Students of pharmacy must first learn the technical operations in the manipulation of balances and weights. This proves the importance of
metrology. Metrology is a science that deals with the study of measurement as applied to extension, volume and weight. In the study and general practice of pharmacy both the avoirdupois and apothecaries systems of weight measurements are used in the handling of medicines. To avoid confusion the pharmacist must clearly establish in mind that avoirdupois system of weight is used to buy and sell all goods other than on prescription; and apothecary system is used to dispense prescription and sometimes to compound medicines.
One drop is often considered as equal to about one minim, but the
size of drops varies with characteristic of the liquid, the
temperature and the surface from which it is dropped. In order to
promote accuracy and uniformity of dosage, the USP specifies that a
medicinal dropper shall have at its delivery end an external diameter
of 3mm and deliver 20 drops of distilled water, weighing one gram, at
a temperature of 15
oC. This standard medicine dropper must
be accurate to within 10 percent.
