Andrew Gabriel wrote:
In article ,
writes:
Andrew Gabriel wrote:

I bought one of the older ones Maplin stocked a few years ago,
and that's hopeless on non-resistive loads (out by a factor of
3 on SMPSU's -- too high, which is a puzzling way for it to be
in error).

If it does not sample fast enough, then it may well overestimate
current consumption.

Yes, I guess if the spike width at the voltage peaks is small
compared with the sampling rate (hello Mr Nyquist;-) and the
sampling is somehow always catching it (prehaps due to synching
with the voltage waveform), then that would account for it.

Isn't it as much to with low power factor as anything. The
cheap/simple power meters simply measure average (and I mean average,
not RMS) voltage and current, then multiply the two together and apply

If it did this, it would under estimate rather than 3 times
over estimating. That's why I said the form of the error was
puzzling.

No it wouldn't. It could have lots of amps flowing at 240 volts but
with (say) 10% power factor the Volts*Amps would be way more than the
power.

the right factor to convert to RMS and say the product is power. It
matters not one whit how fast your samples are if you do this, the
answer will always be wrong.

Yes, although if you don't sample fast enough to see the general
form of a rapidly changing value, you will get a completely
misleading impression (lookup Nyquist's Theorem).

I was thinking of 'averaging' as in a moving coil meter and rectifier,
that doesn't sample in the sense you mean at all, it genuinely
averages the current (or voltage).

--
Chris Green