field Ml . Alternatively, the arrangement may be such that the follower 34, or at least part of it, is located below the field Ml (but the field M2 remains above Ml). In either case, the opposing magnetic fields Ml, M2 support the follower 34 in the tube 20.

Hence, the repelling effect of opposing magnets (as opposed to the coupling effect of magnetic attraction) is used to float the follower 34 inside the tube 20. As the level 16 of the liquid 14 varies in the tank 12, the float device 32 moves up and down (in a direction generally parallel with the longitudinal axis of the tube) accordingly because of its buoyancy in the liquid 14. Consequently, the follower 34 is also moved up and down by a corresponding amount by the magnetic fields Ml, M2 (Ml moves as the float device 32 moves and the repelling force between

Ml and M2 causes the follower 34 to move correspondingly). Hence, the follower 34 is maintained in the tube 20 at a level that corresponds to the level of the liquid 14. The apparatus 10 may take fluid level measurements in conventional fashion, i.e. by measuring the time taken for an emitted signal to be returned to the transceiver 18 after having been reflected from the reflecting surface 64. It is preferred that the reflecting surface 64 is maintained at substantially the same level as the liquid surface 16. This may be achieved by selection of the lengths of the follower 34 and/or the float device 32 and depends on the specific density of the liquid 14 (which determines how low the float device 32 sits in the liquid with respect to the surface 16). Alternatively, the position of the reflecting surface 64 may be offset from the level 16 of the liquid 14 and the apparatus 10 may be calibrated to account for this.

The apparatus 10 is suitable for use in either a pressurised or un-pressurised vessel 12 and is suitable for use with convention measurement equipment (including the transceiver) since sealing the tube 20 separates the tank environment (which may be pressurised, aggressive, corrosive and/or vapour heavy) from a controlled and normal environment in side the tube 20 in which accurate measurements can be made.

The float device 32 and follower 34 experience significantly less frictional resistance than would be present when magnetic attraction is used to couple a float and follower. In addition, since the follower 34 falls under gravity and is raised by the repellent effect of opposing magnetic fields, the follower 34 is not coupled to the float 32 as it would otherwise be if magnetic attraction were used. The preferred arrangement is such that the mass of the follower 34 is insufficient for it to force its way past the magnetic field Ml created in the tube 20 by the float magnets 36, but sufficient to always fall (under gravity) inside the tube 20 when allowed to do so by the position of the float 32. As a result, the follower 34 maintains a substantially fixed position with respect to the float 32 and cannot " become de-coupled. The apparatus 10 allows fluids of different specific density to be measured accurately. The buoyancy of the float 32 should be sufficient to overcome both the mass of the float device 32 and also the mass of follower 34 inside the tube 20. This is relative to the specific density of the fluid being measured. The lower the specific density of the fluid, the greater the buoyancy required. For example, for use in a vessel containing petroleum, the same float will settle lower in the liquid than it would in water. By adjusting the length of the follower, e.g. employing a slightly longer follower 34 inside the tube 20, the level at which the level measurement is taken can be adjusted to match the fluid level in the tank, still using the same float mechanism.

To cause the follower 34 to float inside the tube 20 while avoiding it being subjected to adverse wear caused by repetitive travel along the same path, the magnets are preferably positioned as follows. Both sets of magnets 36, 60 are positioned around the circumference or periphery of the float 32 and follower 34 respectively, the arrangement being such that they oppose magnetically. The North (or South) poles of the magnets 36 in the float 32 are positioned so that the magnetic field Ml created passes through the non-magnetic surface of the tube 20 (copper or stainless steel for example). Conveniently, this is achieved by positioning the magnets 36 such that the relevant poles face the tube 20. The magnets 60 in the follower 34 are positioned so that their North (or South) poles face downwardly (as viewed in Figure 1). Hence the resultant magnetic field M2 is located below the follower 34, or at least below the part of the follower 34 that carries the magnets 60. The follower 34 is positioned inside the tube 20 such that the field M2 is located above the field Ml . Hence, the magnetic field Ml inside the tube 20 serves as a barrier or net to prevent the opposing field M2, and therefore the follower 34, from passing through the field Ml . It is preferred to provide an odd number of magnets in one of the float 32 or follower 34 (for example 5 magnets 60 in the follower 34) and an even number in the other of the float 32 and follower 34 (for example 4 magnets 36 in the float 32). This encourages the follower 34 to float generally centrally inside the tube 20 and not to favour any specific position.

In alternative embodiments, in which the transceiver 18 and associated components are located at the bottom end 28 of the tube 20 (in which case the tube may emanate from the bottom surface of the tank) measurements may be taken from the follower's lower surface to the bottom of the vessel (or tube) as opposed to from the follower's top surface to the top of the vessel (or tube). More generally, any surface of the follower may be used to reflect signals for measurement purposes.

The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.

Claims

CLAIMS:

1. An apparatus for measuring the level of fluid in a container, the apparatus comprising a measuring chamber; a fluid level indicator comprising a float device located outside of the measuring chamber; and a float follower located inside the chamber, the float device and the float follower each being provided with at least one magnet, the or each magnet of the float device creating a first magnetic field inside the measuring chamber, and the or each magnet of the follower creating a second magnetic field inside the measuring chamber, wherein the first and second magnetic fields are mutually opposing to create a repelling magnetic force that is capable of supporting the float follower.

2. An apparatus as claimed in Claim 1, wherein said second magnetic field is located, in use, above said first magnetic field.

3. An apparatus as claimed in Claim 1 or 2, wherein said float device comprises a sleeve-like body adapted to fit around said measuring chamber.

4. An apparatus as claimed in Claim 3, wherein said sleeve-like body is substantially cylindrical.

5. An apparatus as claimed in any preceding claim, wherein said float device comprises a plurality of spaced-apart magnets, the magnets being, in use, spaced- apart around said measuring chamber.

6. An apparatus as claimed in Claim 5, wherein each magnet is aligned such that a respective one of its magnetic poles faces said measuring chamber, each chamber-facing magnetic pole having the same polarity.

7. An apparatus as claimed in any preceding claim, wherein said float device includes at least one bearing positioned for engagement with the external surface of the measuring chamber.

8. An apparatus as claimed in Claim 7 when dependent on Claim 3, wherein said at least one bearing maintains said body in a spaced-apart relationship with the external surface of the measuring chamber during use.

9. An apparatus as claimed in any preceding claim, wherein said float device includes an annular carrier adapted to carry said at least one magnet.

10. An apparatus as claimed in Claim 9 when dependent on Claim 7, wherein said annular carrier is adapted to carry said at least one bearing.

11. An apparatus as claimed in any preceding claim, wherein said float device is movable with respect to the measuring chamber in a direction substantially parallel with the longitudinal axis of the chamber.

12. An apparatus as claimed in any preceding claim, wherein said float follower includes a plurality of spaced-apart magnets, the magnets being spaced-apart around the longitudinal axis of the chamber.

13. An apparatus as claimed in Claim 12, wherein said magnets are aligned such that magnetic poles of the same polarity face in substantially the same direction.

14. An apparatus as claimed in Claim 13, wherein the magnetic poles of said magnets face in a direction substantially along, or parallel with, the longitudinal axis of the measuring chamber.

15. An apparatus as claimed in any preceding claim, wherein said float follower includes an annular carrier adapted to carry said at least one magnet.

16. An apparatus as claimed in any preceding claim, wherein the at least one magnet provided on the float follower is orientated to create said second magnetic field, in use, below itself/themselves.

17. An apparatus as claimed in any preceding claim, wherein at least the part of the float follower that carries said at least one magnet is located above the second magnetic field during use.

18. An apparatus as claimed in Claim 17, wherein all, or substantially all, of the float follower is located above said second magnetic field during use.

19. An apparatus as claimed in any preceding claim, wherein said float follower includes a reflecting surface disposed substantially, or generally, perpendicular with the longitudinal axis of the measuring chamber.

20. An apparatus as claimed in any preceding claim, wherein said float follower comprises an elongate body having its longitudinal axis disposed substantially, or generally, parallel with the longitudinal axis of the chamber.

21. An apparatus as claimed in Claim 20, wherein the length of said elongate body is as least equal to, and preferably greater than, the width of the measuring chamber taken in a direction substantially perpendicular with the longitudinal axis of the chamber.

22. An apparatus as claimed in any preceding claim, wherein the width of the float follower, at least at its widest point, is less than but substantially equal to the width of the measuring chamber.

23. An apparatus as claimed in any preceding claim, wherein each of said float device and said float follower comprises a respective plurality of magnets, and wherein one of said float device and float follower comprises an odd number of magnets, the other of said float device and float follower comprising an even number of magnets.

24. An apparatus as claimed in Claim 23, wherein each of said respective plurality of magnets is arranged in a substantially evenly spaced-apart and annular manner.

25. A fluid level indicator comprising a float device for location outside of a measuring chamber; and a float follower for location inside the chamber, the float device and the float follower each being provided with at least one magnet, the or each magnet of the float device being arranged to create a first magnetic field inside the measuring chamber, and the or each magnet of the follower being arranged to create a second magnetic field inside the measuring chamber, wherein the first and second magnetic fields are mutually opposing to create a repelling magnetic force that is capable of supporting the float follower.

26. An installation comprising a container and an apparatus as claimed in Claim 1, wherein the measuring chamber extends into the container and is positioned and dimensioned to be, in use, at least partially surrounded by fluid to be measured.