Measurement

66 Measurement

everywhere else. It is theuser's responsibility to select equipment that provides measurements that agree with everyone else.

Reliability requires theability to consistently read a given measurement device and also requires a given measurement device to perform equally well, test after test. It is the user's responsibility toknow how to achieve repeatable data from the equipment being used.

Accuracy refers tohow well a measurement device is calibrated and how many significant figures onecanreliably expect. It is theuser's responsibility to know how to read hisequipment andnotinterpolate data to be anymore accurate (i.e., significant figures) than they reallyare.

Laboratory research is dependent onreliable quality measurements andthe use of uniformity, reliability, and accuracy to achieve this goal and this cannot be emphasized enough. Poor or inaccurate measurements can only lead to poor or inaccurate conclusions. A good theory can be lost if the experimental data are misread.

Commerce is equally dependent onuniformity, reliability, and accuracy of measurement systems. The potential economic liabilities of mismeasurements and misunderstanding areeasy to understand. In fact, it was theeconomic advantages of uniformity that ledto themetric system's expansion after the Napoleonic wars. What canupset and/or confuse consumers and businesses is when thesame word (which may have varying meanings) is applied to different weights. Westill confuse the weight value of a ton and the volume of a gallon: In theUnited States,the ton is equal to 2000 lb; however, in Great Britain it is equal to 2240 lb. Similarly, in the United States, the gallon is established as 231cubic inches; however, in Great Britain it is 277.42 cubic inches. If you areaware of these differences, you can make theappropriate mathematical corrections. But realistically, it should not be a problem to be dealt with. Rather, theproblem of different measurement systems should be avoided in the first place. That is specifically was what themetric system was designed to do.

2.1.2 History of the Metric System

Overcoming the incongruities of inconsistent measurement systems on an international basis wasconsidered for centuries. Thebasics of the metric system were first proposed by Gabriel Mouton of Lyon, France, in 1670. This vicar (of St. Paul's Church) proposed three major criteria for a universal measurement system: decimalization, rational prefixes, andusing parts of the earth as a basis of measurement (length was to be based on the arc of one minute of the earth's longitude). There wasalso a desire to find a relationship between thefoot and gallon (i.e., a cubic foot would equal one gallon). Unfortunately, these measurement units were already in use, andbecause there was nobasis for these measurements

68 Measurement

surement system for the first time. Official prototypes of the meter and kilogram were made and stored in Paris.

Because of Napoleon's conquests, the metric system spread rapidly throughout Europe. However, it was not in common usage in many areas (even in France) until international commerce took advantage of its simplicity and practicality. By the mid-1800s, it was the primary measurement system in most of Europe. In 1875 the International Bureau of Weights and Measures was established near Paris, France. It formed a new international committee, called the General Conference on Weights and Measures (CGPM), whose goal was to handle international matters concerning the metric system. The CGPM meets every six years to compare data and establish new standards. Every member country of this committee* receives a copy of the meter and kilogram prototype with which to standardize their own country's measurement system.

Over the years there has been ongoing fine-tuning of the measurement system because the greater the precision with which our measurement units can be ascertained, the better we can define our universe. A new era in the measurement system came in 1960, at the 11th meeting of the CGPM, when the International System of Units (SI) was established. This system established four base units: the meter, kilogram, second, and ampere. They are collectively known as the MKSA system. Later, three more base units were added: kelvin (for temperature), candela (for luminous intensity), and mole (for the amount of substance) (see Table 2.1). In addition, two supplementary base units (which are dimensionless) were added: radian (plane angles) and steradian (solid angles) (see Table 2.2).

From the nine base units, over 58 further units have been derived and are known as derived units. There are two types of derived units: those that have special names (see Table 2.3) and those that have no special names (see Table 2.4). An example of a derived unit with a special name is force, which has the unit newton (the symbol N) and is calculated by the formula "N = kg-^s2." An example of a derived unit that does not have a special name is volume, which has the unit of cubic meter (no special symbol) and is calculated by the formula "volume = m3."

Table 2.2 Supplementary SI Units"

" From the ASTM document E380-86, Table 2, with permission.

"The National Institute of Standards and Technology (formally the National Bureau of Standards) represents the United States at the CGPM. They store the United States' copies of the original measurement prototypes.