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Reading logarithmic plots.


Reading logarithmic plots.

Example resistance vs. temperature plot for an NTC resistor.
Fig.1: Example resistance vs. temperature plot for an NTC resistor.

Reading a value from a logarithmic plot with reasonable accuracy can be difficult ( assuming the plot is accurate in the first place ).
In the example above, we are interested in the resistance value at 37.5 °C.
This value is called yv. The values at the bottom and the top of the plot are called yb and yt.
You can use any value for yb and yt that are easy to read on the plot.
Using a ruler ( in this case an on-screen ruler ), you can measure the positions of yb, yv and yt. These are called pb, pv and pt.
In this example, pb = 466 pixel, pv = 217 pixel and pt = 0 pixel ( after resizing the plot ).
yv = yb * 10 ^ ( ( pv - pb ) / ( pt - pb ) * log ( yt / yb ) )
yv = 0.01 Ω * 10 ^ ( ( 217 px - 466 px ) / ( 0 px - 466 px ) * log ( 100 Ω / 0.01 Ω ) ) = 1.37 Ω

You can download a spread-sheet with the calculations here.

A logarithmic on-screen ruler would be very useful, but I have not found one?

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