As we discussed in our previous blog that the Calibration of a weighing balance is a critical process to ensure accurate and precise measurements. The selection of the weights used in calibration is an important factor that can greatly affect the accuracy of the calibration results. Weights are divided into E1, E2, F1, F2, M1, M2 &M3 accuracy classes based on their maximum permissible errors.
Now the question is Out of so many accuracy classes and capacities which nominal weight of which accuracy class is suitable for the verification of a particular weighing balance?
Before jumping to the solution we need to understand why a weighing balance required calibrated weights for verification. The answer is simple weighing balance is an electronic device that may lack in performance from time to time, also the performance of an electronic balance depends on many parameters such as environmental conditions, as environmental conditions remain unstable so the performance of a weighing balance, for accurate routine weighing it is necessary, that a weighing balance is verified by using a calibrated weight of known mass value, before the use.
The selection of a weight manufacturer or calibration laboratory for weight procurement or recalibration of weights is of the same importance as the overall accuracy of a weighing balance is to be derived from the weight itself. For the selection of weights, the following points are required to consider.
Accuracy class: The weight class is a measure of the accuracy of the weight. The weight class is usually indicated on the weight box itself and is based on the accuracy of the weight compared to the nominal weight. The weight class should be appropriate for the resolution and accuracy of the balance being calibrated.
Material: The material of the weight can affect the accuracy of the calibration. Calibration weights made of austenitic stainless steel, having low magnetic properties and high corrosion resistance are best.
Traceability: The weights used for calibration should be calibrated from a reputed laboratory and should be traceable to a national or international standard. This ensures that the weights have been calibrated to a known and recognized standard and can be trusted to provide accurate results.
Selection of Accuracy class:
“In principle, the standard weights or standard masses used for the type examination or verification of an instrument shall meet the metrological requirements of OIML R 111. They shall not have an error greater than 1/3 of the maximum permissible error of the instrument for the applied load. If they belong to class E2 or better, their uncertainty (rather than their error) is allowed to be not greater than 1/3 of the maximum permissible error of the instrument for the applied load, provided that the actual conventional mass and the estimated long-term stability is taken into account.” - OIML R 76-1: 2006 (E) 3.7.1
Selection of weight range:
The selection of the nominal value of weight is based on the test to be performed and the maximum and minimum capacity of the weighing instrument.
For the repeatability test, two weights one of maximum load capacity of weighing balance and one of half of the maximum capacity of the balance are required, for example, to perform a repeatability test for a 100g capacity weighing balance weights required is one 100g and one 50g.
For the eccentricity test a load recommended by the manufacturer of the balance if it is not known, or a load between one-third and half of the maximum capacity of the balance may be used, the basis of selecting a weight for the eccentricity test is that surface area of the weight should be between one third and half of the pan area.
For the departure of indication from the nominal value or the linearity of the weighing balance, the weight range should be sufficiently and equally spaced over the full ranges of the weighing balance. Usually, a minimum of ten such points are taken.
The hysteresis test is carried out between the mid-range to the full range of the weighing balance; two weights of approx. half of the balance capacities are to be used, to make the capacity of the full range of the weighing balance.