Vac Pot

A vacuum brewing device uses the temperature changes in water and the resultant vaporizing of water to both brew and filter coffee. The way it works is fairly simple. A vacuum brewer has two globes, one on top of the other. They are connected with a syphon tube running between them.

When assembled and heat is applied, the water in the bottom vessel heats up, expands, and creates water vapor. Vapor (or gas) takes up more physical space than liquid, so as the vapor increases, the pressure on the glass grows, and the remaining liquid seeks to escape. The only route it can take is up the syphon tube to the top globe, through the filter and into the coffee grounds.

Once the device is at full boil, all the water that can go up the syphon does. At this point, vapor is escaping up the tube instead of water, and this in turn continually heats the water/coffee mixture in the top globe, and also creates a gentle rolling action in the mixture. This agitation helps the process, and keeps the top coffee at near perfect constant brewing temperatures.

After about 2-3 minutes of brewing , you remove the heat source, and almost immediately, all these heat-expanded gases in the bottom globe start to contract and turn back into liquid. This contraction contains a fair amount of force. As the vapors contract, something is required to fill the space being left in the bottom globe, and that something is your brewed coffee liquid in the top globe. It gets pulled back down the syphon tube through the spent grounds and the filter (gravity isn't much of a help to get through those obstacles), and it is pulled down with sufficient force that it actually "vacuums" your coffee grounds once all the water has gone "downstairs". The finish is usually a bubbling action in the bottom globe for a few seconds as air is pulled through the spent grounds.

The "vacuum" name of this brewing method doesn't come from that particular action; it comes from the fact you are creating a partial vacuum in the bottom globe that uses the coffee liquid to fill said vacuum. The entire process takes about 12 minutes, including water heating, brewing, and finishing, and is very fascinating to watch.

Information Source: I Need Coffee and Image Source: Bodum