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New WELMEC-Guide for automatic weighing instruments (AWI) adopted

For several decades now, electronic weighing instruments have been tested and approved for verification at first as nonautomatic weighing instruments (NAWI), while subsequently the approval as automatic weighing instruments (AWI) is being applied for. In the past it became obvious that European notified bodies partly followed different philosophies when handling such applications. It was alarming that the specific metrological problems of AWIs had partly not been taken into account at all.

In order to ensure a uniform metrological treatment of AWIs after implementation of the European Measuring Instruments Directive 2004/22/EG [1] (generally known as MID) a subgroup to WELMEC (WELMEC: European Cooperation in Legal Metrology) working group WG2 had been established in January 2004 (with Denmark having the chair), aiming at attaining that goal.

As the name already reveals, automatic weighing instruments normally work without an operator being present who might supervise the weighing process. Moreover, weighing processes often run so fast that a visual inspection of the weight displayed would hardly be feasible. From this the following problems result:

The zero of the weighing instrument (weight display of the unloaded instrument) may drift off due to warm up effects of the electronics after the instrument has been switched on. Since no operator is present who could correct the zero error, zero drifts fully enter into the measuring result.
The zero of the weighing instrument may drift due to the influence of varying ambient temperatures (change of the weight display of the unloaded instrument due to variations of ambient temperature). Also in that case the error directly enters into the weighing result.
In case of a NAWI, the influence of the zero error on the measuring result may not exceed 0.25 scale intervals (d) after a zero setting or a tare operation (OIML R76 [2]; OIML: Organisation International de Metrologie Légale). The scale interval is defined as being the difference between two consecutive indicated values with digital indication. Especially when filling small amounts of mass, using automatic gravimetric filling machines (AGFI), the deviation of 0.25 d may have such a significant influence on the actual fill in comparison to the preset fill that verification and in-service error limits may be exceeded. Thus, OIML R61 [3], dealing with AGFIs, fixes absolute lower limits of fills depending on different scale intervals and accuracy classes.
Transient disturbances because of electromagnetic impacts, that would be recognised by the operator of a NAWI, will not be detected in case of AWIs. With AGFIs, e.g. temporary changes of the displayed measuring value fully enter into the fill because such a rise of the displayed result is interpreted by the AGFI as having reached the preset value. As a consequence the filling is aborted before the preset fill has been filled into the package.

The first edition of WELMEC Guide 2.8, intended to deal with the problems mentioned above, had been published on the WELMEC homepage in May 2008, under the leadership of PTB. In addition to the general rules to be applied and the mathematical mechanisms, the Guide shows, by means of examples especially for AGFIs, how notified bodies should act. As a draft the Guide initially contained a lot of redundant information from other WELMEC Guides (e.g. for testing of indicators) so that a very extensive document (of more than 100 pages) had been drawn up. PTB had a tremendous interest in making the Guide available to other European notified bodies in order to facilitate their job and in order to achieve an equal treatment of manufacturers. Hence, the chair of the WELMEC working group in charge was de facto taken over by PTB working group ”Dynamic Weighing” after the former chairman had stopped his activities. In cooperation with the other members a completely new version of the Guide was drawn up which contained no redundant information from other Guides at all. Up to now no experiences from third parties using the new Guide are available.

Especially AGFIs require an extended calculation time (recursive procedures) to determine the possible minimum fills (Minfills), due to the special error regime. So a large part of the Guide is dedicated to these AWIs. The results are normally determined on basis of the smallest signal per d of the indicator of the weighing instrument.
With the signal level increasing the relative effect of, for example, zero drift is decreasing because from a technical point of view it represents a change of the offset voltage of the electronics. The variation (ΔU) remains constant while the signal voltage per d increases and thus ΔU causes a smaller shift of the displayed value (expressed in d). This, in turn, theoretically permits calculation of new, smaller Minfills. Correspondingly programmed Excel sheets to perform the necessary calculations are available at PTB.

The Minfills, stated in the type examination certificates, obtained on the basis of the calculations mentioned above can also be determined for higher signal levels. In case a corresponding calculation tool is available, this task is significantly facilitated. This tool is planned to be published on the internet pages of PTB or WELMEC in the medium term. In addition, type examination certificates according to MID shall be supplemented by explanatory information. So for under more favourable technical conditions, that is at a higher signal level per d, smaller Minfills can be calculated. In practice the need for such a subsequent re-calculation of the Minfills determined will not arise very often since the Minfill of the actually realised instrument is normally strongly influenced by the quality of the feeding devices, of the properties of the material to be filled (adhesive and caking materials) and of the ambient conditions (e.g. vibrations). The figures stated in the type examination certificates are intended to prevent – as reference values – in the first place that instruments are realised on site of which the accuracy classes are higher and of which the Minfills are lower than those theoretically determined on basis of laboratory test results, e.g. of temperature and warm-up tests.

[1]	Directive 2004/22/EC of the European Parliament and the Council of 31 March 2004 on measuring instruments
[2]	Non-automatic weighing instruments. Part 1: Metrological and technical requirements – Tests, Ausgabe 2006
[3]	Automatic gravimetric filling instruments. Part 1 : Metrological and technical requirements – Tests, Ausgabe 2004

Contact person:

Karsten Schulz, FB 1.1, AG 1.13, e-mail: karsten.schulz@ptb.de