boren wrote:I think you may have misread my question.

This might be the case.

boren wrote: If in both cases the grind level doesn't allow building enough pressure to hit the OPV pressure, where would it make a difference if the OPV is set to 9 bar (0.5 bar more than extraction pressure) or 12 bar (2.5 bar more)?

As i wrote, the OPV is having a linear area and is not "switch-like" on or off. You can retry your experiment, but set the OPV to something like 12bar (if even possible, your bypass valve might prohibit that).

boren wrote:If in both cases the grind level doesn't allow building enough pressure to hit the OPV pressure, where would it make a difference if the OPV is set to 9 bar (0.5 bar more than extraction pressure) or 12 bar (2.5 bar more)?

Are you referencing your panel pressure gauge or the gauge on your group?

I would be mightily surprised if the panel gauge were reading less than your bypass setpoint whilst delivering a flow rate appropriate for espresso. The rotary vane pump in your Bianca is likely no smaller than 50L/hr, which is around 13.8ml/s in the absence of any restrictions. Dropping that down to 2ml/s will most certainly activate the bypass valve.

My guess is that your pump bypass is open, but there is a slight pressure drop through the plumbing of your Bianca on the way to the group (and possibly a slight drop on the way to the panel pressure gauge). The remaining drop occurs over your flow control needle valve, giving you the final pressure at the group that is impacted by the flow rate through the group - which is set by the flow resistance of your puck - which is set by your grind size, dose and basket selection.

Remember that any time there is flow through a system, there is a corresponding pressure drop. No pressure drop, no flow. So what you are likely seeing is how different grinders impact the shape of the flow curve, with the grinder that delivers the longest dwell time delivering the highest peak pressure, and the grinder that delivers the most steady flow throughout the shot yielding the lowest peak pressure.

Cheers!

- Jake

boren wrote:BTW, I can only tell for a fact that you can get the grouphead pressure to reach 8.5 bar by adjusting grind level. I didn't measure what happens to the flow rate if in an 8.5 bar extraction I adjust the maximum pressure of the machine from 9 bar to 12. If you or anyone else here knows please share.

For a vibe pump, the flow will decrease with with the pressure. The flow is caused by the spring. The springmoving the pump's core back to resting position is moving the water ahead of it out through the pump's outlet. The magnetic field is only "cocking the gun", pulling the core back to prepare for the next magnetism-free moment. Of course, once you get to the point where there is no flow making it through the OPV, the pump will just run at whatever rate the coffee prep allows with no changes as the (No longer mattering)OPV is adjusted higher and higher.

For a rotary pump, the amount of water through the vanes is the same every rotation, the higher the pressure the more electrical current the motor will draw to put the excess flow over the bypass.

@Jake_G - I'm referring to the pressure in the group. For all intent the pressure results between the different grinders are almost identical. I would expect multiple shots with the same grinder to result in similar fluctuations due to puck prep imperfections.

boren wrote:@Jake_G - I'm referring to the pressure in the group. For all intent the pressure results between the different grinders are almost identical. I would expect multiple shots with the same grinder to result in similar fluctuations due to puck prep imperfections.

Maybe you should explain more what you want to know or prove. All I can say is, that the statement from your first post:

boren wrote:I just realized that by adjusting grind level and being meticulous about puck prep one can achieve the target brewing pressure without relying on the OPV.

implies that you think this is possible without the OPV (or bypass valve) but it isn't due to the large amount of water your pump delivers. Whenever somebody tries to explain this to you, it seems that you don't even know what your topic is about, as you imply with such statements:

boren wrote:why does it matter what happens elsewhere in the machine?

Fair enough. Let me give you some background (I also updated the OP with it). Some machines are frequently recommended against because their OPV is set too high and is not adjustable. The Nuova Simonelli Oscar and Oscar 2 are such examples, and there are other entry level machines that even come with a pressure gauge and a PID, but have an OPV that's set to 11 or 12 bar and is not easily accessible (e.g. Lelit PL41TEM). Following my experiment my theory is that this is less of an issue than commonly believed, and that users should worry less about how the OPV is set in these machines and more about getting optimal grind level and puck prep. Is this assumption wrong? Should machines with non-adjustable OPV (set to 12 bar) be avoided in favor off otherwise-less capable machines? I think not, but I could be wrong.

boren wrote:Following my experiment my theory is that this is less of an issue than commonly believed, and that users should worry less about how the OPV is set in these machines and more about getting optimal grind level and puck prep. Is this assumption wrong?

Yes.

Set your pump pressure to 12 bar and then adjust your grind until you have 8.5 or 9 bar at the puck.

You will find that the shots need to be in the 12-17s range to get anywhere close to this low of a brew pressure when the pump is set that high.

Cheers!

- Jake

What about the machines I mentioned in the previous comment, where pump pressure is not adjustable but there's an OPV? In those cases the pump is also not a rotary one, if that makes a difference.

Same deal with those.

You see posts come up relatively regularly where someone comments that in order to get their brew pressure gauge "in the green" they have to grind so coarse that their shot time is much too short.

There answers are almost always there same:

1. "Ignore your pressure gauge ans sweety your grind to give you your desired short time"
2. "Install an adjustable OPV and set your brew pressure independent of your grind setting."

Cheers!

- Jake