Calibrating Non-Automatic Weighing Instruments - Recent Developments with Regards to Global Harmonization

摘要

The metrology of measuring instruments is a critical component of any organization's quality operations. The standout prerequisite for traceable and accurate weighing is the effective calibration of weighing instruments, covered under the company's quality management system. It seems, however, that not all organizations have a thorough understanding of current metrological science. Unless specifically addressed by metrologists, it is still a widespread belief that calibrating a weighing instrument mainly consists of placing reference masses on the weighing platform with the objective to assess the deviation between the indication and the mass value of the reference. Not everybody is aware that calibration is a process that establishes a relation between quantity values provided by measurement standards and corresponding indications, which is only complete if the contributing measurement uncertainties are taken into account. On a national level, there are manifold calibration guidelines for weighing instruments, which are based on the concepts described in the Guide of the Expression of Uncertainty in Measurement (GUM). However, on a global level, only one document remains: The EURAMET calibration guideline cg-18 "Calibration of non-automatic weighing instruments". This document is currently under revision and might also act as a basis for a possible development of a US calibration guideline. This article focuses on the EURAMET cg-18 calibration guideline and its recent revision, specifically highlighting its practical implications for applications in the laboratory and the production area. One appendix that is added to the guideline introduces the so-called "minimum weight", i.e. the smallest sample quantity required for a weighment to just achieve a specified relative accuracy of weighing. The concept of minimum weight was also introduced during recent developments in the regulated industry in regards to the revision of the USP General Chapters 41 and 1251, thus also putting emphasis on key properties of weighing instruments that strongly affect the quality of weighing results.