Both of my E61 machines with Flow Control devices have their overpressure (aka bypass) valves set to 10 bar by their respective dealers. Both dealers say they do this to achieve a brew pressure of 9 bar. However, I can see on both machine's group pressure gauges that the brew pressure often reaches 10 bar, especially if I grind finer for a ristretto or slow extraction.

Would there be any difference in extraction if I use the Flow Control valve to limit pressure to 9 bar (or lower) vs. adjusting the OPV lower? To achieve 9 bar with the FC devices, I need to open them only a small amount, around 1/4 turn, so the flow rate is very low. If I were to lower the pressure with the OPV and keep the FC valve fully open, would I get a faster flow, or would the flow be limited by the rate water flows through the puck?

I noticed that 1st-Line's description of the Lelit Bianca says they deliberately set the Bianca's OPV to 11 bar, and recommend lower pressures be obtained by using the FC paddle. So they seem to imply there is no difference.

Can someone with a better knowledge of fluid dynamics than I have advise me if I should bother lowering my OPV, or just stick with the FC device to achieve 9 bar or lower?

Thanks for the help!

Depends on the pressure pick off location. Typically e61 brew pressure gauges are pre gicleur. 10 on the gauge can be 8.5 on the puck.

The group gauge on the Bianca should match the pressure gauge when there is no flow with a blind basket and be lower with espresso being pulled.

I have the same exact question as OP.

reduce via OPV or does reducing bar by flow control do the same thing?

Jim answered your question in the previous comment. There is no difference. The advantage of lowering your OPV is that you will have a greater range of motion on your flow control device. Both of my setups have OPV set to 10 bar, and the useful range of motion on my FC is 1/4 to 3/8 of a turn from completely closed. Opening it any further will raise the pressure to above 9 bar.

Thanks, Bernie.

Could you elaborate on range.

That it's enhanced or greater with smaller absolute pressures.

What you've described feels even more constricted/catastrophic.

Those are tiny rotating values that are considered useful.

Wouldn't you prefer your range to be from closed to, say, a full 360° turn ?

Thank you.

The varying flow rates you measured occur when there is no resistance to water flow from the coffee puck. Once you are forcing the water through the puck, it's a whole different ball game. I find that once I open the valve 1/4 to 3/8 of a turn, then the the group pressure gauge peaks at 9 bar. Opening the valve any further will increase the pressure over 9 bar, which I don't want to happen.

In my case the effect may be exacerbated, because I like to make ristrettos which are very finely ground so I can get a longer extraction time, with a short pull. With a "normale" grind, there will be less resistance from the puck, so you may get a larger useful range of the FC knob.

As I said, my OPV is set for 10 bars. If it were set for 9 bars then I could use the entire range of the FC knob without the pressure getting too high. But I would expect flow rate to be limited by the puck, not the FC knob, once it was opened a bit.

You are correct that I would get more range on my FC knob if I lowered my OPV. So far I have been too lazy to do so.

I know I'm about 6 mos late to the party, but I just received my new maching, a Quick Mill QM67 with a Lelit FCD, the same one that's on the Bianca. While the QM67 might (will?) have a different flow rate as the Bianca or other machines should they not have a FCD installed, I did some "mapping" that I think helps understand how much water could be pushed through if the valve were totally closed, or totally open with the 540° rotating abilty of the Lelit FCD.

On my QM67 with the Lelit FCD and paddle fully left and adjusted to close the water flow, the range of water debit went from 0mL/sec to 5.6mL/sec w/paddle fully to the right. Obviously resetting the paddle position on the spindle will make it so you can't close off the water, but can open up the higher flow rate; in my case all the way up to 7.8 mL/sec. I think the take-away is that if you look at the flow at the initial point throught 1/2 way or 3 3/4 turn (past paddle limit) that the water flow increases more in that short throw-space than 3/4 through 1.5 turns (540°). It seems the valve's release of water relative to degrees of turn is not linear but "declining progressive" (if the term makes any sense. So as I look at the chart below, comparing Lelit FCD with WLL's flow control and water debit rate, while not the same (and of course with different machines, both different than the Bianca) it also appears that the rate of water flow increase lessens the more you open the valve. So it would seem that on a practical level the extra 360° that you get by having a free spinning knob may have limited value and that the Lelit paddle seems, at least on the surface, so have practical value combined with simplicity and ease of use and consistency. If one were crafty, I think the range could be increased of one were willing to "scupt" the paddle on both sides to slight increase the turning range. Doesn't seem worth it, does it?

Getting to the subject of water flow and pressure, OPV adjustments vs FCD restrictions, I'm still a bit lost.

If you think about a typical double shot where at most 60 ml is brewed in around 30 seconds, that equates to a flow rate of 2 ml/sec. So you can see that the flow rate will be drastically limited by the water resistance of the coffee puck. Opening the flow control valve to a higher flow rate will allow the puck to be wetted faster, but once that is done, you will be doing all of your work with the valve nearly closed. For that reason, you want to adjust your paddle so that it can be fully turned off. That will limit your maximum flow rate, but it won't matter much.

That is an interesting comment and observation. I can see how being able to slow the flow rate down to nothing has value. At a flow rate of 5.6mL/sec the resistance of the puck would certainly have to be a factor if one is trying to get into the 20-30 second times (or longer). So it seems that my paddle, set to be able to close off the water flow, combined with the beans and grind I'm using gets me about 25 seconds wide open and longer with a declining flow. I am beginning to see the merit in the Lelit FCD design. My question then is how is it that the guy at Whole Lotte Love is demonstraing pulls with the full 540° rotation? THAT seems like a lot of work and one of the reasons I was drawn to the Lelit FCD.

I don't recall if I'd said this before, but a handy fellow could sculpt a few more degrees out of each side of the paddle if desired. But it seems there is no reason to do so.