Why Does My Espresso Machine Hiss When Unsealed?

[jkg0]

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.

 

[Simon Bridge]

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.