GB2176290A

GB2176290A - Apparatus for monitoring fluid level

Info

Publication number: GB2176290A
Application number: GB08511295A
Authority: GB
Inventors: John Linklater Hutton
Original assignee: GRAMPIAN REGIONAL COUNCIL
Current assignee: GRAMPIAN REGIONAL COUNCIL
Priority date: 1985-05-03
Filing date: 1985-05-03
Publication date: 1986-12-17
Also published as: GB8511295D0

Abstract

Apparatus for monitoring the level of an interface between the two phases in a multi-phase fluid system in which the phases are separated by gravity, the apparatus comprising a plurality of pairs of electromagnetic radiation emitters (60) and electromagnetic radiation receivers (62) which are mounted in succession along an elongate member (48) so that each emitter opposes and is spaced from the corresponding receiver of the respective pair, the elongate member being immersible in the fluid system so that each emitter and receiver pair us positioned at a respective height in the fluid system and so that a respective portion (58) of the fluid system at each respective height fills the space between the respective emitter and receiver, each emitter being adapted to transmit a signal of electromagnetic radiation via the respective portion towards the respective receiver, and means for detecting whether each respective received signal is above or below a particular threshold value, the means for detecting being arranged to produce a plurality of electrical signals each of which is representative of whether the interface is above or below the respective pair. Infra red radiation is transmitted via light guides. The apparatus is used in sewage treatment plant. <IMAGE>

Description

SPECIFICATION Method and apparatus for monitoring the level of an interface The present invention relates to a method and apparatus for monitoring the level of an interface between two phases in a multi-phase fluid system in which the phases are separated by gravity. In particular, the present invention relates to the detection and control of sludge blanket layers in water, sewage and industrial waste treatment processes.

Some known water and waste treatment processes involve a sequence of discrete processing stages one or more of which may involve gravity settling in a clarifier liquid containing solid particles. In wastewater treatment practice, liquor and particulate matter in suspension continuously enters the clarifier via a feed pipe or inlet channel into a stilling zone designed to dissipate the kinetic energy of the incoming fluid. The inflow displaces over the outlet weir clarified supernatant liquor already in the tank, the particulate matter having settled to form a sludge as the liquor has moved progressively from the stilling zone to the outlet weir. The time taken for the liquor to so move is known in the art as "nominal retention time" and is a function of tank capacity and rate of inflow to the tank.The settled sludge is conveyed by a cleaning mechanism which may operate continuously or intermittently to a sump for removal. The flow of sludge from the bottom of the tank is known in the art as the "sludge underflow". To ensure maximum clarifier efficiency a carefully controlled balance between inlet flow, supernatant overflow and sludge underflow is essential. The depth of the sludge blanket layer must be carefully controlled to ensure in particular that the blanket does not rise to the level of the overflow. In some processes in which the sludge blanket may be a submerged floating layer effectively acting as a filter the thickness of the layer may have to be preserved between limits to ensure processing efficiency.

It is an object of the present invention to provide a facility for controlling the rate of rise or fall of the sludge blanket without the need of moving mechanical parts or live electrics within the body of liquid in the clarifier.

Accordingly, the present invention provides apparatus for monitoring the level of an interface between two phases in a multi-phase fluid system in which the phases are separated by gravity, the apparatus comprising a plurality of pairs of electromagnetic radiation emitters and electromagnetic radiation receivers which are mounted in succession along an elongate member so that each emitter opposes and is spaced from the corresponding receiver of the respective pair, the elongate member being immersible in the fluid system so that each emitter and receiver pair is positioned at a respective height in the fluid system and so that a respective portion of the fluid system at each respective height fills the space between the respective emitter and receiver, each emitter being adapted to transmit a signal of electromagnetic radiation via the respective portion towards the respective receiver, and means for detecting whether each respective received signal is above or below a particular threshold value, the means for detecting being arranged to produce a plurality of electrical signals each of which is representative of whether the interface is above or below the respective pair.

The present invention further provides a method for monitoring the level of an interface between two phases in a multi-phase fluid system in which the phases are separated by gravity, the method comprising the steps of: (a) transmitting a plurality of signals of electromagnetic radiation through the fluid system at a succession of respective heights in the fluid system; (b) receiving the transmitted signals at respective corresponding heights in the fluid system; and (c) detecting whether each respective received signal is above or below a particular threshold value so as to produce a plurality of electrical signals each of which is representative of whether the interface is above or below the respective height.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a part-sectional schematic diagram of part of a waste treatment clarifier incorporating an apparatus in accordance with the invention; Figure 2 is sectional schematic diagram of a part of the apparatus in accordance with the invention which is illustrated in Fig. 1; and Figure 3 is a perspective view of a portion of the apparatus in accordance with the invention which is illustrated in Figs. 1 and 2.

Referring to Fig. 1, there is shown a partsectional view of a waste treatment clarifier 2, such as an activated sludge final clarifier. The clarifier 2 comprises a tank 4 which is generally cylindrical in shape and is open at the top. The tank 4 consists of a cylindrical upright side-wall 6 and a floor 8 which is slightly inclined downwardly from the side wall 6 towards the axis of the cylindrical tank 4 so as to be frusto-conical in shape. In the region of the axis of the tank 4, the floor consists of conical portion 10 which points downwardly to form a sump 12 in the bottom of the tank 4. Around the outside of the tank 4 there is provided an annular channel 14 which is formed by an annular wall 16.When the tank 4 is full of water and waste to be treated, any excess liquid can spill over the upper edge 18 of the side-wall 6 into the channel 14 from which it will flow under the influence of gravity to another place. To facilitate the overflow of the liquid into the channel 14, the upper edge 18 of the side-wall 6 is inclined downwardly towards the channel 14 so as to form an overflow weir.

An inlet pipe 20 for water and waste to be clarified passes through the conical portion 10 into the sump 12 and then upwardly along the axis of the tank 4. The inlet-pipe 20 discharges into an open-ended cylinder known in the art as a stilling drum 22. The stilling drum 22 is open at the top and bottom and the cylindrical upper edge 24 of the cylindrical side-wall 26 of the stilling drum 22 is positioned slightly above the height of the upper edge 18 of the side wall 6 of the tank 4. An outlet pipe 28 is provided in the bottom of the conical portion 10 and extends away from the tank 4 to a discharge tank (not shown).

The exit of the outlet pipe 28 is provided with a sludge draw-off control valve 30 for controlling the output of matter from the outlet pipe 28.

After initial filling, the tank 4 operates continuously. In operation, water and waste to be clarified passes along the inlet pipe 20 into the stilling drum 22. The stilling drum 22 dissipates the kinetic energy of the inflow and directs it downwards and outwards into the body of the tank 4. Gradually the solid particles in the waste settle out by gravity to form in the tank a sludge blanket layer 32 beneath a supernatant (clarified) liquid layer 34. In operation the sludge blanket layer 32 is continually being formed, and the sludge is removed through outlet pipe 28 in a controlled manner while the supernatant liquid passes over the overflow weir into the channel 14.

Further water and waste to be treated are fed in through inlet pipe 20 at a rate varying between limits controlled by the hydraulic design. The rate of input of water and waste and the rate of withdrawal of the sludge underflow are carefully controlled so as to give a continuous clarification process while maintaining a suitable depth of sludge blanket layer 32 The time which any given amount of water and waste to be clarified spends in the tank is controlled by the input and sludge underflow rates. The time is sufficient to ensure that settiing of the solid particles from the given amount of liquid has occurred before the liquid is displaced from the tank by fresh influent.

For the removal of sludge which settles on the floor 8 of the tank 4 there are provided a number of scraper blades 36 which are mounted in a line along a radius of the tank 4.

Each scraper blade 36 is attached to a respective leg 38 which depends from a bridge -40. The bridge 40 is mounted above the cylindrical tank 4 and extends along a radius thereof. The bridge 40 is pivotally mounted about the axis of the tank 4. The bridge 40 is provided with a wheel 42 at its outer end which can run along an upper edge 44 of the annular wall 16 whereby the bridge 40 can be rotated about the axis so that the scraper blades 36 scrape the whole surface of the floor 8 of the tank 4 in one- rotation. The sludge is conveyed down the incline of the floor 8 into the sump 12 by the continuous rotation of the scraper bridge 40.

In accordance with the invention, a detector 46 for detecting the level of the sludge blanket layer in the tank 4 is mounted on bridge 40 and depends therefrom into the tank 4.

Thus as bridge 40 rotates so that the scraper blades can scrape the floor 8 of the tank 4, the detector 46 also rotates through the liquid and sludge in the tank 4. Alternatively, the detector 46 may be mounted in a stationary position in the tank 4 provided that the detector 46 does not interfere with the sludge removal apparatus.

As is shown more clearly in Figs. 2 and 3, the detector 46 has an elongate ladder-like construction. The detector 46 comprises two elongate tubes 48 which are parallel and a series of rung-like connections 50 joining the two tubes together at a plurality of spaced locations. Each rung-like connection 50 comprises an elongate cylindrical detector head 52, each end 54 of which is joined to a respective one of the tubes 48 by means of a T-shaped tube connector 56. The lower wend 57 of each tube 48 is sealed, and the Tshaped tube connections 56 are sealed to the tubes 48 and the detector heads 52 so that when the detector 46 is submerged or partly submerged into the liquid in the tank 4, no liquid from the tank can enter the tubes 48.

Each detector head 52 is formed as a cylindrical plug which has a transverse cylindrical cavity 58 extending across a diameter thereof.

The detector head 52 is arranged in the detector 46 so that- when the detector 46 is upright, the cavity 58 is also upright. Preferably the diameter of the cylindrical cavity is about 10mm and the diameter of the cylindrical detector head 52 is about 20 mm. Preferably, the edges of the cavity 58 are smooth so as to minimise the risk of fouling of the cavity 58 by sludge and to facilitate any cleaning of the detector head 52 which may be required.

Preferably, the detector head 52 is formed from a plastics material such as perspex (RTM). The perspex detector head 52 can readily have the cavity 58 drilled therethrough.

An emitter 60 and a receiver 62 of electromagnetic radiation are mounted in each detector head 52, with the emitter 60 and receiver 62 being mounted in the wall of the cavity 58 so as to oppose each other across a diameter of the cavity 58. The emitter 60 and receiver 62 are aligned along the longitudinal axis of the detector head 52.

In the illustrated arrangement, the emitter 60 and receiver 62 each comprise the end or ends of one or more respective optical fibres 64, 66. The ends of the optical fibres 64, 66 are firmly mounted in the detector head 52 so as to align accurately the ends of the emitter and receiver optical fibres 64, 66 thereby to provide efficient transmission of a signal of electromagnetic radiation therebetween. The cavity 58 permits liquid to be detected to pass between the emitter 60 and receiver 62 and ensures accurate spacing between the ends of emitter and receiver optical fibres 64, 66. The optical fibres 64, 66 pass up the tubes 48 to a control box (not shown) which may be integral with the detector 46 or may be situated away from the detector 46 in a convenient location.The control box includes one or more sources of electromagnetic radiation which can send electromagnetic radiation along the emitter optical fibres 64 and also one or more sensors for sensing electromagnetic radiation which is received by and transmitted along the receiver optical fibres 66.

Preferably, the electromagnetic radiation is infra-red radiation.

The control box further includes a microprocessor unit for controlling the operation of the detector 46 and the control valve 30. The operation of the detector and the microprocessor unit will be described hereinbelow.

In operation, the detector 46 is submerged upright in the liquid and sludge to be clarified which is in the tank 4.

The detector 46 is provided with a desired number of detector heads 52 and the detector heads are spaced a desired distance from each other, the number of detector heads and the distance depending on the particular application to which the apparatus of the invention is to be put. In the illustrated arrangement, six detector heads 52 are employed which are equally spaced from each other in a series to give a six-channel detector 46, each channel corresponding to a respective detector head 52. Typically the uppermost detector head is about 1 metre below the maximum level 68 of liquid in the tank 4. The lowermost detector head is situated just above the floor 8 of the tank 4. The interface 70 between the top of the sludge layer 32 and the supernatant liquid layer 34 is monitored between the uppermost and lowermost detector heads 52.

The apparatus is arranged so that in each detector head 52, infra-red radiation can be sent from the respective emitter 60 to the respective receiver 62 through the liquid and/or sludge which fills the respective cavity 58. The sensitivity of each channel of the detector 46 is adjusted so that when the cavitity 58 is filled with the clarified supernatant liquid 34, transmission of the infra-red radiation through the supernatant liquid in the cavity 58 can occur and when the cavity 58 is filled with sludge 32, transmission of the infra-red radiation through the sludge 32 cannot occur due to the opacity of the sludge 32 to infrared radiation. The absence of sludge 32 in cavity 58 results in a binary zero signal, whereas the presence of sludge 32 in the cavity 58 results in a binary one signal.

Each detector head 52 is arranged to detect the presence or absence of sludge 32 in its respective cavity 58. A display of light emitting diodes on the control box indicates the presence or absence of sludge 32 at the detector heads 52 in response to the binary signals emitted by the respective channels. The light emitting diode display may consist of six light emitting diodes in a row. For example, when the interface 70 is between the third and fourth detector heads 52 (the first detector head 52 and the sixth detector head 52 being the lowermost and uppermost detector heads respectively), the first three light emitting diodes (corresponding to the first three detector heads) are ON and the last three light emitting diodes (corresponding to the other three detector heads) are OFF. Thus the display gives a visual indication of the approximate height of the interface 70 to an operator.The operator can accordingly alter the rate of sludge removal (sludge underflow) from the clarifier 2 by adjusting the control valve 30 in order to maintain the interface 70 at a desired level.

The detector apparatus may also be arranged to give automatic on line control of the sludge underflow rate by automatically adjusting the sludge draw-off control valve 30 and thereby the height of the interface 70. This is achieved by storing the signals produced by the detector heads 52 so that they can detect the rate of rise or fall of the interface 70 at the top of the sludge layer 32. The several channels are arranged to send a sequence of signals to the microprocessor unit which analyses the signals and automatically adjusts the control valve 30. The signals sent from the microprocessor unit to control the control valve 30 may be sent either by direct line or by radio transmission. The interface 70 may be automatically contained between specified limits by varying the rate of sludge underflow from the clarifier 2.

The detector heads 52 are operated in sequence fron the first (lowermost) detector head (dl) to the sixth (uppermost) detector head (d5) to give a six-bit binary number S representative of the position of the interface 70 relative to the detector heads 52.

S=d6 d5 d4 d3 d2 d, where di=O if detector head i is clear of sludge, 1 if detector head i is covered with sludge.

The only possible valid values of S are 0, 1,3, 7, 15, 31 and 63 representing the sequential covering by the sludge of the six detector heads from the lowest to the highest.

The control logic in the microprocessor unit employs the value of S to determine the position of the interface 70 relative to the series of detector heads 52. The value of S is stored in the memory of the microprocessor unit.

After a period of time t, the detector heads 52 are again sequentially operated to give a second value of S (which is S, at time t1). The microprocessor unit compares S, with S to determine whether the interface 70 has risen above or fallen below one of the detector heads 52 during time interval t and, in response thereto, to control the height of the interface 70 by operation of the control valve 30 to increase or decrease the sludge underflow rate. After a further period of time, a third value of S is determined and that value is used to control the height of the interface 70 in a similar manner. The control of the height of the interface 70 is thus continual.

In the illustrated arrangement, the period of time between which successive values of S are determined is preferably 5 minutes. However, when conditions are critical, such as at high sludge level conditions, the frequency of operation may be increased so that the se quentiai determinations of S occur at two-minute intervals.

The control logic in the microprocessor unit may operate in the following manner for a sixchannel detector.

The main "inner" control band is between detector heads numbers 2 and 4. If detector head number 2 is covered with sludge but detector head number 4 is not covered with sludge (i.e. S=3 or 7 and the interface 70 is between detector heads numbers 2 and 4), no action is taken by the microprocessor unit to vary the sludge underflow rate which is at a pre-set "normal" value.

If the interface 70 is above detector head number 1 but below detector head number 2 (i.e. S=1) the sludge underflow rate is reduced by x% to a pre-set minimum value, - x being a predetermined parameter which is dependent upon the particular clarifier system with which the detector apparatus is being employed. If the interface 70 is below detector head number 1 (i.e. S=O), the sludge underflow rate is reduced to zero until the interface 70 rises again above detector number 1 when the sludge underflow rate is restored to the pre-set minimum value. However, if the detector head number -1 is clear of sludge for four consecutive readings, an alarm is triggered to inform the operator.

If the interface 70 is above detector head number 4 (i.e. S=15), the frequency of the determination of S is increased so as sequentially to operate the detector heads every two minutes. If the interface 70 is above detector head number 5 (i.e. S=31), the sludge underflow rate is increased by x%. If the interface 70 is above detector head number 6 (i.e.

S=63), the sludge underflow rate is increased by 2x%. If the interface 70 remains above detector head number 6 for four consecutive readings, the alarm is triggered.

When the interface 70 drops below detector head number 4, the sludge underflow rate is reduced by x%. If the interface 70 drops below detector head number 3, the sludge underflow rate is restored to the pre-set normal value and a 5 minute scanning interval is resumed.

The control box including the microprocessor unit may be arranged to control a number of detectors 46, each of which is associated with a respective clarifier 2. When a number of clarifiers 2 are being controlled the control box may be situated at a convenient location remote from the clarifiers 2.

The preferred embodiment of the present invention provides a method and apparatus for controlling the rate of rise or fall of the upper level of the sludge blanket layer without the need for moving mechanical parts or live electrics within the body of the liquid in the clarifier. The advantages of the present invention are that no live electrics are submerged in the liquid to result in a safety hazard, and that there are no moving parts in the detector assembly which may disturb the carefully controlled hydraulic balance within the clarifier, or which may fail through corrosion.

The preferred embodiment of the present invention is essentially fail-safe in character since failure of any one channel above the interface 70 to allow transmission of a signal between the respective emitter 60 and the respective receiver 62 would result, when the interface 70 is above a desired level, in an increased sludge draw-off rate which would cause the upper level of the sludge blanket layer to be lowered.

The preferred embodiment of the present invention is designed so that failure of the invention will not in a short time seriously affect the operation of the treatment process.

It will be apparent to those skilled in the art that the detector apparatus and the detection method of the present invention are not limited to the detection and control of a sludge layer in a sewage clarifier in a domestic waste water treatment works. The method and apparatus of the present invention may be employed in any system where an interface is formed between two-phases in a multi phase fluid system in which the phases are separated by gravity. For example, the invention may be employed in other water, sewage and industrial waste treatment processes in which two separable or immiscible liquid phases separate under the action of gravity to form a boundary therebetween. The invention may also be employed in a fluid system in which one of the two phases is gaseous. For example, the invention may be employed to monitor the level of a liquid ip a tank which is open to the atmosphere. The type of electro magnetic radiation utilized in the detection method and apparatus, and the sensitivity of the detector heads, may be varied in accordance with the types of fluid phases which are being monitored. Furthermore, the number and spacing of the detector heads may be varied as desired.

In some multiphase systems, a sludge blanket may be a layer which floats on a lower layer but is also submerged beneath an upper layer, with the sludge layer effectively acting as a filter between the upper and lower layers. The detection method and apparatus of the present invention may be arranged to control the thickness of the layer and maintain the top and bottom of the layer within prescribed limits.

Claims

1. Apparatus for monitoring the level of an interface between the two phases in a multiphase fluid system in which the phases are separated by gravity, the apparatus comprising a plurality of pairs of electromagnetic radiation emitters and electromagnetic radiation receivers which are mounted in succession along an elongate member so that each emitter opposes and is spaced from the corresponding receiver of the respective pair, the elongate member being immersible in the fluid system so that each emitter and receiver pair is positioned at a respective height in the fluid system and so that a respective portion of the fluid system at each respective height fills the space between the respective emitter and receiver, each emitter being adapted to transmit a signal of electromagnetic radiation via the respective portion towards the respective receiver, and means for detecting whether each respective received signal is above or below a particular threshold value, the means for detecting being arranged to produce a plurality of electrical signals each of which is representative of whether the interface is above or below the respective pair.

2. Apparatus according to claim 1 wherein each emitter and each receiver is an end of a respective optical fibre.

3. Apparatus according to claim 2 further comprising a source of electromagnetic radiation for transmitting electromagnetic radiation along the emitter optical fibres and an electromagnetic radiation detector for detecting electromagnetic radiation received by the receiver optical fibres.

4. Apparatus according to claim 3 wherein the source of electromagnetic radiation and the detector are arranged to act in succession on each emitter and detector pair.

5. Apparatus according to any foregoing claim wherein the electromagnetic radiation is infra-red radiation.

6. Apparatus according to any foregoing claim wherein each of the phases has a respective opacity to the electromagnetic radiation and the emitter and receiver pairs are adapted so that when the respective portion of the fluid system is comprised substantially of one of the two phases, transmission of the signal of electromagnetic radiation can occur through the portion between the emitter and the receiver and so that when the respective portion of the fluid system is comprised substantially of the other of the two phases, transmission of the signal of electromagnetic radiation cannot occur through the portion between the emitter and the receiver.

7. Apparatus according to any foregoing claim wherein the electrical signals produced by the means for detecting are binary signals which are indicative of which of the two phases is between the respective emitter and receiver pair.

8. Apparatus according to any foregoing claim further comprising a visual display comprising an array of lights, each of which is associated with a respective emitter and receiver pairs and is switched on or off in response to the value of the respective electrical signal produced by the means for detecting thereby to give a visual indication of the level of the interface relative to the emitter and receiver pairs.

9. Apparatus according to any foregoing claim further comprising means for controlling the level of the interface by varying the relative volumes of fluids in the fluid system, the means for controlling being qperable in response to the electrical signals produced by the means for detecting.

10. Apparatus according to any foregoing claim further comprising means for producing from the electrical signals a composite signal, the magnitude of which is representative of the level of the interface relative to the emitter and receiver pairs, the composite signal being operable to give a visual indication of the level of the interface and/or to control the level of the interface by varying the relative volumes of fluids in the fluid system.

11. Apparatus according to any foregoing claim wherein the elongate member is a ladder-like member having a pair of upright members which are joined togethwer by a plurality of rung-like cross-members each of which is associated with a respective emitter and receiver pair, each cross-member having a transverse cavity therein for receiving the respective portion of the fluid system when the elongate member is immersed in the fluid system and the emitter and receiver being mounted in the side or sides of the cavity.

12. Apparatus for monitoring the level of an interface between two phases in a multiphase fluid system in which the phases are separated by gravity substantially as hereinbefore described with reference to the accompanying drawings.

13. A method for monitoring the level of an interface between two phases in a multi phase fluid system in which the phases are separated by gravity, the- method comprising the steps of: (a) transmitting a plurality of signals of electromagnetic radiation through the fluid system at a succession of respective heights in the fluid system; (b) receiving the transmitted signals at respective corresponding heights in the fluid system; and (c) detecting whether each respective received signal is above or below a particular threshold value so as to produce a plurality of electrical signals each of which is representative of whether the interface is above or below the respective height.

14. A method according to claim 13 wherein each of the phases has a respective opacity to the electromatnetic radiation and at each respective height when the fluid system at that height is comprised substantially of one of the phases, transmission of the signal of electromagnetic radiation can occur through the fluid system at that height and when the fluid system at that height is comprised substantially of the other of the two phases, transmission of the signal of electromagnetic radiation through the fluid system at that height cannot-occur.

15. A method according to claim 13 or claim 14 wherein each electrical signal is a binary signal which is jndicative of which of the two phases is present in the fluid system at the respective height.

16. A method according to claim 13, 14 or 15 wherein each electrical signal is arranged to switch on or off a respective light in an array of lights thereby to give a visual indication of the level of the interface relative to the respective heights in the fluid system.

17. A method according to any one of claims 13 to 16 wherein the electrical signals are employed to control the level of the interface by varying the relative volumes of fluids in the fluid system.

18. A method according to any one of claims 13 to 17 wherein a composite signal is produced from the electrical signals, the magnitude of the composite signal being representative of the level of the interface relative to the respective heights in the fluid system, the composite signal- being operable to give a visual indication of the level of the interface and/or to control the level of the interface by varying the relative volumes of fluids in the fluid system.

19. A method according to claim 18 wherein the magnitude of the composite signal is determined at successive periods in time, each magnitude is stored in a memory, the next successive magnitude is compared with the preceding magnitude, and the difference between the two magnitudes is employed to control the level of the interface by varying the relative volumes of fluids in the fluid system.

20. A method according to claim 18 or claim 19 wherein if a respective one of the plurality of signals is not received at the respective corresponding height due to a failure, the magnitude of the composite signal represents that the height of the interface is at least that of the respective corresponding height.

21. A method according to any one of claims 13 to 20 wherein the signals of electromagnetic radiation are transmitted- in- succession at the respective heights in the fluid system.

22. A method according to any one of claims 13 to 21 wherein each signal of electromagnetic radiation is transmitted from an end of a respective transmitter optical fibre and received by an end of a respective receiver optical fibre.

23. A method according to any one of claims 13 to 22 wherein the electromagnetic radiation is infra-red radiation.

24. A method for monitoring the level of an interface between two phases in a multiphase fluid system in which the phases are separated by gravity substantially as hereinbefore described with reference to the accompanying drawings.