Precision can be defined as the closeness in agreement of results during a series of repeat measurements under presumed identical conditions. These results are often expressed as a standard deviation. Precision is often mistaken as a measure of accuracy; it is distinctly different from accuracy, and the confusion can lead to significant regulatory violations associated with weights and measurements.
The fact is, precision between instruments or labs can be very good, but the accuracy can be off by orders of magnitude. Poorly maintained or low-quality balances can provide good repeatability, while at the same time their accuracy can be very poor. Simply put, precision describes the scatter of results achieved when repeatedly measuring the same weight.
Precision is not a factor of the true value of the weighed item and should not be used to describe accuracy. It should be noted that when measuring small quantities of a substance on an analytical balance (i.e., a few percent of the balance’s capacity), repeatability is the dominant contributing uncertainty of measurement, accounting for greater than 90% of the total uncertainty.
Accuracy can be defined as the closeness of agreement between the result of a measurement, or series of measurements, and the true value of a traceable, certified standard. Accuracy is establishing a relationship between values displayed on the balance and values known. In essence, accuracy is obtained by comparing values obtained on the balance against the value of the standard as reported on the weight’s certificate of calibration.
Having a clear understanding of the difference between precision and accuracy will allow you to maintain your weights and measurements in a compliant manner. The quality-management functionality within Mettler Toledo’s LabX 2010 Balance Excellence software contains all necessary components to completely support establishing measurement uncertainty (minimum weight) for determining the precision of your lab balance (Figure 1). LabX balance software also contains all necessary calibration and weight traceability support for determining the accuracy of your lab balance.
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?