Sensitivity Requirement

A “Class A” (also referred to as class III) balance is commonly used for weighing drugs and other pharmaceutical substances required in prescription compounding. All pharmacies are required by law to have one. This balance is a torsion-type balance, has two pans and uses internal and external weights. It requires use of external weights for measurements exceeding 1 gram.

All balances, including the class A prescription balance have a point below which they cannot measure accurately. A class A prescription balance can measure amount of a drug down to 6 milligrams. More sensitive electronic balances can measure as little as 100 micrograms. The cutoff weight below which a balance cannot measure accurately is termed as it’s sensitivity requirement (SR) . In other words, SR is defined as the minimum weight that causes the pointer of a balance to move by one division. A class A prescription balance must have a sensitivity requirement (SR) of 6 mg or less.