Voltmeters – Precision Notation Explained


From the technical details of a multimeter:

AC Voltage: – Range and Accuracy: 200mV ± (1.0%+2); 2V ± (0.7%+3); 20 ± (1.0%+3); 200/600V ± (1.2%+3)

What does the notation "± (0.7%+3)" mean? Specifically, what does "+3" mean?

Best Answer

What does the notation "± (0.7%+3)" mean? Specifically, what does "+3" mean?

The "+3" in that context, is the number of least-significant digits (LSD) on that range, which can be in error. This notation is one of a few "industry standard" ways of specifying measurement accuracy.

The accuracy specification you quoted is from the 2V range of whichever meter is in the question. Therefore using that as an example, the accuracy on a reading on the 2V range is between:

(reading +0.7% +3 LSD) and (reading -0.7% -3 LSD)

I found the Etekcity MSR-A600 meter has the same AC voltage specification as you quoted, so I'll use that one to illustrate LSD a little more. That is a 3.5 digit meter i.e. maximum count = 1999. The maximum count value is important for this next calculation.

For a 3.5 digit meter on the 2V range (actual maximum value shown is 1.999V), the LSD value is 1mV (0.001V).

So the accuracy calculation on that 3.5 digit meter's 2V range, is:

reading ±0.7% ±3mV
(where the 3mV is the 3 LSD, on that 2V range, on a 3.5 digit meter)

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