AndyS wrote:Looking at the graph makes me think that a declining pressure profile (DPP) might keep the mass flow rate fairly even through the middle portion of the extraction. At the end, the puck is nearly spent, so there is less material being extracted and the mass flow drops. With a DPP the mass flow would drop even faster, but this is when you'd typically cut off the shot anyway.
I would like to underscore Jim's remarks about preinfusion
because preinfusion is critical to the pressure profiling.
One thing that Illy talks about is the migration of fines and its relationship to flow. According to the experiments discussed in the Illy book, as fines migrate from the top of the puck, they collect downstream and slow the flow. Excess pressure can cause fines "dams" and a resulting flow restriction well before the puck has been "spent". As a corollary, in the lever machine pressure mod made by Alchemist, we see that by easing off
on the lever, a dammed flow can be made to flow freely again.
There are three very different kinds of "resistance" offered by the puck:
1) The good resistance when preinfusion has caused the puck to swell
2) The bad resistance caused by dammed fines
3) The bad resistance from overtamping and/or too fine a grind
The latter two are actually inhibiting the extraction.
Yet to an eye trained on the the numbers and the graphs and plots alone, the three extractions might appear "to present" the same; the difference might not be apparent in the numbers. A brew pressure of, say, 8 bar when the resistance is caused by dammed fines or by too fine a grind|too dense a tamp is different from a brew pressure of 8 bar that results from a properly preinfused and swollen puck.
If the numbers included preinfusion duration and pressure, as well as some number representing the fineness of the grind|weight of the tamp, the data would be completer, but that could be a hassle. Elsewhere Jim has remarked about doing postmortems on pucks preinfused to varying degrees, and this would be good to bear in mind for these pressure profilings. The profiler should determine the threshold point where the puck has been thoroughly preinfused. A very gentle preinfusion pressure (too gentle for channeling to occur) and then looking for the first drops is a good seat-of-the-pants approach if you want to avoid the hassle of post-mortems. But gentle preinfusion pressure is paramount in that case.
Depth of roast and the bean itself also can have significant effects on the porosity of the puck. Since different beans have different makeups (e.g. robusta more solids than arabica has) would it be advisable to run these tests with an S.O. arabica bean instead of a blend?
P.S. Judging from the spring characteristics and piston diameters of those lever machines we have taken apart and measured here on H-B.com, they produce maximum brew pressures of 6 to 7 bar. I haven't had the pleasure of reading a photo essay on a heftier spring lever.