Let's see if I have it by way of summary. Please correct me if I am wrong. There seem to be two sets of issues here, (1) the one concerning the mechanics of measuring brew pressure, and (2) the closely related issue concerning the experience of what's happening with brew pressure on espresso.
1. Mechanical Concerns. Thanks for your replies and references Dan and Ian. Reading more of the threads you have highlighted, I understand there to be a difference in brew pressure between the brew pressure measured with a blind PF, which is "static," and the brew pressure that is "dynamic," that is, taken with the puck that itself can vary according to grind, amount, and tamp. We are concerned with dynamic pressure ultimately, and when setting the expansion valve, for example, we are really after the dynamic brew pressure even if we do it (or have to do it) according to a static read.
There may also be important differences between the Group Brew Pressure and the Machine Brew Pressure. The former, the Group Brew Pressure, can be measured by the Scace 2 device, and might be called (for lack of a better term) an "absolute" measurement because it delivers a reading right there on the group head. So now a question: In the case of an HX machine like my own, an Andreja Premium, will the Machine Brew Pressure (for lack of a better term) be more "relative" because it is gauged just before it goes into the heat exchange system? In any case, the Machine Brew Pressure might be called "relative" because the readings can vary from machine to machine (say, an HX and double boiler) and may not correspond to the reading from the Group Brew Pressure.
As I understand it, the actual numbers of brew pressure from the gauges are not important in themselves, but are significant insofar as they give us a consistent point of reference. Assuming that one's own machine is consistent—and speaking of one machine only—the reading of the brew pressure is arbitrary but functional so long as it can serve as a stable point of reference. But if we want to speak "across" machines, and as Ian remarks, help another diagnose a problem or give a "social" reference point for dialing in a shot with a specific blend, then the matter becomes more difficult. We would want a consistent way of measuring brew pressure across machines, and it seems that the Scace device could serve this purpose—assuming they are calibrated the same way (of course, we would all have to own one
So, I take from this a few things: First, when I am speaking with others about brew pressure, it is important to know how they are measuring it. Second, it may be possible that my 10 or 10.5 bar might be a "group" 9.0 or 9.5 bar brew pressure. Again, this only matters as a point of reference. Third, once we have this straight, it can be meaningful to speak of optimal dynamic brew pressures for such and such a blend, or perhaps to help diagnose an extraction problem as a "brew pressure" problem. Does this sound right?
2. Experiential Concerns. I realize that none of this can replace an experienced barista and a trained palate—it's all about the taste!--and that's the point of the brew pressure measurements. Chris's comments about texture and flavor were really helpful.
I am familiar with Hairbender (originally from Portland), and when I was in Portland last month on my migratory trip to the Division St. Stumptown, I had a couple ristrettos. I sensed (in my newbie way) what was probably more on the higher brew pressure side (though I realize there are a lot of variables)—actually it seemed a little nuttier (almost almond-like). In any case, I have been fixed on B and B's Dancing Goats, and have a couple more lbs coming in shortly. I will try to experiment with brew pressure with the newly arrived beans. Do you have any suggestions concerning what I should be attentive to with this blend?
I am eager to learn as an amateur more about this variable, brew pressure, general telltale signs of high or low brew pressure, and from those more experienced than I, what brew pressures do with different origins and blends.