Several varieties of dispensing scales and weight boxes are available. Three to four varieties of balances capable of weighing different quantities and of desired accuracies should be available in a pharmacy and a pharmacist exercises his choice for use while dispensing a prescription depending on the quantity to be weighed and accuracy desired.
The balances should be installed on a heavy, stable platform in a corner free from drift of air or fumes. The balance meant for daily use may be installed at a suitable place on the working table. The balance should be adjusted in level with the help of a spirit level and checked for the null point and oscillations of the beam before use.
While commissioning a new balance, the pharmacist should carefully read the literature supplied by the manufacturer and understand its operation and accuracy.
Quantities lesser than 50 mg of ingredients and lesser than 100 mg of potent medicaments should be weighed on an analytical balance. After testing a balance, pieces of white paper should be placed under each pan.
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The weights should be handled and picked with the help of forceps and kept in the left pan of the balance. Solid is slowly added to the right pan or removed there from using a spatula of suitable size.
Greasy and sticky substances are weighed by placing them on a greaseproof (butter or waxed) paper and then weighing using a counterpoise. After the weighing operation, the weights are rechecked while returning them to the weight box. The solid is then removed from the right pan, which is usually made of glass or stainless steel.
Corrosive and oxidizing agents should not be put directly in a stainless steel pan but weighed in a watch glass providing a suitable counterpoise in the left pan. It may be borne in mind that weighing operation, if executed without care, may lead to serious errors.