Determination of the position error in automatic mass comparators

• A measurement model is developed to quantify position errors in automatic mass comparators. • The method enables correction of systematic errors without degrading measurement uncertainty. • The approach provides a robust solution for maintaining measurement accuracy in automatic mass comparators. • Validation demonstrates the method’s effectiveness for four-position mass comparator systems. In automatic mass comparators, weighing is carried out automatically without operator intervention, eliminating effects associated with manual handling of mass standards. This allows for significantly more repeatable measurements compared to those obtained manually. Comparisons are performed by automatically exchanging mass standards on the weighing pan, with the standards typically placed on turntables or magazines. Automatic mass comparators may be affected by a particular systematic error inherent to the weighing system, where the measured mass difference between two mass standards depends on their placement position within the turntable or magazine. This effect is commonly referred to as “position error”. This error can increase over time and lead to significant measurement errors if not properly evaluated, making regular comparator assessment a recommended practice. The evaluation of such errors is particularly challenging when the number of positions is large, and methods for assessing this effect are poorly documented in the literature or calibration guides. This paper proposes a model to determine these position errors by performing comparisons between mass standards placed at different positions, and applying the weighted least squares method for their estimation. The proposed method provides a practical solution for correcting position errors, leading to improved measurement accuracy and reliability in automatic mass comparators.