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Re: Metal manifold
Posted By: SVP Neon Equipment In Response To: Re: Metal manifold (Don Miner)
Date: Friday, 13 February 2009, at 2:58 p.m.
Other than what Don so eloquently said (LOL), there are some other inherent problems. Mercury will chemically combine with brass and more than likely bond to it. Obviously if you are only doing new clear red tubes mercury is not an issue, but that is probably not the case. Even if you never actually sucked liquid mercury into the manifold, small amounts of mercury vapor will find their way into the manifold and condense on the interior surface, thereby causing the contamination. Removing it is difficult at best. There may be other means to do so, but the easiest way I know of is to wash out the manifold with nitric acid. This in and of itself has its own set of problems. Too strong of a nitric acid solution, or leaving the solution in the manifold for too long will literally "eat" the brass, thereby etching the interior surface of the manifold. This will tremendously increase the surface area on a microscopic level. This may not sound like a big deal at first thought, but it will increase the time required to evacuate the manifold to a high vacuum and it will make the surface more prone to collect contaminants, which will increase evacuation time even further. On the other hand, using too weak of a solution will not remove the mercury contamination, but may still attack the brass to some extent.
Another potential problem is how it is constructed. Simple solder joints will eventually develop micro pipes (microscopic leak paths) through the soldered joint. Once this happens, the manifold will never be able to maintain a high vacuum and finding the leak(s) will more than likely require a helium leak detector. (The other option is to just resolder all the joints and hope you resolve the problems). If it is actually welded together instead of soldered, it will present fewer problems in the long run relative to leaks (assuming there were none to start with). In either case, the inside of the manifold should be thoroughly cleaned before first use.
It may also be difficult to find good quality high vacuum valves that are truly rated for high vacuum use (hardware store grade valves cannot be used). Even if there is a way to adapt aluminum or stainless high vacuum valves to the brass, there is still the problem of the brass itself. Copper exhibits similar problems, even if it is high grade oxygen free copper (OFC).
It's for these reasons and more that the Townsend-type brass/copper manifolds were never a viable long-term choice. Typically after a few years they would get to the point of not being able to achieve and/or maintain a clean high vacuum, and often produced (in my opinion) poor quality units.
If you are dead set (no pun intended) on a metal manifold, stainless is the only choice. But to do it right won't be cheap. And keep in mind that, like all neon manifolds, it will have to periodically be cleaned. So consideration as to what will be required to clean it and whether or not you can do it yourself should be a concern.
Mark
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