Established Regulatory Requirements
A standard that is used by a huge variety of industries around the world, ISO 9001, states “Measuring equipment shall be calibrated or verified at specified intervals…against measurement standards traceable to international or national measurement standards.”
Another important regulation often requiring compliance, 21 CFC pat 211.68 (a), which also affects weighing operations, states, “Automatic, mechanical or electronic equipment…shall be routinely calibrated, inspected or checked according to a written program designed to assure proper performance.” USP section 41 also succinctly outlines measurement uncertainty requirements by stating “measurement uncertainty is satisfactory if three times the standard deviation of not less than 10 replicate weighings divided by the amount weighed, does not exceed 0.001 (0.1%).”
What is measurement uncertainty? VIM defines measurement uncertainty as “a parameter characterizing the dispersion of the quantity values being attributed to a measurement.” When the repeatability test is executed, you can then establish the minimum weight for each balance, regardless of readability. The minimum weight is the lowest amount of sample mass that can be weighed on a balance with a high degree of confidence while complying with the required weighing accuracy.
Many environmental influences affect measurement uncertainty, including: temperature, airflow, electrostatics, vibration, and magnetism. As a result, measurement uncertainty must be established on the balance in situ. Once a minimum weight is established, it must be monitored and controlled. So how do you know you are minimizing your weighing risk?