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Vacuum Pump Selection

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High Vacuum Pumps

For high vacuums in the sub-microTorr range you will need a three stage pumping system that will cost as much as an economy car.

The first stage is a "roughing pump" that takes the chamber from atmosphere down to the milliTorr range. The next stage (closer to the vacuum chamber) is a "diffusion pump" that boils oil into a vapor and in a cooled section the oil rain traps gases which can then be removed by the roughing pump. The third stage is a "cold trap" which is just chamber off the main vacuum chamber cooled by liquid nitrogen or liquid helium. Any remaining gases freeze to the walls of this chamber.

The pump down starts with roughing as far as it will go then turning on the diffusion pump and letting get as much of the remaining gas in the chamber. This can take hours. Just before an experiment the cold trap is filled to take the vacuum down to minimum the apparatus can achieve.

A nice little article on a high vacuum system from the UC Berkeley Microlab site is here.


Medium Vacuum Pumps

Plasma work typically needs 1 to 10 Torr so you can just use just about any vacuum pump or compressor to reach that vacuum including some hand pumps.

A roughing pump for a vacuum system will cost you about $1000 and probably need its own circuit breaker. It is probably overkill for a small plasma lab but if you can get one cheap as a surplus unit it will probably last forever. If you find a used pump make sure it has never been run without pump oil. Like a car engine it only takes a short while for the pump surfaces to become scratched if they are not well oil. Repairing a damaged roughing pump will cost you as much as buying a new one (think engine job).

At the low end are hand pumps (for those who want forearms like Popeye) and scavenged compressors. Some (not all) refrigerators and air conditioners have compressor units that can be run as vacuum pumps. In general newer appliances have cleaver designs that work fine as refrigerators but cannot be used as a vacuum pump. Do not buy a new A/C from WallMart and pull the compressor out expecting to get a vacuum pump. You will have more success twenty year old unit collecting dust in someone's basement you can get for free. For the ecofriendly removing the compressor will release Freon which damages the planetary ozone shield. See if you can get a repair shop to recycle the Freon for you.

The problem with using compressor is that pump a pitifully small volume and you will need to put fittings on the flimsy tubing to make them useful. It is very likely you will invest a lot of time making the thing work and spend even more time as you suffer with reliability problems trying to keep the thing working.


Automotive A/C Maintenance Pumps

The best price/performance for the home inventor is a vacuum pump used in the auto repair business to replace the Freon in automobile air conditioners. Car A/Cs run in pretty rough conditions and moisture can get into the Freon because of imperfect seals and large hot/cold cycles. This moisture combines with the degraded Freon to make an acid that eats away at the metal A/C tubing.

When you get your car A/C repaired they suck out the acidic Freon and vacuum test the system looking for leaks. The pumps they use are rated at 50 milliTorr and may go lower to achieve that rating. The pumps are designed to be portable and take a fair amount of abuse so they are ideal for a small lab. If get a new pump or a well maintained used pump and you keep a proper oil level it should last forever with practically zero trouble.

My pump is a Robinair 15234 which pumps 1.2 cubic feet per minute and consumes about 2 amps when struggling with a maximum vacuum in the chamber. A fact sheet from Robinair for my class of pump is here. There are many other manufacturers of these pumps: CPS, Mastercool, Mountain are other major brands. Low volume pumps like mine cost from $150 to $400 new and Robinair is typically the more expensive. I got mine used but in extremely good shape for $100.

Using a 4" PVC end cap vacuum chamber my Robinair 1.2 CFM pump achieved a full vacuum in very quick 25 seconds. For the rest of the vacuum system continue reading on the "Vacuum Chambers" link.