Graphs and charts are pictorial representations of data. They enable you to quickly visualize certain relationships, complete complex calculations, and predict trends. Furthermore, charts allow you to see the rate and magnitude of changes.

Information is presented graphically in many different forms. Graphs are often found in the form of bar graphs, pictographs, broken-line graphs, continuous-curve graphs, and the circular graph or pie chart. [Figure 1]

Figure 1. Bar graphs, pictographs, broken-line graphs, continuous curved-line graphs, and pie charts are all ways of graphically representing numerous calculations |

## Nomograms

The need to show how two or more variables affect a value is common in the maintenance industry. Nomograms are a special type of graph that enable you to solve complex problems involving more than one variable.

Most nomogram charts contain a great deal of information and require the use of scales on three sides of the chart, as well as diagonal lines. In fact, some charts contain so much information that it is very important for you to carefully read the instructions before u sing the chart. On the other hand, some charts are simple to use.

## Electric Wire Chart

An example of a nomogram chart that is used extensively in the maintenance industry is the electric wire chart. This chart is made up of vertical lines that represent the American Wire Gauge (AWG) wire sizes. Horizontal lines represent the length of wire in feet that produces an allowable voltage drop for each electrical system listed. Drawn diagonally across the chart is a series of parallel lines representing current flow. A common use for this chart is to find the wire size required to carry a given amount of current without exceeding the allowable voltage drop.

For example, determine the minimum size wire of a single cable in a bundle carrying 125 amps 25 feet in a 28-volt system. [Figure 2]

Notice that the three curves extend diagonally across the chart from the lower left corner to the right side of the chart. These curves represent the ability of a wire to carry the current without overheating. Curve one represents the continuous rating of a wire when routed in bundles or conduit. If the intersection of the current and wire length lines are above this curve, the wire can carry the current without generating excessive heat.

If the intersection of the current and wire length lines fall s between curve one and two, the wire can only be used to carry current continuously in free air. If the intersection falls between curves two and three, the wire can only be used to carry current intermittently.

## Brake-Horsepower Charts

Another common type of graph you will encounter as a technician is the performance chart. One common performance chart is the brake-horsepower chart. These charts represent many hours of calculation by engineers but are presented so that you can quickly determine if the performance being observed is acceptable. For this sample chart, assume you have an engine that has a 2,000 cubic-inch displacement and develops 1,500 brake-horsepower at 2,400 rpm. [Figure 3]

## Fuel Consumption Charts

The fuel consumption chart is another type of performance chart that you must be familiar with. For this sample chart, assume that you are trying to determine how much fuel an engine consumes when it is operating at a cruise of 2,400 rpm. [Figure 4]

## Engine Horsepower/Altitude

This chart represents the relationship between engine horsepower and altitude. For this sample chart, assume you are doing an engine run-up at an altitude of 7,000 feet. [Figure 5]

There are many other ways of presenting information with graphs. Pie or circular charts can show the percentage of an item to the whole. Graphs show the relationship of two or more variables.

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