WO2001019170A1

WO2001019170A1 - An arrangement for automatically milking animals

Info

Publication number: WO2001019170A1
Application number: PCT/EP2000/009028
Authority: WO
Inventors: Gunnar EDERSTÅL; Jan Eriksson; Krister SJÖBLOM
Original assignee: DeLaval Holding AB
Current assignee: DeLaval Holding AB
Priority date: 1999-09-15
Filing date: 2000-09-15
Publication date: 2001-03-22
Anticipated expiration: 2002-03-15
Also published as: SE9903284D0; SE515217C2; AU7419700A; SE9903284L

Abstract

An apparatus for milking animals is proposed which enables the presence of discontinuous or transitory elements in the milk, such as flocculent masses, or streaks of blood or slime indicating mastitis or other udder disorders to be detected. This is achieved with a monitoring arrangement (3) including two optical sensors disposed in spaced relation along a conduit and each including an optical emitter (33) and detector (34) facing one another across the conduit (2). The arrangement further includes a monitoring unit (32) coupled to the detectors of the sensors with means (35) for determining the difference between the detector signals. By determining the difference signal, the monitoring arrangement (3) is effectively self-calibrating. Any variation in the state of the conduit, for example due to the gradual build-up of deposits or roughening or discolouring of the surface, will be cancelled out by the difference function.

Description

An arrangement for automatically milking animals

Field of invention The invention concerns an arrangement and method for automatically monitoring the milk obtained from an animal, for example, in order to determine the animal's health or the quality of milk.

Background art The composition of milk obtained from an animal such as a cow can provide valuable information about the health of the cow. This is the case, for example, when the cow is suffering from a disorder of the udder, such as mastitis. The condition of mastitis is indicated when blood, pus or flocculent masses resulting from the coagulation of suspended particles are present in the milk. Information of this kind is important for the subsequent treatment of the cow, but is also useful for gauging the quality of the milk and determining whether it is fit for human consumption. It is advantageous to monitor the composition of milk in an automatic milking arrangement since this permits the cow's health to be monitored automatically and in a regular and routine manner. The milk can also be separated immediately on milking which saves both time and space.

EP 0 516 546 describes an arrangement for measuring the conductivity of a cow during milking. A conductivity sensor is incorporated in the milking robot and measures the conductivity of milk taken from each teat. While variations in the conductivity of milk indicate the presence of mastitis, regulations in force in many countries require that a visual check of the milk be carried out to ascertain whether flocculent masses are present. Thus in those countries where such regulation are imposed, the conductivity measurement would have to be supplemented by a visual check. However, it is believed that the use of an optical sensor would satisfy the present regulations and enable this check to be performed automatically. In this context, the term 'optical' is intended to refer to sensors that utilise electromagnetic radiation to 'illuminate' the milk. The use of an optical sensor to measure the transparency of milk is described in US 5,116,119. The described arrangement consists of a double array of optical emitter and detector pairs which is used to determine the velocity of milk flow. A further optical sensor consisting of an infrared emitter and detector determines the relative transparency of the milk. This latter reading is used to calibrate the double array of sensors. However, by measuring the absolute attenuation of light transmitted through the milk it also provides an indication of the composition of the milk, for example of the relative proportions of fat and protein. A problem with this arrangement is that the transparency measurement is sensitive to the state of the tube in which the milk is transported, as well as the amount of liquid flowing through the tube. Hygiene regulations require that the tube be regularly cleaned by flushing through cleaning fluid, however, this may be inadequate to prevent deposits from collecting on the tube walls and gradually masking the emitter or detector. Furthermore, the cleaning fluids commonly used will corrode the tube with time, particularly when the tube is plastic. The result may be the discolouring of the tube or even abrasion, which causes the deflection or reflection of light. With time, therefore, the reading would become unreliable unless regular calibration is performed.

It is therefore an object of the present invention to provide an arrangement capable of automatically monitoring the composition of milk taken from an animal which is highly effective and reliable over time and does not require regular calibration. SUMMARY OF INVENTION

This object is achieved in an apparatus for milking animals including an arrangement for milking and/or cleaning at least one teat of an animal, a conduit for transporting liquid away from the milking arrangement and a monitoring arrangement coupled to said conduit for monitoring the composition of the liquid. The monitoring arrangement includes two optical sensors each including an emitter and a detector of electromagnetic radiation positioned to oppose one other across the conduit. The sensors are furthermore disposed in spaced relation along the conduit such that milk travelling in the conduit is irradiated first by one sensor and then by the other sensor. The arrangement further includes a processing unit coupled to the detectors of the sensors for receiving from each detector a signal indicative of the level of radiation received, wherein the unit has means for generating a monitoring signal indicative of the difference between the detector signals.

By determining the difference between the signals obtained from the optical sensors, the monitoring unit is effectively self-calibrating. Any variation in the state of the conduit, for example due to the gradual build-up of deposits or roughening or discolouring of the surface, will affect both sensors to essentially equivalent degrees, and will accordingly be cancelled out by the difference function. Even when one sensor is affected by some build-up in the tube more than the other sensor, this can be readily identified in the difference signal and therefore ignored. Furthermore, variations in the density of milk produced by different animals or the same animal at different milking sessions will also have no effect on the evaluation. Only the presence of discontinuous or transitory elements in the milk, such as flocculent masses, changes in colour or streaks of blood or slime, for example, will be registered.

Mastitis may affect an animal's whole udder or conversely be present in only one or some of the teats. In this case milk extracted from a healthy teat is generally fit for human consumption. In accordance with a preferred embodiment of the invention therefore, the milking arrangement includes a single teat cup. In this way, the monitoring arrangement is coupled to a conduit that is connected to a single teat cup only. The milk produced by each udder quarter can then be checked separately and if necessary discarded. The single teat cup may be utilised for all the teats, in which case it will be attached to each teat of the animal in turn. In the case where a cluster of teat cups is used, the monitoring arrangement is preferably coupled to a conduit that is connected to one or possibly two teat cups.

However, the monitoring unit may also be useful when connected to a single milk conduit carrying all the liquid from a cluster of teat cups.

The milking arrangement is preferably an automatic pre-milking arrangement. Flocculent particles that result from the coagulation of the milk, or blood or pus, which indicate the inflammation of a teat, collect in the teat prior to milking. As milking proceeds, these components are swept away. If the presence of these transitory manifestations is to be used as an indication of the health of a cow, or of a particular udder quarter, it is preferably to search for such anomalies in the first milk obtained. This milk is commonly referred to as the pre-milk.

It is particularly advantageous when the milking arrangement is also adapted for cleaning the teats of an animal. Such an arrangement will generally operate in two distinct modes, a first cleaning mode and a second pre-milking mode. However, the process of cleaning a teat may sometimes stimulate the production of milk. Pre-milk will then be present in the cleaning fluid. This is useful to know since time can then be saved by shortening or even skipping the pre-milking sequence. Thus, the apparatus preferably also includes a milk sensor coupled to the conduit and including an emitter and detector of electromagnetic radiation disposed to oppose each other on opposite sides of the conduit. It is useful to determine when the monitoring unit should be operational. Accordingly, in a preferred embodiment of the invention, the milk sensor is coupled to the processing module of the monitoring arrangement and in this way can relay a signal indicative of the presence or absence of milk to the monitoring arrangement.

For pre-milk to be detected in cleaning fluid, the milk sensor must be capable of differentiating between an empty conduit, a conduit full of essentially clear cleaning liquid and a conduit containing both cleaning liquid and pre-milk.

The radiation transmitted through an empty conduit is taken as a control value. A conduit containing just cleaning liquid generally magnifies the radiation compared to an empty conduit and the presence of pre-milk in the cleaning fluid will attenuate the transmitted radiation. In order to determine reliably the contents of a conduit, the control reading for an empty conduit must be obtained. Conventionally, such a sensor was disposed vertically, that is with the emitter and detector positioned above and below a horizontal conduit portion. However, after cleaning the teats and/or pre-milking, some fluid may remain in the base of the conduit, filtering the beam between emitter and detector. This gives a false reading of the empty state and disrupts the auto- calibration function.

In order to prevent such an erroneous reading, the emitter and detector of the milk sensor preferably define an axis of radiation that intersects both the emitter and detector normally and lies substantially in a plane cross-sectioning the conduit and are arranged such that the radiation axis is inclined at an angle to the vertical in said plane.

By suitably selecting the angle of inclination, allowance can be made for a desired tolerated level of remaining liquid without compromising the detection accuracy.

Preferably, the monitoring arrangement sensors and the milk sensor are not coupled to the main part of the conduit, but instead to a parallel branch portion connected at both ends to the conduit and extending over at least part of the length of the conduit. This conduit branch preferably has a smaller diameter than the main part of the conduit, which ensures that the milk sensor operates on a conduit branch full of liquid. The milk sensor may be coupled to the same conduit branch as the monitoring unit or to a separate one.

The portions of the conduit or conduits to which the monitoring unit and the milk sensor are coupled may be of any material that is substantially transparent to the radiation used. Preferably, this material is glass or plastic.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the present invention will become apparent from the following description of the preferred embodiments that are given by way of example with reference to the accompanying drawings. In the drawings:

Fig. 1 schematically depicts an apparatus for cleaning and pre-milking a cow with a differential sensor and milk sensor in accordance with the present invention,

Fig. 2 schematically depicts a detail of Fig. 1 showing the differential sensor,

Fig. 3 schematically shows a detail of Fig. 1 showing the milk sensor,

Fig. 4 schematically depicts a cross-sectional view of the milk sensor of Fig. 3 through the line A- A, and

Fig. 5 schematically depicts a further embodiment of the apparatus.

DETAILED DESCRIPTION OF THE DRAWINGS

A cleaning and pre-milking apparatus shown in Fig. 1 includes a cup 1 of a shape and size designed to fit snugly around a teat of a cow. A conduit 2 is connected to the base of the cup and carries waste cleaning fluid and water and also any pre-milk away from the cupl. The expression 'pre-milk' is used to denote the first milk that is released when an animal, in this case a cow, is milked. It is released at a slow rate and is distinguished from the full milk flow, which generally follows at an increased flow rate. The conduit 2 is connected to a waste water tank 6 and through this to a vacuum source indicated in Fig. 1 by the words 'vacuum pump'. A second conduit 5 transports cleaning fluid to the cleaning cup 1. While this second conduit 5 is shown separate from the first conduit 2, those skilled in the art will recognise that it may be incorporated in the first conduit 2. A third conduit (not shown) for carrying an alternating, or pulsating, vacuum to the teat cup will also be provided. This conduit also could be bundled together with the first conduit 2.

A differential sensor 31 is disposed on the conduit 2 and, together with a monitoring unit 32, forms a monitoring assembly 3 for detecting discontinuities in the milk composition and thus assessing whether the respective udder quarter and teat is infected or inflamed, such as with mastitis or other disorder. A typical indication of mastitis is the presence of flocculent particles caused by the coagulation in the milk. These flocculent particles are thus of higher density than the milk and will attenuate light to a higher degree than milk alone. Other discontinuities may be streaks of blood or pus in the milk. These change the colour of the milk, which can likewise be detected by radiation of a suitable wavelength. The monitoring assembly 3 is illustrated in more detail in Fig. 2. As shown in this figure, the differential sensor 31 comprises a housing that encloses a portion 22 of the conduit 2 carrying waste cleaning liquid and pre-milk. The portion of the conduit 22 enclosed in the housing is made of glass, plastic or any other substantially transparent material that will permit radiation from the differential optical sensor 31 to pass through it. The material for the conduit portion 22 does not have to be particularly resistant to the cleaning fluids used. The conduit portion 22 may be connected in a sealed fashion to the conduit 2 proper, or alternatively the conduit 2 may be of the same material as the conduit portion 22, requiring no connection. Within the housing there are mounted two optical sensors positioned a little distance apart. Each sensor comprises an emitter 33 for emitting electromagnetic radiation and a detector 34 for detecting electromagnetic radiation. The emitters 33 and detectors 34 of each sensor are positioned to oppose one another, respectively, on either side of the conduit 2 such that liquid travelling in the conduit 2 is illuminated by the radiation emitted by the emitter 33 of first one sensor and then the other. The detectors 34 receive the radiation passed through a first wall of the conduit portion 22, the liquid in the conduit, and a second wall of the conduit portion 22.

The emitters 33 and detectors 34 are selected according to the wavelength of radiation required. In the present embodiment, visible red light is used since this is adequate for detecting flocculent particles, blood and pus. However, infra radiation may also be utilised to good effect for detecting these anomalies. The emitters 33 preferably include light emitting diodes (LEDs), and the detectors 34 photodetectors, such as photocells or photodiodes. The voltage produced by the detectors 34 upon receipt of the illumination through the conduit is relayed to the monitoring unit 32 through two separate leads. In the monitoring unit 32, the signals from each detector 34 are supplied to a separate input of a difference generator 35. The difference generator 35 generates the difference signal from the two input signals. The difference generator 35 may be appropriately implemented with a differential amplifier. The difference signal is then passed on to a processing module connected to the difference generator 35. The processing module 36 preferably includes an

A/D converter, a microprocessor with associated memory for processing the signal received from the difference generator 35, and possibly also for evaluating the signal to determine whether flocculent particles, blood, pus or similar transitory components are present in the pre-milk. Storage means may also be provided for storing the results. Alternatively, the calculation of the difference signal may be implemented by the processing module 36 after the A/D conversion, for example by performing a subtraction of the two resultant digital signals. The monitoring unit 32 may also include a screen for displaying the difference signal to enable an operator to deduce directly from this signal the condition of the milk or cow. Other graphical or alphanumerical information regarding the condition of the milk or cow could additionally or alternatively by provided on the screen. The monitoring unit 32 is preferably connected to the main control (not shown) of the milking and cleaning apparatus, for example to enable information regarding the condition of the cow and udder quarter to be collated with the determined condition of the milk. Alternatively, the monitoring unit 32 may form part of the main control of the milking and cleaning apparatus.

By utilising a difference value obtained from the two sensors, variations in the transparency of the conduit 21 with time, such as due to deposits collecting inside the conduit, or abrasion or discolouring caused by the cleaning liquid, will be present in substantially the same amounts at both sensor locations and therefore not be registered by the monitoring unit. Only variations in the liquid in the conduit that are present at either one or other of the sensors will cause a variation in the monitoring signal. Such a variation could be caused, for example, by a flocculent particle passing between the emitter 33 and detector 34 of one sensor and then the other. While it should be ensured that the spacing between the two emitter detector pairs and the sampling rate of the A/D converter are selected to ensure that the various transitory components will enable a difference reading to be registered, the sampling rates available with present day microprocessors are so high that the spacing is not critical.

Returning now to Fig. 1, a further optical sensor 4 is also coupled to the first conduit 2. This sensor 4 is for detecting the presence of pre-milk in cleaning liquid contained in the conduit and is shown in more detail in Fig. 3. As for the differential optical sensor 31, the milk sensor 4 is coupled to a portion 23 of the conduit that is made of glass, plastic or another suitable, substantially transparent material. The sensor 4 includes a housing enclosing part of this conduit portion 23 and an emitter 41 and detector 42 of electromagnetic radiation arranged on opposite sides of the conduit 23 and disposed to face each other. As for the first optical sensor assembly, the emitter 41 may include, but is not limited to, an LED while the detector 42 may include a photocell, photodiode or other suitable optical detector. The detector 42 is connected to processing means, which in the present embodiment are provided by the processing module 36 of the monitoring unit 32 (see Fig. 1). This processing module 36 evaluates whether pre-milk is present in the cleaning liquid.

As mentioned above, the emitter 41 and detector 42 of the milk sensor oppose each other. This is illustrated in Fig. 1 by an axis of radiation denoted by R which extends substantially normally between the emitter 41 and detector 42 and represents the centre of a beam of radiation emitted by the emitter 41 and incident on the detector 42. The radiation axis R preferably lies in a cross- sectional plane through the conduit portion 22. However, this radiation axis R does not lie in a vertical plane. This is better illustrated in Fig. 4, which shows a cross-sectional view through the emitter 41 and detector 42 of the milk sensor and the conduit portion 23. In this figure it is apparent that the radiation axis R is tilted at an angle α to the vertical.

In order to be able to detect pre-milk in the cleaning fluid, the milk sensor 4 must be capable of differentiating between the empty state of the conduit portion 23, i.e. when it contains only air, when it is partially or completely full of cleaning liquid, and when it contains an amount of pre-milk in the cleaning liquid. The reading for an empty conduit is typically used as a control value. The presence of clear cleaning liquid in the conduit generally magnifies the radiation transmitted, and the presence of pre-milk in the cleaning fluid will attenuate the transmitted radiation. By measuring the degree of transmission of light and comparing this measured value with that for an empty conduit and for a conduit full of cleaning fluid only, it is possible to determine whether pre-milk is contained in the conduit. However, to auto-calibrate the sensor, it is important that the reading for an empty conduit is correct. After use of a cleaning arrangement, some liquid may remain at the bottom of the conduit, as denoted by reference numeral 50 in Fig. 4. There is a risk that this liquid will obscure the field of vision between the emitter 41 and detector 42 and thus falsify the subsequent reading for an empty conduit 23. However, by disposing the emitter 41 and detector 42 of the milk sensor 4 such that the radiation axis R is at an angle to the vertical, it is ensured that the field of view between the emitter 41 and detector 42 is not obscured by liquid remaining in the conduit

23. A reliable control reading can thus be obtained. The angle a preferably lies in the range of between 10° and 60°. However, it will be understood that the angle may be selected to compensate for the amount of remaining waste liquid that can be tolerated and can be as much as 90° to the vertical. Fig. 5 shows an alternative embodiment of the invention wherein the differential sensor 31 and milk sensor 4 are not coupled to the conduit 2 directly, but are instead coupled to a branch 221 of the conduit. The branch conduit 221 is joined at both ends to the conduit 2 and runs substantially parallel to it. It is preferably of a length adequate to support one or both of the sensors 31, 4. Some of the liquid travelling in the conduit 2 is diverted into the branch conduit 221 and thus passes into the fields of operation of the two sensors 31, 4. The branch conduit 221 is preferably of smaller diameter than the main portion of the conduit 2. This ensures that the branch conduit 221 will be substantially full of liquid even when the main portion of the conduit 2 contains some air. In this respect, it should be noted that it is preferable for both the differential sensor 31 and the milk sensor 4 that the conduit be full of liquid, i.e. that air be excluded from the conduit. In the case of the milk sensor, this ensures that a reliable, repeatable value for transmission through a conduit containing only cleaning fluid can be obtained for calibration. For the differential sensor 31 this prevents air bubbles in the liquid from distorting the evaluation of the milk composition. However, it should be understood that it is not essential for the differential sensor 31 to operate on a full conduit. The analysis of the difference signals in the monitoring unit 32 may be adapted to recognise variations caused by air bubbles, and ignore such variations in the evaluation of the health of a cow or the quality of the milk. It will be understood by those skilled in the art that the two sensors 31, 4 need not be coupled to the same branch conduit 221, but could be disposed in separate branch conduits, possibly of different diameter and preferably arranged in series along the conduit 2.

In use, the cleaning cup 1 is placed over the teat of a cow and held there by the application of a vacuum through the conduit 2. The teat is then cleaned by cleaning liquid, which may include water, supplied through the second conduit 5. The cleaning liquid may be applied under pressure to increase the cleaning force. Excess cleaning liquid is drained away through the conduit 2 into the waste water tank 6. After the cleaning process is complete, the teat is stimulated to produce milk by applying a pulsating vacuum to the cup 1 through the conduit 2. The pre-milk secreted at the start of milking is passed through the conduit 2 and also discarded in the waste water tank 6. After a predetermined time, the stimulation is stopped, the vacuum released and the teat cup moved on to the next teat. The process is repeated for the remaining three teats until all four teats have been cleaned and the pre-milk extracted from the respective udder quarters. It sometimes occurs that the cleaning process stimulates the production of milk in the respective udder quarter. In this case, pre-milk will be released into the cleaning cup before cleaning is terminated. This pre-milk will be mixed with the waste cleaning liquid and transported through the conduit 2. Here it will be detected by the milk sensor 4. This enables the pre-milking process to be shortened so that the cup 1 can be moved on to the next teat more rapidly. During the pre-milking phase, the differential sensor 31 takes a reading from the pre-milk and sends its signals to the processing module 36 via the difference generator 35. The former determines whether flocculent particles, blood, pus or other transient elements indicative of a diseased teat or udder quarter are present in the milk. The processing module 36 may also retain a record of the state of the milk from each udder quarter for each cow in its storage memory (not shown) for reference purposes.

It will be understood that the differential sensor 31 is not limited to a cleaning apparatus but may also be implemented in a milking apparatus. Many milking apparatus are utilised to stimulate and extract pre-milk prior to the main milking process. In this case the cleaning of the teats will have been performed previously, either manually or automatically. The differential sensor 31 would then be coupled to a milk tube or conduit connected to a teat cup and be operational at least during the pre-milking phase. The arrangement would be similar to that shown in Fig. 1 with the teat cup being replaced by a milking teat cup and the waste water tank being replaced by a milk collection tank.

Since milking apparatus commonly utilise a cluster of teat cups for milking all four udder quarters simultaneously, a differential sensor 31 would preferably be coupled to each of the milk conduits connected to each teat cup, so that the milk produced by each udder quarter could be assessed separately. However, for reasons of practicality or cost the sensor 31 could be connected to a single conduit carrying the milk away from teat cup pairs or even to a conduit adapted to carry the milk from all four teat cups. In this way at least an indication of the health of the udder halves or whole udder, respectively, may be obtained.

Claims

Claims:

1. An apparatus for milking animals including an arrangement (1) for milking and/or cleaning at least one teat of an animal, a conduit (2) for transporting liquid away from the milking arrangement and a monitoring arrangement (3) coupled to said conduit for monitoring the composition of the liquid, characterised in that the monitoring arrangement (3) includes two optical sensors each including an emitter (33) and a detector (34) of electromagnetic radiation positioned to oppose one other on opposite sides of the conduit (2, 22, 221), the sensors being disposed in spaced relation along the conduit such that milk travelling in said conduit is irradiated by first one sensor and then the other sensor, and a processing unit (32) coupled to the detectors (34) of the sensors for receiving from each detector a signal indicative of the level of radiation received, wherein said unit (32) further includes means (35) for generating a monitoring signal indicative of the difference between the detector signals.

2. An arrangement as claimed in claim 1, characterised in that said milking arrangement includes a single teat cup (1).

3. An arrangement as claimed in claim 1 or 2, characterised in that said milking arrangement (1) includes one or two of a cluster of teat cups adapted to be connected the teats of the animal.

4. An apparatus as claimed in any previous claim, characterised in that the portion of the conduit (22) coupled to said monitoring unit is of glass or plastic.

5. An apparatus as claimed in any previous claim, characterised in that said milking arrangement (1) is an automatic pre-milking arrangement.

6. An arrangement as claimed in any previous claim, characterised in that said pre-milking arrangement (1) is adapted to automatically clean the teats of the animal.

7. An apparatus as claimed in claim 6, characterised by including a milk sensor (4) to detect the presence of milk in the conduit (2, 23, 221) during cleaning.

8. An apparatus as claimed in claim 7, characterised in that the milk sensor (4) is coupled to the processing module (32) of the monitoring arrangement (3) to relay a signal indicative of the presence or absence of milk to said monitoring arrangement.

9. An apparatus as claimed in claim 7 or 8, characterised in that said milk sensor (4) is coupled to said conduit (2, 23, 221) and includes an emitter (41) and detector (42) of electromagnetic radiation disposed to oppose each other on opposite sides of said conduit and defines an axis of radiation (R) that intersects both the emitter and detector normally and lies substantially in a plane cross-sectioning the conduit, wherein the emitter and detector are arranged such that the radiation axis is inclined at an angle (α) to the vertical in said plane.

10. An apparatus as claimed in claim 9, characterised in that said angle ( ) is between 10° and 60°.

11. An apparatus as claimed in any one of claims 7 to 10, characterised in that the portion of the conduit (23) coupled to said milk sensor (4) is of glass or plastic.

12. An apparatus as claimed in any one of claims 7 to 11, characterised in that said conduit (2) includes a parallel branch (221) that is connected at both ends to the conduit (2), extends over at least part of the length of the conduit and has a smaller diameter than said conduit (2), and that said milk sensor (4) is coupled to the conduit branch.

13. An arrangement as claimed in claim 12, characterised in that said monitoring arrangement (3) is also coupled to said conduit branch (221) with the emitters (33) and a detectors (34) of said sensors respectively disposed to oppose each other across said conduit branch.

14. An apparatus as claimed in any one of claims 1 to 8, characterised in that said conduit (2) includes a parallel branch (221) that is connected at both ends to the conduit (2), extends over at least part of the length of the conduit and has a smaller diameter than said conduit (2), and that said monitoring arrangement (3) is coupled to said conduit branch with the emitters (33) and a detectors (34) of said sensors respectively disposed to oppose each other across said conduit branch.

15. An apparatus as claimed in any previous claim, characterised in that the sensors (33, 34) of the monitoring arrangement (3) operate with red light in the visible spectrum.