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Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Lots of people seem to build and like the RQ-style spark gaps, where multiple short sections of copper water pipe are placed inside a large PVC tube with optional air blast. While admittedly not RQ-style, the first gap I ever made was with parallel sections of 3/8" tungsten rod placed over an air blast. My observation was that, particularly when the air blast was running, the sparks would only jump at the very ends of the rods. Presumably there are a few factors working against such a gap:
1) The air pressure is going to be somewhat lower at the ends of the pipe sections than directly in between them.
2) The breakdown voltage is somewhat less at the ends of the tubes because they're sharp-edged.
Whilst searching for something unrelated today, I ran into Liberty Brass, a company that sells brass half-spheres and spheres:
The half-spheres, in particular, looked like just the thing for solving this problem. I.e., you stick these on the ends of your gap pipes (or large-diameter solid brass rod, if you want to be really cool ) and I would think that should solve the problems listed above.
Has anybody else observed this problem with RQ-ish gaps and air blasts? It really seems like the one of the principle downfalls of such gaps, since then the fouling and heating of the metal is concentrated in one place.
Tragically, I no longer have such a gap to play with, but it would make for a nice experiment...
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
If the gaps are nice and parallel, mine are generally pretty regular.
So you want to use the spheres to make the ends of the segments round? Seems like a hammer/grinder (for hollow/solid) would do the job just fine, although if you need to add too much of a bend it might look sorta nasty...
Geometrically Frustrated Registered Member #6
Joined: Thu Feb 02 2006, 04:18AM
Location: Bowdoin, Maine
Posts: 373
That might not be a bad idea, but it seems that most people are doing fine without, so you might want to try some alternatives first. Make sure there are no burs on the end of your pipes, check that they are parallel (maybe put something flat in between adjacent tubes to test), and if that fails, try adding more gaps (and reduce gap size accordingly). The arcs should begin somewhere in the middle of the pipes and get blown out towards the end. If the gaps are getting too hot, it's possible that the arcs could be migrating to and staying at the end.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
In my design, the blast was coming from behind the pipes, rather than from one end as in an RQ gap. That is why I said "RQ-ish". In an end-blown RQ gap, perhaps the air pressure is more regular, however it never seemed to me as though the classic "RQ" was an actively-quenched gap; rather, the air flow was small and mainly for cooling. In my case, I needed the arc to be blown out. An RQ didn't see too good for this, because as you say, the arc gets blown along the pipes.
I no longer have the gap fully built, but I do still have the tungsten rods, and they are clean and polished. No burs that I can see.
Increasing the number of gaps makes some sense; the stresses on the air will be less, and perhaps then the disparity between the air stresses in the middle and on the ends of the tubes will be insignificant. I don't know.
If I build a lower-powered or table-top coil again soon, I'll have to play with this some more. Ideally I'd find a machinist friend to just machine me some "copper lozenges" out of solid copper or brass or something. That would be "cool"
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Capsule-shaped electrodes for static gap?
) and I would think that should solve the problems listed above.
