Ken Fox wrote:Times change, grind and shot pulling technologies change. The classic lever preinfusion of 1.25 bar for 10 seconds may produce drips and a completely soaked puck on a lever machine, using coffee ground appropriate to such equipment, but it doesn't do anything except ruin the puck on a pump machine. Our observations make sense of Andy's not being able to get a low pressure preinfuse to make good shots at his recent Gimme stint. In fact, given that the puck is left half dry and half wet, the shot is likely to be worse than a no preinfusion shot. I believe Barry once posted that he was unable to get good shots (on an LM?) using the classic 1.25 bar preinfusion. Again this result explains it.

Going to to 2.25 bar got us a surprise. The timing is close to the 3 bar one, but poor wetting was exactly like the 1.5 bar case. Again, the seepage was down the sides.

At 3 bar, good stuff is happening, the water has enough pressure to go through the whole puck in a reasonable time.

We have no idea whether 6 seconds at 3 bar will work for all machines like it works for the Junior; other design factors, such as the distribution of holes in the machine's diffusion disk, may play a role. However, if you're setting up a preinfusion, you might want to answer this question - is the puck completely wet before the pump kicks in and it starts dripping? This is most likely a prerequisite for a positive impact of preinfusion on the quality of shots in the espresso cup.