First let me say that I get an excellent tasting espresso from this machine. Obviously this is the desired end result.
The question I have is the pressure setting. I am getting my shots in about 15 seconds. After reading the reviews on this machine I noticed that it was reccommended by one knowlegable person to increase the pressure setting to maintain 1.5 bar. This is what I have done.
Then I noticed that another kowledgable reviewer said he reset his to 0.9 bar to get good results.
My question is should I leave well enough alone? Should I try to reduce the pressure setting to get a longer shot time, that is closer to the desired 20-24 seconds? I seem to have the right tamp pressure and grind, because I am getting excellent quality as far as crema and color and taste.
I am no expert, but here's some advice (for what it's worth):
The BOILER pressure is what regulates the brew temperature. Higher pressure means higher temp. water. My boiler is cycles between 0.9 bar and 1.1 bar and that works out very well for darker roast coffee. Some like higher temps for lighter roasts.
The PUMP pressure is generated when the machine's pump forces the hot water through the coffee grounds. Ideal pressures for this are between 8.5 bar and 9.5 bar. My Isomac was set up to limit the pump pressure to about 10 bar.
What controls the pump pressure is the RESISTANCE made by the coffee "puck" (that's the term for coffee grinds when they are packed into the filter basket). If you grind finer and/or tamp with more force then your draw pressure will go up and the shot will take longer to extract. Ideally a double shot uses ~15 grams of coffee and gives ~2.5 oz. in ~25 seconds.
(Search "golden rule" in these forums). In the case where your shot is 15 seconds, try to grind finer to get it to take around 25 seconds. On your machine, with everything right (correct pressure and temperature, grind and tamp and of course fresh espresso beans roasted less than 10 days) you should be able to pull a double shot that comes out at least 1/2 crema and be smooth and creamy in the mouth when sipping (not too bitter and not sour at all). When using REALLY fresh beans my pour is almost ALL crema that settles to about 1/2 by the time I drink it. Pure heaven!!!
Also, as a note, on the Millenium you should preheat it for at least 30-45 minutes (with the portafilter installed) and then run a "blank" shot before making espresso to get the overheated water out of the brew path. Many use a timer to make the machine turn on an hour before they will use it. I'll try to post some pictures this afternoon or tomorrow showing what my pour looks like using the Relax (it's a similar system to yours in many ways).
I think Jon pretty much said all that's needed, but I was having some seriously short extraction times from my Tea for the first day or so, because I was using that horrible plastic tamper that came with it (the RB I ordered hadn't arrived yet). My brew pressure went from ~9 BAR to almost 0 right after the preinfusion.
I'll try to post some pictures this afternoon or tomorrow showing what my pour looks like using the Relax (it's a similar system to yours in many ways).
Sorry this took so long. Who knew that taking pictures of coffee brewing would be so challenging? Anyway, here are a series from this morning's espresso fun....
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