bartb wrote:
Hi Jim, jimlux wrote:One of the main points that Paschen showed was a curve identifying breakdown voltage with gap distance or pressure. It's a curve because of the molecular density of air and the probability of electrons getting from point A to B.Ahem... Paschen actually didn't take any data down low enough to get to the minimum. His data fits pretty well on a straight line, and says only that p*d = constant for a given voltage. I just got a copy of the graphs from his paper, and I'll put them up in a day or two. (the paper's already out there.. send me an email and I'll send you the link).Yes, your correct. I was speaking of Pachens Curve as shown in the North report. I already have copies of Pachen's paper in German and a transcript in English, but the data tables are still in German. I would like to see the actual data graphs when you get those up. I know there's not enough there to show everything and it would tend to be linear in the narrow window he made actual measurements in air.
Paschen only made measurements from about 2 Torr on up. One of the guys at work thinks he made measurements lower down, but had a lot of variability, so didn't include the measurements in the data.
I think he just didn't do the measurment because of pump limitations. Paschen was a great experimentalist and very careful: Doing the measurements over many months, he noted that they changed with the seasons.. whether it was due to different sensitivity of the Righi electrometer he used for voltage, or a change in breakdown strength due to humidity, I don't know.
Also important to remember that for small gaps and low pressures, the electrode material becomes quite important (photo ionization, secondary emission, etc.)
In the "straight line" region (where Paschen actually worked), you're looking at avalanche breakdown. But when the gap gets small or the density gets low, (i.e. the mean free path starts to be comparable to the gap), other factors come into play. There's some well known data for small gaps where the breakdown does NOT occur at the shortest distance.I agree. This is the area in Norths report shown on a graph (Ch-7, pg-55). And it makes sense. As /d /reduces to the point where free electrons become far a few between, density is reduced and thus it becomes more difficult to cause breakdown.
A coworker has an interesting graph from a recent (last 10 years) paper showing breakdown field vs gap, and it looks like field is inversely proportional to gap. That is, the breakdown voltage is constant.
> Maybe terminology is a problem here. For me, breakdown voltage is the arc voltage (that is what I meant as arc voltage).Do you have another definition for arc voltage?
Arc voltage, to me, is the drop across the arc, after it's already broken down and is in steady state. Basically the sum of resistive losses in the arc column and cathode drop.
I'm speaking of a typical sphere style gap with normal TC spacing. For example, 2 spheres of 1"D with a gap distance of 0.25". In this example, should you double the pressure leaving everything else the same, I think you have almost doubled the arc voltage as well.
Yep.. that's a moderately non uniform, and I'd expect breakdown voltage to be proportional to pressure/density.
I'm sure in reality, a hyperbaric gap is not doubling the pressure, simply increasing it by some margin. But, that margin is increasing the breakdown voltage. If you set a standard atmosphere blown gap (increasing the distance to accommodate the equivalent hyperbaric gap breakdown voltage), only then would you have a true comparison. The hyperbaric gap may "win" at the end of the day, but at least it would be an apples to apples comparison. From a mechanical build standpoint, I think the hyperbaric gap has already won.
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