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21 November 2003

Sorption Pump Vacuum Systems

by Shawn Carlson
Excerpted from "Scientific American's The Amateur Scientist", first published October, 1996.

For many applications, sorption pumps are the vehicles of choice for creating a good vacuum. They have no moving parts; instead they work by chilling a type of substance, called a sorbent, to a temperature at which it absorbs gases. Activated charcoal works, but a molecular sieve is better. Molecular sieves are little pellets with so many microscopic nooks and crannies that they have fantastically large surface areas; a one-gram pellet may have more than 1,000 square meters of surface.

When chilled, air molecules get caught in these microchasms. A 50-gram supply can pump a one-liter volume down to 10 millitorr in 20 minutes. (Atmospheric pressure is about 760 torr.) Half a gallon of molecular sieve from Duniway Stockroom sells for about $35.

To hold the sorbent, you need to obtain a Pyrex bulb approximately one inch in diameter and three and a half inches long, with a one-quarter-inch glass tube neck. A local glassblowing shop will probably make you one for less than $30. Fill it with the sorbent, then stuff in a little glass wool on top to keep the molecular sieve in place. Over the neck of the glass tube, slip a short length of flexible tubing, called Tygon tubing (check your local hardware store).

Before it can be used, the molecular sieve must first be activated--that is, it must be baked. Wrap the bulb with heating tape, available from Omega Engineering in Stamford, Conn. (800-826-6342 or 203-359-1660; model no. FGS0031-010). The 12-inch-long piece sells for $20. Or cannibalize an old toaster for its heating element. In either case, be sure that the heater does not cross over itself and that all of it touches the bulb. Wire in a dimmer switch to control the temperature of the heater.

You will need to chill the sorbent with liquid nitrogen. Don't worry--liquid nitrogen is inexpensive (less than $1 per liter) and easy to obtain (try the Yellow Pages under "Welder's Supplies"). It can be safely handled if you exercise some common sense. Store it in a large plastic drink cooler--10 liters will last a weekend. Make sure the container does not have a spigot at the bottom. Do not put the lid on tight, or else pressure from the boiling nitrogen will build up inside and burst the container. To pump the air out of the canning jar, immerse the Pyrex bulb in the liquid nitrogen. The molecular sieve will suck the air out of the glass chamber, producing a vacuum as low as 10 millitorr.