GB2307043A

GB2307043A - Liquid level determination

Info

Publication number: GB2307043A
Application number: GB9522752A
Authority: GB
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 Ltd; Boehringer Ingelheim International GmbH
Current assignee: Boehringer Ingelheim Ltd; Boehringer Ingelheim International GmbH
Priority date: 1995-11-07
Filing date: 1995-11-07
Publication date: 1997-05-14
Also published as: AU7319496A; GB9522752D0; WO1997017594A1

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 determinatlon 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 in a non-intrusive 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.

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. However, this may not be entirely reliable in achieving the desired deflection.

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 22e to 260, and most preferably approximately 240. 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.

If an upwardly directed beam misses the liquid upper surface edge region by passing above it, no signal is received by the detector. This indicates that the level is too low. If however the beam misses the edge region by passing below it, the beam may in some circumstances be internally reflected by the main, central region of the upper surface, in which case the detector receives a signal. Thus the method reliably indicates when the level is too low and thus detects underfills. It is also useful in detecting an empty vessel, unlike the known leak detection system.

However, the method is less reliable if the level is too high. This is acceptable in certain applications, but if it is not then it is preferred to direct a second beam of radiation at the vessel, the second beam being arranged to interact with the upper surface in the event that the upper surface is above a predetermined level. The interaction can be detected by a second detector.

In the preferred embodiment where the target for the first beam is the liquid upper surface edge region, then it is convenient for the same target to be used for the second beam. The second beam may then be parallel to the first beam and at a higher location. If the second beam is deflected by the edge region and detected by the second detector, this indicates 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.

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 line and preferably therefore the liquid level is determined during conveyance of the vessel past the beam(s) of radiation. In practical terms, this enables the second beam of radiation to 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 lOa and detector lob 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 O.Smm 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 lOb includes a light sensor connected to a l.Omm fibre optic the tip of which is disposed at a height below that of the emitter iCa 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 in a cylinder 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 1Oa. A similar adjusting arrangement is provided for light detector lOb 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 into 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 lOmm 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 within 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 defines 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 in 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 lOb. 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 in 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 conveying 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 in claim 1, comprising detecting the beam if it is deflected by the upper surface.

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

4. A method as claimed in claim 1, 2 or 3, 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.

5. A method as claimed in any preceding claim, comprising directing a second beam of radiation at the vessel, the second beam being arranged to interact with the upper surface in the event that the upper surface is above a predetermined level.

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

7. 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.

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

9. 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.

10. Apparatus as claimed in claim 9, comprising a second emitter for directing a second beam of radiation at the vessel, the second beam being arranged to interact with the upper surface in the event that the upper surface is above a predetermined level.