Basic Units of Measurement and Conversions - Basic Mathematics for Water and Wastewater Operators - page 10

TABLE 2.1
Commonly Used Units

Quantity SI Units USCS Units

Length Meter Foot (ft)

Mass Kilogram Pound (lb)

Temperature Celsius Fahrenheit (F)

Area Square meter Square foot (ft 2 )

Volume Cubic meter Cubic foot (ft 3 )

Energy Kilojoule British thermal unit (Btu)

Power Watt Btu/hr

Velocity Meter/second Mile/hour (mile/hr)

In the study of water/wastewater treatment plant math operations (and in actual
practice), it is quite common to encounter both extremely large quantities and extremely
small ones. The concentrations of some toxic substance may be measured in parts per
million (ppm) or parts per billion (ppb), for example. To describe quantities that may
take on such large or small values, it is useful to have a system of prefixes that accom-
pany the units. Some of the more important prefixes are presented in Table 2.2.
Note: For comparative purposes, we like to say that 1 ppm is analogous to a full
shotglass of water sitting in the bottom of a full standard-size swimming pool.

CONVERSION FACTORS
Sometimes we have to convert between different units. Suppose that a 60-inch piece
of pipe is attached to an existing 6-foot piece of pipe. Joined together, how long
are they? Obviously, we cannot find the answer to this question by adding 60 to
6, because the two lengths are given in different units. Before we can add the two
lengths, we must convert one of them to the units of the other. Then, when we have
two lengths in the same units, we can add them.

TABLE 2.2
Common Prefixes

Quantity Prefix Symbol

10 -12 Pico p

10 -9 Nano n

10 -6 Micro µ

10 -3 Milli m

10 -2 Centi c

10 -1 Deci d

10 Deca da

10 2 Hecto h

10 3 Kilo k

10 3 Mega M