A possible design flaw? I hope I'm wrong about this...
In the course of trying to figure out the source of some recently sour
shots, in the past few days I've been through a tumult of long
sessions on the Brewtus running various experiments with brew temps,
offsets, thermocouples and thermometers, switching roasters and
espresso blends, posting to boards, formulating various hypotheses,
and even discussing with my fellow but non-java oriented engineering
colleagues. Through it all I've formulated a hypothesis that though
so far I've been able to confirm by experiment, I still hold out some
hope that I've overlooked something that invalidates my conclusion.
Here's the conclusion that I've reached re. my Brewtus I, but should
also apply to the whole Brewtus series, and, in fact, most other
similarly designed dual boiler espresso machines. The conclusion that
I've reached: given a thermal drop of x deg C from the pump to the
portafilter (noting that x is negative), the maximum possible brewing
temperature at the portafilter is 100+x deg. C.
So given an offset of, say, -6 deg C, this implies a maximum possible
brewing temperature at the portafilter of 100 + -6 = 94 deg. C.
A corollary to this is that the thermal drop x, which is the offset
that we program into our temperature controller as C0, can easily be
mistaken/mismeasured to be greater than the "intrinsic" thermal drop
across the grouphead, given a key point in the hypothesis. According
to the hypothesis a meaningful thermal drop x can only be measured
under certain operating conditions.
So here's the hypothesis:
The water that will brew your espresso is the very first volume of
water which exits the pump. As we pump out the water for brewing our
espresso, it is first being pumped out into an empty (and therefore
non-pressurized) brew path whose volume I estimate to be around 1.135
ounces. Thus barring any significant losses at this point the water
at the output of the pump will have a maximum temperature of 100 deg C
or Tbrew_boiler, wichever is less. This is because once the water
leaves the pressureized confines of the brew boiler it no longer can
reach temperatures above 100 deg C. Any water which was kept in the
boiler above 100 deg C will instantly lose its excess heat as steam
and drop to 100 deg C.
Now this water, as it travels from the output of the pump to the
portafilter, will have dropped x degrees by the time it reaches the
puck of compressed coffee.
Thus, noting that x is a negative quantity, the brew water is
determined by:
Tpump_out = 100 deg C : for Tbrew_boiler > 100 deg C
= Tbrew_boiler : for Tbrew_boiler <= 100 deg C
Tbrew_water = Tpump_out + x deg C
Thus:
Max(Tbrew_water) = Max(Tpump_out) + x deg C
= 100 + x deg C
Ouch! Not a good situation when our desired brew temps are so close
to 100 deg C and our offsets, irrespective of any debate, is at least
(-) 5, the original factory default for the Brewtus I.
So here's the corollary which follows the hypothesis:
The "offset" that we program into our temperature controller x is only
a meaningful offset from the boiler temperature Tbrew_boiler when
Tbrew_boiler <= 100 deg C
This can be seen by the relation for Tpump_out above. Under any other
condition the offset x is only an offset off of a fixed 100 deg C.
Thus if one tries to estimate their offset x via a measurement of the
brew boiler temperature, it is important to do so only when
Tbrew_boiler <= 100 deg C.
(I suspect that the tendency for users to suggest offsets greater than
(-) 5 might be due to brew boiler and portafilter measurements taken
under conditions where the brew boiler exceeds 100 deg C. Under these
conditions every degree increase in brew boiler temperature will not
increase the brew water temperature at the portafilter and hence
result in a larger "apparent" offset.)
I've verified the hypothesis via a simple experiment that's documented
here: http://groups.google.com/group/brewtus/msg/107c4e427e109637?hl=en
Basically I take repeated measurements of the brew water under a fixed
target brew temperature but varying offsets. I found the brew water
temperature not to change for any condition where Ttarget_brew_temp -
x > 100 deg C (again noting that x is negative). As soon as
Ttarget_brew_temp - x <= 100 deg C, the output brew water temperature
started to change accordingly.
Note that the hypothesis applies to only the initial volume of water
sufficient to fill the total volume of the brew path, which my
experiments estimate to be around 1.135 oz. As any water pumped
subsequent to this initial volume will be pressurized due to the
purging of any air in the brew path, subsequent flow can break the
temperature restrictions implied above. However the initial 1+ oz.
for even double espressos represents a sizeable portion of the total
brew volume, and in my case of super short ristrettos this initial
volume alone is enough to generate several shots worth of brew water
under these temperature-compromised conditions.
Note that temperature flushes will not relieve us from these limits,
as the temperature flush still leaves the brew path empty of any brew
water. (However temperature flushes will help us get as high a
temperature of brew water possible while still being limited by these
constraints.)
I wouldn't have gone as far as to post this if my experiments did not
support the hypothesis. Unfortunately so far they do just that.
Believe me this is a case in which I'd enjoy being wrong!
Anyone else out there notice the same? Any findings or experiences
supportive or contradictory to these findings? Any alternate
hypotheses for the observed behavior? Any of you input or feedback
would be most welcome.
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