Maitlands Coffee Siphon_

How a Coffee Siphon works:

A siphon coffee maker works on the principle of expansion and contraction of gases - actually one gas, water vapour - is what allows the device to brew a full infusion style of coffee and filter the grounds efficiently, leaving a generally clean, pristine cup.

Siphon coffee makers are made up of four parts: the bottom container where the water initially sits and the brewed coffee eventually rests; a top container that has a siphon tube attached to it (and a hole in the bottom of the vessel), where the coffee brewing takes place; a type of sealing material (usually a rubber gasket) to help create a partial vacuum in the lower vessel while brewing is taking place, and a filter, which can be made of glass, paper, metal, or cloth.

There is also a heating source, and there’s usually three types - a cloth-wick alcohol burner (slowest), gas or electric stovetop (faster), or a specialty butane burner (fastest). There are additional heating devices, including the halogen burner system that is part of the $20,000 setup that Blue Bottle Cafe in San Francisco has for their siphon coffee makers.

Once the siphon coffee maker is assembled, heat is applied to the lower container. As it heats up, some of the water is converted to a gas - water vapour. A gas occupies a lot more space than its liquid or solid variant, and it can expand as more heat is applied. Gas can be compressed, but only to a point, whereas liquids do not compress. The water vapour continues to expand and it seeks some relief from all the compression it’s starting to have. The only escape route out of the bottom vessel is the siphon tube traveling up to the top, but the problem is, there’s a lot of liquid blocking its way. So what does the gas vapour do? It pushes the water up the siphon tube!

This is how the brewing water “defies gravity” and gets up top past the installed filter and starts saturating the coffee grounds. Heated (though not boiling) water will continue to force itself up the siphon tube until the vapour gas in the bottom vessel can have direct access to the siphon. By design, all vacuum coffee makers do not have siphons that sit flush with the bottom vessel: there’s always a couple of millimetres of clearance (or more). This leaves some water in the bottom vessel which serves two purposes - protects the glass (if it boiled dry, glass would superheat and could crack), and has a continuing source of water to turn into - you guessed it - more steam, vapour, gas!

This is a desirable thing. Once the water in the bottom vessel is lower than the siphon tube, water vapour starts pouring up the siphon into the top chamber. This does two things - it keeps the liquid all up in the top vessel where the full-saturation brewing is taking place, and it continually heats the top vessel, maintaining near-ideal brewing temperatures (90C and 95C (185F and 204F)). As the vapour bubbles pass through the top liquid, it may appear that the active brewing is “boiling” but in fact it isn’t under normal brewing conditions. If you leave the siphon coffee maker long enough, eventually the top vessel water temperatures will reach 100C, but this takes a very hot heat source and five or more minutes of brewing time in most cases.

Once the brewing time is completed (different sized siphon coffee makers require different brewing times), the heat source is removed and more simple physics take over. Remember that gases expand when heated and that gases also compress under pressure. When heat is removed, the gases start to contract and shrink. Eventually all the gas in the bottom vessel contracts enough that it starts to create a partial vacuum of pressure (negative pressure), and it start pulling the brewed coffee back down, through the filter.

Another thing is going on as well - phase change. Some of the gas vapour starts changing back into its liquid form: water. This is almost like a miniscule turbo boost for the contraction of gas and vacuum forming, aiding the pull of the brewed liquid back down to the bottom vessel. At first it starts slowly, but as you watch, things will pick up speed and unless the coffee filter has been excessively clogged, the last half of the water travel down south will happen twice as fast as the first half did.

The effect is so efficient that once all the liquid has passed back down to the bottom vessel, air is drawn through the grounds up top, drying them out, to fill the vacuum void in the bottom vessel and balance out the pressure to normal room atmospheric pressure. For some, this is how the siphon coffee maker got one of its names - the ground coffee is literally “vacuumed” dry.