Most measuring instruments used for commercial purposes are governed by legislation aimed at promoting fair trade and protecting consumers. As a consumer, it is obviously very difficult to verify whether the 5 kilograms of potatoes you have paid for actually weighs 5 kilograms, for example. The same applies to a fuel dispenser. Did you really put 50 litres in your tank? And how can you be sure that a valid brake tester was used to check your brakes when your car was serviced?
Legislative compliance must be demonstrated by means of tests before a measuring instrument may be offered for sale on the market. This is called legal metrology. Measuring instruments may be subject to both national and international requirements.
A few standard steps form the basis for enforcing requirements on measuring instruments. These principles underlying market access are briefly explained below.
A model of a newly designed measuring instrument must undergo extensive testing by an independent testing laboratory to ensure that it meets all the requirements. The model is then described and recorded. If it meets all the requirements, a national or European type examination certificate is issued and this type of measuring instrument may then be offered for sale on the market.
Once the model has been approved, the measuring instrument goes into production. The initial verification is to confirm that the measuring instruments emerging from the production process are the same as the approved model. The initial verification is carried out by designated organisations, although occasionally the manufacturer may do it, subject to a certified quality system. The owner may then start using the instrument.
Some measuring instruments must be regularly retested by a designated or accredited organisation. This often depends on national regulations, but re-verification is always required if the seal of a measuring instrument has been broken (after a repair for example). This procedure guarantees that, even after the passage of time, the meters still meet all requirements and therefore generate measured values that come within the specified measurement uncertainty.
The verification and re-verification of measuring instruments take place by appointment. However, a meter user may modify the system in order to deliberately obtain other measured values (fraud). For this reason, there are also unannounced inspections to check that the seals are intact and that there are no other irregularities. For most of the legislation in the Netherlands, this is the task of Verispect.