metrology Re: [TCML] Re: Dummy load for optimum cap size
sparktron01 at gmail.com
Sun Oct 3 21:49:40 MDT 2010
Metrology is absolutely a valid word AND science.
I was responsible for procurement, training, and annual calibration of lab
instruments for a proprietary project with net worth of close to $1,000,000.
But when you are running lab work on new circuitry, and the systems
under test are more precise and accurate then your measurement
equipment (measure the accuracy of a micrometer with a straight edge
experiment); it becomes real tricky to develop absolute and relative
accuracy to a standard and measurement uncertainty to same.
Usually requires thousands (or millions) of automated measurements
in a temperature controlled, RF tight, non florescent light illuminated
environment. I have had to run several ANOVA Gauge R&R studies
on new measurement systems as compared to commercial equipment
performing the same measurement but using different measurement
methodologies to determine fitness for use and uncertainty to a
standard engineering unit.
Mind numbing and notoriously anal retentive to determine all error terms
in measurement process...
On Sun, Oct 3, 2010 at 5:02 PM, dave pierson <dave_p at comcast.net> 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.)
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Dave Sharpe, TCBOR/HEAS
Chesterfield, VA USA
Sharpe's Axiom of Murphy's Law
"Physics trumps opinion!"
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