This is an interesting thought, and I don't have an answer. As mentioned above, the transparent portafilter videos somewhat dispel the notion of the puck expanding until after the shot, but they show a conventional extraction (perhaps even an unusually fast ramp, by the look of it) rather than what I described above. I suspect that changes in the nature of the so-called "fines migration" are at the heart of why this increased resistance happens with a faster pressure ramp, but I don't know-and I don't have the means to establish a causal link here. The main goal, to me, is to investigate how to leverage the dynamic rather than to explain why exactly it happens.Chert wrote:I might be repeating points you made earlier in other discussions, but isn't this a result of how much the puck can expand?
A more gradual pressure rise allows the puck volume to expand more, therefore less resistance. But a more rapid pressure increase keeps the puck volume smaller and particles tighter, hence greater resistance.
I used a pressure regulator. It's set at about 20 psi (1.4 bar), the lowest pressure the regulator can deliver to the pump without causing cavitation (or something similar that sounds awful) at high flow rates, like when flushing the group. That's not to say that a different regulator than mine would have the same minimum pressure.Peppersass wrote:Nicholas, how did you reduce line pressure to 1 BAR? Did you dial down your regulator or did you add an in-line needle valve?
In a related question, have you been able to determine the minimum line pressure required by your gear pump to attain maximum shot pressure?
When the pump is off at the beginning of the shot, it appears to give enough flow resistance by itself that the pressure I see on the puck eventually reaches just below 1 bar. I haven't experimented with a needle valve yet, although I have played with using the paddle as a choke valve at the beginning of the shot by setting it in the middle position. This works to curtail the initial flow even more, except it's very difficult to exactly reproduce shot-shot.