The biggest challenge for weighing instruments is weighing small samples with reliable accuracy. Knowing the minimum weight of the instrument solves this issue. In addition, regular performance verification ensures the continuous validity of weighing results. Both minimum weight determination and performance verification are fundamentals of successful quality assurance of weighing processes.
Assured Accuracy – Determination of the Minimum Weight and Regular Calibration
Accurate results are results that fulfill quality requirements and stay within acceptable limits. In weighing, this means that the measurement uncertainty when weighing a sample should always be smaller than the required weighing accuracy. This rule is particularly important when weighing small samples, since relative measurement uncertainty increases with smaller sample sizes (figure 1). Every weighing instrument has such an accuracy threshold – the so called minimum weight. The determination of the minimum weight is the most crucial factor in the quality assurance of weighing processes. As long as the user weighs above the minimum weight, the accuracy requirements are met.
Measurement uncertainty and thus the minimum weight are affected by various factors, such as the location of the balance or its environment. Therefore, it is important that the minimum weight is assessed in regular intervals. Changes in environment and other unforeseen effects in-between two calibrations can be eliminated by applying a safety factor for the calculation of the minimum weight.
Regular Performance Verificati on of Weighing Equipment
In addition to calibration and minimum weight determination, simple user tests need to be performed at regular intervals to continuously monitor and document the weighing performance of the equipment. This ensures the validity of weighing results and minimizes unexpected deviations.
Performance Verificati on – GWP® Verificati on
To ensure the validity of weighing results, a sound calibration and control scheme for weighing equipment should be established. METTLER TOLEDO’s GWP® Verification service enables you to optimize the performance of your weighing equipment and make your operations audit-proof.
A safe and effective performance verification of weighing equipment defines:
- The accuracy limit of the instrument – the minimum weight
- The test methods and frequencies – based on risk and process requirements
- The test acceptance limits – based on accuracy and safety factor
- The recommended test weights – based on balance characteristics
GWP® Verification is available for all models and brands of weighing instruments and helps you assure accurate weighing at the lowest costs. Visit www.mt.com/na-auditproof for additional details.