 What is minimum weight and why is repeatability so important

Weighing is the most commonly performed task in a laboratory. For almost every research process or analysis, something needs to be weighed, whether it is a single sample for further processing or a compound or agent used in a recipe. A balance is always needed to establish the weight of a specific substance.

The weighing topic is covered by USP chapter 41 “Balances” and further recommendations are described in chapter 1251 “Weighing on an Analytical Balance”. The regulation applies to accurate measurements focusing especially on the fact of calibrating the balance before using it for any weighing. This regulation defines also repeatability and accuracy test, including acceptance limits that are still permissible when weighing samples accurately. US Pharmacopeia describes Minimum Weight as the minimum load that is being measured within the permissible error limits. This starting point, the minimum weight value, is calculated based on the repeatability test of a minimum of 10 measurements, using the same weight. The weight in question is not specified, but a load of a few per cent of the capacity of the tested instrument is recommended. For instance, a 220g capacity balance could be tested using a 10g weight corresponding to 5% of the instrument’s capacity. Repeatability is considered satisfactory if two times the standard deviation of the weighted value, divided by the desired smallest net weight does not exceed 0,10%. Where:
SD – Standard deviation
N – Desired smallest net weight

If the standard deviation obtained is less than 0,41d, where is the readability of the instrument tested, the value should be replaced by 0,41d. In such a case, the repeatability is satisfactory if two times 0,41d divided by the desired smallest net weight does not exceed 0,10%.

The minimum weight value, being the starting point of operating range of the balance can be determined the following way: Where:
SD – Standard deviation
N – Desired smallest net weight

If SD is less than 0,41d it should be replaced by 0,41d and the final minimum weight value is calculated as follows: This means that the minimum weight of a balance with good repeatability starts at 820d, where d is the readability.

This defines the optimal minimum weight, a starting point from which any weighing will comply with the USP requirement.

 Balance’s readability d (g) Optimum minimum weight (820d) 0,01mg 0,00001 0,00820g 0,1mg 0,0001 0,0820g 1mg 0,001 0,820g 0,01g 0,01 8,20g 0,1g 0,1 82,0g

It is the ideal situation, however the reality might show that SD is greater than 0,41d,  and in such case, the minimum weight value calculated for a particular device might differ from the optimum shown in the table above.
The weight used for the accuracy test is suitable if its nominal mass falls between 5% and 100% of the balance’s capacity. Accuracy of such a balance is considered satisfactory if the results are within 0,10% of the test weight value and the MPE (maximum permissible error) of the test weight should not exceed one-third of this 0,10%. This means that only calibrated weights can be used for performing such measurement and class of the weight that fits must be checked accordingly. OIMLR111-1: 2004 (E) regulation, chapter 5.2.3. Table 1 is a great tool to select the right class of the weight meets the accuracy requirement above.

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