chpt 6

Systems of Measurement (Romich, Chapter 6)

I. Systems Used in the US

  • Metric System

    • Not the official system of weights and measurements.

    • Commonly used in science, medicine, and technology.

    • Base units:

      • Length = meter (m)

      • Volume = liter (L)

      • Weight (technically mass) = gram (g)

    • Based on factors of 10 with prefixes indicating size:

      • Micro = 1/1,000,000

      • Milli = 1/1,000th

      • Centi = 1/100th

      • Kilo = 1,000 units

  • Household System (based on the Imperial (British Empire) system)

    • Familiar to most people in everyday use.

      • Common units include teaspoon, cup, fluid ounce, etc.

    • One household unit still used in medicine is the drop (gt) (drops = gtt).

    • Not precise/standardized.

    • Base units:

      • Length = inch (in)

      • Volume = gallon (gal)

      • Weight = pound (lb)

  • Apothecary System

    • Mainly used by pharmacists.

    • One unit still in use is the grain (gr).

      • Derived from a wheat grain.

      • Approximately 60 mg.

      • Some medications like phenobarbital may still be expressed in grains.

II. Converting Between (or Within) Systems

  • Conversion Factor:

    • The relationship between two units.

    • Examples:

      • Imperial to metric: 1 inch = 2.54 cm.

      • Metric to metric: 1 L = 1,000 mL.

      • Also relevant for concentrations or sizes of tablets/capsules.

        • Example: Clavamox drops = 62.5 mg/ml (1 ml = 62.5 mg).

        • A 50 mg tablet or capsule can be expressed as 1 tablet = 50 mg.

  • Dimensional Analysis

    • Employing conversion factor(s) for calculations.

    • Ensure that the unit desired is positioned “on top” (numerator) for accuracy.

    • Canceling units:

      • Units that are “on top” of one another cancel out (e.g., 1200/1200 = 1).

  • Converting Within the Metric System

    • When using the same units, divide or multiply by factors of 10:

      • To go from smaller unit to larger: divide.

      • To switch from larger unit to smaller: multiply.

      • Can utilize dimensional analysis or simply move decimal places.

        • Dividing = move decimal left.

        • Multiplying = move decimal right.

      • For different units (e.g., mg to ml):

        • Same rule applies, but ensure units are compatible for mathematical operations.

        • Use conversion factors as necessary (refer to II. A.3).