UNITS AND CONVERSION FACTORS - Water Quality Engineering in Natural Systems - page 405

APPENDIX A

UNITS AND CONVERSION FACTORS

A.1 UNITS

Candela (cd) . Luminous intensity of 1/600,000 of a
square meter of a radiating cavity at the tempera-
ture of freezing platinum (2042 K).
Mole (mol) . Amount of substance that contains as
many specified entities (molecules, atoms, ions,
electrons, photons, etc.) as there are there are
atoms in exactly 0.012 kg of carbon-12.

The Système International d'Unités (International
System of Units or SI system) was adopted by the 11th
General Conference on Weights and Measures (CGPM)
in 1960 and is now used by almost the entire world. In
the SI system, all quantities are expressed in terms of
seven base (fundamental) units. These base units and
their standard abbreviations are as follows:

In addition to the seven base units of the SI system,
there are two supplementary SI units: the radian and
the steradian. The radian (rad) is defined as the angle
at the center of a circle subtended by an arc equal in
length to the radius, and the steradian (sr) is defined
as the solid angle with its vertex at the center of a
sphere that is subtended by an area of the spherical
surface equal to the radius squared. The SI units should
not be confused with the now obsolete metric units ,
which were developed in Napoleonic France approxi-
mately 200 years ago. The primary difference between
the metric and SI units is that the former uses centi-
meters and grams to measure length and mass, while
these quantities are measured in meters and kilograms
in SI units. The United States is gradually moving
towards the use of SI units; however, there is still wide-
spread use of the “English” system of units, which are
also referred to as “U.S. Customary” or “British Gravi-
tational” units.
In addition to the seven SI base units, there are
several derived units that are given special names. These

Meter (m) . Distance light travels in a vacuum during
1/299,792,458 of a second.*
Kilogram (kg) . Mass of a cylinder of platinum-
iridium alloy kept in Paris.
Second (s) . Duration of 9,192,631,770 cycles of the
radiation corresponding to the transition between
two hyper fine levels of the ground state of the
cesium-133 atom.
Ampere (A) . Magnitude of the current that when
flowing through each of two long parallel wires of
negligible cross section separated by one meter in
a vacuum, results in a force between the two wires
of 2 × 10 −7 N for each meter of length.
Kelvin (K) . Defined in the thermodynamic scale by
assigning 273.16 K to the triple point of water
(freezing point, 273.16 K = 0°C).

* “Meter” is the accepted spelling in the United States; the rest of the
world uses the spelling “metre.”