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Re: measuring true self capacity of a coil.
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> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: measuring true self capacity of a coil.
> Date: Tuesday, May 30, 2000 7:07 PM
>
> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
>
> Hi Galvin, All,
>
> Now that we are worrying about percentages in the single digits when it
> comes to measuring or predicting Fo, I guess we should worry about "how"
to
> make the measurements.
> <<<<snip>>>
> It is truly strange that we are now almost to the point of being able to
> calculate Fo, with a number of different methods, more accurately than we
> can measure it!
>
This isn't all that unusual. On an antenna range, it is very difficult to
get measured uncertainty better than 0.1 dB (or even 0.3 dB), yet we know
that the gain of the antenna is known to 0.01 dB by theoretical
computation. (say, for "standard gain horns", where the gain is entirely
determined by dimensions, and we can calculate the effect on the gain from
manufacturing tolerances, etc.). There are also "3 antenna" measurement
schemes with carefully balanced levels (i.e. you match a level reading,
rather than using the reading itself, and use geometry to change the gain
by changing the distance in the far field.)
For that matter, how accurately are you measuring the dimensions of your
test object? A 1% error in the diameter of a sphere will give a 1% error
in the capacitance. While it is fairly easy to measure a 1 meter sphere to
an accuracy of 1 cm, measuring it to 0.1% (or 1 mm) is somewhat trickier,
and even trickier is assuring that it is truly spherical to that accuracy,
and that the surroundings are uniform to that accuracy. (unless you are
measuring in a far field range.. like on top of a tall tower with a finite
ground plane under the coil.)
I would say that the uncertainty in the ACTUAL fRes introduced by HV
discharge in the form of corona, leader, and spark formation is much
greater than that from C modeling or measurement error. What we need to do
is think about ways to measure the actual characteristics of the spark
channel as it forms. I've been thinking that now that LOTS of computer
power is available cheaply, we might be able to use a technique like that
in Efield 5.x, except backwards... Put a bunch of e field probes and
digitizers around the coil. Let 'er spark. Now, calculate the charge
distribution in the leader and spark that corresponds to the measured
e-fields (lots o' computation, there..)
Or even something as simple as measuring the spark channel current from a
deliberate breakout point on the toroid. You could put one of those $50
radio shack battery ProbeScopes inside the toroid measuring the current
through a shunt or a toroidal transformer.
So, given that sparks develop in a timescale of nanoseconds, has anyone got
a clever idea on how to build a cheap high speed multichannel digitizer?
12 bits at 100 MSPS or 1 GSPS would be nice... The video A/D's exist...