Different types of calibration certificates are available on the market in order to verify the accuracy of measuring instruments. One frequently asked question in this context is the difference between an accredited calibration and an ISO calibration. In order to assess the validity of the different certificates, it is important to understand what traceability means and which metrological bodies exist in the calibration field. This article provides a basic overview of the hierarchy of calibrations and explains the special features of the respective calibration certificates.
The term traceability describes a process by which the measured value displayed by a measuring device can be compared, in one or more steps, with a national standard for the physical quantity in question. These steps must form an unbroken chain of calibrations. In each case, a measuring device is compared with a standard the metrological characteristics of which have in turn been determined by a comparison with a higher-ranking standard. In this way, a calibration hierarchy or hierarchy of test equipment is created (see figure on the right).
National Metrology Institutes (NMI) are at the top of a state's metrology system and represent the top level of the calibration traceability chain. They hold the national etalons (measurement standards) and ensure the link between the measurements and the international SI system of units. The national standards must be compared with other national standards by means of "key comparisons" in order to be recognised internationally in accordance with CIPM-MRA (international agreement on mutual recognition). NMIs can entrust designated institutes (DI) with this important task, provided these institutes have the necessary metrological competence and equipment.
In Austria, E+E Elektronik has been commissioned by the Federal Office of Metrology and Surveying (BEV) as the designated institute to provide the national etalons for humidity, dew point, air velocity and CO2 gas concentration. For these physical quantities, the E+E Calibration Laboratory is the highest metrological authority in the country.
Calibrations by NMIs and DIs are very time-consuming and not geared to everyday calibration tasks in industrial metrology. They are therefore primarily of interest in international comparisons and to accredited calibration laboratories.
Accredited Calibration according to EN ISO/IEC 17025
Accredited calibration laboratories are subordinate to the NMIs / DIs and carry out operational calibration activities. The essential characteristic of an accredited calibration is the traceability to a national standard and therefore typically also to an international standard. A calibration is only considered traceable if it also includes information on the measurement uncertainty of the measurement process, and the suitability of the procedure is confirmed by independent experts.
Under the ILAC (International Laboratory Accreditation Cooperation) multilateral agreement (MRA), only calibration bodies accredited to EN ISO/IEC 17025 can perform traceable calibrations, thereby ensuring full international comparability of the calibration results.
Accredited calibrations are required for test devices which in turn serve as reference standards for monitoring other test equipment, or where a company's quality standards (e.g., IATF 16949) require it.
"ISO or factory certificates" are often offered on the market as a more cost-effective alternative to accredited calibration certificates. ISO calibrations are comparative measurements of external test specimens with internal reference devices (factory standards). The reference instruments used are traceable to national standards. The comparative measurement is carried out according to internal procedures which meet the requirements of ISO 9001 or ISO IATF 16949.
ISO calibrations are subject to the self-commitment of the calibration laboratory to a correct statement of measurement uncertainty, traceability and diligent laboratory work. The calibration laboratory itself is responsible for conformity with EN ISO/IEC 17025. Unlike an accredited calibration, ISO calibrations are therefore not considered traceable, as the process is not monitored and audited by an external accreditation body. ISO calibration certificates are therefore also not internationally comparable.
Need an accredited calibration?
E+E Elektronik is accredited as a calibration laboratory according to EN ISO/IEC 17025 with the identification number 0608 by Accreditation Austria / Federal Ministry for Digital and Economic Affairs. The scope of services of the accredited laboratory includes manufacturer-independent instrument calibrations for the physical quantities humidity, dew point, CO2, air velocity, mass flow, temperature and pressure.