Physikalisch-Technische Bundesanstalt

Standard and standard are not the same things – but almost the same, since the different standards within the calibration hierarchy are regularly compared according to a definite pattern. Right at the bottom of the hierarchy there are the many working standards of industry. In the majority of cases, these standards are just measuring instruments such as calliper gauges, micrometer gauges or dial gauges. As they are subject to wear during continuous use, they must regularly be linked up with a reference standard. In the case of length measurement, the reference standard is a set of gauge blocks kept in a cupboard, well protected from ambient influences, and taken from it only if such calibrations are to be carried out. The designation "reference standard" simply means that this set of gauge blocks is in turn regularly checked against its counterpart at the next higher hierarchy level (for example a DKD laboratory). This counterpart may in fact be a working standard as well, which is linked up with the next higher reference standard, and so on. Right at the end of the unbroken chain is the national standard – in length measurement a laser maintained at PTB. PTB maintains standards for every quantity and these standards in turn are regularly compared with the corresponding standards in the national metrology institutes of other countries.

The more important international contacts between individual firms become, the more often the question arises whether we in fact use the same measures and whether our measuring instruments are of similar accuracy. It is, therefore, not surprising that traceability to the national standards is more and more becoming the rule. The question at which level of the calibration hierarchy one begins essentially depends on the accuracy of measurement which is to be attained. The rule of thumb is: The farther below, the more comparison steps between the standards have already been made. The standard is less accurate, but calibration normally is cheaper and takes less time because it is less sophisticated.