How much is really in it?
What does the customer expect concerning the fill level when for example he takes a crate of mineral water or a high-quality spirit off the shelf and puts it into his shopping trolley at the drinks cash-and-carry? Without doubt he will unconsciously make a visual check of the fill height (i.e. take a measurement from the base of the bottle); in other words assume that bottles with a greater fill level also contain more of the product.
By comparison the use of templates is a widespread measuring method used in bottling plants (please see fig. 1). The upper edge of the closure is used as the point of reference in this case. The advantage of this method of measuring is that it can be carried out quantitatively quickly and is non-destructive.
The legislator has his own ideas: if 1 litre is stated then it must contain 1 litre; the legal marginal conditions are quite clear-cut (please see the October Internet report) and only orientate themselves to the internal volume of the container.
In this way there are three measured values for a production container which should state the same but are different in principle. How good the correlation between these measuring methods is varies from container type to container type.
But how can this be put into practice?
At any rate it must be taken into account in the case of fast lines that there is only approximately 30 ms available for measuring and evaluating each container. It is difficult to determine the internal volume of the container and therefore the fill height is generally checked. The most frequent methods for this are measuring with X-rays, an electrical high-frequency field or optically using a camera. In all cases heights are fixed for the minimal or maximum fill level and all containers the fill level of which is within this fill height range are evaluated as good and those without are removed from the production flow.
Measuring the fill level height is carried out on the conveyor. It is not a favourable situation when this measurement clearly contradicts the regular check measurements carried out by the laboratory personnel using templates. This is particularly irritating when the height tolerances of the container are great. The height tolerances of returnable PET bottles can easily be between 5 and 10 mm due to the fact that the containers shrink with increasing use and every time they are cleaned.
When taking two containers with fill levels of the same height, but one bottle for example is 8 mm taller, the fill level checking device along the line will produce the same value for both bottles but the measurement of the laboratory personnel using templates will have a value of approximately 10 ml less for the taller bottle. This is a not an inconsiderable difference and fundamentally an unfavourable combination between the on-line measurement of all the bottles and the check measurement by the laboratory personnel.
In order to avoid such discrepancies right from the beginning the SPECTRUM fill level detections supplied by HEUFT SYSTEMTECHNIK GMBH can be equipped with a height compensation module. The height of the bottles can be measured with this independent of the fill level and then the fill height, which is actually determined as the fill level along the conveyor, is converted into a fill height which uses the upper edge of the closure as its point of reference. In this way the on-line measuring method agrees perfectly with the check measurement carried out in the laboratory and unnecessary discussions are avoided.
One should be aware of these aspects of the fill level measurement which at first appear to be trivial and to keep in mind the properties of one´s own production containers as well as the resulting consequences relating to quality control.
We have many years of experience in order to give you competent advice concerning the optimum solution for your individual case.