This is a subject that really gets people confused. If a balance reads in 0.001 g (milligram) , then why did the manufacturers put “Minimum Load” on the data sheet at several milligrams. Why not a milligram?
Please note: this article is describing a situation common to all balances. We are not criticizing any particular balance or manufacturer.
All balances have slight errors mentioned in their performance rating and this means that your results will therefore vary. For instance, if we read that the linearity error (See our post on Linearity) as ± 0.002 g and the repeatability as ± 0.002 g, our theoretical error is ± 0.004 g if the stars were all lined up against us.
When weighing a 200 g sample on this milligram balance, the 0.004 g (max error) represents 200.000 g ÷ 0.001 g which is 0.002% error, perhaps very acceptable for most people (note: these errors quoted are maximized to provide clarity). If however, we attempt to weigh 12 mg or 0.012 g on the same balance, the repeatability and linearity errors are the same whatever the value the balance reads, but the percentage of error will increase to where it is no longer acceptable. The math now reads 0.012 g ÷ 0.004 g which as we can all see is 33% potential error. Remember of course that this is a worst case scenario, but even so, just 50% of this type of error could seriously affect your results.
There is a simple rule of thumb that says that to weigh 1 milligram of sample, you should be using at least a 0.0001 g (Four Place) balance. Even this may not be sufficient. We recommend a minimum load on our 4 place balances as 10 milligram (mg). Or 0.0100 g to be sure of your readings.
To show you the seriousness of the problem, consider the pharmaceutical arena . The Minimum Sample Quantity (MSQ) is defined in USP 41 and USP 1251 for a milligram balance as 0.0820 g. This practice (U.S Pharmacopeial Convention) sets standards for the strength and quality of medicines and food ingredients worldwide. It is designed to ensure that weighing is conducted accurately. Although you may not be weighing medicines, the very fact that there is such a strong standard existing, shows you that there is a serious problem with weighing too small a sample on too large a balance.
We hope this post is of help to you. If you ever want to discuss balance or scale accuracies, or balance do’s or don’ts, please call or email us and one of our staff will try to help you.
Thanks to Mike Tyce for the correction