Page 813 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

768 Appendix A: International System of Units

7. Time: The SI unit of time is the second. This unit is j. Adding to symbols: Attachment of letters to a unit
preferred, especially when technical calculations are symbol to give information about the quantity is
involved. In cases where time relates to life customs incorrect. For example, do not use psia or psig to
or calendar cycles, the minute, the hour, and the day distinguish absolute and gage pressure. Instead,
may be advisable. For example, vehicle speed is state
expressed as kilometers per hour. ‘‘ . . . at a gage pressure of 13 kPa,’’ or ‘‘.. .. at an
8. Angle: The SI unit for the plane angle is the radian. absolute pressure of 13 kPa.’’ Where space is
The degree and its decimal submultiples is permis- limited, one may write, ‘‘13 kPa (gage),’’ or ‘‘13
sible when the radian is not convenient. kPa (absolute).
9. Area: The SI unit of area is the square meter. The k. Conversions and rounding: The number of sig-
hectare is a special name for a square hectometer nificant digits in conversions should be neither
2
(hm ). Large land or water areas are expressed in diminished nor exaggerated. For example, 125 ft
2
hectares or in square kilometers (km ). converts exactly to 38.1 m. If, however, the 125 ft
10. Volume: The SI unit of volume is the cubic meter length had been obtained by rounding to the near-
3
(m ). This unit or one of the common multiples, such est 5 ft, the conversion is 38 m.
as cubic centimeter, is preferred. The special name
liter (L) has been approved for the cubic decimeter. A.6 MASS, FORCE, AND WEIGHT
The prefixes milli- and micro- may be used with
the liter. The SI system uses distinct units for mass and force. The
11. Energy: The SI unit of energy is the joule, with its kilogram is the unit of mass and the newton is the unit of
multiples, and is preferred for all applications. force. The term, kilogram-force should not be used.
12. Pressure: The SI unit of pressure and stress is the In the SI system, the term weight, means that the force, if
pascal (Newton per square meter) and should applied to body, would give it an acceleration of free fall.
be used, with the proper prefixes. Other units are When non-SI units are used, a distinction should be made
discouraged. between force and mass.
13. Other units: The CGS units are to be avoided. The
poise and the stoke should not be used, along with the
A.6.1 NEWTON
calorie, kgf, mm mercury, standard atmosphere, etc.
14. Rules for symbols: The force required to accelerate one kilogram at one meter
a. Orientation: Unit symbols should be upright per second is defined as the Newton (N). The acceleration due
2
regardless of surrounding text. to gravity at the earth’s surface is 9.8066 m=s . The gravity
b. Case of symbols: Letter unit symbols are written force exerted by (weight of) 1 kg mass at the earth’s surface is
in lower case unless the unit name comes from a therefore
proper name, in which case, the first letter of the
symbol is capitalized, e.g., W, Pa. The exception F ¼ M g (A:1)
is the symbol for liter, L. 2
¼ 1:0 kg 9:81 m=s (A:2)
c. Spaces: When a quantity is expressed as a numer-
ical value and a unit symbol, a space should be left ¼ 9:81 N
between them, e.g., 35 mm, not 35mm. An excep-
tion is that no space is left between the numerical A.6.2 CONVERSION FACTOR, g C
value and the symbols for degree, minute, and
The relationship between force and mass causes some con-
second of a plane angle and degree Celsius, e.g.,
sternation because the units are not homogeneous between the
use 458,208C.
two sides of the F ¼ ma equation. To account for this prob-
d. Prefix space: There is no space between the prefix
lem, an ‘‘artifice,’’ g c , was invented. The term g c is contrived
and the unit symbols.
to cancel units on the right side, to give
e. Multiplication product: Products are expressed
with a dot, e.g., N m for Newton meter.
1
f. Quotients: Quotients are expressed as m=s, or F ¼ ma (A:3)
m g c
1
m s ,or .
s
g. Digit separations: Digits are separated into groups
Example A.1 Show the Dimensions of g c
of three, counting from the decimal from the left for SI Units
and the right, e.g., 2 141 596 73 722 7372 0.1335.
h. In numbers of four digits on either side of the 1. Applying SI dimensions gives
decimal place, the space is not necessary.
i. Thousands symbol, M: The use of M to indicate 1 m
kg (ExA:1:1)
thousands is deprecated. g c s 2
1N ¼