All mechanical and electronic devices operate within tolerances,
electronic and balance scales included. Even check weights are made
within tolerances since it is not possible to make all weights
exactly the same. My electronic scale states accuracy within 2%, or
2 grams in 100, .2 grams in 10, .02 grams per gram. The key thing is
that the weights will all be relatively accurate (compared with each
other) even if they are not absolutely accurate (compared to a
standard weight). So, even a scale that weighs 2% light will weigh
all things 2% light and the weights will be accurate enough for
baking purposes.
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