Anthony wrote: Let me ask a question that seems so evident as to be naive, but it is a matter of confirmation. When I am pulling a ristretto, my brew pressure is right around the static setting (like I would have with a blind PF), and when I am pulling a normal, it is around 9.0 bar, so 0.5 bar less. Does that sound right?.
Yep. That sounds right. There are two things at play here. First, when no water is flowing through a plumbing system, the pressure is the same throughout. In order to get water to flow through a pipe you have to have a pressure differential across the water. Imagine that you are holding your finger over a garden hose so that no water squirts out. The force of your finger pressed on the end of the hose exactly opposes the water mains pressure and the net force on the water is zero, meaning that no water moves down the hose. Once you release your finger, the pressure at the open end of the hose is at atmospheric pressure, while the pressure at the water mains connection is the water mains pressure. Now there's s big force imbalance and water flies out of the pipe. In the case of your espresso machine, the flow when pouring a ristretto is pretty small, so the measured pressure is gonna be similar to the static case (when pumping against a blind filter). As soon as substantial water flow occurs, such as when brewing normal espresso, the pressure drop along the flow path increases, and the pressure at the group is reduced. The magnitude of the pressure drop that is reported by the gauge in your machine depends on where in the flow path the gauge is tied into the system. If it's close to the OPV you may not see much difference (but read the next section). If it's near to the group you'll for sure see a substantial difference.
Second, we have to consider the behaviour of pressure relief valves that rely on spring pressure and a plunger to regulate the pressure of a system. Spring force on the plunger is given as k*X, where k is the spring constant, given in pounds per inch of displacement (or Newtons per meter in SI units), and X is the amount that the spring is compressed from its free, uncompressed length. The important concept is that the more you compress the spring, the more force the spring exerts. When you are pumping on a blind filter, or pulling a ristretto, the spring is compressed more than when you are pulling a normal shot, because more water is being forced through the OPV. As a result the spring puts more pressure on the plunger and the OPV regulates at a higher pressure than when less water is flowing through the OPV, the case when relatively more water flows through the group. Depending on the OPV design,and the capacity of the pump, the difference in regulated pressure can be pretty large. I once measured a system of vibe pump and OPV that produce 4 bars difference in regulated pressure between pumping on a blind filter and actually brewing coffee.
For espresso machines, the flow-dependent pressure drop through the plumbing system and the difference in regulated pressure due to relative changes in spring force imparted to the OPV plunger work in the same direction, unfortunately toward more pressure variability. As relatively more of the total water flow goes to the group, the regulated pressure (from the OPV) goes down, and the pressure is further reduced as the water flows down the flow path to the coffee. The effect is greater with vibe pump machines than with rotary pump machines. In rotary pump machines, the internal pressure relief valve in the pump has a very large bypass port and the pump itself has a very high flow capacity compared to a vibe pump. The difference in flow between a ristretto and a normal espresso does not change the position of the plunger much, resulting in less difference in regulated pressure compared to vibe pump / OPV systems.
Constant pressure regulation requires alternative regulation techniques, such as a system of variable speed pump, electronic pressure transducer, and process controller. One of the advantages of the system that Eric S referred to in his light reading suggestions is that the regulated pressure at the pump is highly reproducible, resulting in greater ease in maintaining consistency.
Getting to the real nut of the matter - The idiosyncrasies of espresso machines are interesting if you are so inclined, or if you are a machine designer or engineer. It's often useful to know why things work the way that they do and to be aware of how and why variability is introduced into the brewing process. I don't know what you're capable of mechanically or electronically, or if you're a super-taster or what, but at some point you'll prolly wanna decide for yourself what you can and can't taste, how much of an effect pressure variability is on your particular setup, and how much money and effort you wanna spend on seeing if you can do better. Generally pressure variability is not the tallest nail sticking out of the board. Whack on the tall ones first, taste carefully, draw your own conclusions. You seem pretty smart.