E61 Preinfusion confusion
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I see a lot of discussion on forums about pre-infusion on the E61 and the E61 middle position. This tends to come up on a regular basis and usually receives conflicting and confusing responses. I think it's time to examine this area a little and try to clear up some of the confusion.
Firstly, the E61 has built in pre-infusion, in the lower chamber of the group there is a preinfusion chamber and while this fills the pressure to the coffee puck is lower measured with a pressure measuring portafilter. This chamber is sealed with a weak spring seal that lets by at around 4 bar. The chamber fills very quickly a matter of a few seconds for a rotary and a little longer for a vibration pump. The idea of this is to prevent the shock of 9 bar water hitting the coffee puck in the portafilter and allow a more gradual ramp up of pressure. this is by design, this is correct and it works well.
The "Middle Position" preinfusion is really opening the top brew valve before the pump runs and wetting the puck.
- line pressure water in the case of a plumbed in machine (should be 2 bar if you have regulated pressure correctly)
- * or passive flow from the hot water expanding out of the HX unit in a non plumbed machine (hot dribble). I personally don't believe either is correct because of the way that these machines function.
Then there is the passive dribble from the twin boilers brew boiler (not the same problem as in the first 2 scenarios, but still not correct). This puck wetting has advocates, I am not one of them and have not tasted any significant shot difference after setting up various machines to do this "puck wetting".
The middle position on an E61 will only let line pressure water through when the top brew valve is open and the pump is not engaged….this is purely a function of how far the micro-switch that turns on the pump protrudes through the front fascia, as this directly impacts how soon the lever cam is able to actuate it during movement of the lever. If the micro-switch pokes out a long way, then the pump will engage slightly before the top brew valve opens, hence . Or the reverse with the switch set too far in and the group opens before the pump engages, then it's possible to carefully raise the lever to a specific point where line pressure water flows through the group but the pump does not run. A typical brew switch is shown below, both the front and the back (inside the case), adjustment is via the two nuts either side of the front fascia panel.
In the Goldilocks scenario…."it's just right" when the pump runs almost exactly as the group opens. This is how most manufacturers set it up and by design they are not expecting anyone to do any passive "puck wetting", which is why it's never covered in the instructions.
The E61 that may be the exception to the rule with respect of built in pre-infusion are some automatic solenoid operated E61's which sacrifice the preinfusion chamber for the electrically operated group exhaust solenoid and it's often a reason why some of the semi automatic machines might be best avoided. Some other non E61 machines simulate the E61 preinfusion using a separate expansion chamber in the brew circuit e.g. La Spaziale Vivaldi S1 and Mini Vivaldi (not sure if it's standard now or available as an add on)