HomeRoast Digest


Topic: Variable and Fixed Resistors--Coffee Pucks (37 lines)
1) From: Mark Neuhausen
Bear with two paragraphs and rambling thoughts and I get to my confusion
path to draw the G-d shot.
Regarding the resistor analogy, a fixed value resistor has only half the
resistance if you had a way to tap off the middle.  A coffee puck sees 9
bars of pressure at the top, and 0 bars (or close to it) at the bottom.  The
pressure at any point within the puck would range from 0-9 bars depending on
where one took the measure.
As I said before, the more I think about this, the more confused I become.
I have a thicker puck in my 53 mm basket portafilter than someone would have
in a 58 mm basket.  Assuming we both have 9 bar entering, I would have to
grind slightly coarser or tamp slightly looser to get the same 25 second
extraction.  If I grind slightly coarser, wouldn't that mean that the
optimal extraction time actually lengthens slightly (as dust grind over
extracts)?  So, my new thought process (if you can call these thoughts) is
that I should tamp about 20% harder, 40 lbs. instead of 30 lbs. to be able
to use a similar grind as a commercial portafilter.  (58 mm diameter to 53
mm diameter is about 20% more surface area and hence, 20% less thickness for
the same weight/volume of coffee.)
This thought process leads me down the confusion path, as now I start
thinking that a 25 second extraction with a coarser grind and harder tamping
should give a different result than a finer grind with lighter tamping.  And
if that is true, it would mean that there really is an optimal tamping
pressure/grind size for the G-d shot.  And if so, that would then say that
the idea of just leveling off a basket and leveling it and finishing it with
a 25 second extraction is different than a 30 lb. tamp.
I think I drank too much caffeine today.
-Mark
homeroast mailing listhttp://lists.sweetmarias.com/mailman/listinfo/homeroast


HomeRoast Digest