Units and Measurements

Measure wfiat is measuraBCe and make measurakCe what is not so. 

This is US Paralympian Marlon Shirley competing in the men s F44 long jump final at the IPC 2002 Athletics World Championships. 

A measurement is always reported as a value, a quantitative description that includes both a number and a unit. For example, before a 100-meter race is run, the distance must be measured as precisely as possible. Its value is 100 meters. In this value, the unit is meters (defined below), and the number of units is 100. 

As measuring techniques became more precise and the demand for accuracy increased, the standards on which people based their units were improved. In the 18 century, the French invented the metric system, based on a more consistent, systematic, and carefully defined set of standards than had ever been used before. For example, the meter (or metre, from the Greek metron, a measure ) became the standard for length. The first definition for the standard meter was one ten-millionth of the distance from the North Pole to the Equator. This became outdated as the precision of scientist s measuring instruments improved. Today, a meter is defined as the distance light travels in a vacuum in 1/299,792,458 second. Technical instruments for measuring length are calibrated in accordance with this very accurate definition. 

The International System of Measurement (SI for Systeme International d Unith), a modern elaboration of the original metric system, was set up in I960. It was developed to provide a very organized, precise, and practical system of measurement that everyone in the world could use. The SI system is constructed using seven base units, from which all other units are derived (Table 1.1). The chemist is not usually interested in electric currents or luminous intensity, so only the first five of the base units on Table 1.1 will appear in this text. The meaning of mole, the base unit for amount of substance, is explained in Chapter 9. Until then, we will use the first four base units meter (m), kilogram (kg), second (s), and kelvin (K). 

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In this chapter, we work primarily with pressures in atmospheres or torr, but we use pressures in bars in later chapters when we work with standard chemical conditions. Example illustrates pressure measurement and unit... 

Compare different measurements and units of radiation dosage. (Section 21.9)... 

This group, created by Congress, collects, analyzes, develops, and disseminates information and recommendations on radiation quantities, measurements, and units. NCRP publishes maximum permissible levels of external and internal radiation. The major handbooks are entitled Maximum Permissible Body Burdens and Maximum Permissible Concentrations of Radionuclides in Air and Water for Occupational Exposure and Review of the Current State of Radiation Protection Philosophy. The NCRP suggests an annual permissible whole-body dose of 5 rem/year, with 3 rem permitted within a 13-week period. 

See also Barometry Climate Engineering Climate Modeling Climatology Measurement and Units Meteorology Remote Sensing Temperature Measurement. 

See also Maps and Mapping Measurement and Units Quality Control Temperature Measurement Time Measurement Weight and Mass Measurement. 

Measurements and units are most useful when they are standardized so that measurements mean the same thing to everyone and are comparable. One method of solving the problem of variability in measurement is to establish a standard value for each unit and to regularly compare all measuring devices to this standard. 

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