bluesman wrote:That's not exactly correct. Pressure is force per unit area, e.g. pounds per square inch. And because liquid is incompressible, the pressure of a liquid is the same throughout and in all directions. Fluid at a pressure of 10 psi will exert a force of 10 pounds on the blind end of a tube with a 1 square inch cross section. If you cut the size of the tube to 1/10 of a square inch, the force exerted on whatever's closing off its end will only be one pound - but the pressure is still 10 psi.
Along with force, the other important factor here is volume flow per time unit, which (unlike pressure) is controlled by the cross sectional area of the narrowest point in the fluid path, along with several less significant (to us) parameters like resistance of the walls of the conduit to flow and flow patterns within the path (eg laminar, turbulent, etc). Once the critical area of the path is reduced below that which will pass the full volume of the pump's output at the specified rate, the volume of water per second drops as does the maximum force it can exert on whatever's in its way (which is the puck in this case).
I'm pretty sure that's how these needle valves affect the process. They reduce the force with which the brew water hits the puck and they reduce the volume flow. But this all happens at the same pressure unless the pump can't maintain it against the restriction, which is not likely to happen with the kinds of machines we're discussing.
EDIT: Having said all this, I just reread the descriptive literature on these kits, and they do say that the pressure is reduced. I don't understand how this is possible unless the needle valve is so tight that the brew path is almost completely closed or there's a bypass somewhere in the new mushroom that bleeds some of the pressurized brew water out of the brew path. Anybody have more input?
Sorry I didn't come back to this sooner. My analogy is technically full of holes, but I hoped someone might find it useful. Let's drop the analogy and consider what happens physically. Warning: I do not have an intimate knowledge of the workings of an E61 grouphead, so the following may contain wrong assumptions about how some components work.
What you've described in the first paragraph is static pressure. When there is flow involved, pressure is not the same everywhere. For instance, pressure drops along the length of a pipe when a fluid is flowing. A more extreme example is in air relative to an aircraft wing; the different flow rates on the lower and upper sides result in different pressures, which produce lift on the wing.
The pump does not, strictly speaking, "pump" pressure, it delivers a rate of flow by pushing water through it. When the flow from the pump meets resistance, the result is pressure. In our espresso machines, the pump is not directly connected to the grouphead, it has to go through the heat exchanger and/or boiler. Let's assume in normal usage these water paths are already filled with water, all the way up to the gicleur in the grouphead. (I have not seen a schematic, but I assume that a machine with flow-profiling has its needle in the top part of the gicleur. The important thing is that water flows first to the part of the gicleur that seals off the rest of the grouphead.)
When you lift the lever, two things happen: the gicleur opens and the pump activates. The flow from the pump pushes against the water already in the circuit. Since that water is incompressible, it also tries to flow. If we removed the entire grouphead, it would meet little resistance and gush like crazy. Instead, it meets resistance at the gicleur (the part of it that opens or closes flow), whose opening has, as you point out, a rather smaller cross-sectional area compared to the rest of the circuit. The water cannot flow through this opening as fast here as the pump flow rate, which creates pressure between the pump and the gicleur. If there is no puck, i.e., no second large resistance, this would result in a maximum flow rate in grams per minute out of the portafilter. The grouphead does not fill and so there is practically no overpressure in the grouphead, although there are around 9 bars of pressure before the gicleur, depending on the pump setting.
If, on the other hand, there is a puck in the portafilter, the water can flow through the gicleur faster than it can flow through the puck-the puck has a higher resistance to flow than the gicleur. Once the chambers in the group head fill up, there is very little flow through the puck, and the situation is close to a static pressure, both between the pump/boiler and the gicleur and in the grouphead up to the puck. You would measure close to 9 bars everywhere.
With flow profiling, we introduce a needle somewhere close to the gicleur opening. As I understand it, the opening of the needle is even smaller than the gicleur opening, so that its max flow rate without puck is less than a normal E61 grouphead. The novelty is that the user can now continuously adjust the area through which water can flow by changing the needle (via the paddle or equivalent control), which is what you were saying. How does this relate to pressure?
Pressure is force per area (N/m2 or psi). When the volume of the liquid is fixed, as in the grouphead, then the area is also fixed. It can't expand like a balloon. You can only change pressure by changing force. In the grouphead, the force component of pressure is a result of flow. The pump tries to create a high flow, but the flow-profile needle (I'll assume the gicleur now has a larger area than the needle, so that the puck and needle dominate flow) allows only a small flow through. The pressure before the needle (and maybe gicleur opening) is thus 9 bar.
How pressure develops in the grouphead depends on the puck and the needle, similarly to before with the puck and the gicleur. Some water flows into the puck, saturating it, and eventually flows out of the puck. So there is some flow from the grouphead to the puck. If the puck and the needle have about the same resistance, no overpressure* will result in the grouphead, and the manometer will show 0 bar. This is when the needle is almost completely closed. If the needle is fully open, then the puck has much greater resistance. As the shot starts, water flows from the needle, fills the chambers and starts to saturate the puck. Since the puck resists flow, and more water can flow through the needle than through the puck, pressure builds inside the grouphead. If we close the needle part way, the flow that can be passed through the needle is closer to the flow of the puck, and less pressure builds up. So you really are controlling the flow in flow profiling, which results in different pressures. In pressure profiling, you instead regulate the flow of the pump by adjusting it; so technically you are controlling flow in both instances, either at the pump or with a needle. There is nevertheless a difference.
As a thought experiment, what happens to the pressure in the grouphead if we put in a blind basket, preventing flow out of the portafiliter? I just did this experiment by backflusing with the paddle in different positions on my Bianca, and got the following times to achieve max pressure (about 9 bar):
fully open: 12 s
6 o'clock: 15 s
~8 o'clock: ~65 s
The whole time, the manometer for the boiler showed about 9 bar. For each operation, the grouphead manometer initially read zero. When fully open, it stayed there while the chambers filled up, then after several seconds started to move and then quickly went to about 9 bars. Nearly the same happened at the 6 o'clock position, although it took slightly longer to fill the chambers. The ramp up was only a little slower. At the 8 o'clock position, it took a very long time to fill the chambers-I remember more than 30 seconds-and the ramp up was very slow, too. But it did eventually reach full pressure, since nothing flowed out. Once enough water flowed through, the full force of the pump pushing on the water was achieved in the grouphead.
bluesman, this is all a long-winded way of saying you're right. The needle restricts flow by changing its cross-sectional area. And pressure IS reduced if the needle closes partly, as a result of the difference in flow rates into and out of the grouphead.
*The manometer on the grouphead of my Lelit Bianca measures overpressure, not absolute pressure. It subracts the ambient pressure of about 1 bar, so 10 bar reads as 9 bar. I'm pretty sure of this, since it shows 0 under normal conditions, and my house is not in a vacuum.