The picture below will help illustrate how the Presso works. Mechanically it is a simple machine, employing an O-Ring to seal the inner chamber and build up pressure. This makes tamping critical, as its a closed system except for what pressure can escape through the coffee puck. Its somewhat like an Aeropress with handles attached to the top for extra leverage and a portafilter instead of a paper filter at the bottom, but don't take that comparison too far.
The O-ring is the clever part of the machine's design, it sits in a wide groove approximately twice its diameter. The groove holding the O-ring is at the top of the pressure chamber. The top half of the groove has air/water holes in side wall of the cylinder and bottom plate of the fill chamber. These holes are sized so that when the O-ring is moved to the bottom of the groove water can flow from the fill chamber into the brewing (pressure) chamber. When the O-ring is moved to the top of the groove, the holes are sealed and pressure can then build up. And since an O-ring is round it easily rolls up and down as soon as the plate direction is reversed, until it is caught by the opposite groove edge and then gets pulled along, scraping the side wall of the inner cylinder.
The picture below will help illustrate how the Presso works. Mechanically it is a simple machine, employing an O-Ring to seal the inner chamber and build up pressure. This makes tamping critical, as its a closed system except for what pressure can escape through the coffee puck. Its somewhat like an Aeropress with handles attached to the top for extra leverage and a portafilter instead of a paper filter at the bottom, but don't take that comparison too far. The O-ring is the clever part of the machine's design, it sits in a wide groove approximately twice its diameter. The groove holding the O-ring is at the top of the pressure chamber. The top half of the groove has air/water holes in side wall of the cylinder and bottom plate of the fill chamber. These holes are sized so that when the O-ring is moved to the bottom of the groove water can flow from the fill chamber into the brewing (pressure) chamber. When the O-ring is moved to the top of the groove, the holes are sealed and pressure can then build up. And since an O-ring is round it easily rolls up and down as soon as the plate direction is reversed, until it is caught by the opposite groove edge and then gets pulled along, scraping the side wall of the inner cylinder.
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