This
is a subject that really gets people confused. If a
balance reads in 0.001g (milligrams), then why do
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 as ± 0.002 g and the repeatability
as ± 0.002 g, our maximum theoretical error is ± 0.004
g.

When weighing a 200 g sample on this milligram balance,
the 0.004 g (max error) represents 0.004g ÷
200.000g, 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
0.004g ÷ 0.012g, 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.