How to Handle Units in Custom Nomograms

Nomograms are usually created using the set of units most appropriate and convenient for the expression or relationship of interest. The best way to create a nomogram in the units of choice is to start with an algebraic relationship expressed in terms of these units. As an example, consider the area of a circle A of radius R expressed in SI units.

A = 3.14159 * R2

where R is in units of meters and A is in units of meters squared. But say we wish to use centimeters for radius R and square feet for area A. To do so, we convert the expression as follows:

.0929  * Asq ft = 3.14159 * (0.01 Rcm)2

where we have employed the conversions: 1.0 sq ft = .0929 m2 & 1.0 cm = 0.01 meters. By combining the constant factors, the above equation reduces to the form:

Asq ft = 3.382 * 10-3 * Rcm2

Now, entering a radius value in centimeters into the equation gives the value for area directly in square feet. This unit conversion method may be employed to convert any expression into a more convenient form for a nomogram with any set of units. It is only necessary to input the correct coefficient in the dialogue box when the nomogram is initially created. Note that it is also very important to provide the correct units for all variables in the dialogue box.