Every week I hear this fight. "I need to measure........".I mentioned this back in post #6 of this thread, but note the exception i also warned about.
The question was measuring only DC in the presense of AC. So, he wants an average AC meaurement, not RMS. For example, a sine wave on a DC level would average to zero, leaving only the DC level.
try to measure the average DC value of a full wave rectified waveform without a smoothing capacitor with some meters and you'll see a display that bounces all over the place. I know this because i've tried it several times.
Any meter that uses digital averaging will see this same problem because it cant sync to the waveform. Could be the cheaper ones because that's what i was referring to, one Radio Shack and one Velleman.