Revised metric system units

For many years scientists recorded measurements in metric units, which are related decimally, that is, by powers of 10. In 1960, however, the General Conference of Weights and Measures, the international authority on units, proposed a revised metric system called the International System of Units (abbreviated SI, from the French 5ysteme /ntemationale d Unites). Table 1.2 shows the seven SI base units. All other units of measurement can be derived from these base units. Like metric units, SI units are modified in decimal fashion by a series of prefixes, as shown in Table 1.3. We will use both metric and SI units in this book. 

We use the same procedure to convert between systems of units, for example, between the English (or American) unit system and the International System (a revised metric system discussed fully in the next section). Suppose we know the height of Angel Falls in Venezuela to be 3212 ft, and we find its height in miles as... 

Almost everything we own—clothes, house, food, vehicle—is manufactured with measured parts, sold in measured amounts, and paid for with measured currency. Measurement has a history characterized by the search for exact, invariable standards. Our current system of measurement began in 1790, when the newly formed National Assembly of France set up a committee to establish consistent unit standards. This effort led to the development of the metric system. In 1960, another international committee met in France to establish the International System of Units, a revised metric system now accepted by scientists throughout the world. The units of this system are called SI units, from the French Systeme International d Unites. 

SI unit A unit composed of one or more of the base units of the Systeme International d Unites, a revised metric system. (13) side reaction An undesired chemical reaction that consumes some of the reactant and reduces the overall yield of the desired product. (93)... 

The two systems of units with which you are probably most familiar are the English system (foot, gallon, pound, etc.) and the metric system (meter, liter, kilogram, etc.). Although there has been an increase in the use of metric units in the United States in recent years, English units still are used commonly. For many years scientists recorded measurements in metric units, but in 1960, the General Conference on Weights and Measures, the international authority on units, proposed a revised metric system for universal use by scientists. We wiU use both metric and revised metric (SI) units in this book. 

For centuries, units of measurement were fairly inexact. A person might mark off the boundaries of a property by walking and counting the number of steps. The passage of time could be estimated with a sundial or an hourglass filled with sand. Such estimates worked for ordinary tasks. Scientists, however, need to report data that can be reproduced by other scientists. They need standard units of measurement. In 1795, French scientists adopted a system of standard units called the metric system. In 1960, an international committee of scientists met to update the metric system. The revised system is called the Systeme Internationale d Unites, which is abbreviated SI. 

The ISA Standards and Practices Department is aware of the growing need for attention to the metric system of units in general, and the International System of Units (SI) in particular, in the preparation of instrumentation standards. The Department is further aware of the benefits to USA users of ISA standards of incorporating suitable references to the SI (and the metric system) in their business and professional dealings with other countries. Toward this end, this Department will endeavor to introduce Si-acceptable metric units in all new and revised standards, recommended practices, and technical reports to the greatest extent possible. Standard for Use of the International System of Units (SI) The Modern Metric System, published by the American Society for Testing Materials as lEEE/ASTM SI IQ-97, and future revisions, will be the reference guide for definitions, symbols, abbreviations, and conversion factors. 

Almost everything we own is made and sold in measured amounts. The measurement systems we use have a rich history characterized by the search for exact, invariable standards. Measuring for purposes of trade, building, and surveying used to be based on standards that could vary a yard was the distance from the king s nose to the tip of his outstretched arm, and an acre was the area tilled in one day by a man with a pair of oxen. Our current, far more exact system of measurement began in 1790 when a committee in France developed the original metric system. In 1960, another committee in France revised it to create the universally accepted SI units (from the French Syst me International d Unites). 

You probably know your height in feet and inches. Most people outside the United States, however, measure height in meters and centimeters. The system of standard units that includes the meter is called the metric system. Scientists today use a revised form of the metric system called the SystSme Internationale d Unit6s, or SI. 

---