Jake_G wrote:As long as the OPV is closed, the pressure and flow relationship during a flush are driven by your pump curve, which is a little on the low side (flow should be more like 220mL/30 seconds @ 3.5 Bar or pressure should be more like 6 Bar at 180mL/30s) but this could be due to the PI chamber "stealing" some of the water during the test. @ 3.5 Bar, the PI valve should be cracked, so that's a definite possibility... However, the shape of the chart above is based on the Cv of your gicleur. If you get a smaller gicleur, the pressure drop will increase, and the pump will follow its curve to deliver less water. The shape of the curve above will get steeper (higher pressure) and shorter (less water debit means we stop at maybe 4.5 mL/s instead of 6).
For some reason I thought we were calculating the size of the gicleur. What is the equation behind this curve? Also, my pressure here at 6000' is under 0.82 bar so the delta P would be higher (not sure if you included that or how much difference it could make).
This assumes a constant pump pressure of 9 bar.
My pump pressures during a shot are 9.3-9.4 bar and during a blind flush, 9.6-9.7 bar.
So what this is saying is that IFF your puck is so porous or eroded that it flows 5mL/s (Super Lungo!), the pump will only be able to muster around 6.8 bar to the inlet of the gicleur. The gicleur will then have a pressure drop associated with this flow of just under 2.5 bar, so the resulting pressure at the puck is only 4.4 bar. You can see how the OPV flattens out the pump curve for flow rates of less than 4.5mL/S and leads to much better (or at least predictable) performance in the typical range of espresso brewing.
Never wanted a Super Lungo (always ristretto), but I suppose that if I did I'd want the pressure to reduce anyway when I'm drawing blonde, right?
Thanks for the crunching, and the results, but I'm skeptical by nature until I see what is behind the curtain so to speak.
Cheers and Thanks!!