- 33,700 pounds
- 134,800 pounds
- 18 metric tons
- 72 metric tons
- None of the above

The solutions to this problem are
relatively straightforward. The volume
of a full tank of water is pi times the square of the radius times the tank
height (3 x 3^{2} x 20) or approximately 540 cubic feet. The density of water is approximately 62.4
pounds per cubic foot so the water in the tank weighs approximately 33,700
pounds.

The solution to the example in
metric units is (3 x 1^{2} x 6) or approximately 18 tons (18,000
kilograms) because the density of water is approximately 1 metric ton per cubic
meter. Therefore, Answer A and Answer C
are correct.

As mentioned, this problem is
relatively straightforward from a technical perspective. The point of performing the calculation in
both the English and metric systems is that the English system is more prone to
error. The metric system is more elegant
--- even though both will yield the same answer for the same size tanks. The metric system is less prone to
calculation error because many of its conversion factors are 1 or multiples of
10. (If you have already attended one of
my seminars you know that I am good at multiplying and dividing by 1.)

This exercise may not seem important, but small errors
can make a big difference in results. I
seem to recall that a space probe crashed into a planet (instead of orbiting)
due to a misunderstanding regarding the measurement units used during design.

Additional Complicating Factors

The above calculations in the English system can
be made even more complicated by using the weight of water per gallon and the
number of gallons per cubic foot. In
addition, the liquid could be different from water and have a density that is
different from that of water so the tank contents could be heavier or lighter
than calculated in either system.

This article originally appeared in Flow Control magazine.