Let’s consider the basic physics behind each step of vacuum or siphon brewing. Initially, heat is applied to water in a globe—as shown in the lead photo—and this system is “open” to the environment. Because you are applying heat to the system, some water is converted to steam. But that water vapor is free to exit the globe.
By installing the funnel, as above, you effectively “close” the system. The gasket at the neck of the funnel keeps steam from escaping the globe. As long as heat is applied to the system, water continues to be converted to water vapor. Since a gas takes up more space than a liquid, more and more water is displaced as the amount of steam in the globe increases. In effect, the water vapor pushes or “kicks up” the column of water into the funnel.
This process continues until the level of the steam in the globe reaches the bottom of the neck of the funnel, as shown in the photo above. At this point, any additional water vapor escapes the system by bubbling up through the column of water or coffee slurry in the funnel. The liquid remains in the funnel as long as you maintain an adequate amount of steam pressure in the globe. A gentle simmer in the globe will do the trick, and ensure that the slurry does not overheat.
When the brewing cycle is complete, you initiate “drawdown” by turning off the heat source. Once you stop applying heat to the globe, you can actually trigger drawdown by simply blowing across the globe. Cooling the globe causes water vapor to convert back to water. Because water takes up less space than water vapor, a low pressure system forms in the globe as the steam condenses, which sucks or pulls the brew in the funnel back down into the globe. This is the vacuum stage from which the brewer gets its name.
If you listen very closely at the end of drawdown, you will hear a sucking sound as the partial vacuum in the globe pulls the last of the liquid out of the bed of grounds, and some air rushes in behind it to equalize the atmospheric pressure between the globe and the environment. In many cases, you will see a transient crema-like bloom on the bed of grounds, some of which gets vacuumed through the filter and into the globe. You will also notice that the seal at the neck of the funnel has been sucked more tightly into the mouth of the globe. Simply rock the funnel and pop it loose.
While the eponymous “vacuum” gets all the credit, steam is actually the primary motive force behind the siphon brewing process, which is perfectly fitting given that this is a steam age brewing system.