Quality management of your weighing process begins with selecting the right instrument. After the weighing instrument has been selected and purchased, the manufacturer or service professional will install it and perform an IQ, OQ, PQ. The purpose of this initial three-step validation process is to make sure that the weighing instrument is properly installed in a suitable environment, that its overall functionality operates in accordance with the manufacturer specifications, and that it performs adequately for its intended use. In order to ensure continued verification of the weighing instrument’s performance, routine user testing should be performed.
This routine user testing should be incorporated into your quality management system as regular performance verification checks. The goal of these performance verification checks is to detect problems in your weighing process before they occur. The frequency and nature of the tests performed can be determined through a risk-based approach. The risk-based approach allows you to utilize knowledge of your weighing process, such as your accuracy requirements, the costs of testing, and the risks associated with making an incorrect measurement, to decide which tests should be carried out more often and which ones are unnecessary based on their impact on the weighing process. At a minimum, routine performance verification checks generally include a repeatability test and a sensitivity test. Repeatability is determined by taking repeated measures of the same weight and calculating the standard deviation of the readings. This is an indication of the precision of the weighing instrument. Sensitivity is a measure of the accuracy of the weighing instrument and is determined by calculating the difference between a calibrated weight’s value and the reading displayed when that weight is put on the weighing pan. Since a weighing instrument’s sensitivity offset is proportional to the mass on the weighing pan, it is best tested with a weight of nominal value at or near the instrument’s capacity. Other tests commonly executed in performance verifications are tests of eccentricity and linearity. Eccentricity refers to the effect of loading a weight off-center of the weighing pan, and linearity refers to the deviation of readings throughout the weighing range from the ideal straight line formed between the sensitivity point near capacity and the zero-point reading. In the end, the risks and requirements of your specific weighing process will dictate which tests are performed and how often.
Many quality management systems and regulations such as ISO 9001, GLP, and GMP require routine testing to be performed; however, they do not specify which tests should be performed or how frequently. Instead, they emphasize a risk-based approach where the user decides the frequency and nature of the routine tests that will best verify the performance of their instrument to meet the requirements of their specific process and mitigate both the risks of incorrect measurements due to inadequate testing and the costs in time, money, and resources due to unnecessary testing.