TomC wrote:Yes, but if you're after accurate temps in the basket, it doesn't serve any purpose to have the pre-infusion gush immediately thru the group and into the basket.

That's why you do it in concert with John B's method of covering the Scace outlet during pre-infusion. There is no flow until you remove the cloth and raise the lever, at which time you can restrict its motion to achieve the appropriate flow rate without using an adjustable valve.

I don't think hot steam plays a role here. A thermosyphon fills the group with a loop of boiler water, not steam. The only air between the piston face and the puck is just at around group temperature. As soon as the lever is activated the group fills with water, pushing this air out. Dan's tests with the Lusso look exactly as I would expect. If one fills the group then immediately allows the lever to rise the boiler water should have cooled to about what he found, 206-207F. When I let it cool for 10 or more seconds in the group it is perfect for brewing.

The Prestina group is rapidly filling with 230F water - since there is no dwell time for this water to cool I don't find 217F very surprising. Wait ten seconds, however, and that massive bit of brass should have taken the edge off (and the taste of the espresso suggests that it has). Simple tests with a non-pressurized lever easily demonstate that brew temps above 203-205 taste bitter.

Bob_McBob wrote:That's why you do it in concert with John B's method of covering the Scace outlet during pre-infusion. There is no flow until you remove the cloth and raise the lever, at which time you can restrict its motion to achieve the appropriate flow rate without using an adjustable valve.

Totally agree. When I read your first reply, it read like you meant it (spring restriction) as an optional approach. Not a combination of the two.

peacecup wrote:I don't think hot steam plays a role here. A thermosyphon fills the group with a loop of boiler water, not steam. The only air between the piston face and the puck is just at around group temperature. As soon as the lever is activated the group fills with water, pushing this air out. Dan's tests with the Lusso look exactly as I would expect. If one fills the group then immediately allows the lever to rise the boiler water should have cooled to about what he found, 206-207F. When I let it cool for 10 or more seconds in the group it is perfect for brewing.

The Prestina group is rapidly filling with 230F water - since there is no dwell time for this water to cool I don't find 217F very surprising. Wait ten seconds, however, and that massive bit of brass should have taken the edge off (and the taste of the espresso suggests that it has). Simple tests with a non-pressurized lever easily demonstate that brew temps above 203-205 taste bitter.

If you use preinfusion time to adjust water temperature, will you not simultaneously be causing other effects on the puck? For example, I've read that well-infused pucks let the brew water run through more quickly. I've also read that longer preinfusions "brew" the puck, thereby adding brewed-coffee flavours to the final mix. I've also read that lighter- and darker-roasted beans require different preinfusion times. (There are probably other claims as well.) How is one to balance all of these considerations when determining preinfusion pressure and timing?

Thanks for asking the question. At the end of the day you must judge the results with your taste buds, try different variations to home in on the optimal set of parameters -- especially with a new coffee -- and build enough experience to predict the likely result of changing one variable or another.

In my experience (with pump machines), preinfusion actually regulates flow, rather than increases flow rate. This is because a small amount of water with no or minimal pressure causes the grounds to swell gently, eliminating the tiny air pockets that would otherwise allow free flow, and when water under pressure is applied during the pull, it extracts the coffee molecules more deeply and effectively.

Thank you for your reply. That seems like sound advice.

Matt