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Re: HELP!!! I Screwed-UP

Posted By: John Anderson
Date: Saturday, 30 December 2006, at 2:15 a.m.

In Response To: Re: HELP!!! I Screwed-UP (Mr. Neon Signs)

> As for my gauge..it was error on my part of
> not follwing routine as I normally do...I
> left the stop-cock open while using a spark
> coil to test for leaks after
> cleaning/re-fitting my manifold and had a
> moment of brain freeze.

As is common in a busy neon shop after a long day. Mistakes DO happen. A safety ground path helps minimize the damage when they do IMO.

> Apparently, it was enough spark to fry the
> gauge. I sent the sensor tube and gauge to
> Hastings or repair..Hopefully they can fix
> it..

Your gauge fried because is has a ground potential through which the dischage completed its path. Electriacally isolated, battery powered gauges will help - but still have a small amount of ground-potential. I feel an earth-grounded gauge tube provides more reliable protection. I have seen both bombarder flashbacks and spark-coil dischages safely dissapate though a grounded gauge tube.

Unless you are going to enclose your entire system and gauges in a vacuum or dielectric-oil - you can't escape capcitive coupling to ground. That's why a neon tube still flickers if you remove one electrode wire. You can also inadvertantly become the ground path yourself by touching a metal Robbins valve, gauge tube, or other conductive part (of your glass manifold) - with dangerous consequences.

> I always close the stop-cock when
> processing..For many years I did this with
> no problem. I rarely get any flash-back but
> at one time I grounded the pyrex manifold
> with and electrode to ground. It created too
> many problems.

Everyone has to adjust the pressure to the 'trode manufacturers recommended level while processing (usually around 1 torr). Sure, you can repeatedly cut the bombarder on and off as you do this... but IMO that's a lot of trouble and it slows down the pumping process. We never do that here - bombarding contuniues while we momentarily open the stopcock to maintain the proper pressure. Flash back only occurs if something goes wrong, like the pressure gets too high or too low by lapse of attention. In these cases, it is safely carried off to ground. This very, very rarely happens. But when it does - I'm sure glad the ground path is there. So are the sensitive electronics peppering my manifold.

If you have problems with a ground path in the manifold - it means the path of resistance is less the bombarder pole to the ground than it is inside the tube. This shouldn't be - even with a mid-point grounded pole pig. Pressure is too low or too high. Or, the grounding electrode is too close to the tube. I think an ideal place for the safety ground is after the tubulation glass (going toward the manifold), where the manifold is back to large diameter tubing and before any gauges or stopcocks. You can also reduce the tendency to flashback by removing the emission coating from this electrode - or, as I now do just have a small dumet wire hang down.

I don't think anyone here wants to see two people duke it out over 180-degree opinions on grounding electrodes. There are many shops all over the world making quality neon using both methods. I'll happily discuss it further with anyone off-line - but I've already said more than I intended in this long thread. This is my last post on it.

Peace to all - and I wish everyone a prosperous 2007

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