15.11.2018
Automatic weighing instruments are an important element of industrial production and are used, for example, to check the content of prepackages. Currently, however, no specifications exist for the calibration of these instruments. Over the past three years, proposals for calibration guides for these instruments have been drafted within the scope of the AWICal project (see also 
www.awical.eu).
To enable accurate weighing, weighing instruments are regularly calibrated and their measurement uncertainty is determined. At the European level, however, there is only a guide on the calibration of non-automatic weighing instruments for static weighing (EURAMET calibration guide 18). In legal metrology, however, automatic weighing instruments are mainly used and their functionality and accuracy are regularly checked within the scope of verification. To make traceability by means of calibration possible also for automatic weighing instruments with a dynamic function, the AWICal project was launched by the EU in 2015 within the scope of the EMPIR support programme. The aim of this project was to develop calibration procedures designed specifically for dynamic weighing systems and to verify their use.
The project consortium, which has, over the past three years, collaborated to reach this goal, is composed of nine national metrology institutes, namely MIRS (Slovenia), BEV-PTP (Austria), CMI (Czech Republic), GUM (Poland), IMBiH (Bosnia-Herzegovina), Metrosert (Estonia), DMDM (Serbia), TUBITAK (Turkey) and PTB, representing Germany. As the industrial partner, Mettler-Toledo participated in the project.
Since there are many different dynamic automatic weighing instruments, the project activities have focused on three categories, namely automatic catchweighers, automatic instruments for weighing road vehicles in motion, and automatic gravimetric filling instruments. In the project, PTB has focused on developing and testing calibration procedures for automatic catchweighers. Such weighing instruments are frequently used as automatic checkweighers in industrial production and for the packing of foodstuffs.
Together with five other institutes and the manufacturer, tests were performed on the calibration sequence which was proposed in the consortium and is based on the EURAMET guidelines on the calibration of non-automatic weighing instruments (EURAMET calibration guide 18) and on specifications of the OIML Recommendation R51. This was undertaken within the scope of individual tests carried out on different instruments by seven of the project participants. After adjusting the calibration sequence, a comparison campaign was carried out at PTB. The manufacturer Mettler-Toledo, as the project partner, provided an automatic checkweigher for this purpose (Fig. 1). The other partners came to PTB to perform in situ measurements to calibrate the weighing instrument in accordance with the draft calibration guide. The participants all entered their results into a spreadsheet that had been developed within the scope of the project. Calibrations had to be performed at least at two previously defined belt speeds (20 m/min and 40 m/min) and with commercially available products as well as with test masses manufactured exclusively for this purpose (approx. 100 g and approx. 500 g) (Fig. 2). The test masses were selected so as to provide different mass distributions, varying density and also loose material in order to determine their influence on the measuring instrument. For this reason, different kinds of chocolate (milk chocolate and chocolate with nuts), pasta, and cardboard boxes with equal weights – but with differently distributed contents – were used.
Figure 1: Automatic checkweigher of Mettler-Toledo for comparison purposes.
Figure 2: Test masses.
The mass of the test masses used as a reference was determined by the participants in accordance with the specifications of the guide just before the measurements took place; they were then used to calibrate the automatic weighing instrument. In principle, the measurements allowed the calibration procedure to be validated. For instance, the results obtained confirmed the different weighing results that had been expected for the two different belt speeds. However, the results all in all showed better agreement than expected.
Finally, the calibration guide on automatic catchweighers thus drafted was submitted to EURAMET, together with two further drafts that had been developed within the scope of the project (on automatic instruments for weighing road vehicles in motion and for automatic weighing instruments), for further elaboration and for – possible – confirmation as calibration guides at a later date.
Link:
Contact:
Dorothea Knopf, FB 1.1, E-Mail: 
dorothea.knopf(at)ptb.de