Page 812 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Appendix A: International System of Units 767

TABLE A.3 TABLE A.4
Common Derived Units SI Prefixes
Quantity Unit Symbol Multiplication Factor Prefix Symbol
18
Absorbed dose rate Gray per second Gy=s 1 000 000 000 000 000 000 ¼ 10 . exa E
15
Acceleration Meter per second squared m=s 2 1 000 000 000 000 000 ¼ 10 . peta P
12
Angular acceleration Radian per second squared rad=s 2 1 000 000 000 000 ¼ 10 . tera T
9
Area Square meter m 2 1 000 000 000 ¼ 10 . giga G
6
Concentration Mole per cubic meter mol=m 3 1 000 000 ¼ 10 . mega M
3
Density, mass Kilogram per cubic meter kg=m 3 1 000 ¼ 10 . kilo k
2
Energy density Joule per cubic meter J=m 3 100 ¼ 10 . hecto h
1
Entropy Joule per kelvin J=K 10 ¼ 10 . deka da
1
Exposure (x and gamma rays) Coulomb per kilogram C=kg 0.1 ¼ 10 . deci d
2
Heat capacity Joule per kelvin J=K 0.01 ¼ 10 . centi c
3
Heat flux density irradiance Watt per square meter W m 2 0.001 ¼ 10 . milli m
2 6
Luminance Candela per square meter cd m 0.000 001 ¼ 10 . micro m
9
Molar energy Joule per mole J=mol 0.000 000 001 ¼ 10 . nano n
12
Molar entropy Joule per mole kelvin J=(mol K) 0.000 000 000 001 ¼ 10 . pico p
Molar heat capacity Joule per mole kelvin J(mol K) 0.000 000 000 000 001 ¼ 10 15 . femto f
Moment of force Newton meter N m 0.000 000 000 000 000 001 ¼ 10 18 . atto a
Power density Watt per square meter W=m 2
Radiant intensity Watt per steradian W=sr Source: Abridged from ASTM, Standard Practice for Use of the
International System of Units (SI) (The Modernized Metric
Specific heat capacity Joule per kilogram kelvin J=(kg K)
System), Designation E380-91a, PCN 03-543191-34, American
Specific energy Joule per kilogram J=kg
Society for Testing Materials, Philadelphia, PA, 1991.
Specific entropy Joule per kilogram kelvin J=(kg K)
3
Specific volume Cubic meter per kilogram m =kg
Surface tension Newton per meter N=m
Thermal conductivity Watt per meter kelvin W=(m K)
1. Application of prefixes: First, the SI form of the
Velocity Meter per second m=s
metric system is preferred for all applications. In
Viscosity, dynamic Pascal second Pa s
2
Viscosity, kinematic Square meter per second m =s some cases, units outside SI are appropriate.
Volume Cubic meter m 3 2. Prefixes for multiplication: Table A.4 lists the pre-
Wave number 1 per meter 1=m fixes associated with multiplication factors.
3. Elimination of zero digits: In general, the SI prefixes
Source: Abridged from ASTM, Standard Practice for Use of the should be used to indicate the orders of magnitude,
International System of Units (SI) (The Modernized Metric eliminating nonsignificant digits and leading zeros in
System), Designation E380-91a, PCN 03-543191-34, American
decimals and giving an alternative to the power of
Society for Testing Materials, Philadelphia, PA, 1991.
ten notation. Examples are
a. 12,300 mm is 12.3 m
3
b. 12.3 10 m is 12.3 km
SI system over a wide range of circumstances. For example, the c. 0.00123 mm is 1.23 nm
kilopascal (kPa) is convenient because one atmosphere of 4. Prefixes to control decimal location: When express-
pressure is 101,325 pascals (Pa) and it is easier to say 101 kPa. ing a quantity by a numerical value and unit, a prefix
should be chosen so that the numerical value is
between 0.1 and 1000. In expressing area and
A.4 UNITS IN USE
volume, the prefixes hex-, deck-, deci-, and centi-
Some units that have been used commonly are depreciated may be required, e.g., square hectometer, cubic
under the new system. Examples include the calorie, kilo- centimeter.
gram-force, langley, metric horsepower, millimeter of mer- 5. Deviations from SI: For certain applications, one
cury, standard atmosphere. The CGS system is to be avoided, particular multiple is used by custom. For example,
e.g., the erg, the dyne, the poise, etc. the millimeter is used for linear dimensions in engin-
eering drawings, even when the values are far out-
A.5 CONVENTIONS side the 0.1–1000 mm range. The centimeter is often
used for body measurements and clothing sizes.
A number of rules were adopted by the CGPM, which help in 6. Calculations: Errors in calculations are minimized if
facilitating communication, i.e., in providing a common lan- the base and the coherent derived SI units are used
guage. These are taken from ASTM (1991) and are enumer- and the resulting numerical values are expressed in
ated in the following paragraphs: powers of 10 notation instead of using prefixes.