We are having some issues with balance minimum weights (smallest net weights). The problem is with balances where the number of decimal places (readability) can be selected by the user. Guidance we have gotten from USP indicates that each readability setting (for example to 0.0001 g, coarse range, and 0.00001 g, fine range, for an analytical balance) should have a separate minimum weight established. Since a standard deviation of replicate weighings is used to set the minimum weight, and since there is a minimum standard deviation based on the balance readability, the minimum weight will, more than likely, be different for the 4 and 5 place settings. The balance manufacturer, who is also performing the USP General Chapter 41 calibration for us (which includes the minimum weight determination), has performed minimum weight determinations for us for both readability settings, but now seems to be balking at using the USP minimum standard deviation for the coarse range and is using the value for the fine range which is a factor of 10 less than what is calculated for the coarse range.

These balances are not what used to be called Deltarange balances, where the readability would automatically change when the weight reached a specified value. These allow the user to select the balance readability from zero.

While the balances in question are using the same load cell for both fine and coarse ranges, it finds stability faster for the coarse range than the fine. So it would seem that there is something different in the balance operation for the two ranges. It would also seem that if the calibration paperwork states USP General Chapter 41, it should be done in accordance with USP's interpretation of what is needed.

Has anyone else dealt with this issue? Thanks for any assistance / comments.