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Tables, Charts, Axes & Scales

Tables, Charts, Axes, and Scales

Engineering decisions are driven by data. Much of this data—such as material properties, standard pipe sizes, or fluid viscosities—is not calculated from scratch every time but is instead retrieved from established reference tables and charts. Reading them correctly is a mandatory skill.

Engineering Tables

Tables organize discrete data points into rows and columns.

  • Lookup Process: To use a table, you must identify your "entry variables" (e.g., Temperature and Pressure) to find the corresponding output variable (e.g., Density).
  • Footnotes are Critical: Engineering tables frequently contain asterisks and footnotes. These often contain critical limits (e.g., "Do not use for temperatures above 200°C"). Ignoring them can result in unsafe designs.

Engineering Charts and Graphs

Charts provide a visual representation of continuous data relationships.

Linear Scales

On a linear scale, the physical distance between marks is proportional to the mathematical value. The distance from 1 to 2 is exactly the same as the distance from 10 to 11. Most everyday graphs use linear scales.

Logarithmic Scales (Log Scales)

Many engineering phenomena cover a massive range of values. For example, the frequency response of an electrical filter might range from 10 Hz to 10,000,000 Hz. Plotting this on a linear scale would compress the low frequencies into an unreadable sliver.

A logarithmic scale spaces values out based on their order of magnitude (powers of 10).

  • On a log scale, the distance from 1 to 10 is the exact same physical distance as the distance from 10 to 100, and from 100 to 1,000.
  • Log-Log Plot: Both the X and Y axes use logarithmic scales.
  • Semi-Log Plot: One axis is linear, and the other is logarithmic.
  • Reading Caution: When reading a log scale, remember that the tick marks between 10 and 100 (e.g., 20, 30, 40) are not evenly spaced. They get closer together as you approach the next magnitude. You cannot simply estimate the halfway point between 10 and 100 as 55 by looking at the physical middle.