Re: metrology Re: [TCML] Re: Dummy load for optimum cap size Experimentation

[Gary Lau <glau1024@xxxxxxxxx>]

The load on the (15A) Variac, as measured by an analog Watt meter, is
between 500 & 1500W.  I didn't have an Ammeter hooked up.

I just tried comparing readings between the Weston analog voltmeter, the
Wavetek 27XT, and a Fluke model 75 DMM.  With the unloaded output of the
Variac, all 3 agree within about 0.5V from 110-140V.  But when the Variac is
loaded, the Wavetek reading is significantly lower than the other two.  The
Fluke and Weston track identically, loaded or not.

But this says nothing about which one is right!  Looks like I need to invest
in a true-RMS DMM!

Regards, Gary Lau
MA, USA



On Sun, Oct 3, 2010 at 5:02 PM, dave pierson <dave_p@xxxxxxxxxxx> wrote:

>
> > 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
>    real word.)
>
>    best
>     dwp
> _______________________________________________
> Tesla mailing list
> Tesla@xxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla