It is absolutely vital for Hospital Corpsmen to thoroughly understand the principles and applications of metrology in pharmacy. Errors in this area endanger the health-even the life-of the patient.

The Metric System
The metric system is the official system of weights and measures used by Navy Pharmacy Departments for weighing and calculating pharmaceutical preparations. The metric system is becoming the accepted system throughout the world. Hospital Corpsmen need to be concerned primarily with the divisions of weight, volume, and linear measurement of the metric system. Each of these divisions has a primary or basic unit and is listed below:

Basic unit of weight is the gram, abbreviated "g"
Basic unit of volume is the liter, abbreviated "l"
Basic linear unit is the meter, abbreviated "m"
By using the prefixes deka, hecto, and kilo for multiples of, respectively, ten, one hundred, and one thousand basic units, and the prefixes micro, milli, centi, and deci for one-ten thousandth, one-thousandth, one-hundredth, and one-tenth, respectively, you have the basic structure of the metric system. By applying the appropriate basic unit to the scale of figure 6-1, you can readily determine its proper terms. For example, using the gram as the basic unit of weight, we can readily see that 10 g equals 1 dekagram, 100 g equals 1 hectogram, and 1000 g is referred to as a kilogram. Conversely, going down the scale, 0.1 g is referred to as a decigram, 0.01 g is called a centigram, and 0.001 g is a milligram.

The Apothecary System
Although fast becoming obsolete, the apothecary system for weighing and calculating pharmaceutical preparations is still used and must be taken into consideration. It has two divisions of measurement: weight and volume. In this system, the basic unit of weight is the grain (abbreviated "gr"), and the basic unit of volume is the minim (abbreviated "m").

The Avoirdupois System
The avoirdupois system is a system used in the United States for ordinary commodities. The basic units of the avoirdupois system are dram (27.344 grains), ounce (16 drams), and pound (16 ounces).

Table of Weights and Measures
See table 6-1, a table of weights and measures; study it thoroughly.

Converting Weights and Measures
Occasionally, there are times when it will be necessary to convert weights and measures from one system to another, either metric to apothecary or vice versa. Since patients can hardly be expected to be familiar with either system, always translate the dosage directions on the prescription into a household equivalent that they can understand. Household measurements are standardized, on the assumption that the utensils are common enough to be found in any home. Table 6-2 is a table of household measures, with their metric and apothecary equivalents.

CAUTION: For the conversion of specific quantities in a prescription or in converting a pharmaceutical formula from one system to another, exact equivalents must be used.