Precision notation for voltmeters

Question

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?

Answer 1

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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• The value of the LSD will vary on other ranges. For example, on a 3.5 digit meter's 200V range (maximum value shown of 199.9V) the LSD value on that range is 0.1V.

• The "percentage of reading" value is the gain accuracy; the "number of LSD" value is the offset accuracy. They relate to different parts of the measurement process and so have different units.

• Some more explanations, for future reference and showing more "worked examples" for different meters:

  1. BK Precision: How to Read Accuracy Specifications
  2. Tektronix (Keithley): Specs: How Can I Apply an Accuracy Specification on a Data Sheet to My Specific Measurement (PDF download)
  3. EE.SE: What does it mean when multimeter accuracy is marked as: ±0,03%+10Digit?

Answer 2

A bit supplemental to the question, but that spec is incomplete. As the voltage being measured is AC (alternating current), bandwidth limitations apply. As they do to any circuit or oscilloscope.

I have a shiny new PROSTER VC99 meter. AC accuracy is stated as ±(0.8% + 5). It's cheap. But, that is only officially for frequencies of 40 - 400Hz. Whilst you think of your meter as for measuring the mains voltage (50-60Hz), that's fine. It's alluring though to think of the meter as a kinda substitute for an oscilloscope and use it to measure all sorts of AC voltages. That's wrong, as the metered AC voltage will drop into the higher frequencies. For reference, the VC99 response stays surprisingly flat till 4kHz (tested with function generator and oscilloscope). At 40kHz, it's quickly rolled off to only 15% of the true peak to peak.

So the moral of this answer is: stick it into your wall sockets by all means, but make sure to read the full spec if you're going to stick it into your HI-FI or radar installation.