By: Fred Algieri Jr., Atlantic Scale Company
USP Chapter 41 has updated their testing requirements to determine the suitability of balances. Here is a quick reference on the important aspects and updates:
Chapter 41 is a REQUIREMENT for QC analysis measurements.
There are two required tests in Chapter 41, Accuracy and Repeatability. Both tests have tolerances of 0.10%.
The Accuracy requirement is applicable between 5% and 100%.
Calibration weights being used in the Accuracy check must have an accuracy of NMT 1/3 of the balance tolerance for that respective test point. This can be the tolerance of the nominal value of the weight or it can be the uncertainty of measurement for the weight when applying the actual weight value from the certificate.
The Repeatability test consists of 10 replicate weighings. The resulting standard deviation is to be used in the following calculation:
(2 × SD) ÷ Desired Minimum Sample Weight ≤ 0.10%
The coverage factor is now 2. Previously it was 3.
The denominator in the calculation is now stated as the “Desired Minimum Sample Weight”, not the weight applied as previously stated.
There is also now a “standard deviation floor” of 0.41d (d being the scale interval or the readability of the unit). This means if the determined standard deviation from the replicate test is less than the 0.41d value then the 0.41d value will need to be used in the assessment calculation.
The 0.41d (standard deviation floor) limits the low end measuring range to the below:
- 8.2 mg minimum sample weight (lowest possible) 5-place (0.00001 g) analytical balances
- 82 mg minimum sample weight (lowest possible) 4-place (0.0001 g) semi-analytical balances
- 820 mg minimum sample weight (lowest possible) 3-place (0.001 g) toploading balances
- 8.2 g minimum sample weight (lowest possible) 2-place (0.01 g) toploading balances
Again, the above values are the lowest possible minimum weights. The determined standard deviation needs to be at or lower than the 0.41d value to obtain the above results. This usually requires a very good environment. Most test results for on-site repeatability testing is as follows:
- 10 mg – 20 mg minimum sample weight (commonly determined) 5-place (0.00001 g) analytical balances
- 100 mg – 200 mg minimum sample weight (commonly determined) 4-place (0.0001 g) semi-analytical balances
- 1 g – 2 g minimum sample weight (commonly determined) 3-place (0.001 g) toploading balances
- 10 g – 20 g minimum sample weight (commonly determined) 2-place (0.01 g) toploading balances
Successful repeatability testing mostly depends on the balance quality, the controlled environment it is in, and also the balance operator’s dexterity and experience in performing the repeatability test.