Balance minimum weight calculator (USP <41> and <1251>)

How the calculation works

1. 01Repeatability is satisfactory when twice the standard deviation of replicate weighings, divided by the net weight, does not exceed the required accuracy: 2s / m ≤ 0.001 for the 0.10 % in the compendial text. Rearranged, m_min = 2s / 0.001 = 2000 s.
2. 02USP <41> replaces s with 0.41 d when s is smaller than 0.41 d, where d is the readability, because a balance cannot be more repeatable than its display can resolve however favourable ten readings were.
3. 03The compendial method uses at least ten replicate weighings of a test weight; pasted data are checked for that count, and a standard deviation you already have can be entered directly.

Limitations

• A minimum weight belongs to a balance in its working location. Draughts, bench vibration and an unlevelled pan raise s, and it should be remeasured after the balance moves.
• It uses the standard deviation of the test weight, which should be near the load you will actually weigh, not an unrelated one.
• 0.10 % is the compendial figure. Your own process may require a different accuracy, which can be entered; whether it is acceptable is your quality system's decision.

Frequently asked questions

What is the minimum weight of a balance?

The smallest net weight that can be weighed with a stated accuracy, usually 0.10 %. Below it, the balance's own repeatability is a larger fraction of the sample than the accuracy allows, so the result cannot be trusted to that standard.

How do I calculate balance minimum weight under USP <41>?

Weigh a test weight at least ten times, take the standard deviation s, raise it to 0.41 × readability if it is smaller, and calculate 2000 × s. For s = 0.0002 g that is 0.4 g.

Why is the minimum weight larger than the readability suggests?

Because readability is only the display step. Real repeatability, from air movement and the weighing system itself, is usually several times larger, and the minimum weight follows the repeatability, not the last digit.

How often should minimum weight be determined?

When a balance is installed or moved, after repair, and at intervals set by your quality system. It is also commonly re-checked when the environment changes enough to affect repeatability.