I called Heather right after her original post. Here's what I told her:
The main issue is the grinder. In conjunction with a smaller gicleur'ed machine, you'll see different things than you might be used to.
In a dry (pre-extracted) puck, I can identify three distinct elements that restrict the flow:
- the insoluble solids
- the solubles
- and a subset of both: the fines.
Remember that the coffee is the final flow-restrictor in the system. In an unscientific sort of way, I like to say that an in-line flow-restrictor (gicleur) helps espresso quality by relieving some of the flow-restriction burden from the coffee itself, letting the coffee focus more on extraction instead of flow-restriction. A bit of anthropomorphism here for sure, but if the coffee can speak, the coffee can focus.
Based on burr design and RPM's, I'd hypothesize that flat-burr grinders (super jolly) create more fines than conical-burr (robur) grinders. Also, newer/sharper burrs would create more fines than older/duller burrs as well (Heather told me that she was using a Robur, and that the burrs were almost a year old).
Generally speaking, as the extraction progresses, a number of things happen, with a couple of them relevant to our discussion: solubles are extracted from the puck, and fines are pushed downward (often into what I like to call a "floor").
In an extraction environment with a sufficient quantity of fines, the fines will be pushed downward (through the matrix of larger particles), and with the bottleneck created by the basket holes, grid-lock into that "floor." As the extraction progresses, the flow will actually slow down as that grid-lock gets more packed and more backed-up.
If, however, there is a relatively low-quantity of fines, that floor won't be estabished sufficiently, in which case the flow rate through the coffee will increase as the solubles are carried away by extraction into the water.
We'd always been a flat-burr (super jolly) shop, only experiencing Roburs at competition. It was always weird adjusting to the different flow that the Roburs created (that, and the unrestricted flow on the machines prior to Charlotte USBC). Now we have a Robur in-house (as well as a Kony), and it's interesting to compare the conical vs. flat side-by-side.
BTW, hand-in-hand with this line of thinking is my belief that the proliferation of conical-burr grinders encouraged people to up-dose more and more. I (again) hypothesize that flat-burr grinders create more fines. With fewer fines (proportionally) from a conical-burr grinder, you make up for it by grinding more coffee. Just a theory.