metrology Re: [TCML] Re: Dummy load for optimum cap size
dave_p at comcast.net
Sun Oct 3 15:02:38 MDT 2010
> I'm making slow but steady progress in my experiment to determine the
> best cap size based on using a halogen lamp dummy in place of a
> primary, and monitoring the light intensity to indicate how much power
> is being processed.
> I'm recording the power at 120V and at full Variac setting. The full
> Variac setting is typically ~135V under load. It changes slightly
> depending on the cap value, but it is what it is. When I set the
> Variac to get 120V, I monitored the voltage with both a
> Wavetec 27XT DMM, and an old lab-quality Weston analog meter,
> calibrated at the 120V point with the 27XT, with 120V from the Variac,
> under no load.
> But when the Variac is under load from the NST,
How much load roughly....?
> there can be a significant difference between the two meters
> (113V vs. 120V). I'm speculating that this is due to the loaded
> Variac output waveform being distorted,
Quite Possible. Once saw my lab instructor get fooled Real Good
by similar. I Omit the details as nonTC, tho its a dandy tale of
the potential pitfalls of metering...
> and the two meters see this differently.
Just so. By Definition (and physics...) moving coil meters
are _average_ responding. Scales may be marked 'RMS', however
if the fine print be read:
RMS ASSUMING A SINE WAVE is being measured.
If not sine wave: all bets are off. Period.
(Exceptions, obviously for fancy electronic, or hotwire
converters, to give 'RMS'.)
> I don't own a true-RMS meter, so I have to decide which of these two
> readings to believe. I have been using the analog meter. Can anyone
> suggest which one might be closer to a true-RMS value?
Hard to say. The DMM might get goophy with strays from the
spark gap. I'd tend to go with the DMM here.
(I THINK Metrology (as distinct from meterology....) is even a
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