Environmental Engineering Reference
In-Depth Information
derived units are used for convenience rather than
necessity, and are listed in Table A.1 (Szirtes, 1998;
Wandmacher and Johnson, 1995).
When units are named after people, like the newton
(N), joule (J), and pascal (Pa), they are capitalized when
abbreviated but not capitalized when spelled out. The
abbreviation capital L for liter is a special case, used to
avoid confusion with one (1). In the SI system, the unit
of absolute temperature is the degree kelvin, which is
abbreviated K without the degree symbol. The units of
second, minute, hour, day, and year are correctly abbre-
viated as s, min, h, d, and y.
In using prefixes with SI units, multiples of 10 3 are
preferred in engineering usage, with other multiples
such as centimeter avoided if possible. It is conventional
practice to separate sequences of digits into groups of
three by spaces rather than commas.
EXAMPLE A.1
The height of a water control structure is reported as
19.3 ft. Convert this dimension to meters.
Solution
The conversion factor is given in Table A.2 as 1
ft = 0.3048 m, hence,
19 3
.
ft
=
19 3 0 3048
.
×
.
=
5 88264
.
m
Since 19.3 ft could have resulted in rounding any
number between 19.25 and 19.35 ft, the maximum pos-
sible rounding error is ±0.05/19.3 = ±0.26%. Similarly,
rounding 5.88264 to 5.88 m gives a maximum rounding
error of ±0.005/5.88 = ±0.085%, and rounding to 5.9 m
gives an error of ±0.05/5.9 = ±0.85%. Hence, accuracy is
lost by taking 19.3 ft as 5.9 m, while 5.88 m more accu-
rate than indicated by 19.3 ft. It is usually prudent not
to discard accuracy, so take 19.3 ft = 5.88 m.
A.2 CONVERSION FACTORS
In most cases, application of unit conversion factors
result in converted numbers that have more digits than
the original numbers. In these cases, the converted
number should be rounded off such that rounding error
is consistent with the rounding error of the converted
number (Taylor, 1995).
A good rule of thumb is that the converted number
should have the same number of significant digits as the
original number, assuming that the conversion factor is
always more accurate than the original number.
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