It sometimes happens that a formula is needed to calculate some required quantity. Unfortunately, the available formula uses a system of units that is not convenient. The problem is to convert the available formula to the units that one wishes to use.

Since a mistake in converting the formula's units will inevitably lead to calculating the wrong value for the required quantity, the procedure that is used should be logical and as error proof as possible. First of all, consider these two statements, both correct, which follow.

It is the second of these two statements that is needed to convert a formula correctly. Specifically, conversion factors are needed that will convert the desired units into the units required by the formula. Correct conversion of the formula depends, therefore, on coming up with the right conversion factors.

The recommended procedure is to set up the conversion statements, similar to the two above, with the desired units on the left side of the equals sign, and the units required by the formula on the right side of the equals sign. The whole process will be illustrated best by working an example.

If the inside diameter of a pipe, the velocity of the flowing stream, and the density of the stream are known, then the mass flow rate (W) of the stream in the pipe will be

In this expression, v is the velocity in per second, d is the inside diameter of the...