I just ran a series of tests taking the temperature of the brew water
at the point where it passes the 5mm E61 grouphead bolt. Note that
these measurements were taken with a blind portafilter and the 5mm
grouphead bolt removed where a 0.2 degree accurate temperature probe
was loosely inserted. In this way the measurements were essentially
taken at atmospheric pressure, and not at the typical brewing
pressures during an extraction.
This is justified per the reasoning in my last, last post above. That
is the initial brew-water which leaves the pump at the very start of a
shot, which is the water that will largely contribute to the
extraction of small volume shots such as the ristretto, will
essentially be nearly at atmospheric pressure as the air in the brew
path has not yet been displaced. In fact any shot whose volume is
smaller than, or close to that of, the volume of the entire brew path
may be subject to this effect.
It is suspected that for all operating points whose combined desired
brew temp and temperature offset exceeds 100 deg C, then the brew
water will instantly cool to 100 deg C (and give up some steam in the
process), resulting in a cooler shot than expected. Note that this is
the case even if this brew water were to subsequently repressurize due
to the presence of tamped coffee in the p/f. The initial "fill" of
water in the brew path is not likely to regain much, if any, of this
lost temperature.
So to test this hypothesis I ran a series of tests with the target
temp set to 96 deg C, but with an offset that was varied from -1 to -9
deg C. As expected the data shows that there was no change to actual
depressurized water temps until the combined desired brew temp and
temperature offset dropped below 100 deg C.
Note that before running each test I waited until the brew boiler
light switch ON, then swtiched to OFF. This insures that the brew
boiler temperatures were at their maximum possible temperature (an
entire hysteresis step, in my case 1 deg C, above the target boiler
temperature).
Expected Estimated
Boiler Temp at Temp at
Setpoint Offset Temp G/H bolt P/F
96.00 -9.00 105.00 99.00 96.78
96.00 -8.00 104.00 99.00 96.78
96.00 -7.00 103.00 99.00 96.78
96.00 -6.00 102.00 99.00 96.78
96.00 -5.00 101.00 99.00 96.78
96.00 -4.00 100.00 99.00 96.78
96.00 -3.00 99.00 97.39 95.17
96.00 -2.00 98.00 96.78 94.56
96.00 -1.00 97.00 94.17 91.94
Note that the temperature measured at the G/H (where the 5mm allen
bolt normally resides) does not change until the boiler temperature
drops to 100 deg C or below.
This perhaps explains my inability to eliminate sourness in the cup by
increasing either the set point or the offset temps, as my typical
setting of 96 with an offset of -8 already placed the brew water
boiler to exceed 100 deg C.
This left me with no more "room" to affect an increase in brew water
temps for such a short shot. I would have to expel more than a brew
path's volume of water before I'd get the benefit of any 100+ deg C
brew boiler water.
Note, however, that this hypothesis predicts that even a flush will
not take care of this effect, as a flush occurs practically at
atmospheric pressure.
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