11.06.2011
According to the Units and Time Act, PTB's task is, amongst other things, to disseminate the legal units. This task is fulfilled with the support of accredited laboratories. Since 1 January 2010, the accreditation of these calibration laboratories has been the responsibility of the Deutsche Akkreditierungsstelle GmbH (DAkkS - the German accreditation body). Pursuant to DIN EN ISO/IEC 17025, calibration laboratories are expected to take all the necessary measures to ensure the quality of their calibration results. In order to comply with this requirement and to be assured of the quality of calibrations performed by accredited laboratories in the field of weighing instruments, comparison measurements were performed and the results were analysed.
All Twenty-seven accredited German calibration laboratories and one calibration laboratory which is in the accreditation process took part in this round robin. Since weighing instruments are usually not calibrated at the calibration laboratory's site but at the place of installation, a corresponding procedure was agreed upon for the round robin test. Therefore the weighing instrument to be calibrated was set up in one of PTB's laboratory rooms and all 28 participating calibration laboratories came to PTB to perform the calibration. Finally a calibration certificate was issued by each calibration laboratory.
Both the calibration of the weighing instrument and the calculation of the measurement uncertainty were carried out in accordance with the specifications of EURAMET's Calibration Guidelines CG 18 for non-automatic weighing instruments. In principle, the measurement results, which were defined within the scope of this round robin test, show a good agreement. In addition the round robin tests identified where a need of improvement of the Calibration Guidelines CG 18 exists. The establishment of the uncertainty budget is left to the calibration bodies themselves, for instance influences of eccentric loading or convection effects of the mass piece may be neglected under certain conditions. Also other European Member States confirm that there is the need of a harmonisation of the determination of the measurement uncertainty. Thus, the competent EURAMET Committee TC3 (Mass and Related Quantities) has decided to revise the Guidelines CG 18.
Figure 1: Example of a non-automatic weighing instrument with mass standards
Contact person:
Michael Denzel, Dept. 1.1, WG 1.12, e-mail: michael.denzel@ptb.de