allon wrote:One thing that the lever group has over a pump machine in this regard is that it allows a hot preinfusion followed by a cooler extraction, because the water for the extraction is pulled from the boiler and cools during preinfusion. Trying to do a hot preinfusion with an unflushed pump HX machine would do a hot preinfusion followed by a hot extraction. The temperature of a lever group isn't constant, that is clear - it would be interesting to log the temperature during a shot as well, and graphing the changes during the whole process.
I believe that allon is dead on with this idea. I was testing my Conti Prestina the other day to determine the deadband of the original Sopac PSTAT installed on the machine and determined that I do want to replace it with a Sirai PSTAT.* I built a basket with an inserted thermocouple (TC) to measure the temperature in the coffee cake. When taking my measurements I noticed something consistent and surprising. The temperature would start at my usual target and then decline 20 to 30 degrees F through the course of the shot. This seemed like a large decline but it was there on multiple shots. I'd pretested the TC for accuracy and have been using it along with an Amprobe data logger into my Mac computer running Artisan roasting software. Even though a pull is much briefer than a roast I found that Artisan has a zoom feature that allows one to highlight a small part of the curve and magnify it. This revealed very readable shot profiles. You'll see the shots run long, well over a minute, because I'm dialing in my machine and I was really testing for starting temperature. But if you look at the first 30 - 40 seconds of the shot you'll see this temperature decline. Here are a couple of those traces. The first of these is run at the low end of the PSTAT range, 0.45 bar as measured by an Orphan Espresso steam wand gauge. I began each pull with about 15 seconds of pre-infusion using 19 gm of coffee ground on a Rio Super Jolly and firmly tamped.
The second trace is pulled at 1.0 bar.
Here's my basket with probe inserted:
Okay, that's a geeky description. But here's what I think it reveals. Anyone who's tried a Conti Prestina raves about the shot quality.** I find the shots reveal many layers of flavor blending seamlessly one to the next. This is different than what I've been able to accomplish on my home lever machines, although my Elektra Microcasa a Leva (MCAL) comes close, but it doesn't stand apart in taste as much as the Prestina. I might measure the MCAL while retarding the boosted spring to see if there's a similar temperature drop on the pull. But back to commercial levers. Doug Garrott wrote this in my Owner Experience with Conti Prestina thread:
Gary....over on the restoration thread I actually did laugh out loud when you began to experience some espresso from your new commercial group for the first time...HA! I seem to recall a conversation with you before you bought it (for the sum of 300 bucks or so) [$200 FWIW but that's not what it cost to rebuild it, for sure -- GS] warning you that once you tried a commercial lever that it would ruin you for the smaller home machines....and now even your sweet wife is clearing counter space!!
So yeah I can now say I TOLD YOU SO!!
But to the point, you are going to be able to see some design quirks on the Prestina that are not shared on other commercial groups...but a lot of this may have to do with Conti-specific concepts....short(ish) lever plus strong spring using the linkages for force multiplication and piston centering...that odd choke down at the end of the piston effectively reducing the piston bore then expanding the diameter again in the filter basket....I am sure you will come up with a few more concepts unique to the Prestina...but to comment on your question..yes. I do think that by and large all commercial lever groups/machines produce a very similar shot as far as consistency and quality.
They all have some little or large design idea that sets them apart...some of these ideas are in the user preference area and some are obviously patent avoiders, but the espresso turns out to be very similar since the groups are by and large the same....Thermosiphons, direct bolt on dippers, HX setups etc....all produce by and large a very similar shot as far as the quality that you are now experiencing, though on each and every machine one has to learn what the machine likes to do and what it simply will not do (La San Marco is very dose specific, but comes with 6 different basket sizes to very dose).
So to expand on allon's idea, what if:
- Steady temperature and pressure decline reveal many more layers of flavor than a narrow temperature range?
- The declining pressure softens the effect of declining temperature so those flavor differences are subtly revealed?
- The large range of temperature decline reveals one after another layer without fixing on any one, again creating a subtle effect instead of spikes of flavor?
- Thermosyphons and other devices to warm commercial groups change the profile somewhat but don't change the overall principle of running the group temperature lower than the boiler temperature?
* The Sopac showed 0.55 bar deadband and the Sirai is said to start with 0.2 bar before that deadband begins to widen as the membrane stiffens. This is one reason that Sirai sells a rebuild kit for their PSTAT.
**For those pulls at very different starting temperature there was a strong taste difference. The low pressure pull of an underdeveloped roast was quite sour but not intensely so. The pull at 1.0 bar was slightly sour and also softened.