High-voltage transformers for the measurement of electrical energy are tested at PTB in a special room which has been completely copper-plated.
All measuring instruments used in accordance with the 
Verification Act for measurements subject to legal control must be verified. In most cases, prerequisite for verification is a type approval, i.e. the type – and not each individual instrument but a typical specimen – must be approved by PTB. The approval procedure is as follows:
The manufacturer files an application for approval with PTB and submits the necessary technical documents and, normally, a sample instrument. The approval documents and the sample instrument are tested by the competent PTB section in compliance with the provisions of the Verification Ordinance. Essential criteria here are the instrument’s correctness and stability. The applicable requirements and maximum permissible errors must be complied with, and this for a period prescribed by law, the so-called period of validity of verification. The approval test covers metrological, technical and administrative tests. Among the metrological tests are tests performed under the impact of specific influence quantities such as temperature, humidity, vibrations, variations of the voltage supply, and electromagnetic fields. Within the scope of the technical tests which to an increasing extent also comprise software tests, one checks whether the functions of operation, display and printing meet the requirements and whether the instrument is adequately protected against faulty operation and manipulation.
About 20 PTB working groups are concerned with the approval of measuring instruments and carry out the necessary tests. Normally, such a test takes three months at most. However, if problems arise in the course of the test, the procedure may take more time and in the end it may become necessary to improve measurement features of the sample instrument submitted. In rare cases, instruments must even be rejected.
Are petrol quantity and price correct? This is one of the questions when petrol dispensing pumps are type-tested.
The approach is optimum when the manufacturer contacts PTB at an early stage and informs himself about the legal requirements already during the development phase. PTB is readily prepared to give advice well in advance, before approval is applied for, because it sees itself as a customer-oriented, competent partner of industry.
If the approval test was a success, PTB issues an approval certificate for the applicant and lays down an approval mark which must be affixed conspicuously to all measuring instruments of this type. There are approvals under national law and European approvals. In the case of the European approvals, a distinction is made between conventional EEC approvals and EC type approvals in compliance with the new approach of the European Union. EC type approvals are to date possible only for non-automatic weighing instruments for which national approvals (approvals under national law) are no longer granted.
The approval marks (for approvals under national law and EEC approvals) and the approval numbers (for EC type approvals) comprise references to the Verification Ordinance, the measuring instrument type, the year of approval and a consecutive number; to the EEC approval mark and the EC approval number the letter “D” for Germany is added (example: D01-09-012).
The type approvals granted are published every three months in the PTB-Mitteilungen, the official bulletin of PTB.
After approval
Attention: speed check! PTB is not, however, interested in traffic offenders but in testing.
After the instrument type has been approved by PTB, each single instrument must be verified by the responsible verification authority before it may be used, for example, in commercial transactions.
General approval
For simple, mechanical measuring instruments, the Verification Ordinance provides a simplified procedure: the general approval. This kind of approval is in certain cases also possible under EEC and EC directives. The manufacturer may apply for the verification of such an instrument directly with the responsible verification authority, without having obtained type approval before.
© Physikalisch-Technische Bundesanstalt, last update: 2011-11-17, Volker Großmann
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