Traceable calibration of automatic weighing instruments operating in the dynamic mode

The Measuring Instruments Directive (MID) details the requirements an instrument must possess prior to sale or use in Europe with the aim of creating a single market in measuring instruments for the benefit of both manufacturers and consumers. The MID covers a range of instruments including Automatic weighing instruments (AWIs) which are used to determine the mass of objects where the intervention of an operator is not required.

The majority of AWIs operate in a dynamic mode, measuring the weight of objects that are in motion. Whilst Non-Automatic weighing instruments (NAWIs) are calibrated according to the EURAMET Calibration Guide, methods for dynamic AWIs are generally devised in-house by calibration laboratories. This is because no standardised approach or method for calibrating these instruments to SI units are available, only testing procedures to confirm their functionality.

Calibration laboratories are themselves assessed by external accreditation bodies to ensure competence in accordance with the international standard ISO 17025. Frequently, National Measurement Institutes (NMIs) provide the technical assessor to the accreditation body but in many emerging EU member states NMIs lack the necessary skills and experience to do this.

Closing the gap between emerging and developed NMIs by developing best practice in automatic weighing instrument calibration will create greater harmonisation in trade across the EU. Thus, there is a pressing European need for validated, reproducible calibration methods for different groups of dynamic AWIs.

This project brought together nine NMI’s across Europe to address the lack of standardised methodology in this area.

Three types of instruments were selected, representative of three AWI categories which are also covered by OIML recommendations R51, R61 and R134; ‘automatic catchweighers’, ‘automatic gravimetric filling instruments’ and ‘automatic instruments for weighing road vehicles in motion’. Based on existing methodology a draft good practice guide was drawn up for each instrument type detailing measurement methods, error models and measurement uncertainty budgets. These procedures were validated by inter-comparison exercises performed on-site at manufacturing production lines or at vehicle weighing stations.

Results were used to further improve the content of the draft guides - which  will be submitted to EURAMET for consideration as an AWI calibration guide. In parallel, individual strategies for the long-term development of research capabilities were developed for the less experienced NMIs, including priorities for establishing quality schemes and accreditation. The new guides aim to provide measurement harmonisation that is currently lacking for AWIs operating in the dynamic mode, assuring the traceability of instrument measurements.

Work from the project has already helped development of AWIs protecting rural infrastructure by supporting police in Estonia and in increasing the pool of technical assessors available to national accreditation bodies.

Furthermore, the improved expertise developed by emerging EURAMET members will increase reliability and confidence in the performance of AWIs for the end-user community, increase competence in conformity assessments and help demonstrate compliance to relevant legislation in these countries.