Probabilistic decision making for the implementation of the Measuring Instrument Directive

The approval of the European Measuring Instrument Directive on April 2004 renews completely the legal metrology sector, proposing a "new approach" aiming to technical normalization and harmonization and toward a "global approach" to evaluation and conformity. According to the principles of the "new approach" regulatory model, the Directive leaves to the manufacturer itself to adopt proper technical solutions, giving only very essential requirements, without going in any detail. Moreover new certification procedures based on Quality Systems are available and there is no indication regarding the procedure to verify product conformity. In the present paper, after a brief discussion regarding the MID philosophy, a probabilistic approach to conformity assessment is presented. The proposed approach covers a set of open issues such as the measurement uncertainty due to the influence of operating conditions, the management of the risk due to conformity decision based upon a set of measurement results, the costs of instrument uncertainty and eventually non conformity for the manufacturer and instrument user. The approach is supported by a set of software codes enabling an easy and user friendly application. All these aspects are presented with reference to the case of water meters, considering both the end of production acceptance tests and the operative situation of the meter.