Metrological standards and calibration of measuring instruments

However precise a measuring instrument may be, it must be checked periodically against a known reference: only in this way can its measurements be declared correct. The reference is the metrological standard, an object of known and certified value (a thickness, a diameter, a mass) against which the instrument is checked and, if necessary, corrected.

This process — calibration — makes measurements traceable, that is, linked through an unbroken chain to national and international reference standards, and documented by a certificate stating the value and the uncertainty.

What a metrological standard is

A metrological standard embodies a quantity with a value that is known and stable over time: gauge blocks for thickness, rings and plugs for diameters, masses for weighing, and dedicated calibrators for specific instruments. Its quality depends on dimensional stability, surface hardness and certification of the value.

For instruments that measure large quantities, dedicated calibrators are used: for railway wheel gauges, for example, there are calibrators with a certified nominal diameter, complete with a measurement report, which allow the instrument to be checked and zeroed before use.

Calibration, verification and traceability

Calibration compares the readings of the instrument with the standard and documents the deviation; verification establishes whether the instrument is still within the permitted tolerances. Both rest on traceability: the standard used must itself be calibrated with traceability to national standards, in a documented chain.

• It improves the reliability of quality control data.
• It ensures the consistency of measurements between operators, shifts and different sites.
• It satisfies the requirements of quality systems (e.g. ISO 9001, IATF).
• It reduces the risk of accepting non-conforming parts or rejecting good ones.

Good calibration practice

It is advisable to calibrate or check the instrument with a standard of the same type of quantity and in stable ambient conditions (temperature), to observe the calibration intervals established according to use, and to keep the certificates for traceability. A daily check against the standard, before measuring, immediately catches any drift in the instrument.

RODER calibrators and references

Together with the instruments, RODER supplies the corresponding calibrators and check standards with a measurement certificate — for example the calibrators for the DIACHECK-CTFR railway gauges. For the instruments and their calibration accessories, please refer to the linked catalog pages.

Related products

RODER instruments for this topic. Go to the catalogue pages for models, technical data and documentation.

Frequently asked questions