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Subject: Re: Question on multimeters

Date: 01/07/01 at 8:21 AM
Posted by: Dean Huster
E-mail: dhuster@pb.k12.mo.us
Message Posted:

In Reply to: Re: Question on multimeters posted by Dean Huster on 01/05/01 at 5:22 PM:

Regarding the true-RMS function. A true-RMS meter isn't any more accurate than a meter that bases its ACV readings on the average value of the voltage. If you feed a sine wave to both types of meters, both with 0.1%,1 count accuracy, both will give you the same reading with the same accuracy. It's when you start measuring non-sinusoidal waveforms that the true-RMS meter "shines". Since the display is in RMS volts on either meter, the true-RMS meter will be providing that actual RMS voltage. The other meter will now be giving you an erroneous reading, for it will be measuring the average value and incorrectly converting that value to RMS using an incorrect factor based upon a sine wave -- which is not what is being measured at the time.

Therefore, a true-RMS meter with an accuracy of 3% ACV is not nearly as accurate as an averaging meter with an accuracy of 0.5% on a sine wave. On a severely non-sinusoidal waveform, the less-accurate meter may actually be more accurate.

Dean


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