Vac pots


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Siphon brewers, vacuum coffee makers, or just vac pots, as they are most often called, are very probably the coolest sort of coffee gagdetry you can find. The glass contraptions look intriguing by themselves, but watching them brew is the real fun. It's like chemistry class revisited.

Want to know how it works? Please read the info below.
If you are interested in buying a "vac pot", have a look at the "Vac pot shopping" page. Have one already, but want to know how to get the best tasting coffee, do give the "Vac pot brewing" section a read. Lastly, if you're interested in a little history, there might be some interesting info in my "Vac pot collection" pages.

How's it work?

Easy. Let's have a look at a vac pot & what happens during brewing.

The simplest vac pots consist of just five parts, a bowl with a handle, or stand, a funnel with a gasket & a filter. The funnel sits in the bowl, with the gasket sealing the mating parts. The pic on the right is my Bodum Domingo with the gasket (left) removed from the siphon tube on the funnel (top). On the right you can see the filter which clamps to the bottom of the tube with a spring. Parts of a vac pot
Just started... You start by filling the lower bowl with water, insert the funnel & scoop the ground coffee in the funnel. Then put the vac pot on the stove & enjoy the show.
As the water gets warmer, it slowly rises up the siphon tube, through the filter into the upper bowl & mixes with the coffee.
There's two effects at work here. First is the expansion of the water as it gradually heats up. This is a pretty weak effect, but it does let some water reach the coffee when it's only about 50°Celcius.
Water is about 50 degrees
Coffee & water is well mixed The second effect is a magnitude stronger & takes place at the phase transition. As you may know, at around 100°C (or 373 K) water transforms from its liquid form into steam, or as physicist say, the water changes phase. Since the volume of steam is several times that of liquid water, the pressure rises quickly & pushes the liquid upwards through the funnel, until a new equillibrium is found. Some water will remain behind, just under the siphon tube. This water will slowly be turned into steam, causing the bubbling in the upper vessel. This steam ensures that the water "upstairs" will be kept at a fairly constant temperature of about 95°C
When the heat is removed again, the reverse happens: the steam left in the lower bowl will transform back to water again, but since the volume of water is, as explained, much smaller, the pressure drops again. This, then, causes the mixture in the funnel to be sucked back, through the filter, into the lower bowl - separating the grounds from the brew & leaving you with a delicious cup of coffee waiting to be poured in. Downstairs again
Apart from the route down south again, the process is pretty similar to what happens in moka pots.
It's a bit of a long, boring description, but when you actually *see* it happen, you'll understand what's so cool about it.


Curious? Want to know more? Ask!