Converting Within and Between Systems

CHAPTER 8


Converting Within and Between Systems



Objectives


After reviewing this chapter, you should be able to:


1. Convert a unit of measure to its equivalent within the same system


2. Convert approximate equivalents among the metric, apothecary, and household systems of measure



EQUIVALENTS AMONG METRIC, APOTHECARY, AND HOUSEHOLD SYSTEMS


As noted in earlier chapters dealing with the systems of measure, some measures in one system have equivalents in another system; however, equivalents are not exact measures, and there are discrepancies. Several tables have been developed illustrating conversions/equivalents. Sometimes drug companies use different equivalents for a measure. As mentioned previously, a common discrepancy is with grains, which is an apothecary measure. Some sources indicate 60 to 65 mg = gr 1, but remember that 60 mg = gr 1 is used in most medication administration. However, 65 mg = gr 1 is the best conversion factor for some common oral medications, notably aspirin, Tylenol, and iron.



In the health care system it is imperative that nurses be proficient in converting among all three systems of measure (metric, apothecary, household). Nurses are becoming increasingly responsible for administration of medications to clients and teaching clients and family outside of the conventional hospital setting (e.g., home care). Nurses have become more involved in discharge planning and are responsible for ensuring that the client can safely self-administer medications in the correct dosage. Table 8-1 lists some of the equivalents between systems. Memorize these common equivalents!





CONVERTING


The term convert means to change from one form to another. Converting can mean changing a measure to its equivalent in the same system or changing a measurement from one system to another system, which is called converting between systems. The measurement obtained when converting between systems is approximate, not exact. Thus certain equivalents have been established to ensure continuity.


It is important that nurses be able to make conversions because they are often called on to convert medication dosages among metric, apothecary, and household systems. The nurse therefore must understand the systems of measurement and be able to convert within the same system and from one system to another with accuracy.


Before beginning the actual process of converting, the nurse should remember the following important points that can make converting simple.




NOTE


Equivalents indicated in this table are those used most often. The accurate and preferred term for liquids is mL.




METHODS OF CONVERTING


Moving the Decimal Point


Moving the decimal point is discussed in Chapter 6. Because the metric system is based on the decimal system, conversions within the metric system can be done easily by moving the decimal point. This method cannot be applied in the apothecary or household system because decimal points are not used in either system. Remember the two rules in moving decimal points:



RULE


To convert a smaller unit to a larger one, divide or move the decimal point to the left.






Example:

0.85 L=(larger)___mL(smaller)image



Solution:

After determining that L is the larger unit and you are converting to a smaller unit (mL), recall the conversion factor that allows you to change liters to milliliters (1 L = 1,000 mL). Therefore 0.85 is multiplied by 1,000 by moving the decimal point three places to the right, indicating 0.85 L = 850 mL.


image


Note the addition of a zero here to allow movement of the decimal point the correct number of places.



image Practice Problems


For additional practice in converting by decimal movement, convert the following metric measures to the equivalent units indicated.



Answers on p. 607



Rather than remembering the right to left rule or left to right rule, you might find it easier to use ratio and proportion as a method of conversion.



Using Ratio and Proportion


Using ratio and proportion is one of the easiest ways to make conversions, whether within the same system or between systems. The basics on how to state ratios and proportions and how to solve them when looking for one unknown are presented in Chapter 4. To make conversions using ratio and proportion, a proportion must be set up that expresses a numerical relationship between the two systems. A proportion may be written in colon format or as a fraction when making conversions. Regardless of the format used, there are some basic rules to follow when using this method.



RULE


Rules for Ratio and Proportion






When using the method of ratio and proportion to make conversions, as with any method used, the known equivalents must be memorized. Stating the proportion in the fraction format may be a way of avoiding confusion with the terms (means and extremes). However, regardless of the format used, the terms must correspond to each other in value and have a relationship. Division should always be carried at least two decimal places to ensure accuracy.




Example:

8 mg = __________ g







CONVERTING WITHIN THE SAME SYSTEM


Converting within the same system is often seen with metric measures; however, it can be done using the other systems of measurement, such as one apothecary measure being converted to an equivalent within the apothecary system. Any one of the methods discussed can be used, but movement of decimal points is limited to the metric system. Ratio and proportion can be used for all systems.





DIMENSIONAL ANALYSIS


Dimensional analysis is a conversion method that has been used in chemistry and other sciences and will be discussed in more detail in Chapter 16. Dimensional analysis involves manipulation of units to get the desired unit. This method can be used for conversion in all systems. As with other methods discussed, you must know the conversion factor (equivalent).


Steps:


Related posts:

  1. Apothecary and Household Systems
  2. Heparin Calculations
  3. Reading Medication Labels
  4. Calculation of Oral Medications

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Feb 11, 2017 | Posted by in PHARMACY | Comments Off on Converting Within and Between Systems

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