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# What is Measurement and What are the Units of Measure

In modern industry, uniformity for interchangeability of parts is vital for cost-effective manufacturing. If a machine part fails or wears out, a replacement must be available and must fit. It will if the parts are made according to specific and accurate measurements. Data must be analyzed on parts and processes to determine conformance to product specifications, and data must be fed back to the manufacturing process to prevent production problems. The quantification of data on parts and processes involves defining standard units, calibrating instruments to these standard units, and using these instruments to quantify parts and processes. This quantifying is called measurement.

## What is Measurement?

The term measurement has several meanings. It can be defined as the process of quantification, comparing an unknown magnitude to a known magnitude. Measurement can also mean the resulting number. In the case of quantification (comparing an unknown magnitude to a known magnitude), it is the act of obtaining specific data about a characteristic of a part or a process, i.e., “The measurement was done in the assembly plant.” A resulting number is the specific data on a part or a process, i.e., “The measurements on part A all fell within the specified tolerances.” The science of measurement is called metrology.

## UNITS OF MEASURE

To allow the quantifying of data on parts or processes, defined standard units must be used. These standard units are called units of measure. They are definitions of standardized units that are used to quantify characteristics about which we are interested. Metrologists have developed systems of international units of measure for the purpose of international commerce. The primary systems in use today are the English system, the metric system, and the Systeme International d’Unites (SI). The more preferred system is now SI. Table 7.2.1 gives examples of units of measure for the three systems.

## How the English system was retained

The English system was retained by the American colonies when they separated from England in 1776. This system consists of units of measures that were developed before the industrial revolution, i.e., a foot is 12 inches, a yard is 3 feet, etc. Only part of the English system is based on decimal multiples.

## How the meter was defined

In 1799, a committee of French scientists, under the direction of the French government, established a system of measures and weights. A basic unit, called the meter, was defined and is the basis for the metric system. This system of measures, primarily concerned with length, area, volume, and mass, is based entirely on a decimal system. See Table A-6 in the Appendix for a list of the units of measure used in this system.

By the 1970s, all industrialized countries adopted the metric system, with the exception of the United States. However, most U.S. companies with international involvement tend to use the metric system. The metric system is preferred to the English system on a purely technical basis and has been widely accepted by the scientific community. Most of the world is now adopting the SI system, including the United States and the United Kingdom.

## How the SI system evolved

The SI system has evolved more recently from the metric system and is an international system. It consists of seven basic units of measure: length, mass, time, temperature, light intensity, electric current, and amount of substance. All are fully compatible with the metric system. Two supplemental units are used for solid angles and planes. There is a long list of units of measure derived from the seven basic units of measure, and standardized terminology for subdivisions and multiples of units of measure. Latin prefixes are used to indicate subdivisions, and Greek prefixes to indicate multiples of any standard unit. Table 7.2.2 lists the SI system multiples and subdivisions. Tables A-7 and A-8, respectively, in the Appendix list the SI system units of measure and conversion charts for the different systems.