What Is Minimum Weight and How Accurate Weighing Can Be Ensured
The “minimum weight” refers to the minimum quantity of a sample required when the measurement uncertainty(potential error) of the balance used is taken into account.
If the sample quantity to be weighed is too small, uncertainty will take up too large a portion of the weighedvalue, and consequently make the result of the analysis unreliable. Therefore, if a balance has the minimum
weight set to it, operators must pay attention so that the sample quantity (net weight excluding the tare) does
not fall below the minimum weight.

USP
In the pharmaceutical industry, the general chapters (GC) <41> and <1251> of the United States Pharmacopoeia(USP) are widely recognized as standards for managing balances, and how a minimum weight is calculated isalso stipulated in these chapters.
According to the GC <41> “Balances”, a balance used for samples that necessitate accurate weighing must be
calibrated over the operating range and meet the following requirements defined for repeatability andaccuracy:

Repeatability requirements
1) Repeatability is assessed by measuring one test weight not fewer than 10 times.
2) Repeatability is satisfactory when two times the standard deviation (S.D.) of the weighed values, divided
by the desired smallest net weight (the smallest sample quantity that users actually intend to measure withthe balance), does not exceed 0.10%.
These conditions can be expressed as:(2 × S.D. of not fewer than 10 repeated weighing) / smallest net weight ≤ 0.10%
In other words, the following can be said:

Minimum weight ≥ 2000 × Repeatability (S.D.)
Further,3) When the S.D. obtained is smaller than 0.41d (d = scale interval), that S.D. is replaced with 0.41d.
The reason why the minimum weight is determined from repeatability is that when the measurement
quantity is small, repeatability becomes dominant as uncertainty component (sensitivity, non-linearity andeccentricity can be disregarded).



Repeatability is not greatly affected by the value of the test weight. Accordingly, when determining theminimum weight, it is unnecessary to use a weight that is close to the expected minimum weight (i.e. toosmall and difficult to handle). On the other hand, if the weight is excessively large, there rises the risk ofeccentricity and other factors that affect the measurement. A few percent of the balance capacity is therefore
considered appropriate.

Repeatability can vary depending on the day-to-day environmental conditions (draft, vibration,temperature/humidity changes, etc.) and the weighing skills of operators. It is thus recommended that thelower limit of the actual sample quantities to be measured (i.e. smallest net weight) be sufficiently, not justbarely, above the set minimum weight.

Accuracy requirements
1) Using a suitable weight(s), accuracy is satisfactory when the measured value does not deviate by more than
0.10% of the test load value.
2) A suitable test weight has a mass between 5% and 100% of the balance capacity.
3) The maximum permissible error, or alternatively the calibration uncertainty, of the test weight is not morethan 1/3 of the applied test limit of the accuracy test (0.10% of the test load value).

Examples of minimum weights (for reference)

Readability (d)0.001 mg

0.01 mg

Model

Specifications

Minimum weight (typical)*2

Capacity

Repeatability (S.D.)*1

BA-6TE

6.2 g

1 μg (for 1 g)

1.8 mg

BA-6E

6.2 g

1 μg (for 1 g)

1.8 mg

BM-5

5.2 g

1.2 μg (for 1 g)

2.0 mg

BM-20

22 g

2.5 μg (for 1 g)

3.0 mgBA-225TE

220 g

0.015 mg (for 50 g)

20 mg

BA-225220 g

0.015 mg (for 50 g)

20 mg

BM-252250 g

0.03 mg (for 100 g)

20 mg

GH-252101 g (fine range)

0.03 mg

24 mg

GH-20251 g (fine range)

0.02 mg

24 mg

HR-202i51 g (fine range)

0.02 mg

24 mg

GR-20242 g (fine range)

0.02 mg30 mg

0.1 mg0.001 g0.01 gBM-500520 g

0.2 mg

120 mgGH-200220 g

0.1 mg

120 mgGR-200210 g

0.1 mg

120 mg

GX-324AE

320 g

0.1 mg (for 200 g)

120 mg

GX-324A

320 g

0.1 mg (for 200 g)

120 mg

GF-324A

320 g

0.1 mg (for 200 g)

120 mgHR-250AZ

252 g

0.1 mg

140 mgHR-250A

252 g

0.1 mg

140 mg

GX-203A

220 g

0.001 g

1.4 g

GF-203A

220 g

0.001 g

1.4 g

FZ-200i

220 g

0.001 g

1.4 g

FX-200i

220 g

0.001 g

1.4 g

GX-2002A

2200 g0.01 g

14 g

GF-2002A

2200 g0.01 g

14 g

FZ-2000i

2200 g0.01 g

14 g

FX-2000i

2200 g0.01 g

14 g*1 Repeatability specifications in accordance with brochures/data sheets. Corrected sample standard deviation (thedenominator is n-1).
*2 Based on actual repeatability measured using a weight of not more than 5% of the balance capacity, which wasproperly calibrated (adjusted) and operating in a favorable environment.

In order to determine the minimum weight for a balance, repeatability needs to be assessed in the actualenvironment where the balance is used. For high precision balances, especially those with a readability of 0.1
mg or finer, it is important to ensure a good environment as repeatability is influenced by environmental
conditions as well as the balance settings. For repeatability (S.D.) measurement, it is recommended to use asingle weight whose mass is not more than 5% of the balance capacity but not less than 100 mg.