Measurements and Conversions
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CH 5 MEASUREMENTS AND CONVERSIONS LEARNING OBJECTIVES INTRODUCTION List the metric equivalents of 1 fluid ounce, During and after the Renaissance, the study of science and 1 tablespoonful, and 1 teaspoonful. medicine began in earnest. At that point in history, no consistent, international system of weights and measures List the metric equivalent of 1 pound, 1 ounce, existed. Measurements of mass (usually referred to as and 1 grain. weight) were based on a commonly weighed material, such Convert a patient’s weight in pounds to as a grain of wheat or a penny. Different systems of measure- kilograms or in kilograms to pounds. ments were used for different applications. For example, Explain why it is important for technicians to be people who worked in trades such as carpentry or smithery completely comfortable using and converting used a different system of measurement than jewelers or to the metric system. apothecaries. An apothecary was a member of the healing arts who was the predecessor of the modern pharmacist Use dimensional analysis to convert between units. and a specialist in the preparation of medicines and reme- dies. Apothecaries dispensed remedies, performed surgery, and assisted in childbirth. To prepare remedies consistently, OUTLINE apothecaries needed a systematic method for weighing and measuring ingredients. INTRODUCTION The apothecary system of weights and measures was related to the Roman system of measurement. This system HOUSEHOLD AND AVOIRDUPOIS WEIGHTS was based on the grain (gr) as a measurement of weight and AND MEASURES the fluid ounce as a measurement of volume. There are some Units of weight and volume similarities among apothecary measurements, common Measuring doses accurately household measurements, and avoirdupois measurements Solving problems using household measurements (the system of weights and measures historically used in the United States and Great Britain) but also some significant CONVERTING BETWEEN SYSTEMS OF differences. The apothecary pound, for instance, is divided MEASUREMENT into 12 oz, but the avoirdupois pound is divided into 16 oz. Conversion factors The dry ounce is a measure of weight. To avoid confusion, Problems that require conversions between systems this text discusses conversions for ounces and pounds from DOI 10.37573/9781585286393.005 37 38 PHARMACY CALCULATIONS: AN INTRODUCTION FOR PHARMACY TECHNICIANS TABLE 5-1. Conversion Factors of Volume TABLE 5-2. Common Household in the Apothecary System Measurements of Volume and Weight Pint Fluid Ounce Fluid Dram Measurements of Volume 1 pt 16 fl oz 128 fl dr 1 gal 4 qt 8 pt 16 c 128 fl oz 256 T 768 tsp 1 fl oz 8 fl dr 1 qt 2 pt 4 c 32 fl oz 64 T 192 tsp fl = fluid; pt = pint. 1 pt 2 c 16 fl oz 32 T 96 tsp 1 c 8 fl oz 16 T 48 tsp the household and avoirdupois systems of measurement only. 1 fl oz 2 T 6 tsp The minim, fluid dram (fl dr), and fluid ounce are the measure- ments of volume in the apothecary system. The word “fluid” 1 T 3 tsp indicates a measure of volume. For cases in which the apoth- ecary system is used to measure volume, symbols may repre- Measurements of Weight sent these units. Table 5-1 shows the conversion factors in the 1 lb 16 oz apothecary system. Although the apothecary system of measurement was the standard for the practice of pharmacy into the 20th century in the United States, it was officially replaced by the metric system in 1971. Use of apothecary measurements is discouraged because they are not universally recognized. It is not unheard of, however, for older physicians to write prescriptions using the apothecary system. To be adequately prepared for work in various pharmacy practice settings, students must be familiar with the more common apothe- cary measurements. HOUSEHOLD AND AVOIRDUPOIS WEIGHTS AND MEASURES Units of weight and volume Household measurements of weight (mass) are based on the avoirdupois system, which, like the apothecary system, is an antiquated system of measurement. The avoirdu- pois system is only used to measure weight, and the basic FIGURE 5-1. Volume variations in common household unit is the ounce. Household measurements of volume teaspoons include the teaspoon (tsp), tablespoon (T), fluid ounce (fl oz), cup (c), pint (pt), quart (qt), and gallon (gal). (See measuring medications, especially for children, for whom a Table 5-2.) dose variation of this magnitude can be dangerous. To prevent inaccuracy of dosing measurements, it is Measuring doses accurately important that patients have appropriate measuring devices In some instances, doses of liquid medications in outpa- in their homes and are instructed how to use them. Choosing tient prescriptions are written as teaspoons or table- the correct device to measure volume depends on the age of spoons because these measurements are more familiar to a patient. Infants do better taking medication from a cali- many people than metric units. The problem with direc- brated (marked with measurement lines) oral syringe or tions of this type is that household teaspoons and table- dropper, whereas toddlers are able to use a dosing spoon spoons vary widely in the actual volume they hold based (see Figure 5-2). Adults taking liquid medications can use a on the measuring device used (see Figure 5-1). The volume calibrated medicine cup. These devices are labeled with both of a teaspoon, for instance, can range from 4 to 7 mL. This household and metric measurements. In retail and other difference means that a person trying to measure a 5-mL outpatient pharmacies, it is good practice to include both (1-tsp) dose could get a dose that ranges from 20% less to metric and household measurements in the directions on a 40% more than desired. This outcome is unacceptable for prescription label for a liquid medication..