9
$\begingroup$

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)

$\endgroup$
15
$\begingroup$

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 !

$\endgroup$
  • $\begingroup$ 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? $\endgroup$ – John H. K. Feb 18 '15 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 '15 at 10:05
  • 1
    $\begingroup$ @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 '15 at 10:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.