Generation Change in the Field of the 50 kg Mass Comparators

After more than 40 years of use, the 50 kg beam balance of the PTB which has been used in the mass range from 20 kg to 50 kg for the realisation of the mass scale and dissemination of the unit of mass has been replaced by a mass comparator of the latest generation. So now a weighing technology in accordance with the state of the art is also applied in this mass range. The result of this generation change is a clear increase in the efficiency together with a decrease in the uncertainty of measurement.

The 50 kg comparison balance of the PTB is of special importance for the reali-sation of the mass scale and dissemination of the unit of mass in the range from 20 kg to 5 t. The mass calibration of the PTB's 50 kg primary standards is performed on this balance. One hundred 50 kg standards are used for the realisation of the mass scale up to a total mass of 5 t. In the years from 1963 to the end of 2003, an equal arm balance was used as 50 kg balance (Fig. 1). In the past 20 years, the standard deviation of this balance could be decreased from 10 mg to approx. 1 mg on the basis of various technical improvements. An unsolved problem was, however, the determination of interchange errors which was impossible due to the construction and the mass determination of bodies with larger dimensions as, for example of weights for force standard machines. Additional error influences resulted from the positions of the interchanging facility arranged one above the other because of differences in temperature, air density and acceleration due to gravity.

Fig. 1: 50 kg beam balance (August Sauter KG) with load exchange facility

Fig. 2: 64 kg comparator AX64004 (Mettler-Toledo GmbH) with load exchange facility

Since the end of 2003, the PTB has now a new 64 kg comparator (Fig. 2) at its disposal. For the specified applications, this comparator offers essential improvements as, for example, an automatic interchanging facility with 4 positions, the possibility of interchanging the positions, a maximum capacity of 64 kg, a modern and fast weighing system with built-in weights (250 g steps), a reduced standard deviation down to less than 0.4 mg and the possibility of weighing measurement objects whose dimensions are considerably larger compared with the conventional 50 kg standards.
The new comparator allows weights, which are used in force standard machines for the realisation and dissemination of the force scale, to be determined up to a diameter of 350 mm and, together with auxiliary means, up to a diameter of 800 mm.
To allow mass determination in the range from 20 kg to 5 t in the future to be performed in the same building, the new 50 kg measuring facility was established together with a volume and susceptibility measuring arrangement as well as a crane facility in immediate vicinity to the PTB's 5 t balance. This avoids costly and time-consuming transports of the one hundred 50 kg standards. The more efficient weighing system and the considerable simplification of the operational procedures allowed a reduction by more than 60 % of the time required for the performance of regular re-calibrations of the one hundred 50 kg standards which are required within the scope of quality assurance.

Contact person:

M. Borys, FB 1.1, AG 1.11, E-Mail: michael.borys@ptb.de