Re: [TCML] measure VDG voltage

[resonance@xxxxxxxxxxxx]

Using Terry's E-field antenna and also calibrating this against a spark
gap (14" sphere discharging to grounded 7 inch sphere, and also a 30 inch
sphere discharging to a grounded 14 inch sphere), it works out to 26.5
kV/cm.  Both methods yielded nearly identical results.

Happy holiday sparks,

Dr. Resonance



> Hi Bart,
>
> Sorry for the delay in getting back with you. Jim and Antonio have
> already responded with excellent answers. Unlike most home experimenter
> setups, the voltage breakdown tables reflect measurements made using
> rigorous setup, scrupulously clean spheres, gaps that are a small
> fraction of sphere diameter, and precision DC voltage sources or impulse
> voltages with well defined leading and trailing parameters. Typical home
> experimenter setups use smaller spheres, dirty or unpolished electrode
> surfaces, low impedance circuits and oscillatory waveforms, and minimal
> E-field control. It's not too surprising that the results may differ
> from the "ideal". I would agree that, under these conditions, a more
> typical breakdown threshold may indeed be in the range of 26 - 27 kV/cm.
>
> Best wishes to all for a very Merry Christmas and only good things in
> 2008!
>
> Bert
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> Barton B. Anderson wrote:
>> Hi Bert,
>>
>> I measured a 1" ball gap back when doing TSSP measurements for Paul. I'm
>> at a very low altitude of about 100 ft above sea level here in a CA. I
>> measured 26kv/cm at that time. I think the 30kv/cm is a nice roundabout
>> number, but I think 26kv/cm is a more accurate number and not just from
>> my own measurements. I know where the 30kv/cm is from, but from my own
>> measurements and others who have measured (including papers on the
>> task), I've seen values between 26 and 27 and "never" higher. That
>> 4kv/cm does make the terminal voltage little lower.
>>
>> When corona inception voltage is reached, I have to assume breakout at
>> that point in this application. Although the corona itself "grows" the
>> ROC a tad bit, it's not enough to change the voltage significantly.
>>
>> The ROC is the best method of predicting top volts. Far lower than
>> energy calcs and certainly more real.
>>
>> Take care,
>> Bart
>>
>>
>>> You can estimate the corona-limited maximum voltage for the VDG based
>>> on the radius of curvature of the top terminal. However, this only
>>> provides the maximum that the VDG generator could achieve, not
>>> necessarily what your generator is achieving. The following
>>> relationship assumes an air breakdown voltage of 30 kV/cm at sea level
>>> and a polished sphere. The maximum voltage will be reduced for
>>> locations with higher elevations or if the sphere's surface was not
>>> polished. The Radius of Curvature (ROC) is in centimeters. So, a
>>> spherical topload 20 cm in diameter would have a ROC of 10 cm, and an
>>> estimated maximum VDG voltage of 300,000 volts.
>>>
>>> Vmax = 30*ROC
>>>
>>> You can also approximate the actual maximum voltage by using a spark
>>> gap and a suitable table that converts gap distance to voltage. This
>>> has the advantage of not loading down the VDG until actual spark-over
>>> occurs. You can use the VDG top terminal as one gap electrode and a
>>> similarly curved grounded spherical terminal (a gazing globe or a
>>> metal "float" ball or even a suitably round metal bowl) as the other
>>> gap electrode. Jim Lux's HV Handbook contains sphere gap construction
>>> hints and a gap-voltage table:
>>> http://home.earthlink.net/~jimlux/hv/spherev.htm
>>>
>>> Bert
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>
>
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Dr. Resonance

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