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Re: [TCML] voltage
On 16:59, Bob Svangren wrote:
Hi guys,
I notice that there is a lot of conversation on measuring the terminal voltages on these fantastic coils.
I have read a lot of books on the subject over the years and found that it takes very complicated and costly equipment to measure these very high voltages. It seems that everything has a bearing on the accurate measurements such as the electrostatic field, barometric pressure, humidity, AC- DC. frequency and many other factors.
Just these days I have been making some investigations on how to
accurately measure voltages with spark gaps. The spark mechanisms are
really quite complicated, and the many academic works about the subject
not always agree.
Of course we all like boasting rights of our coil output and it's great to say that we are putting out a million volts but in reality, no one is sure of this number
You will almost surely conclude that a "million volts" is at most a few
hundreds of kV.
The best experts cant agree on what an inch of arc is equal too but it ranges from 10,000 volts per inch to 50,000 volts per inch of arc. The makers of stun guns claim anywhere from 100,000 volts to a million volts for less than a three inch arc as one example. Who would believe any of this to be fact.
40 kV at most.
We do need our boasting rights and I for one would be in favor of adopting our own voltage standard which we could all understand and would do away with all this guess work.
I have determined that 20,000 volts would be a good and close to accurate standard for us to use and we would all be on the same page. Be it right or be it wrong the 20KV per inch should be close for our use.
If we should adopt the 20KV volts per inch standard, I would be in favor of naming our new standard after the late and great Mr. Harry Goldman who gave much of his life to making coiling fun and interesting for all of us to enjoy. We could simply call this the Goldman standard. A 50 inch arc would equal a million volts. Doesn't that have a nice ring to it?
So, the big question is, what do you fellows think of this plan.
Spark formation depends essentially on the electric field at the
electrode surfaces, with the field, much smaller, between the electrodes
mostly defining if a spark forms or just corona appears. The spark
length is not directly proportional to the voltage, unless,
approximately, in the simple case of a gap between large flat surfaces.
Even in this case the breakdown voltage is a nonlinear function of the
distance. A formula that works well for this case is:
k = 293*p/(760*T) (correction factor for pressure and temperature)
Vbreakdown = 24.22*k*d+6.08*sqrt(k*d) kV
p = pressure in Torr (mm Hg)
d = distance in cm
T = Temperature in Kelvins
This tends to 24.2 kV/cm for large gaps (between flat surfaces) and
results in larger values for small gaps (30 kV/cm at 1 cm). This formula
is in the book by Meek and Craggs "Electrical breakdown of gases". I
tested this formula with sphere gaps, and it works, but produces
somewhat lower values than my experiments. A formula that agrees very
well with my experimental setup, with R being the radius of the spheres, is:
E=27200*k*(1+0.54/sqrt(k*R)) (maximum electric field)
Vbreakdown=E*4*d/(d/R+1+sqrt(sqr(d/R+1)+8)) (approximation for spheres)
This formula can be traced to the old book by Peek "Dielectric phenomena
in high-voltage engineering". The first formula seems better for small gaps.
Antonio Carlos M. de Queiroz
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