Introduction to Minimum Weight
All measurement devices including balances are subject to measurement uncertainty, which represents the variance of measured values by an instrument or the degree to which the measured value deviates from the true value (i.e. the accuracy of the device). A balance’s readability does not necessarily equal its accuracy (although typically a balance with a smaller readability will offer greater accuracy than a balance with a larger readability (for example, a balance capable of reading by 0.1mg will be more accurate than a 1mg balance).

In the beginning of the weighing range the main source of error or uncertainty is the non-repeatability of the balance, or the inability of a balance to provide the same result for repeated measurement of the same load under the same measurement conditions. This error is both measureable (typically the result of taking the standard deviation of 10 measurements) and absolute, in that it is not dependent on or relative to the weight of the sample being measured. Therefore as the weight of the sample increases, the relative uncertainty will decrease.

By observing a minimum weight a maximum relative uncertainty can be ensured for that weight and all heavier weight values. The value of this minimum weight is dependent on the required level of accuracy required for a particular application. Adopting a minimum weight requirement is a good weighing practice to ensure that your samples are being weighed according to your acceptable criteria for accuracy.

A balance’s weighing performance is a product of the quality of the weighing instrument itself and the environment and manner in which it is used. Therefore the necessary performance measurements (i.e. repeatability) must be taken after the balance is installed in the location where it will be used. The table below is a reference that can help select the proper balance for their application.

USP Minimum Weight
USP (United States Pharmacopeia) section 41 “states the requirements for balances used for materials that must be “accurately weighed”. Unless otherwise specified, when substances must be “accurately weighed”, the weighing should be performed using a balance that is calibrated over the operating range and meets the requirements defined for repeatability and accuracy.” This is accomplished by defining and requiring the observance of a minimum weight standard as defined in USP Chapter 1251. It is important to note that while this particular guideline is only relevant for pharmaceutical laboratories which must be in compliance with USP standards.

USP standards state that a balance must have a repeatability that is equal to or less than 0.10% (denoted as ‘U’ for Uncertainty) of the smallest net weight that the users plan to use on the balance. Furthermore as balance performance can change over time as a result of environmental changes etc. a ‘coverage factor’ (K) is defined to provide an additional margin of safety in the event that the balance performance changes slightly.

Typical Minimum Weight - uses an average repeatability result from tests done during mass production to calculate the minimum weight for the conditions (U, K) noted.

Maximum Minimum Weight - uses the balance family’s maximum tolerance for standard deviation of repeatability as defined in the product data sheet to calculate the minimum weight for the conditions (U, K) noted. 
 

Product	Readability	Typical Minimum Weights		Maximum Minimum Weights
		(U=0.1%, K=2)	(U=1%, K=2)	(U=0.1%, K=2)	(U=1%, K=2)
Explorer	0.01 mg	12 mg	1.2 mg	20 mg	2 mg
	0.1 mg	120 mg	12 mg	200 mg	20 mg
	1 mg	1.2 g	120 mg	2 g	200 mg
	0.01 g	12 g	1.2 g	20 g	2 g
	0.1 g	120 g	12 g	200 g	20 g
Adventurer	0.1 mg	140 mg	14 mg	200 mg	20 mg
	1 mg	1.4 g	140 mg	2 g	200 mg
	0.01 g	14 g	1.4 g	20 g	2 g
	0.1 g	140 g	14 g	200 g	20 g
Pioneer	0.1 mg	200 mg	20 mg	200 mg	20 mg
	1 mg	2 g	200 mg	2 g	200 mg
	0.01 g	20 g	2 g	20 g	2 g
	0.1 g	200 g	20 g	200 g	20 g

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