WO1997017594A1

WO1997017594A1 - Liquid level determination method and apparatus

Info

Publication number: WO1997017594A1
Application number: PCT/GB1996/002631
Authority: WO
Inventors: John D. R. Jolley; Alan R. Edwards; Christopher F. Wiltcher; David R. Birkbeck; Brian C. Goodwin; David B. Guess; Martin Hutton; John A. Southall
Original assignee: Boehringer Ingelheim Limited
Priority date: 1995-11-07
Filing date: 1996-10-29
Publication date: 1997-05-15
Also published as: GB2307043A; AU7319496A; GB9522752D0

Abstract

The method and apparatus involves directing a beam of radiation from a first emitter (9a) at a vessel (7) containing a liquid, and detecting by means of a first detector (9b) whether the beam is deflected by the upper surface of the liquid. This gives information concerning the level of the liquid. A second emitter (10a) and a second detector (10b), at a higher location than the first emitter (9a) and detector (9b), may be used to give additional information.

Description

Liquid level determination method and apparatus

The invention relates to a method and apparatus for determining the level of a liquid in a vessel .

It is known to fill and seal plastics vessels using a blow fill seal machine. Various liquids for different end uses can be supplied in such vessels, and one particularly useful application of blow fill seal machines is in the production of vials each containing a unit dose of a pharmaceutical or other sterile liquid product. The vials must normally carry a precise quantity of the product so that once the seal is broken the entire contents can be used to provide the correct dose. The vials are typically provided in a strip of e.g. 10 vials, and may for example contain 2, 4, 5 or 10 millilitres of the product. One type of product supplied in this way is a nebulising solution for asthmatics.

It will be appreciated that with certain products it is important that each vial contains the correct quantity of product. Providing the dosing component of the blow fill seal machine is working correctly, correct quantities are achieved. However, this will not be the case if a vial leaks and to deal with this problem a leak detector is commonly employed to check for leaks. This operates by applying a potential difference to the vials and if there is a reduced electrical resistance this indicates that there may be leakage. However, the leak detector only detects leaks and will not indicate any problem if the dosing component has not worked properly and has supplied an incorrect dose or even no dose at all. It is therefore the usual practice to check the level of liquid product in the vials with the human eye. Such a checking procedure is clearly unreliable and inefficient. Viewed from one aspect the invention provides a method of determining the level of a liquid in a vessel, comprising the steps of directing a beam of radiation at the vessel, and detecting whether the beam interacts with the upper surface of the liquid.

Viewed from another aspect the invention provides apparatus for determining the level of a liquid in a vessel, comprising an emitter for directing a beam of radiation at the vessel, and a detector for detecting whether the beam interacts with the upper surface of the liquid.

With such an arrangement the liquid level can be determined automatically and reliably m a non-mtrusive manner. For example, the beam may pass below the upper surface if the liquid level is high, it may pass above the upper surface if the liquid level is low, and it may interact with the upper surface if the liquid level is at an intermediate height. By detecting whether the beam interacts with the upper surface, information concerning the liquid level is obtained. If the level is incorrect an alarm may be operated, but preferably an ejector is operated to eject the vessel. If desired it may then be checked by an operative.

Preferably, the beam is directed at one side of the vessel and the detecting step is carried out at an opposite side of the vessel. The beam need then be deflected by the upper surface through only a small angle, as compared to a large angle deflection which has to be detected if the beam directing and detecting steps are both effected on the same side of the vessel.

The detector may be arranged to detect the beam if it follows a path unaffected by the upper surface of the liquid and not to detect the beam if it is deflected by the upper surface. Preferably, however, the beam is detected if it is deflected by the upper surface.

The beam may be deflected by reflection and/or refraction. In the case of reflection, the beam can be directed from above on to the upper surface which then acts as a mirror or it can be directed from below on to the upper surface which then acts to internally reflect the beam. In the case of refraction, the beam can similarly be directed from above or below so as to be refracted by its passage through the upper surface of the liquid. In either case, it is possible to tell when the liquid is at a predetermined level by detecting whether or not deflection of the beam occurs. The "target" for the beam may thus be substantially the entire upper surface of the liquid. In many cases this will give sufficient accuracy. For example, if the angle of the beam is near to the horizontal the target formed by the entire upper surface may be small, so that the beam will miss the target if the liquid level is below or above a small range of acceptable levels. In addition, the target formed by the entire upper surface may be small if the vessel has a small thickness.

It is preferred for the beam to be arranged to impinge on a region of the upper surface adjacent the vessel wall if the liquid is at a predetermined level (the "edge region") . The curvature of the upper surface may vary sharply in this region from that of the rest of the upper surface, due to the resultant effect of cohesive forces binding liquid molecules together and adhesive forces exerted on the liquid molecules by the neighbouring molecules of the solid forming the vessel wall . In the case of water in a vessel of glass or certain plastics, the adhesive forces are strong so that the angle of contact (i.e. the angle between the solid surface and the tangent plane to the liquid, measured through the liquid) is acute (or even zero with clean glass) and the upper surface rises steeply towards the vessel wall. In the opposite case, where the cohesive forces dominate over the adhesive forces, the angle of contact is obtuse and the upper surface drops steeply towards the vessel wall. In both cases the large and localised deviation of the upper surface from the horizontal in the edge region can be used to provide a reliable target for the beam.

If the angle of contact is obtuse a beam directed generally horizontally will be refracted downwardly by the upper surface edge region. Similarly, if the angle of contact is acute, a beam directed generally horizontally on to at least the lower part of the underside of the upper surface edge region will be reflected downwardly. It is particularly preferred to direct the beam upwardly at the upper surface edge region from below, since the detector can be conveniently arranged to point upwardly in a manner to avoid receiving radiation directly from the emitter. In a preferred embodiment the beam is directed upwardly through the wall of a vessel containing a liquid having an acute angle of contact with the wall, and if the beam impinges on the edge region it is reflected so as to be deflected downwardly where it can be detected by an upwardly directed detector.

Preferably the beam is directed upwardly at an angle of 9° to 15° to the horizontal, more preferably 11° to 13°, most preferably approximately 12°; and preferably the detector is upwardly directed at an angle of 18° to 30° to the horizontal, more preferably 22° to 26°, and most preferably approximately 24°. The radiation emitter may comprise a fibre optic from which radiation emerges in a cone and similarly the detector may comprise a fibre optic which can receive the radiation within a cone. The angles mentioned above then refer to the centre axes of the respective fibre optics at their tips, ie. the central axes of the cones.

By detecting whether or not the beam interacts with the upper surface of the liquid, it can be established that the liquid level is neither too high nor too low, i.e. that it is at an intermediate height withm an acceptable range. Accuracy can however be improved by directing a second beam of radiation at the vessel, and detecting whether the second beam interacts with the upper surface of the liquid. The second beam may be at a different orientation and/or height from the first beam, e.g. parallel to the first beam and at a higher location. The second beam can be used to determine additional information concerning the liquid level.

In effect, the first beam can be used to establish certain information concerning the liquid level, and the second beam can be used independently to establish additional information concerning the liquid level, giving improved reliability and accuracy. For example, the second beam can be used to establish that the liquid level is within a second range of heights, different from the range established by the first beam. The ranges may for example be arranged to overlap, with an acceptable liquid level being within the region of overlap, or within the non-overlapping region of one of the ranges. Alternatively, the top of a lower range may be below the bottom of an upper range, with an acceptable liquid level being m the region between the two ranges.

In a preferred method the first beam is used to determine if the liquid level is above a first height, and the second beam is used to determine if the liquid level is above a second, higher height. An acceptable liquid level may then be defined as any level between the first and second heights. Thus the second beam may interact with the liquid upper surface in the event that the upper surface is above a predetermined level defining a maximum acceptable level.

Whether or not the second beam interacts with the upper surface can be determined by a second detector. As with the first beam, the second detector may be arranged to detect the second beam if it follows a path unaffected by the upper surface and not to detect the second beam if it is deflected by the upper surface. Preferably, however, the second beam is detected if it is deflected by the upper surface.

The use of a second beam is particularly advantageous in the preferred method where the target for the first beam is the edge region of the liquid upper surface. For example, if an upwardly directed beam misses the liquid upper surface edge region by passing above it, this may indicate that the level is too low, e.g. by the detector not receiving a signal. If however the beam misses the edge region by passing below it, the beam may in some circumstances be internally reflected by the mam, central region of the upper surface, indicating that the level is above a certain level, e.g. by the detector receiving a signal. Thus the method reliably indicates when the level is too low and thus detects underfills. It is also useful m detecting an empty vessel, unlike the known leak detection system. However, the method may be less reliable if the level is too high. This is acceptable in certain applications, but if it is not then accuracy can be improved by use of a second beam of radiation. The second beam may then be arranged to interact with the upper surface in the event that the upper surface is above an acceptable level . It is convenient if the liquid upper surface edge region is used as the target for the second beam. If the second beam is deflected by the edge region, this can indicate that the level is too high. A correct level, which in practice is a level within a certain tolerance range, is thus determined only when the first beam is found to have impinged on the edge region and the second beam is found to have missed the edge region. If a second beam of radiation is used, there may be a pause between the first and second beams in order to avoid interference between the two. This can enable a single detector to be used for the two beams, although in the presently preferred method first and second detectors are used.

The vessel may be stationary when the level determination is made, but this is not necessary. It is advantageous to use the method on a production lme and preferably therefore the liquid level is determined during conveyance of the vessel past the beam(s) of radiation. In practical terms, if a second beam of radiation is used, this can be provided downstream of the first beam (or vice versa) , so that the radiation from one test does not interfere with the other test. The method and apparatus of the present invention is suitable for a wide variety of liquids and vessels. The vessels may for example be made of glass or of plastics materials such as low density polyethylene (LDPE) , polyethylene (PE) , high density polyethylene

(HDPE) or polypropylene (PP) . The method is especially useful for determining the level of liquid in a blow fill vessel, and indeed can be used with a strip of such vessels . A preferred embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which:-

Figure 1 shows a side elevation of a machine for conveying strips of vials and determining the level of a liquid in the vials,^•

Figure 2 shows a cross section on the lines II-II of Figure 1 ;

Figure 3 shows a cross section, to an enlarged scale, on the lines III-III of Figure 1, and Figure 4 is a diagram showing the effect of the level of a liquid upper surface on first and second light beams.

The machine 1 shown in Figures 1-3 comprises a first endless belt 2 for receiving strips 3 of plastics vials 7 containing a liquid, a second endless belt 4 arranged to move the strips 3 from a position on their sides into an upright position as they travel along the first belt 2, and a third endless belt 5 for supporting the strips as they are conveyed in the upright position past apparatus 6 for determining the level of the liquid in the vials 7. In this embodiment, the vials are made of low density polyethylene (LDPE) and are 12mm in width. The liquid contained therein is a nebulising solution for the treatment of asthma.

The level determining apparatus 6 comprises an adjustment block 18 including a rotatable cam 8 for coarse height adjustment of the apparatus to cater for vials of different size and filling levels. A first light emitter 9a and detector 9b pair 9 is supported by the block 18 towards the rear thereof and a second light emitter 10a and detector 10b pair 10 is supported towards the front of the block 18. These pairs are basically identical in construction but are provided at slightly different heights.

Figure 3 shows the light emitter and detector pair 10. The light emitter 10a includes a light emitting diode connected to a 0.5mm diameter fibre optic the tip of which is directed upwardly at an angle of 12° to the horizontal. This angle may be adjusted by adjuster means (not shown) if required. The detector 10b includes a light sensor connected to a 1.0mm fibre optic the tip of which is disposed at a height below that of the emitter 10a and is directed upwardly at an angle of 24° to the horizontal. The angle is also adjustable. The light emitter 10a is supported on a vertical slide 13 which is guided m a cylmder 14 and is upwardly biased by a compression spring 15. The slide 13 has a slanted upper face 16 engaged by the end of an adjusting screw 17, whereby rotation of the screw provides fine adjustment of the height of the light emitter 10a. A similar adjusting arrangement is provided for light detector 10b on the other side of the path for the strips 3.

A programmable logic controller (PLC) is provided to process signals received from the level determining apparatus 6. The light emitters are arranged to emit a continuous beam and each time one of the detectors receives a light signal, an output signal is sent to the PLC.

Various proximity switches are provided downstream of the level determining apparatus 6 and are also connected to the PLC. A first proximity switch 19 is arranged to detect the emergence of a strip 3 from the level determining apparatus 6. A second proximity switch 20 is provided to detect the arrival of the strip at a reject chute 31. An air nozzle 30 connected to a source of compressed air is provided to push the strip off the conveyor belt 5 so that it falls mto the reject chute 31. A third proximity switch 21 is provided at the side of the reject chute 31 to check that a strip which should have been rejected has in fact passed down the chute. A fourth proximity switch 23 is provided above the conveyor belt 5 downstream of the reject chute 31 to check that a strip of vials with satisfactory liquid levels has continued on the conveyor belt 5 past the reject chute 31, to reach the end of the conveyer belt 5 as an accepted strip.

In use, when the PLC receives an output signal from the first proximity switch 19, it checks to see if the results of the level determination of all the vials has been found satisfactory by the level determining apparatus 6. If not, once an output signal is received from the second proximity switch 20, the PLC outputs a signal to the compressed air source, whereby a jet of air is discharged from the nozzle 30. The PLC then expects to receive a signal from the third proximity switch 21 to confirm that the strip has been properly rejected down the reject chute 31. If this signal is not received an audible alarm is sounded to alert an operator that intervention is required. If on the other hand the results of level determination are satisfactory then the PLC allows the strip to pass by the reject chute and then expects to receive a signal from the fourth proximity switch 23 to confirm that the accepted strip has safely passed the reject chute 31. Again, if this does not happen then the alarm is sounded.

The operation of the level determining apparatus 6 will now be described with reference to Figure 4. This shows how the beams of light from the first and second emitters 9a, 10a interact with the upper surface, ie. the meniscus, of a liquid 40 contained in a vial 7 having a wall 41.

A first beam of light 50 is shown in solid line interacting with a meniscus 51 (also shown in solid line) which is at a level within the acceptable range. The beam 50 strikes the meniscus 51 at point 52 where it is reflected downwardly at an angle greater than its original angle to the horizontal, because the tangent to the liquid surface at point 52 is itself non-horizontal. In such circumstances the light beam 50 is detected by the detector 9b and a positive signal is sent to the PLC.

There is shown in chain dotted lines a meniscus 61 the level of which is unacceptably low In this case a light beam 60 emitted by the emitter 9a (and initially following exactly the same path as the light beam 50) strikes the meniscus 61 at point 62 where the tangent plane is at an angle to the horizontal sufficiently large for reflection not to occur. Instead the beam 60 is deflected downwardly by refraction as it emerges from the liquid, but not enough to be detected by the detector 9b. Thus, no signal is sent to the PLC which then determines that the liquid level is too low.

If however the liquid level is unacceptably high the first light beam could impinge not on the region of the meniscus adjacent to the vessel wall 41 but instead on the horizontal part of the meniscus, towards the right in Figure 4. This may or may not give rise to a reflection which is detected by the detector 9b, depending on the height of the meniscus. In effect, positive signals may be obtained for vials containing liquid with a level some 10mm or more above meniscus 51. In other words, the first emitter and detector pair 9 may establish a lower limit for the liquid level, but they do not reliably establish an upper limit with an acceptable range. For this reason the second light emitter and detector pair 10 are provided at a higher setting, for example 2-3mm higher. The height difference basically defmes the acceptable range of liquid levels and can be adjusted to suit production requirements. In practice, although the level determining apparatus 6 will function on a close setting this will cause the number of rejects to increase as the liquid in the vials is naturally subject to movement during the measuring operation, because the strips 3 are being conveyed through the apparatus.

The second light beam 70 is shown dotted lines in Figure 4. If the liquid level is below a certain height the second beam 70 will pass through the edge region of the meniscus and will be subject to deflection by refraction but this will not be sufficient for light to be detected by the detector 10b. It will for example emerge in a direction parallel to emerging beam 60. Thus, providing a positive signal is received by the detector for the first beam and no signal is received by the detector for the second beam, then the PLC regards this as a "pass" and the strip is not rejected. When however the liquid level reaches a certain height, shown dotted lines as 71 in Figure 4, then the second beam 70 will be reflected downwardly at point 72 and the detector will receive a signal. If this happens, the PLC- will recognise that the liquid level is too high and generate a signal to reject the strip. For higher levels the second beam may or may not be reflected from the horizontal part of the meniscus to be detected by the detector. However at such heights the first beam is arranged to miss the meniscus altogether, passing below the meniscus during its passage across the vial, so that no signal is received by the second detector. The PLC recognises this as a reject. For extremely high liquid levels both beams of light pass under the meniscus, so that neither detector receives reflected light and again this is a reject.

The PLC is therefore programmed to accept or reject each vial as follows:

Liquid level First detector Second detector Action detects signal detects signal from first from second beam? beam?

Low or empty No No Reject

Acceptable Yes No Accept

High Yes or No Yes Reject

Very high No Yes or No Reject

Extremely No No Reject high

It will be appreciated that each strip consists of a plurality of vials, ten in the preferred embodiment. Since the vials are conveyed past the detectors, light received by a detector is in the form of a discrete pulse. Thus the PLC is programmed to expect a plurality of discrete signals from the first detector for each strip and no signals from the second detector. Once it has received this acceptable set of information, it allows the strip to pass along the full length of conveymg belt 5. If the PLC receives less than the required plurality of signals from the first detector, then when proximity switch 20 detects the strip the PLC generates a rejection signal to the compressed air source. For example, if the expected number of signals is ten and only nine are received then this is a reject strip. Equally, if ten signals are detected by the first detector, but one or more signals are detected by the second detector, then this is also a reject strip. It will be appreciated that the description in relation to Figure 4 shows a liquid which forms an acute angle of contact with the vessel wall, ie. a concave meniscus as viewed from above. It will however be appreciated that the invention is also applicable to a liquid which forms an obtuse angle of contact with the vessel wall, ie. a convex meniscus as viewed from above. In this case the region of the meniscus adjacent to the vessel wall can deflect beams of light in a similar manner to the deflections shown in Figure 4, but by refraction rather than reflection.

Claims

1. A method of determining the level of a liquid in a vessel, comprising the steps of directing a beam of radiation at the vessel, and detecting whether the beam interacts with the upper surface of the liquid.

2. A method as claimed m claim 1, wherein the beam is directed at one side of the vessel and the detecting step is carried out at an opposite side of the vessel.

3. A method as claimed in claim 1 or 2 , comprising detecting the beam if it is deflected by the upper surface.

4. A method as claimed in claim 1, 2 or 3, wherein the beam is arranged to be reflected by the upper surface if the liquid is at a predetermined level.

5. A method as claimed in any preceding claim, wherein the beam is arranged to impinge on a region of the upper surface adjacent the vessel wall if the liquid is at a predetermined level.

6. A method as claimed in any precedmg claim, comprising directing a second beam of radiation at the vessel, and detecting whether the second beam interacts with the upper surface of the liquid.

7. A method as claimed in claim 6, wherein the first beam is used to determme if the liquid level is above a first height, and the second beam is used to determme if the liquid level is above a second, higher height.

8. A method as claimed in any preceding claim, wherein the or each beam is directed at the upper surface from below.

9. A method as claimed in any preceding claim, wherein the liquid level is determined during conveyance of the vessel past the beam(s) of radiation.

10. A method as claimed in any preceding claim, wherein the vessel is a blow fill vessel.

11. Apparatus for determining the level of a liquid in a vessel, comprising an emitter for directing a beam of radiation at the vessel, and a detector for detecting whether the beam interacts with the upper surface of the liquid.

12. Apparatus as claimed in claim 11, wherein the emitter is arranged to be on one side of the vessel and the detector is arranged to be on an opposite side of the vessel .

13. Apparatus as claimed in claim 11 or 12, comprising a second emitter for directing a second beam of radiation at the vessel.

14. Apparatus as claimed m claim 13, wherein the first emitter is arranged to direct the first beam so as to determine if the liquid level is above a first height, and the second emitter is arranged to direct the second beam so as to determine if the liquid level is above a second, higher height.

15. Apparatus as claimed in any of claims 11 to 14, wherein in use the or each beam is directed at the upper surface from below.

16. Apparatus as claimed in any of claims 11 to 15, further comprising means for conveying the vessel past the beam(s) of radiation. 17. Apparatus as claimed in any of claims 11 to 16, in combination with the vessel, being a blow fill vessel.

AMENDED CLAIMS

[received by the International Bureau on 10 March 1997 (10.03.97); original claims 1-17 replaced by amended claims 1-23 (4 pages)]

1. A method of determining if the level of a liquid in a vessel is an acceptable range of levels, comprising the steps of directing a first beam of radiation at the vessel, detecting in a first detection step whether the first beam interacts with the upper surface of the liquid, so as to establish first information concerning the liquid level, directing a second beam of radiation at the vessel, detecting in a second detection step whether the second beam interacts with the upper surface of the liquid, so as to establish second information concerning the liquid level, and using the first and second information to determine if the liquid level in the vessel is in the acceptable range of levels.

2. A method as claimed in claim 1, wherein the first beam is used to determine if the liquid level is above a first height, and the second beam is used to determine if the liquid level is above a second, higher height and wherein the acceptable range of levels includes any level between the first and second heights.

3. A method as claimed in claim 1 or 2, comprising operating an ejector for ejecting the vessel, or operating an alarm, if the liquid level is not in the acceptable range of levels.

4. A method as claimed in claim 1, 2 or 3, wherein the first detection step is carried out using a first detector and the second detection step is carried out using a second detector.

5. A method as claimed in any preceding claim, wherein the liquid level is determined during conveyance of the vessel past the first and second beams of radiation. 6. A method as claimed in claim 5, comprising conveying a plurality of vessels past the first and second beams of radiation, sequentially detecting whether the first beam interacts with the upper surface of liquid in each of the plurality of vessels, so as to establish first information concerning the liquid level in each vessel, sequentially detecting whether the second beam interacts with the upper surface of liquid in each of the plurality of vessels, so as to establish second information concerning the liquid level in each vessel, and using the first and second information concerning the liquid level in each vessel to determine if the liquid level in all the vessels is in the acceptable range of levels.

7. A method as claimed in claim 6, wherein the plurality of vessels comprises a strip of blow fill vessels.

8. A method as claimed in any preceding claim, wherein the first and second beams are directed at one side of the vessel and the detecting steps are carried out at an opposite side of the vessel .

9. A method as claimed in any preceding claim, comprising detecting the first and second beams if they are deflected by the upper surface.

10. A method as claimed in any preceding claim, wherein the first and second detection steps involve detecting whether or not the respective beam is reflected by the upper surface of the liquid.

11. A method as claimed in any preceding claim, wherein the first and second beams are arranged to impinge on a region of the upper surface adjacent the vessel wall if the liquid is at respective predetermined levels. 12. A method as claimed in any preceding claim, wherein the first and second beams are directed at the upper surface from below.

13. Apparatus for determining if the level of a liquid in a vessel is in an acceptable range of levels, comprising means for directing a first beam of radiation at the vessel, means for detecting in a first detection step whether the first beam interacts with the upper surface of the liquid, means for directing a second beam of radiation at the vessel, means for detecting in a second detection step whether the second beam interacts with the upper surface of the liquid, and processing means for establishing from the first detection step first information concerning the liquid level, and for establishing from the second detection step second information concerning the liquid level, the processing means being arranged to use the first and second information to determine if the liquid level in the vessel is in the acceptable range of levels.

14. Apparatus as claimed in claim 13, wherein in use the first beam is used to determine if the liquid level is above a first height, and the second beam is used to determine if the liquid level is above a second, higher height and wherein the acceptable range of levels includes any level between the first and second heights.

15. Apparatus as claimed in claim 13 or 14, comprising an ejector for ejecting the vessel, or an alarm, arranged to be operated if the liquid level is not in the acceptable range of levels.

16. Apparatus as claimed in claim 13, 14 or 15, comprising first and second separate detectors respectively for carrying out the first and second detection steps.

17. Apparatus as claimed in any of claims 13 to 16, further comprising means for conveying the vessel past the first and second beams of radiation.

18. Apparatus as claimed in any of claims 13 to 17, in combination with the vessel, being a blow fill vessel.

19. A method of determining the level of a liquid in a vessel, comprising the steps of directing a beam of radiation at the vessel, and detecting whether the beam interacts with the upper surface of the liquid, wherein the liquid level is determined during conveyance of the vessel past the beam of radiation.

20. A method as claimed in claim 19, comprising conveying a plurality of vessels past the beam of radiation, and sequentially detecting whether the beam interacts with the upper surface of liquid in each of the plurality of vessels.

21. A method as claimed in claim 20, wherein the plurality of vessels comprises a strip of blow fill vessels.

22. A method as claimed in claim 19, 20 or 21, comprising operating an ejector for ejecting the vessel or vessels, or operating an alarm, if the liquid level in the vessel, or in all of the vessels, is not in an acceptable range of levels.

23. Apparatus for determining the level of liquid in a vessel, comprising an emitter for directing a beam of radiation at the vessel, a detector for detecting whether the beam interacts with the upper surface of the liquid, and means for conveying the vessel past the beam of radiation.