Different from approval (where one sample instrument, which represents the whole type series, is sent to PTB), for verification the manufacturer must present every individual instrument to the competent verification authority where it is checked whether the instrument submitted is in compliance with the 
Verification Ordinance and PTB type approval or, in the case of generally approved measuring instruments, with the generally accepted rules of technology. In addition, the instrument must be in compliance with the maximum permissible errors stipulated in the Verification Ordinance. To ensure uniformity in the various federal states, the 
verification authorities accomplish their tasks in close technical cooperation with PTB.
During verification it is determined whether the petrol dispensing pump actually displays the quantity delivered.
Special cases: gas, water, electricity and heat meters
The instruments are mainly verified by state-approved test laboratories. About 340 of such test laboratories on a private enterprise basis assist the State in Germany in the enforcement of the Verification Act. They use measuring facilities tested by PTB and are regularly inspected by the local verification offices.
The accuracy of each measuring instrument changes in the course of time due to wear and tear or aging. The validity of the verification for measuring instruments has therefore been limited in time. Depending on the instrument category, the instruments must be submitted for subsequent verification after a period of one to 16 years.
As both the verification authorities and PTB are required to cover their costs, fees are charged for all services rendered. That is to say, accuracy costs money. It is therefore consistent to put all regulations concerning maximum permissible errors and period of validity of the verification into relationship with the macroeconomic value of the goods measured. Certain measuring instruments are even exempted from legal control as the metrological effort bears no reasonable relation to the intended use.
The problem with the great numbers
Volume standards and stop watches are the instruments used to measure the volume discharged when the measuring system of a road tank vehicle is verified – so the customer can rely on the correctness of the fuel oil supplier’s invoice.
For some measuring instrument categories which are manufactured in great numbers (length measuring devices, barrels or simple weighing instruments), it is sufficient to carry out verification by random inspection. In the case of other measuring instruments which are installed in great numbers, for example gas, water and electricity meters, it is possible to extend the period of validity of the verification if a sampling test has furnished proof of the instrument's measuring stability for a further period of time. Such procedures developed in cooperation with PTB have considerably reduced the test costs of the public utility undertakings.
Gas, electricity and water meters are mainly verified by state-approved test laboratories. They are prepared to efficiently test large numbers of measuring instruments.
© Physikalisch-Technische Bundesanstalt, last update: 2011-11-17, Volker Großmann
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