Output power

[Austin]

It usually reads the average of the rectified voltage and then multiplies by some factor (I forget which) that's right for sine wave. So basically it assumes it's sine.
If you set it to DC, it just reads the average DC

So that would be about the same as rms?

[Steve Conner]

The "form factor" that it multiplies by is 1.1.

Derivation: The RMS of a sine wave is 0.70 of its peak value. The rectified average is 0.63 of the peak. 0.70/0.63 = 1.1.

The implication is that the cheap meter gives the right answer for a sine wave and gets it wrong for all other waveforms. It will tend to read lower than the true RMS values of most of the waveforms you encounter in practice, except for the square wave where it'll read 10% high, as the form factor of a square wave is 1.

[pdf64]

If the wave under test is sine, then the meter should display the rms equivilant.
As the wave deviates from sine, then the error will increase.
Pete

[loudthud]

So that would be about the same as rms?

Pretty close. The average voltage of a (rectified) sine wave is 0.636 times the peak. The Root of the average (Mean) Square is 0.707 times the peak. The meter is calibrated to give the correct RMS number from the average value. Distortion of the wave causes errors.

[Amp Kat]

The main difference of good or accurate meters vs cheap inaccurate meters is accuracy itself. All DMM meters read RMS voltage however some read it more accurate than others. If you have ever seen a Fluke repair facility it is quite amazing to say the least and they use a data machine that has all of the meter software programmed into it and you enter the model number and plug the meter into it and it compares voltage measurememts as to what it's supposed to measure as far as % of accuracy and if it's not right they can turn some pots on the PCB board to bring it back and if that doesn't work they tell you it's unrepairable as circuit drift has deemed it unrepairable. There were some instruments in there that had to be calibrated to as much as .001-25th power which is a bunch of zeroes but I have to say that Fluke set-up was a thing of beauty and very sophisticated and high tech impressive piece of gear