Sets the essential metrological and performance requirements necessary to guarantee effective protection for users and consumers and to lay down certification rules and procedures. Member States must ensure that only those instruments complying with the provisions of the Directive may be placed on the market. Member States shall not impede the placing on the market and the putting into service of instruments meeting the provisions of the Directive. Member States shall presume that instruments complying with national standards implementing the harmonized standards that meet the essential requirements are in conformity with these requirements.