Accuracy of instruments.....

Orn Jonsson

Member
Join Date
Apr 2002
Location
Boise Idaho
Posts
125
I have been looking for some precision pressure and flow transmitters, and am concerned if I fully understans the terminology used in describing the accuracy of the instrument.

% Accuracy of full scale.
% Linearity
% Repeatability

Are these different aspects of the insrument accuracy related?
Does one have to add up (sum) all possible errors to get accuracy or is there a term which describes the ultimate (final) accuracy of an instrument.

I'm looking for 0.25% instrument acuracy

Thanks

OJ
 
Accuracy is a measure of how close the reading is to the actual value, and generally is total error. It should include the effects of non-linearity and so on. It is usually expressed as % of full scale or span. A device with a range of 0-100 °F and an accuracy of ±1% will display a value within 1 °F of actual temperature. A few devices spec accuracy as ±% of reading.

Linearity deifines how much the output signal varies from a straight line.

Repeatability is more important in some cases than accuracy. It identifies how close successive readings are to each other at the same actual value. It takes into account hysterisis and so on.
 
OJ,

Take a look at this. It explains most of what you are asking:
Total Probable Error

Good explanations Tom. (y)

Some manufacturer's will state the total accuracy, but others state it as you have shown, OJ. One of the more important terms that many do not state is stability over time. This states how the accuracy may be affected over time. Stability gives you good insight into how often you ought to be calibrating.

One other thing I have always noticed if a manufacturer says .25% accuracy, such as you did. That means .25% of full scale. A real highly accurate instrument will state either 'accuracy of full scale' or 'accuracy of the reading'.

Also, if you are in an extreme environment take a note of the temperature effects on accuracy.

Good Luck!
 

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