Page 811 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

766 Appendix A: International System of Units

factors as international competitiveness, costs, education, and
TABLE A.1
leadership from the federal government.
Base SI Units
In its efforts to move toward metrication, the ICMP (1991)
reported that the United States has been moving toward met- Quantity Unit Symbol
rication, and that the costs were much less than anticipated. Length Meter m
For example, General Motors converted fully to metric at Mass Kilogram kg
about 1% of the estimated cost. In fact, various industries Time Second s
have found that savings result from carrying fewer parts and Electric current Ampere A
sizes. One of the most compelling arguments for the continu- Thermodynamic temperature Kelvin K
ing move toward metrication has been the problem of inter- Amount of substance Mole a mol
national competitiveness. In working abroad, the construction Luminous intensity Candela cd
industry has used routinely metric in foreign work and
has made the transition for that segment of the industry. Source: Abridged from ASTM, Standard Practice for Use of the
International System of Units (SI) (The Modernized Metric
Most engineering journals have specified the use of metric
System), Designation E380-91a, PCN 03-543191-34, American
for many years.
Society for Testing Materials, Philadelphia, PA, 1991.
a
The mole is the amount of substance of a system, which contains as many
elementary entities as there are atoms in 0.012 kg of carbon-12, as adapted
A.3 LOGIC OF THE INTERNATIONAL SYSTEM
by the 14th CGPM in 1971.
OF UNITS
The original metric system provided a set of units for the
measurement of length, area, volume, capacity, and mass A.3.3 DERIVED UNITS
based on two fundamental units, the meter and the kilogram.
The derived units are formed by combining base units, sup-
About 1900, the meter-kilogram-second (MKS) system was
plementary units, and derived units. Some of the derived units
instituted. Other base units were added over the years to
with special names are listed in Table A.2. Some of the
accommodate other fields, such as the ampere as the unit of
common derived units are listed in Table A.3. Other derived
electric current, the degree Kelvin (later renamed the kelvin)
units include the pascal, the joule, the watt. Of special note is
as the unit of temperature, and the candela as the unit of
the Newton that is used in place of the kilogram-force.
luminous intensity. In 1971, the seventh base unit, the mole,
was added. Also in 1971, the pascal (Pa) was approved as the
special name for the SI unit of pressure, i.e., Newtons per A.3.4 PREFIXES
square meter. The prefixes for decimal multiples and submultiples,
The modernized metric system, the SI system, is a coherent and associated symbols, are listed in Table A.4. These may
system with seven base units. The SI system has only one unit be used as a means to make more convenient the use of the
for each physical quantity, which is the advantage of imple-
menting the system universally. In other words, there is only
one recognized unit for each variable. A second advantage of TABLE A.2
the SI system is its coherence. Such a system requires no Derived Units with Special Names
conversion factors. For example, equations between the units
Quantity Unit Symbol Formula
of a coherent system contain only the number 1 as a conver-
sion factor. In a coherent system, the product or quotient of Frequency Hertz Hz 1=s 2
any two unit quantities is the unit of the resulting quantity. For Force Newton N kg m=s
Pressure, stress Pascal Pa N=m 2
example, the velocity unit is length divided by time, and the
Energy, work, quantity of heat Joule J N m
unit of force is the mass multiplied by the unit of acceleration.
Power, radiant flux Watt W J=s
Quantity of electricity electric charge Coulomb C A s
Electric potential, potential difference, Volt V W=A
A.3.1 BASE UNITS
electromotive force
The SI units are in three classes: base units, supplementary Celsius temperature Degree 8C K
units, and derived units. The base units give the system its Luminous flux Lumen lm cd sr
coherence. The base units, seven in number, are independent, Activity Becquerel Bq 1=s
and are listed in Table A.1. Absorbed dose Gray Gy J=kg
Dose equivalent Sievert Sv J=kg

A.3.2 SUPPLEMENTARY UNITS Source: Abridged from ASTM, Standard Practice for Use of the
International System of Units (SI) (The Modernized Metric
The supplementary units are the radian and the steradian. System), Designation E380-91a, PCN 03-543191-34, American
These are dimensionless units and are the ratios between Society for Testing Materials, Philadelphia, PA, 1991.
two lengths and two areas, respectively.