integrating true RMS wattmeter

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: dest <dest@xxxxxxxxxxxxx>

Hallo.

i`ve found in archives (97) this thing:

http://himplast.ru/father_dest/temp/killawatt.PNG

the author is David E. Sharpe, TCBOR, this is revision 02, 6/26/97.
i have a couple of question to you, Dave:

a) have you upgraded the thing to 10-14KW range as was intended in the
past?

b) is there any more recent revisions here?

c) why did this thing was so nonlinear: -12.66% error at the begin of
scale and +2.69% error at the end? what was the reason of
_nonlinearity_ at all - optocoupler or op amp?

d) http://www.pupman.com/listarchives/1997/march/msg00347.html

"Variation in Yokogawa from calculated due to integration times, manual
data entry, and TCR (temp coeff. of resistance) effects of heater
element(s)."

what? nippon meter was so SLOW??? what does it mean - "manual data
entry"?

e) "The experimental design was basically use the controller to power
a 120VAC,1.56kW floor heater.  Inductance is trival (shaded pole
blower) and 1500W heater."

so you have tested the thing only at 60Hz? and at LINEAR load???
what`s the point in that - anybody can go and buy $5 ferrodynamic
wattmeter for example - it would be more accurate and linear in such
case : ))) my $3 50Hz _mechanical_ meter accuracy rating is 1.5!

f) "The differential voltages (Vshunt) and (Vload) are in fact
MULTIPLIED to the form:
39 X Rshunt/Rmultipler * Vload * Iload"

then why do you advice to reduce Rshunt (to 250 uOhm!) instead of
changing oa gain for example? is it limitation of optocoupler, oa,
something other? there are many other oa & optos here.

10) "The only concern I have with this design is there is minimal circuit
board clearance through the optocoupler (0.1 and 0.3 inches)."

are you speaking about hv spikes in the mains? so strong, that they can
cause breakdown of 2.5mm of air? don`t you think that in that case you
better shut your coil down immediatelly instead of trying to measure
it`s power consumption? % )

11) this circuit is darned simple - maybe we can use a couple of
additional elements to get real _electronic_ accuracy? : )

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I don't doubt that my logic is flawed, could somebody tell me where?
08-08-97 (c) Gary Lau