This question sounds silly.... But what to do? i am very weak in basics tooo .. which type of voltage ( RMS or Aveg) is measured by both analog and digital multimeter? And State the reason for that?

 

This question sounds silly.... But what to do? i am very weak in basics tooo .. which type of voltage ( RMS or Aveg) is measured by both analog and digital multimeter? And State the reason for that?

Usually, multimeters measure RMS values. The reason is because for an AC signal, the average value is zero. Thus, they use RMS to quantify the measured signal. Some special meters may measure average values too.

 

Ok,

Here is the scoop on analog and digital DVMs for AC. Most of them have averaging detectors, but these are calibrated to give the proper RMS value, but only for a sine wave. It doesn't matter if the meter is digital or analog, the detectors are essentially the same most of the time. Now, some more expensive meters, especially DVMs or DMMs, have a True RMS Detector, and will accurately read a non-sinusoidal signal's DC equivalent heating value on a resistive load (that is one definition of RMS). But even for these TRMS meters, they have limits on what's called the maximum "crest factor", which is the ratio of peak to average of an AC signal. Most TRMS DMMs have a limit on crest factor of 5, meaning that if you measure an AC signal with a crest factor higher than 5, it will not read with the instrument's specified accuracy.
The reason that the cheaper averaging detectors can read RMS on a sine wave is that the average of a (half) sine wave is (2*A/pi)= 0.6366. The RMS value of a (half) sine wave is (1/SQRT[2]) = 0.7071. These two results are true for any (half) sine wave. The true average for a sine wave is zero, so we're actually considering the average of a rectified sine wave. Since it is a simple matter to adjust the DMM (or analog meter) reading from 0.6366 to 0.7071 so the meter will read the correct RMS value of a sine wave, that's what is done on the cheaper meters. A true RMS detector is more complex and costs more.

Regards,
Kamran Kazem

 

Excellent response, kkazem!

The only thing I would add is that if you're looking for a "true RMS meter", pay attention to both the bandwidth and the crest factor. For example, relatively inexpensive true RMS meters might have bandwidths of 500 to 1000 Hz. The medium expensive ones might cover the audio range. The better ones will go to 0.1 to 1 MHz. Above that, you get into rather expensive meters. However, you can still buy the great HP 3400A (10 MHz bandwidth) or 3400 B (20 MHz bandwidth) used on ebay (although I don't see many anymore). I was amazed by this instrument nearly 40 years ago the first time I used one and they're still pretty amazing.

But before you get too tied up in knots about the specs, make sure you understand why you need what you think you need.

Edit: I forgot to mention that you'll often see meters with 0.1 or 0.01% or so DC accuracies, but their RMS AC accuracies will still be in the range of 1-3% or so.

 

thanks a lot for ur useful information thank u very much