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I've seen it in several data sheets - it is a measure of error of some kind, of course. The problem is I dont know the exact meaning. I've seen it in the context of repeatability, accuracy and linearity.

An example is the following data sheet: smc data sheet (On page 3)

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FS = FULL SCALE = maximum reading.

It means that the accuracy is such that the reading is probably within + or - 0.5% of the FULL SCALE reading.

This is a very important and easily overlooked qualification of the result.
If I have a reading of 1 Volt and the accuracy is +/- 0.5% it means that the actual result should lie in the range 1 - 0.5% x 1 to 1 + 0.5% of 1
= 0.995V to 1.005 V

However - if I measure the result on the 10V range then 0.5% of 10V = 0.5% of Full Scale
= 0.05V. So 1V +/- 0.5% of FS
= 0.95V to 1.05V.

On the 100V range, 1V +/- 0.5% FS lies in the range
0.5V to 1.50 V. !!!!

The reason for specifying results in this manner is that the error experienced on a given range tends to largely be constant regardless of the actual reading. So, as the input gets smaller the error becomes increasingly large in proportion.

So eg on a 100V range a reading of 0.5V +/- 0.5% FS lies in the range
0 to 1V !

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  • $\begingroup$ Thanks for the answer! How probable is it for the actual value to be in this +- range? Is it 2 or 3 sigma? Or is there no general consensus regarding that? $\endgroup$
    – JHK
    Feb 18, 2015 at 9:55
  • $\begingroup$ @JohnH.K. Octarine, I think :-). If unspecified it should present the outer error limits. For cheaper meters it's more likely to an expression of desire rather than a statement of fact. For eg a Fluke meter they will say what they mean. If they do not state qualifiers then a calibrated meter will lie absolutely in the range stated. They will also say eg +1 LSD due to quantisation error. $\endgroup$
    – Russell McMahon
    Feb 18, 2015 at 10:05
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    $\begingroup$ @JohnH.K. Watch the LSD flick between two adjacent counts ... 5 5 5 6 5 5 6 6 5 6 6 6 5 5 5 5 5 65 ..... Count how many of each over a time period at a constant sampling rate. ie ratio between each. This MAY give you one extra meter digit. Or may not :-). $\endgroup$
    – Russell McMahon
    Feb 18, 2015 at 10:06

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