Compass Coffee wrote:I'm curious why 1 minute recovery post flush seems harsh. To me seems quite lenient. I'd expect to be able to pull back to back shots in well under a minute following post shot flush and next shot. I'd expect with steam boiler on for brew boiler HX pre-heat to be able to pull continuous back to back shots as fast as barista PF build skills allow. My lowly HX Bricoletta only needs ~35sec recovery post shot flush to flush down to temp for next shot's flush and go. Though would agree 3oz stabilizing flush for DB might be more than necessary, temp tests will bear that out.
Three reasons really. Firstly, I am going on the routine used to test the Alexia with PID here on HB, that is:
* Warmed machine with portafilter and Scace thermofilter installed for 60 minutes minimum,
* Portafilter removed and pulled a warming flush of ~3 ounces,
* 2 minute recovery and then pulled a "garbage" simulated shot (data captured this as Shot A)
* Pulled 5 shots with 2 minute recovery between shots, start dump of thermofilter for each shot (Shots B-F),
Secondly, as you say, it is NOT an HX machine and therefore does not run hot. It does not need to be cooled down, and my experience would suggest it also does not need to be warmed up: if it does, a 1oz flush seems perfectly adequate. It is much harder for a thermosyphon to recover lost temp when it is running at optimum brew temp (i.e. small offset). I would have thought that an HX machine would recover from a small over-flush quicker since the thermosyphon will circulate much hotter water through the group.
Thirdly, and you mention this, with preheat on I think you could work much faster (although may notice a slight rise in progressive shot temps, you could perhaps flush to reduce this). The boiler seems to need no time to recover (it does not drop any temp during a shot, except for perhaps the 1 degree temp drop associated with normal cycling operation). It then seems to be perfectly stable, as if the group has no need to recover (in fact, I guess it could need to cool a little, I have no idea).
With preheat off, the temp in the boiler can fall a few degrees during a shot (just like the Alexia) and needs more time to recover. We see in the graphs above, and with the Alexia review, that progressive shots cool a little without preheat. My thinking is that the inrush of cold water cools the boiler. Not a problem, the PID can recover in well under a minute, but it is my belief that in this time the thermosyphon is not maintaining heat in the group as effectively, or possibly even removing heat from the group. I see this backed up by the fact that the boiler temp recovers, then seems to drop a degree and sit there for a while until the group stabilises (I use very conservative PID settings to prevent overshoots, which work well for my routine). When it ticks back up to set temp, I feel the group is now ready. With more aggressive PID settings I may not see this behaviour visually, but I believe it is happening.
This is also why I feel 3oz is just too much. It is much faster flowing (perhaps 10 seconds) than a proper double shot (25-30 seconds) and so would cold shock the boiler more than a normal double shot. The first shot on the Alexia review is taken to be a "garbage shot" since it is cooler, and the Alexia was given 2 minutes to recover after the initial 3oz flush.
With preheat off, about 105 seconds between (single) shots (end of last to start of next), which includes a 1oz cleaning flush at about the 45 second point, seems to work just fine, with my conservative PID settings. With preheat on, far less time seems to be needed, even with the 1oz cleaning flush.
Of course, this is all just my humble opinion.
All the best,