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Suggestion: Pitfalls and quick tets: AC/RMS/power measurements

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rr100:
There are many pitfalls when trying to characterize some non-constant voltage/current with a single value. Some theory from Dave plus some specific tests would be welcome:

- how the cheap multimeter (peak-to-peak divided by 1.41) work and why you can measure some 1A current from a mains powered device that's in standby and obviously doesn't eat hundreds of watts
- why you can't rely on separate current and voltage values to calculate the power even if both are provided by good instruments (I assume some basic DC-DC supply or even DC->nasty waveform might be a good example)
- what are the industrial instruments like metrawatt or whatever doing and how can they be fooled (what we know about how they sample the values and how this can not work so well in a real circuit). I have no idea actually about how they work, although I can just imagine but don't know details; are they sampling with a very high frequency, are they doing some analog filtering/averaging before, etc.

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