Environmental Engineering Reference
In-Depth Information
This relationship is easily seen when a cubic foot of water, which
weighs 62.4 lb, is compared to a cubic foot of aluminum, which weighs
178 lb. Aluminum is 2.8 times heavier than water.
It is not that difficult to find the specific gravity of a piece of metal.
All you have to do is to weigh the metal in air, then weigh it under water.
The loss of weight is the weight of an equal volume of water. To find the
specific gravity, divide the weight of the metal by its loss of weight in
water:
Weight of Substance
Weight
SpecificGravity
=
(2.3)
of EqualVolumeofWater
Example 2.3
Problem:
Suppose a piece of metal weighs 150 lb in air and 85 lb under
water. What is the specific gravity?
Solution:
150 lb - 85 lb = 65 lb loss of weight in water
150
65
SpecificGravity
=
=
2.3
Note:
In a calculation of specific gravity, it is
essential
that the densities
be expressed in the same units.
As stated earlier, the specific gravity of water is 1, which is the stan-
dard, or reference, against which all other liquid or solid substances are
compared. Specifically, any object that has a specific gravity greater than
1 will sink in water (e.g., rocks, steel, iron, grit, floc, sludge). Substances
with a specific gravity of less than 1 will float (wood, scum, gasoline).
Considering the total weight and volume of a ship, its specific gravity is
less than 1; therefore, it can float.
The most common use of specific gravity in water/wastewater treat-
ment operations is in gallons-to-pounds conversions. In many cases,
the liquids being handled have a specific gravity of 1 or very nearly 1
(between 0.98 and 1.02), so 1 may be used in the calculations without
introducing significant error; however, in calculations involving a liquid
with a specific gravity of less than 0.98 or greater than 1.02, the conver-
sions from gallons to pounds must consider specific gravity. The tech-
nique is illustrated in the following example.
Example 2.4
Problem:
A basin contains 1455 gal of a certain liquid. If the specific grav-
ity of the liquid is 0.94, how many pounds of liquid are in the basin?
Solution:
Normally, for a conversion from gallons to pounds, we would
use the factor 8.34 lb/gal (the density of water) if the specific gravity of
the substance is between 0.98 and 1.02. In this instance, however, the
