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.
RELATED POSTS
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 (A), pictographs (B), broken-line graphs (C), continuous curved-line graphs (D), and pie charts (E) |
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]
 |
| Figure 2. Electric wire chart |
To begin, locate the column on the left side of the chart representing a 28 volt system (item 1). Move down in this column until you find the horizontal line representing a wire length of 25 feet (item 2). Follow this line to the right until it intersects the diagonal line for 125 amps (item 3). Because the wire is in a bundle and carries a continuous current, you must be at or above curve 1 on the chart (item 4). Follow along the diagonal line representing 125 amps until it intersects curve 1 (item 5). From this point, drop down vertically to the bottom of the chart. The line falls between wire sizes 1 and 1 /0 (item 6). Whenever the chart indicates a wire size between two sizes, you must select the larger wire. in this case, a 1 /0, or single aught wire is required.
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]
 |
| Figure 3. Brake-horsepower chart |
To calculate the brake mean effective pressure, BMEP, begin by locating 1,500 brake-horsepower on the top of the chart. From this value, drop down vertically until you reach the line representing 2,000 cubic inches of displacement. From this intersection, extend a line horizontally to the right until you intercept the line representing 2,400 rpm. Now, drop down vertically to read the brake mean effective pressure on the bottom line of the chart. The brake mean effective pressure is approximately 248.
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]
 |
| Figure 4. Fuel consumption chart |
To determine fuel consumption for an engine operating at 2,400 rpm, you must first determine the specific fuel consumption. To do this, locate 2,400 rpm on the bottom of the chart and follow the line up until it intersects the propeller load specific fuel consumption curve. From this intersection, extend a line to the right side of the chart and read a specific fuel consumption of .47 LB/BHP/HR. Now, go back to the bottom of the chart and locate 2,400 rpm again. From this point move up to the propeller load horsepower curve. From this intersection, extend a line to the left side of the chart and read the brake horsepower of 127 hp. To determine the fuel burn, multiply the specific fuel consumption by the brake horsepower. The engine burns 59.69 pounds per hour (47 x 127 = 59.69).
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]
 |
| Figure 5. Engine horsepower/altitude chart |
To determine the percent of sea level horsepower developed at an altitude of 7,000 feet, begin by finding the point on the horizontal axis that represents the desired altitude. From this point, move upward until you intersect the horsepower curve. Then move horizontally left to the chart's vertical axis and read the percent of sea-level horsepower available.
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.
