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#2 FAQ - Mechanical Pump - Diffusion pump?

Posted: Wed Apr 15, 2009 2:32 pm
by Richard Hull
The beginner to vacuum within these forums is bewilderd at first. Let me start by saying that we in the fusor world are different from detailed scientific vacuum technology, but not free from many of it basic tenats.

The kind and quality of the amateur fusioneers vacuum system will be determined not only by his wallet but by where he or she wants to go in the effort. Where you want to end up, if you will.

What kind of pump do I need?

#1 The mechanical pump

For the youngest enthusiast looking at making a simple demo fusor for a science project a simple, even rather weak or overused mechanical pump is all that is needed, provided he plans to end the endeavor here.

Completely out of the picture are any type of diaphram pumps used as so called vacuum pumps. Also out are any water pumps or any fluid pumps. Most all piston pumps are out as well.

There are "single stage" and "two stage" mechanical pumps. You really want a two stage pump. The difference is a longer pump down on single stage units and a somewhat higher low end pressure. The cheaper pumps are often single stage. Make sure of what you are buying.

You need, at minimum, an old air conditioner repairman's vacuum pump. These can be found used for anywhere between $50.00 and $150.00. The cheapest new two stage pumps of this type typically run a minimum of $200.00, but a good new one of higher pumping capacity might run nearly $300.00 or more. All of these pumps are called a "direct drive" type, in that the motor's shaft is directly coupled to the pump's shaft. Such units are usually rather light weight, (under 40lbs), and have a carrying handle. Direct drive pumps are usually rather noisey and run at high speeds.

The best mechanical pump might be a used "Scientific Vacuum" pump. While there are many modern scientific direct drive type pumps, the classic scientific pump is termed a "belt drive pump". These are made to run continuously for scientific and educational use. They are all very heavy and weigh, with their metal base, well over 50 lbs with large pumps weighing over 150 lbs. Oddly, these pumps can often outperform a direct drive type and cost far less, provided they are found and purchased, "used".

These pumps are generally very quiet and less rough on the ears that the high speed, often straining, direct drive units. They run at half to one quarter the speed of direct drive pumps. A gentle lub, lub, lub sound is the norm.

The above are the normal mechanical pumps used by virtually 100% of the amateur community.

Note*** If you buy a used pump, be prepared to have to clean and refill it with pump oil at a minimum. Some used acquisitions may need minor to major overhauls and replacement of parts. Be aware of what you are buying. Few, if any sellers can tell you what the lowest vacuum pressure at the pumps head is currently capable of.

The mechanical pump can usually only reduce the chamber pressure to 1/100,000ths of an atmosphere or to about 10-40 microns. (the more modern term is millitorr). This ia perfect range for demo fusors. This is roughly equivalent to the atmospheric pressure found 50-60 miles above the earth. This type of pump in a modern, complex vacuum system is refered to as a "roughing" or "foreline" pump.

What do I look for?

You will need to get at least a 2-4 CFM, (cubic foot per minute), pump with a 5 or more CFM unit being better. There is little need for more than a 15 CFM pump.

This completes the quick rinse on mechanical pumps. While a mechanical pump is a minimal "must have" to play in this game, there is a second pump for those who are serious about going on to do real fusion work..........

A Caveat....................

Real fusion work CAN BE DONE with only a mechanical pump... However, that pump must be PERFECT and fully capable of single digit micron pumping. Almost no used pump is capable of this and none of the cheaper air conditioning repair, direct drive pumps can do this even when new. If you do have a pump that can do this extremely deep pumping, fusion is possible. However, you will be wasting a lot of your valuable deuterium gas as such a system will have to run constantly and a lot of gas will be wasted to bring the fusor's gas atmosphere up to over 90% deuterium. This is needed inorder to do fusion. You are on your own here.

The second pump that most all successful fusioneers have is a diffusion pump.......

#2 The Diffusion pump

The diffusion pump or "diff pump" is an add-on or "following pump" to the mechanical pump. This pump uses high speed jets of special oil to blast the last of the resistant air molecules out of this pumps exhast port into the inlet of the mechanical or "roughing" pump so that they can be passed out of the system, at last.

Pressures attained at the inlet of the diff pump can be as low as 10e-7 torr or about 1,000 to 10,000 times deeper that just the mechanical pump can do alone. This amounts to a pressure in the chamber of about that found 180 miles above the earth which is considered outer space. This pump is referred to as the "high vacuum pump"

Note**** In systems with a diff pump, you will need two high quality valves. One to separate and isolate the mechanical pumps inlet with the diff pump's outlet and another valve between your fusor chamber and the inlet of the diff pump.

Diffusion pumps can be found very cheap, considering what they do. Most all must be purchased used. They can be found on e-bay or at Lab-X auction sites. Some few vacuum companies like Dunaway can supply used pumps that have been reconditioned and cleaned with many new parts in them. On e-bay, expect to pay between $25.00 and $100.00. A rebuilt might cost $300.00. You will need to buy the special diffusion pump oil. Silicone oils made for such pumps can cost about $30.00 for 100milliliters. (Dunaway, Lesker, etc.

What do I look for?

You need to look for a pump between 2" and 4" inlet diameter. You really want an air cooled version, with a fan on it, otherwise, you will be forced into a more common water cooled version where you will have to have a source of running water and a drain.

The ideal is about a 2.5 inch, air-cooled unit.

Your fusion chamber will need some sort of valve between it and the intake of the diff pump. Mating the two with a valve in between can create a bit of a headache. This can often be a daunting task, but if you locate the pump and valve before you build your chamber, the job is a lot easier as you just design the chamber with the proper mating flange to the valve and the valve interfacing flange to the diff pump.

By using a diff pump you will be able to immediately begin to do fusion once you have reached 10-5 or 10e-6 torr since at these pressures, admitting deuterium to raise the pressure to about 10e-2 torr for real fusion work you have a chamber filled with 99%+ deuterium.

Note** There is a pump called a turbo-molecular pump that can be used in place of the diff pump, but few amateurs have them as they are prohibitively expensive. New, they can cost over $3,000.00 with controller. Most offered for sale used are only the pump with no controller. Be very careful here. I suggest you pick up a diff pump rather than burden yourself with possible "error buying" of a "turbo pump" right off at the beginning of your fusion quest.

I will let this conclude this quick rinse buyer's guide to vacuum pumps used by the amateur fusioneer.

Re: FAQ - Mechanical Pump - Diffusion pump?

Posted: Wed Apr 15, 2009 7:14 pm
by George Schmermund

Re: FAQ - Mechanical Pump - Diffusion pump?

Posted: Wed Apr 15, 2009 11:15 pm
by adrian.f.h
The pump on the link looks very similar to mine. I don't know if there are big differences between the different types of these cheepo-pumps. Anyway my pump looks like that and caused lots of trouble with oil vapour (boiling quite fast). It needs to run hot before it can pump down to a demo-pressure-level and this takes about 10-15 min. I always turn the pump off not long after that because of being afraid of it to die from overheat and it contaminating my chamber what happened quite often in the past. Mine works for demoplasma but it's hardly suitable as backing pump.
I’m still working on a phase converter to run one of my other pumps.


Re: FAQ - Mechanical Pump - Diffusion pump?

Posted: Thu Apr 16, 2009 8:50 am
by John Futter
Nice FAQ Richard

maybe you should add that a minimum of a two stage baking pump is required

this in light of the previous two replies to this thread.

Air-conditioning vacuum pumps are usually single stage and the pumping speed is unimportant due to pumping long lines of small diameter. All these pumps have to do is get below the vapour pressure of water. To back a turbo or diffusion pump these would have to be in top condition and even then a diffusion pump of 2" or above will present a load greater than the backing pump of this type is capable of supplying especially the high gas loads presented by a fusor.

Any Welch duoseal (slow rotating) or Edwards {{varian}high speed} two stage pump would be ideal with Leybold tristage pumps {high speed} being the pinnacle of backing pump requirements for fusor. The tristage pumps reach 1 by ten to the minus 4 millibar ultimate vacuum in new condition which would give a high pumping speed in the 1-2 millibar vacuum range as used in a typical fusor.

As for the diffusion pumps
there are two types commonly referred to as either diff pumps or diff stacks

Its easy to tell the difference, in diff pumps the pump body is parallel, in diff stacks the body swells towards the top.
In both cases the body will be covered with a water cooling tube if water cooled-- note Richards comments, the water load for cooling is not insubstantial, a cubic meter of water an hour for a 360L/min pump.

Now for a money cruncher
diff stacks work best with Santovac oil charge -- it is very expensive

diff pumps can use Santovac or ordinary silicon oil such as Dow corning 704 , 705 fluid. The silicon fluids are much hardier at high gas loads (read lousy vacuum) than the Santovac which tends to turn into a nasty very hard black mass when mistreated.

The third type of diff pump that maybe had of the likes of ebay is the mercury diffusion pump. Size for size these offer the best pumping speed vs diameter but because of the mercury they should be avoided at all costs. Some mercury pumps can be cleaned and operated on the silicon pump oils albeit at reduced pumping speed. This type of pump usually has a liquid nitrogen trap above and was very common in older mass specs.

To avoid the mercury pump make sure you see the rating plate on the pump giving oil charge and type of oil. If the plate mentions mercury or grams/ oz's of charge then it is likely a mercury type

Re: FAQ - Mechanical Pump - Diffusion pump?

Posted: Thu Apr 16, 2009 2:53 pm
by Richard Hull
Thanks for all the comments.

I have added a small blurb on the need for a two stage pump in an ideal system. A number of the more expensive air conditioning vacuum pumps are two stage. Even a fairly used one with a new charge of vacuum oil can hit 15-20 microns in a 6" chamber. I have three 5-7 CFM air conditioner pumps that have little trouble doing this.

The little 69 dollar new pump from harbor freight makes no bones about it. It clearly states that it can be expected to do no better than 75 microns, which is pretty poor. You get what you pay for.

Richard Hull

Re: FAQ - Mechanical Pump - Diffusion pump?

Posted: Thu Apr 30, 2009 7:01 pm
by derekm
May I be presumptious to suggest an addition to the FAQ
-A mention of Critical backing pressure - The pressure at which a Diff pump can start and its typical value e.g. 0.4- 0.9 torr. for a small Diff pump (Diffstack 63/150). Thus a newcomer can get a feel for the range a diff pump operates over compared to a mechanical.


Re: FAQ - Mechanical Pump - Diffusion pump?

Posted: Fri May 01, 2009 5:37 pm
by derekm
John Futter wrote:
>note Richards comments, the water load for cooling is not insubstantial, a cubic >meter of water an hour for a 360L/min pump.

Which make of pump is that ... the Edwards Diffstacks seem to quote a much lower figure...

Re: FAQ - Mechanical Pump - Diffusion pump?

Posted: Sun May 10, 2009 6:32 pm
by Squidhat
Hey, when using a diff pump, how do you switch from mechanical to diff? Do you have two separate openings that each pump is attached to, and once you've got the pressure you want with your mechanical, open a valve on the diff pump line and close off the mechanical's line and let the diff pump take over?

Re: FAQ - Mechanical Pump - Diffusion pump?

Posted: Sun May 10, 2009 7:22 pm
by Starfire
Squidhat - the mechanical pump is a fore-line pump - it assists the diff pump and runs in tandem with the diff.

Diffs can not start until the pressure gets low enough.

Re: FAQ - Mechanical Pump - Diffusion pump?

Posted: Sun May 10, 2009 9:32 pm
by Squidhat
Oh, they run at the same time! I knew that you needed to have a low enough pressure to let the diff pump start working, I just thought you would turn the mechanical pump off when the diff pump was running. I was going to get a trap for the mechanical pump before, but knowing that they'll be running at the same time definitely solidifies that need. Thanks John!