Let us assume that you produce screws and that you import the necessary nuts from Brazil – how do you make sure that the nuts actually match your screws?
The best protection from possible liability claims is that both firms – your own firm and the partner firm – voluntarily commit themselves to fulfil the requirements of the same standard. This standard may require, for example, that the measuring instruments used by you for production quality control meet specific accuracy requirements. How do you check this accuracy? By having your measuring instrument calibrated. This can be done in your own laboratory or in an extern accredited laboratory, or even directly at PTB when the requirements are extremely high.
Calibration means the determination of the deviation of the value indicated by the measuring instrument from the correct value of the quantity measured, i.e. the measurand. For this purpose, an object whose values are exactly known – a so-called standard – is measured with the instrument to be calibrated, and the deviation of the reading from the known value is determined. The result and the respective uncertainty of measurement are recorded in a calibration certificate.
In a stricter sense calibration only means that your measuring instrument is put to the acid test – that is to say, its measurement error is determined. If the calibration laboratory has been DKD-accredited, this process covers yet another step: your measuring instrument is linked up with the respective "national standard" of the measurand in question. This is also referred to as "traceability to the national standard".
Traceability works because there is a fixed relation between every standard within the calibration hierarchy and the national standard: Each working standard is regularly compared with the reference standard of the same level and the latter in turn is compared with a reference standard of the next higher level. This leads to an uninterrupted calibration chain which has a decisive advantage for you: You need not send your instrument to PTB for calibration. It is sufficient to use a "lower level" standard, and despite this you know after calibration: traceability to the national standard has been ensured.
And as the individual national standards are also regularly compared among one another, you and your partner firm can finally be sure that your screws in fact match the nuts made in Brazil.
(By the way: this principle works also with other products.)
Both procedures serve to check the accuracy of a measuring instrument. However, while verification has been prescribed by law for certain measuring instrument categories, calibrations are carried out on a voluntary basis – unless you have introduced a quality management system. In this case you must act according to the standards which prescribe, among other things, that your measuring instruments must have been calibrated.