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Topic: Resistors and Coffee Pucks--Really 9 bars? (long and confusing) (9 msgs / 388 lines)
1) From: Mark Neuhausen
I've thought about this mentally and gotten myself all confused.  So, I
thought, why not get the expert help of this forum?
I have an electronics background and was thinking about the coffee in the
portafilter like an electrical resistor.  The water comes in at the top at 9
bars and exits at the bottom at atmospheric pressure (0 bar relative to the
pump measure) where it "dribbles" out the spout on the portafilter.  The
properly ground coffee provides a resistance that results in a current of
about 1 oz. every 12-15 seconds (double shot in 30 seconds) with the 9 bars
of pressure (voltage).
If I go into a resistor that has a 120V drop and measure the voltage
relative to 0 along the resistor, I will find a fairly uniform voltage drop
from the input to the output end.  In other words, if I measured the voltage
at the midpoint of the resistor from one end to the other, I would get about
60 volts.  If I measured 1/3 of the way from the input, I would get 80
volts, and 40 volts at the 2/3 point.  At the output from the resistor, I
would measure 0 volts.  The same thing should be true for the coffee puck
once I got the portafilter filled with water and coffee began flowing.  I
would measure 9 bars at the top of the puck, 0 bar at the bottom, and have a
drop of 9 bars across the depth of the coffee.  The change in pressure as I
do through the puck is more or less linear.
Why does this confuse me?  Because it tells me that the "perfect" cup of
espresso results from having a ground coffee mass with a resistance that
only allows about 2 ounces of 190-200* water at 9 bars pressure to flow
through it in 25-30 seconds.  Using R=V/I, with V being 9 bars and I being 2
oz., I get an optimal resistance of 9/2 equals 4.5 bar/oz.  These are
equivalent to ohms (espresso units?).  I can get this R at 4.5 bar/oz. by
using a certain amount of a certain ground of coffee tamped to the correct
pressure for that coffee volume and grind.  And voila, a perfect extraction
of 25-30 seconds.
Now I get confused, because this also tells me that I really don't need a 9
bar espresso machine.  But then I realize that there is this magic 25 second
extraction time, where under or over is bad for taste.  But that extraction
is not really due to exposure of the delicate ground beans to almost boiling
water, but most likely a combination of that severe environment along with
the flow of water extracting the precious oils.  If I had a 4.5 bar machine,
and put in half the coffee to get half the resistance, I would still get 2
oz. in 25 seconds.  But obviously, I would also get a weak cup of coffee.
Adding more coffee would eliminate the weakness, but increase the extraction
time and that is bad.  If I had an 18 bar machine, I could put in double the
coffee or grind finer or tamp harder, and get the 25 second extraction time.
But in the case of double the coffee, would I not also extract more oils?
What I deduce from all of this is that over time, the rules and
repeatability and portafilter designs have been maximized for an infusion
pressure of 9 bar.  With practice and time, I could make some other
pressures and portafilter designs work, but the goal would be to equal a 9
bar system.  It also leads me to deduce that a 58 mm portafilter is probably
required for the optimal espresso, as the extraction times and 9 bar
pressure and all the other factors can be optimized in this environment, but
probably sub-optimal for a 53 mm portafilter like on my Barista machine.
It also explains in my convoluted mind why there is less repeatability in
other brewing methods.  An espresso maker (person) carefully adjusts the
grind and tamp pressure based on the kind of coffee and degree of roast to
get repeatable flow with constant pressure.  A French press or vacuum pot or
drip pot is more using a standard measure and grind.  This tells me that
with a lot of note taking and care, one could probably get consistent
results with a French press or vacuum pot if they got into the same "fine
tuning" used by an espresso maker (person) of varying the coarseness of
grind and pressure of the tamp.
Bottom line to me.  I should stick with my espresso maker and upgrade to a
58 mm portafilter machine when I can to get that perfect shot at least once.
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2) From: Mike McGinness
You're one sick puppy and fit right in!:-) Fascinating thoughts there. But
didn't you equate the puck to a variable resistance ranging from 9-0 from
top to bottom rather than constant 9 which of course would change your OHMS
Law equation?
Fine tuning grind for all brewing methods is beneficial and doable with a
good grinder IMO.
MM;-) aka Kona Konnaisseur miKe mcKoffee
Dual Variable Transformer Rosto Roasting
Rocky grindin' - Miss Silvia brewin'

3) From: Jim Schulman
Hi Mark,
I think the resistance analogy is only useful up 
to a point. When you get to Bars/oz, you've 
probably gone beyond it.
It's probably more useful to think of pressure 
drop along the puck. In most machines, the single 
basket is tapered, so the single puck has roughly 
the same thickness as the double puck. Perhaps 
it's bars/inch that really counts.
On 16 Jan 2003 at 8:25, Mark Neuhausen wrote:
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4) From: Rick Farris
Mike wrote:
No, it's got to be constant resistance or else the "voltage drop" across the
puck (in your example) would be logarithmic.
If you took a carbon resistor and scraped away the insulation, and connected
one side of your voltmeter to one end and then touched the other end to the
middle of the resistor, you'd find half the voltage.  One-quarter way would
yield 1/4 the voltage, etc.  That's all he's saying.
-- Rick
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5) From: dewardh
As with so many things, the deeper one digs the more complex it gets . . . (up 
to the point where you realize that it probably doesn't matter, anyway ). 
 And "approximations" are just that, "approximations" . . . the starting point 
from which we discover the "hidden gotchas" that render the approximation moot. 
 It is amusing, btw, to see electronics used as the analogy to explain 
"plumbing" . . . it used to be the other way around . . . (how times change 
There are two more-or-less "parallels" to the coffee puck in the world of 
"industrial chemistry", filter cakes and GC/HPLC columns.  They represent the 
range of what you might see in your portafilter.  Filter cakes have a 
definitely non-uniform pressure drop . . . their mechanism of formation 
encourages that.  There is typically a concentration of fines at the bottom of 
the cake, and as the cake forms, and compresses, the lower layers are compre  
ssed more.  A chromatography column, on the other hand, is deliberately made to 
prevent migration of fines, and "pre-compacted" to ensure uniform pressure 
drop.  It is not completely outlandish to draw comparisons to untamped and 
tamped coffee in your basket.  My guess would be that the more you tamp (and 
the higher the tamping pressure) the more uniform (linear) will be the pressure 
drop across the puck during extraction.  Actually measuring it might be an 
interesting project for a graduate student somewhere (if it hasn't been done 
Whether it matters is another question.  There seems to be an "engineering 
consensus" that a 35 lb. tamp (in a 58 mm. basket) is sufficient to if not 
linearize at least "standardize" the behavior of the puck, and to produce the 
extraction result that most users of "pressurized extraction" machines seek.
bar espresso machine.  But then I realize that there is this magic 25 second
extraction time, where under or over is bad for taste.
Of course you don't.  People make perfectly fine coffee at 1 bar in a French 
Press or a Vacupot, and definitely not at any "magic 25 second extraction 
time".  Low pressure extraction just takes longer.  Remember that the original 
*reason* for pressure extraction was to produce a freshly extracted cuppa 
*faster* ('espresso") . . . the resulting "flavor differences" were a 
secondary, relatively minor, and not necessarily an always favored "side 
effect".  There is no one "right way" or "best way" to make espresso, or any 
other kind of coffee.  Maybe there is a "best way" to duplicate, for example 
David Schomer's "style" of espresso . . . but not even he produces, "by 
appointment to the Queen", the only, or even "the best" coffee in the world.
Use the machine you've got, adjusted as you see fit, to produce the coffee that 
you like.  Nothing else matters . . .
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6) From: Jim Schulman
On 16 Jan 2003 at 10:19, dewardh wrote:
This is good stuff!! There's been endless 
discussion about tamping and fines migration on 
alt.coffee, but I don't remember seeing this 
relationship between loose compaction and fines 
migration discussed.
So according to the this, the Italian style, no 
tamp, espresso behaves more like a crema enhancer 
style shot, with most of the pressure drop at the 
bottom of the puck, and the top layers steeping at 
even pressure. Whereas the Schomer style, heavily 
tamped, espresso has a more linear, pressure-
extracted throughout, dynamic.
I wonder if the crema on no tamp espressos is 
frothier but not as long lasting, in line with the 
crema enhanced ones?
Jim Schulman
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7) From: Dan Bollinger
fyi:  My preliminary tests of dry, compacted coffee pucks is that the top of
the puck is more compactr than the bottom.  Not by a lot, about 15%.  Dunno
how this effects what you are doing, but thought I'd let you know.  Dan,
coffee geek
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8) From: rev mark gilstrap
:It is not
        I agree/think that if you tamp once you may get 
        compaction concentrated at the top of the puck.
        I have taken to tamping at least two and usually 
        three times while loading up for a double.  
        However, I do not know how important this is.
        A crema enhancing portafilter restricts the flow 
        at the tail end and a well tamped puck restricts it 
        at the front end (see Jim's comments below).  But 
        does the flavor and body *profile* depend on the 
        pressure so much?  I have heard Barry (I think) 
        say no.   Are some components relatively more (or 
        less) soluble at pressure than others and this is what 
        makes espresso so wonderful, or is it simply the 
        intensity of the increased total dissolved solids not 
        otherwise obtainable except under pressure? 
        It is more complex than either.
        Whether the tamp is uniform or zoned (more dense at 
        top and less so at bottom, or by halves or thirds)... any 
        way you vary the pressure gradient across the puck 
        in a a non-pressurized portafilter, you still end up with 
        the tail end of  the flow seeing ambient prssure.   Ohms 
        Law applies whether it is a linear or tapered resistor.  
        So,...  in a linear model, if equal portions of the puck 
        see high and low pressures, and this gives a great taste, 
        and it does, then what is the benefit - if any - of the 
        higher pressure at one end, vs the lower pressure at the 
        other end.  Are both necessary to get the best profile?
        Or is the pressure just the vehicle for the interaction
        between an increasing concentrated hot aqueous solution 
        of good and bad flavors (mobile phase) passing through 
        a selectively absorptive cellulose-based chromatographic 
        Very good question.  We are thinking along the
        same lines.
        Are no tamp expressos as good/bad as tamped/crema 
        How does the crema enhancer work?   Is it just 
        dropping the pressure all at once to make fizz?  Is
        that why it is less enduring than a tamped espresso 
        that has loads of stable surfactants > 5 minutes.
        It seems to be fairly universally posted that the more 
        uniform pressure in a crema enhancing portafilter does 
        not improve, and probably degrades, the taste profile.  
        If this is so, does it then follow that the pressure does 
        not enhance the flavor by selectively extracting pressure
        soluble species, but simply intensifies the flavor by 
        increasing the solubility of everything?   and that a little low 
        pressure extraction at the tail end may be necessary to 
        selectively extract only the most soluble (more surface 
        active = foamy) species?   
        Then there is the chromatographic effect.  The puck is 
        a column.  Species extracted at the hotter front end
        under the highest pressure are differentially readsorbed 
        as they pass through the bed of grounds.  Elute too long 
        (>25) and the bitter components make it to the detector 
        (your tongue).   If this chromatographic effect is an important
        aspect (and I think it is), then the packing of the column 
        and freedom from channelling is critical to the flavor, and
        to the lower than predicted caffiene content of espresso
        (considering the mass of coffee used).  
        If the chromatographic effect is important to the flavor 
        then low pressure units like my Krups Gusto would need 
        to have longer columns per gram - or smaller diameter
        filters - which they do.  Hmmm...   Also crema enhancing
        units would have less refined extractions.  hmmm again.
        Maybe.  Perhaps no tamps self-organize during 
        pressurization and act more like tamped systems.
        A volumetric pump with enough oomph (EMF) to 
        push through at the same rate (current) no matter 
        what the resistance seems to be part of the answer.
        Someone mentioned single filters being tapered so 
        that it had the same depth as a double.  That is not true 
        for my low end Krups machines.  I can only pull a single 
        if I grind really fine and tamp hard.   Otherwise the 
        pressure doesn't build and the flow rate (channeling?) 
        is huge.  Here again,  a rotary pump that pushes at 
        the same rate no matter the resistance (+/-) is a 
        partial solution.  
        My Gusto just never could quite get to the pressure
        necessary to push through particles of a size small 
        enough to be tastily extracted in 25 seconds.  
        I either had to grind more coarsely or tamp less.  
        Either way I got a too weak and bitter extraction .   
        I  guess the "ideal"  9 bar was derived by experience.  
        I am guessing that there are other combinations that 
        would work,  but would also require redesigning 
        several components in order to keep the delivery of 
        undesired species during the extraction time low .  I 
        can imagine a very high pressure extraction of very 
        finely ground coffee giving *very* quick pulls.  But 
        58 mm has apparently proven to be optimal at 9 bar 
        for the 25 seconds of extraction necessitated by the 
        typical grind . 
        I know that my "15 bar" Gusto probably never got to 
        that pressure (9 bar) and if it did it was only as the static
        pressure behind a choked filter.  My much better Novo
        has a much stronger pump,  but an LM or Techno 
        pump can probably easily deliver more than 9 bar at
        considerably higher flow rates if needed.  I'm starting to
        ramble.  Good night.  
        I will count variables, not sheep
        pr mark
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9) From: David Lewis
At 12:37 PM -0500 1/16/03, Greg Scace wrote:
I love it when Greg takes off his gloves and puts on his duct-taped spectacles!
Less than 0.1 percent of the U.S. population gave 83 percent of all 
itemized campaign contributions for the 2002 elections, according to 
the Center for Responsive Politics.
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