Legal control of measuring instruments is one consequence of the legislator’s intention to protect consumers and to guarantee fair trade and fair competition in commerce. In the simple case of a counter scale, the customer can trust the measured values indicated or printed, because the weighing instrument has been verified as a compact device. In trade and industry there is, however, a trend towards modular and computer-aided weighing systems. These systems can be operated in local or other networks. They can comprise a chain of individual instruments or modules: from a load cell and signal converter to more complex communication equipment (to transmit and receive weighing data) for indicating measured values or further processing in a computer system. A typical application is the weighbridge to determine the net load of lorries and other vehicles.

In principle, to create measurement results which are safeguarded against manipulation and trustworthy would require that all components in the measurement chain are type-approved and verified – a requirement which can hardly be achieved in practice. To meet the commercial wish to use company-own data processing systems and communication networks, which are not subject to legal control, in the processing of weighing data, the European Weighing Instruments Directive 90/384/EEC exempts data processing systems from legal control provided the results of the weight measurements are recorded by a verified data storage device or printer ("alibi printer"). This means that - in order to maintain an unbroken chain of trust - the weighing system must contain at least one indicating unit with a data storage or printer output subject to legal approval and control in order to compare the weight measurement results with those of the non-verified modules in the chain.

The legal requirements for these data storage devices and alibi printers had been formulated in rather general terms in the past. This gave way to a very insufficient harmonization of type approvals and practically no verification technological supervision of the systems could be realized. On instigation by PTB, WELMEC, the European Cooperation in Legal Metrology, attended to the matter. It has now published new, supplementary rules for data storage devices acceptable for verification in WELMEC document 2.5 "Guide for modular approach and testing of PCs and other digital peripheral devices". The technical requirements in the document were decisively affected by the experience PTB had gained in consumer protection exercised in Germany. For the first time there are now Europewide uniform requirements for data storage devices so that the technical progress of complex, computer-based weighing systems is not impeded.