Logo PTB
DKD Masse & Waagen

7. Mass and Weighing Instruments

Steffen Osang

 

The Technical Committee Mass and Weighing Instruments was founded in 1995. It brings together the accredited calibration laboratories for weights and weighing instruments. The Technical Committee currently comprises about 75 accredited calibration laboratories, interested experts from Germany and abroad as well as representatives of PTB.

The SI unit of mass is the kilogram. After the replacement of the international prototype of the kilogram (“original kilogram”) by redefining the ‘accepted reference sample of the physical unit of mass’ (material measure), it is now expressed using the Planck constant h (Avogadro project or watt balance project – the watt balance is nowadays called Kibble balance, renamed after Bryan Kibble who developed the apparatus).

By ensuring the metrological traceability of their calibration results to the SI unit of mass, the calibration laboratories belonging to the Technical Committee Mass and Weighing Instruments guarantee the comparability of their weighing results.

Calibration laboratories for weights calibrate weights of nominal values from 1 mg to 5 t. Mostly, calibrated weights are used as standards for the adjustment and calibration of weighing instruments. The calibration of weights is carried out by means of a high-precision mass comparison, using a reference weight of the same nominal value. For this purpose, mass comparators are used which are capable of differentiating, for example, one kilogram in steps of 1 µg.

Weights are calibrated in accordance with the internationally recognised recommendation OIML R111:2004 issued by the International Organisation of Legal Metrology (Organisation Internationale de Metrologie Légale, OIML for short). Some of the Committee’s calibration laboratories meet the requirements of the highest accuracy class for weights according to OIML R111 (Class E1). These laboratories achieve smallest relative measurement uncertainties of up to 15 x 10–8

For the most part, weighing instruments are calibrated at the customer’s premises to take into account the ambient conditions at the place of use of the instrument. To determine the display deviation of the respective balance, the laboratories use accurate and traceable test weights. The spectrum of calibrated weighing instruments ranges from very sensitive analytical and microbalances to weighing bridges designed for heavy loads (vehicle scales). The smallest relative uncertainty of the calibration results achieved under very good conditions and by means of highly accurate test weights for analytical and microbalances is 1 x 10–6.

The procedures and requirements for the calibration of weighing instruments originate from the guideline EURAMET cg 18 (Guideline on the Calibration of Non-Automatic Weighing Instruments). It should not go unmentioned that the Technical Committee played a major role in the preparation and further development of this guideline. The German translation was published in 2018 as Guideline DKD-R 7-2.

The Technical Committee regularly initiates interlaboratory comparisons for weighing instruments and weights.

A DKD Expert Report on the calibration of non-automatic high-capacity scales by means of the equivalent load method is about to be completed.

 

Steffen Osang, February 2020