Espresso drill-down

These guys caused quite a splash raising a boatload of money for their expresso maker. Pretty cool.

The media has been calling it an “open source” espresso maker… yet only the software is open source, and that is the PID control software for the temperature. From what I can tell they didn’t write it either. Sort of related: check out the $85 osPID which is just a general-purpose PID unit which could be attached to anything, like say your existing coffee maker.

But this got me thinking about open source hardware – how does the pressure part of an espresso machine work?

ShaColby helped me through the anatomy:

  • grind coffee with burrs
    • the time of grind would determine the amount of beans ground, since it’s a hopper above.
    • there may be a secondary hopper for the ground coffee – this determines the amount used in the brew
    • one school of thought has it that ground coffee can sit without ill effects – obviously this is not the policy of e.g. Blue Bottle – they require something like “under 5 minutes sitting”
  • grounds moved into the press
  • grounds are tamped – this presses the grounds into a “puck”.  The higher pressure the tamp, the greater the density the grounds.
    • less density = faster water flow = weak coffee, not enough flavor from oils.
    • greater density = slower water flow = stronger, bitter, acidic coffee.
      • too dense – it will just be mud, and water won’t get through
    • Note: Aeropress was designed about this
    • ideally you want a finer grind (more surface area) with a less pressure tamp ?
  • water enters the head – it is pressurized.
    • It filters through… how long does it take?

Some links on anatomy and “mods”:

From Make Magazine and other mods we can see that a desirable thing to control with a micro controller is the water temperature. What else could we determine?

  • grounds size – burrs are settable.  You’d set the spacing – No sensors needed?
  • bean quantity used per cup
  • tamping pressure (as in Make v.12)
    • …which determines ground density.  Measure both for more accuracy.
  • water volume
  • water pressure – up to 14 bar?!
  • water temperature (as in Make v.04)
  • measure: time at pressure.
    • estimation before brew
    • Display what did happen
Other features
  • instrumented diagnostics based on coffee qualities
  • an easy way to knock non-puck grounds out of the head




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