Least Weighable Amount and Percentage Error

If you need to weigh less than the Least Weighable Amount, you would not be able to weigh the amount with sufficient accuracy: the error in measurement would be greater than the maximum allowed.
You would have to revise your procedure or use a different instrument.

If you need to weigh more than or the same amount as the Least Weighable Amount, you can weigh the amount with sufficient accuracy.

Example

A balance is accurate to 10 mg. A procedure calls for a maximum error of 4 %. You need to weigh 200 mg of substance A and 300 mg of substance B. Can you weigh the substances using this balance and this procedure?

The Least Weighable Amount of the balance with 4 % maximum error is:

Percentage Error =                  Error                  x 100 %
Least Weighable Amount

LWA =   10   x 100 mg = 250 mg
4

You need to weigh 200 mg of substance A. This is LESS than the LWA. Therefore you CANNOT weigh A sufficiently accurately. In fact its error would be: 10/200 x 100 % = 5 %. This is higher than the maximum allowed, 4 %.

You need to weigh 300 mg of substance B. This is MORE than the LWA. Therefore you can weigh B with sufficient accuracy. The error for weighing B would be: 10/300 x 100 % = 3.3 % which is less than the maximum error of 4 %.

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