Environmental Engineering Reference
In-Depth Information
2.2.1 stevin's law
Stevin's law
deals with water at rest. Specifically, it states: “The
pressure at any point in a fluid at rest depends on the distance measured
vertically to the free surface and the density of the fluid.” Stated as a
formula, this becomes:
p
=
w
×
h
(2.1)
where:
p
= pressure in pounds per square foot (psf).
w
= density in pounds per cubic foot (lb/ft
3
).
h
= vertical distance (ft).
Example 2.2
Problem:
What is the pressure at a point 18 ft below the surface of a
reservoir?
Solution:
To calculate this, we must know that the density of the water
(
w
) is 62.4 lb/ft
3
:
p
=
w
×
h
p
= 62.4 lb/ft
3
× 18 ft = 1123 lb/ft
2
(psf)
Wastewater operators generally measure pressure in pounds per
square inch rather than pounds per square foot; to convert, divide by
144 in.
2
/ft
2
(12 in. × 12 in. = 144 in.
2
):
1123 psf
144 in. ft
2
p
=
=
7.8lb/in.orpsi (
rounded)
22
2.3 ProPerTies of WaTer
Table 2.1 shows the relationship between temperature, specific
weight, and density of water.
2.3.1 density and specific gravity
When we say that iron is heavier than aluminum, we say that iron
has greater density than aluminum. In practice, what we are really say-
ing is that a given volume of iron is heavier than the same volume of
aluminum.
Note:
What is density? Density is the
mass per unit volume
of a
substance.
