The Metric System Review for Firefighter Exam Study Guide (page 2)
What Is the Metric System?
The metric system, or Système International d'Unités (SI system as it is commonly known), is the predominant system of measurement in the world. In fact, the United States is one of only about three countries that do not commonly use the metric system. The metric system attempts to eliminate odd and often difficult-to-remember conversions for measurements (5,280 feet in a mile, for example). It is a decimal-based system with standard terminology for measurements of length, volume, and mass (weight). It also uses standard prefixes to measure multiples of the standard units.
The standard unit of length in the SI system is the meter. One meter equals about 3.048 ft. The standard unit of volume is the liter. One liter is a little over a quart. Soda has been sold in 2-liter bottles for many years in the United States, one of the few areas that the SI system has penetrated our daily lives. The standard unit of mass is the gram. Technically, mass and weight are not the same, however, this distinction tends to be ignored often in practice. One thousand grams is about 2.2 pounds.
The most common prefixes used in medicine are kilo, which is 1,000 times the base unit, centi, which is 1/100 of the base unit, and milli, which is 1/1,000 times the base unit. These prefixes are attached to the base unit to give the multiple of the base unit being measured. Therefore, a kilogram is 1,000 grams. A centimeter is 1/100 of a meter. A milliliter is 1/1,000 of a liter.
Common Use in Emergency Services
The SI system is used extensively in the medical field, including emergency medicine, in the United States. Drug dosages are typically measured in either grams or milligrams. Weight (mass) is reported in kilograms. Length such as a patient's height is measured in centimeters. Volumes of fluids are measured in either milliliters or liters, but can also be measured in cubic centimeters (cc). One cubic centimeter is the same as 1 milliliter.
There have been periodic attempts to convert the United States to the SI system. For example, in the 1990s, the federal government mandated that federally funded projects such as road construction projects have to use the SI system. This mandate was reversed after a few years. It is unlikely that a wholesale change to the SI system will occur in the United States in the foreseeable future.
There are two important conversions that you should remember between English units and the SI system. They are:
1 pound = 2.2 kilograms
1 inch = 2.54 centimeters
Volume conversions are not commonly used or needed.
When making these conversions in the field, it is often helpful to make approximations or estimates. One pound is a little more then 2 kilograms, so divide the persons weight in pounds by 2 and subtract an appropriate approximation for the decimal amount. For example, 176 pounds is 80 kilograms: 176/2 = 88.You can approximate to deduct about 8 kilograms for the 0.2 decimal and subtract it to get the result, which is 80. Refer back to the section on dimensional analysis. This technique is very helpful in many of these problems.
Practice using the SI prefixes and units and conversions below:
- 300 milliliters = ____ liters
- 1,200 grams = ____ kilograms
- 350 millimeters = ____ centimeters
- 170 pounds is about how many kilograms? ____
- 65 kilograms is about how many pounds? ____
- A child who is 40 inches tall is about how many centimeters tall? ____
- 2 feet is about how many millimeters (hint: use dimensional analysis)? ____
- 77 (80 would be a good estimate) 100. 610
- 143 (140 would be a good estimate)
- 1,016 (1,000 would be a good estimate)
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