HB wrote:Correct. For vibratory pump, slow flow => high(er) brew pressure, fast flow => low(er) brew pressure. By adding an over-pressure valve that discharges water back to the reservoir, the gross flow rate from the pump is normalized so the pressure is consistent, independent of the flow rate through the brew head.

Well, let's see if I get this right :

let's assume a double is 60ml pulled in 25s : according to the pressure/flowrate chart, it would need at most around a 12bar pressure (60ml/25s = 140 ml/min which corresponds to around 12bar) -> so ~12bar is fine for doubles.

At this pressure, it would take half that time (12sec) to pull a single.
To pull 30ml in 25s you'd need a flowrate of ~72ml/min which corresponds to around 13bar).

So is it correct to assume that we want 9bar not for the flowrate (which is then more than sufficient) but because it is assumed it's the optimal pressure for (taste-wise) good extraction ?
If you've got an OPV set to 9bar, for a single, you'd have water redirected to the reservoir.

The question is then : what's the flowrate *after* the OPV ? How could that fit both single and double shots ?
As a matter of fact, if ~25s extraction time is set, you have to have different flowrates, one for singles, another for doubles ?

Or are we supposed to have 2 OPV settings : one for single shots, one for double shots ?

Yes, but I still don't understand how you solve the flowrate "problem" : if you want to pull during 25 sec or so, whether it's a short or a double, you've got to use different flowrate for each. How do you achieve this, with or without OPV ?

But earlier on this thread, HB said :

Tamping harder/softer doesn't dramatically impact the flow. Try it yourself tamping moderately and then tamping like a gorilla. In my experience, the latter may initially pour more slowly, but will catch up with the pour rate of the more moderately tamped puck. My theory is the tamp pressure can affect the initial wetting of the puck's surface and thus the early pour speed. As I've said many times before, the tamp is way down on my list of important contributors to exceptional espresso. Distribution and the correct dose are significantly more important.

But I've wrote combination of grind, dose and tamping...

Right!

So we might think that playing with such combinations might as well make us reach the right flow rates on a machine without OPV (like the Oscar) for shorts and for doubles too ?

In which case the OPV set to 9 bar purpose would be correct pressure for taste, not for correct flowrate ?