All of the balances described must be used within a degree of error that can be tolerated in prescription compounding and in pharmaceutical manufacturing.
For example, the USP allows a maximum error of 5% in a single weighing operation. Since the sensitivity requirement of a balance represents the absolute error in using that balance, the percent error will depend on the amount of drug weighed and will increase as the amount of drug decreases.
The Minimum Weighable Quantity (MWQ) with no more than 5% error can be calculated for any balance knowing the sensitivity requirement (SR) (i.e., the absolute error) from the following:
MWQ = SR (mg) × (100% ÷ % error)
Examples:
1. Calculate the MWQ with no more than a 5% error for a balance with a sensitivity requirement of 10 mg.
MWQ = 10 mg × (100%/5%) = 200 mg
2. Calculate the MWQ with no more than a 3% error for a balance with a sensitivity requirement of 2.5 mg.
MWQ = 2.5 mg × (100%/3%) = 83.3 mg
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