How to Read Accuracy Spec

[DROBNJAK]

Lets say this is an accuracy specification: ±1.0% ±2.0V and I am measuring exactly 100V, but I don't know that.

So "±1.0%" part means that voltage on the DMM is going to read anywhere between 100 ±1.0% or something between 99 and 101V.

What I do not understand is "±2.0V" part. Does it mean that I need to add it to the above 99-101V range, so I'll get actual 97-103V reading?

[cyr]

Yep...

You may also find the second part expressed as +/- digits or +/- % of range (as opposed to +/- % of reading which the first part refers to).

[DROBNJAK]

So is that interpretation of mine correct?

Practically, reading gets quite far away from the actual value very quickly. I would say that almost one can throw away two least significant digits.

[ohmineer]

You can read this explanation from http://www.ni.com/white-paper/4439/en/ that will help you to understand better the concept of accuracy

Accuracy -- a measure of the capability of the instrument to faithfully indicate the value of the measured signal. This term is not related to resolution; however, it can never be better than the resolution of the instrument. The accuracy is often specified as:

For example, a 5 1/2 digit voltmeter can have an accuracy of 0.0125% of reading + 24 µV on its 2.5 V range which results in an error of 149 µV when measuring a 1V signal. On the other hand, the resolution of this same voltmeter is 12 µV, 12 times better than the accuracy. Keep in mind that the accuracy of your measurement is affected by several factors and we will discuss these factors later in this paper

[Neilm]

You may also find the second part expressed as +/- digits or +/- % of range (as opposed to +/- % of reading which the first part refers to).

 +/- % of range or +/- full scale deflection (f.s.d.) are more commonly seen on analogue instruments. +/- digits are obviously on digital. Manufacturers use it take into account offsets in the measurement circuitry when you are at the very low end of a range. It is usually expressed in digits for a digital meter, so if the 100V range only has a resolution of 1V, +/- 2 digits would be +/-2V. However, if it had a resolution of 0.1V +/- 2 digits would be 0.2V. So your initial interpretation is correct for the statement quoted.

If you look at a cal certificate you will see the uncertainties printed for each value tested (at least you should if it is worth anything). This uncertainty is produced by examining the accuracy statements above.

[alm]

% of range is also used in the specs for some DMMs that can change the number of digits (usually the expensive ones), for example the Fluke 8846A. Specifying digits is inconvenient if a meter can vary its resolution from 4.5 to 6.5 digits.