In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.
Subject: Re: Vacuum troubles
Date: Mar 31, 08:57 am
Poster: Richard Hull
On Mar 31, 08:57 am, Richard Hull wrote:
> Since the last time I talked about vacuum systems, I have a problem. I'm using an oil
>diffusion pump with a hydrocarbon pump fluid
>with a vp of 1.5x10^-7. pumps ok but am not using
>a cold trap. This pump was made to use mercury
>and works very well but you of coures do need a trap. Question!! what's wrong with using mercury
>if you can stop backstreaming. The oil deal seems to contaminate the system, would a cold trap at
>-45 degrees be of help here?
> Could anyone help me here??
> Thanks Bob
You have guessed correctly! Any reduced temperature trap will help! I have no experience with using a mercury diffusion pump with oil, but it should work fine. (as you note)
Many smaller oil diffusion pumps avoid a cold trap with a "cheveron" type of cooled baffle (most often water cooled), although small, 2" throated systems often get by with air cooling the baffles. This arrangement really reduces the oil contamination in small, intermittent use, fast pumped systems.
A cold trap is a must have item if you pump hard over a long period of continuous experimentation.
The cold trap can do a fantastic job with just an alcohol/dry ice slurry in it. If a cold trap can't be found, a snaking (up/down) metal tube trap with its "U" sections in a pan of dry ice will do wonders.
Naturally, LN2 is the ideal situation. The cost of LN2 is fairly low considering how effective it is. Here in Richmond, it is $2.25/liter, which is about the price of good beer! In the open, it disappears about as fast as good beer too!
Most LN2 traps only need a liter or so, but boil off demands that at least a 5 liter quantity be purchased for a good day's experimenting. The cost is not the LN2, it is in obtaining the proper Dewar to efficiently store and transport the stuff! Without a dewar you are just throwing your money away.
In my city there are 15 welding and gas supply houses which will sell 1 -1000 liters of LN2 right off the back dock ala-gas station style to any yahoo who drives up with his own dewar!
On the other hand, we have only two "carbonic houses" who will sell dry ice to John Q. Public. The minimum amount is a 10 lb cake of it for $20.00. It can easily be kept for almost a week in a small, (emphasis on small), styrafoam ice chest! You will have to crush it up as you need it. This exposes you to some cold burn possibilities, but if handled well, will really work out OK for the impoverished, but resourceful "vacuum head". Note, you must make a slurry with alcohol if you are to want to load up a store bought cold trap.
Needless to say, use good ventilation especially if you use cheap wood alcohol (methanol-Solox).
One of your key questions was what is wrong with mercury in a pump? Well, the obvious is the mercury contamination issue of your vacuum system. This is most painful if you are doing electrical discharge work. A micro amount will foul a system up bad and demand complete disassembly and a through and tedious cleaning!!!! If the normal mercury glass diffussion pump gets broken, a real serious helath hazard results. In California, you may be sentenced to death and be subject to $200,000 fine and have your children taken away for breaking an old merury thermometer!
In this day and age, the oil diffusion pump with some of the super pump oils (pentavac- santovac) with some form of trap or baffle are the poor man's answer to deep vacuums.
For simple experimental fusor work, I can't imagine the need for more than a cute little 2" CVC diffusion pump with a small air cooled cheveron baffle. A cold trap on this little system would just be a plus for long term work.