Hi,

I've been recently playing with my OPV a bit and I was surprised with the results. I set it to allow the pressure to rise to around 12bar and tried achieving 9bar with only tamp/grind. I failed miserably, getting nasty 5s blond gushers and still hitting the 12bar point with the OPV opening. It's funny because the flow would start around 5bar, a huge stream would form and than it would ramp up to 12bar for the rest of the extraction.

In the end I decided to go back to my original setting of around 9bar and left it there.

Cheers,
dsc.

Hey,
I played around with my OPV again last night on my VBM after reading this thread. I used a portafilter mounted pressure gauge that was unscrewed just enough to let some water flow through to a measuring cup underneath. This way I could mimic the flow of a shot in 25-27 seconds or so. When tightly screwed on, my gauge read 150 psi (static pressure) whereas with it slightly unscrewed, 60 mls of water resulted in 27 secs, the gauge read 140 psi or about 9.65 bar (dynamic pressure). I then dialed down the OPV to get it at 9 bar dynamic pressure. With the blind basket and onboard VBM gauge, this means about 9.75 bar reading, as reported by others. The shots this morning seemed to have a bit more body and darker crema for what its worth.
Cheers,
Kaelin

You can definitely pressure profile a machine to a blend, that is what the commercial cafes do but they run the same blend all the time. I change blends once a week so I have to find a good generic mid ground. That mid ground varies by machine as well; I run a different pressure on my A3 than I do the Domobar Super. I cannot remember what the actual brew group pressure is on the VBM. There is difference between the onboard gauge and a ScaceII measured grouphead pressure which is common on all machines. I have my VBM set at 9.6 bar on the gauge which I think was around 8.8-8.9 in the portafilter. On the A3 I run around 8.4 bar at the group during a brew if I remember correctly, but it has been a while since I checked it.

I may need to do that again. Pressures will wander slightly over time as the OPV spring relaxes