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Steam in My Espresso Machine

  1. May 26, 2012 #1
    I drink a lot of espresso. From my minimal knowledge of espresso machines, I understand that the water is heated into steam which creates pressure forcing the steam through the coffee grounds. After every batch of espresso I let the machine sit overnight unplugged. Presumably, the water gas mixture returns to room temperature. Every morning when I unseal the heating chamber pressure is released.

    Why? In the ideal gas law P = (k*m*T)/(V*μ*m_u) and T was returned to the starting value. Wouldn’t P also return to the starting value?

    Perhaps some other value in the equation is changing? There is less water which would mean m would be less at the end than the start but this would mean less pressure not more.
  2. jcsd
  3. May 27, 2012 #2

    Simon Bridge

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    The pressure equalized by the steam escaping through the grounds ... thus reducing m in your equation. When it cools, the remaining water changes state. If the chamber is efficiently sealed I guess I'd expect the hiss when you open it is from air entering the low pressure chamber.

    "The thermodynamics of espresso machines" seems to be a much discussed topic.
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