Environmental Engineering Reference
In-Depth Information
62.4 lb
of
water
1 ft
1 ft
1 ft
figure 2.1 One cubic foot of water weighs 62.4 lb.
One cubic inch of water weighs 0.0362 lb. Water 1 ft deep will exert a
pressure of 0.43 psi on the bottom area (12 in. × 0.0362 lb/in.
3
). A column
of water 2 ft high exerts 0.86 psi (2 ft × 0.43 psi/ft); one 10 ft high exerts
4.3 psi (10 ft × 0.43 psi/ft), and one 55 ft high exerts 23.65 psi (55 ft × 0.43
psi/ft). A column of water 2.31 ft high will exert 1.0 psi (2.31 ft × 0.43 psi/
ft). To produce a pressure of 50 psi requires a 115.5-ft water column:
50 psi × 2.31 ft/psi = 115.5 ft
The important points being made here are:
1. 1 ft
3
of water = 62.4 lb (see Figure 2.1).
2. A column of water 2.31 ft high will exert 1.0 psi.
Another relationship is also important:
1 gal water = 8.34 lb
At standard temperature and pressure, 1 ft
3
of water contains 7.48 gal.
With these two relationships, we can determine the weight of 1 gal of
water:
Weight of 1 gal of water = 62.4 lb ÷ 7.48 gal = 8.34 lb/gal
Thus,
1 gal water = 8.34 lb
Note:
Further, this information allows cubic feet to be converted to gal-
lons by simply multiplying the number of cubic feet by 7.48 gal/ft
3
.
Example 2.1
Problem:
Find the number of gallons in a reservoir that has a volume of
855.5 ft
3
.
Solution:
855.5 ft
3
× 7.48 gal/ft
3
= 6399 gallons (rounded)
Note:
As discussed later in this chapter, the term
head
is used to des-
ignate water pressure in terms of the height of a column of water in
feet; for example, a 10-foot column of water exerts 4.3 psi. This can be
referred to as 4.3-psi pressure or 10 feet of head.
