How do vacuum coffee pots work?

largeVacuum coffee pots are probably the most visually appealing way of brewing fresh coffee, as the process is clearly visible at each stage and there’s a lot of movement involved. What actually happens when you put that pot on the heat source, though?

The vacuum pot consists of two glass globes, sitting one on top of the other. When assembled, the lower chamber, which contains the water for brewing, is more or less airtight. As the chamber shouldn’t be filled to the brim, the space above the water will contain a mixture of water and air vapour.

When you introduce the heat source, the pressure exerted on the walls of the container and the surface of the water increases with the temperature. When the pressure exceeds the external air pressure, the water is forced up the funnel into the upper chamber which contains the coffee.

Eventually most of the water in the lower chamber is forced through. Once the liquid level drops below the end of the funnel , the system is no longer closed and the pressurised gas in the lower chamber starts to escape through the funnel and ‘gurgle’ through the coffee, which is when the brewing process occurs.

When the coffee has ‘gurgled’ for a minute or so, depending on how strong you like it, removing the pot from the heat will stop the process. The gases in the lower chamber will start to cool, the volume of air will decrease and the internal pressure reduce, forcing the liquid in the upper chamber back down as brewed coffee.

As the pressure between the upper and lower chambers won’t be completely equal, when you remove the upper globe before serving the coffee you’ll need to use a little effort to break the seal.

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