I still don't get it: Why adjust the OPV?

[buzzmc]

This is stemming from folks helping me "set up" my Andreja Premium some time back, and me getting a new VBM DoubleDomo a few days ago... I still need to go back and find that thread about dialing in a new machine as I think it's pretty helpful... I digress...

I don't believe I ever understood why I needed to do that, and now the new VBM will show pressures up in the 12's (which I know is too high), but I can adjust grind/tamp to get that to read closer to 9, which is what I think I should be aiming for... If I set that OPV to open(?) at 9.5, then I won't really know if my grind/tamp is off unless I time the shots, ya?

[another_jim]

You can adjust your grind and tamp to pull a 9 to 10 bar double; but for singles and ristrettos, you can't. The Italian spec for vibe pumps is to set it to 11 bar on a blind filter. If you can see the pressure during a shot, adjust it while pulling a single or ristretto, so you get 9 to 10 bar.

[HB]

I don't believe I ever understood why I needed to do that...

The OPV serves the same purpose as the bypass valve of a rotary pump: To set the maximum brew pressure. Rotary pumps are capable of moving significantly more volume at espresso brewing pressure than a vibe pump. In the case of a rotary pump, the bypass valve allows water to flow from the outlet side to the inlet side; in the case of a vibe pump, the over-pressure valve allows water to escape to the reservoir, effectively reducing the maximum pressure because of the flow rate / pressure relationship:


From Flow rate of a rotary pump espresso machine

If you do some calculations to determine the flow rate / pressure intersection for the vibe pump (left curves), you'll find that it conveniently works out to around 9 bar -- if you're pulling doubles. As Jim notes, an OPV acts as a "safe release" for ristrettos where the pressure would otherwise rise to 10+ bar given the slower flow.

For even more details, see purpose of adjusting OPV?

[HB]

If I set that OPV to open(?) at 9.5, then I won't really know if my grind/tamp is off unless I time the shots, ya?

Back to your original question, adjusting the over-pressure valve (vibratory pumps) or bypass valve (rotary pumps) to deliver a known maximum pressure just makes life less complicated, even if technically in the case of the vibe pump you could finagle the grind to deliver reasonable espresso brew pressures.

For sake of completeness, not all high-end pump espresso machines have brew pressure adjustment mechanisms. Consider the Elektra Semiautomatica; it has a vibe pump and no OPV. I measured its pressure output with a Scace II (thermofilter + pressure gauge). It's a very reasonable 9.1 bar, underscoring what Jim concluded in his review, i.e., when it comes to double espressos, the Semiautomatica demonstrates that it doesn't need an OPV.

[buzzmc]

For sake of completeness, not all high-end pump espresso machines have brew pressure adjustment mechanisms. Consider the Elektra Semiautomatica; it has a vibe pump and no OPV. I measured its pressure output with a Scace II (thermofilter + pressure gauge). It's a very reasonable 9.1 bar, underscoring what Jim concluded in his review, i.e., when it comes to double espressos, the Semiautomatica demonstrates that it doesn't need an OPV.

Hmm...If I am following this correctly, then the 9bar setting is more important than achieving that through grind/tamp, to the point that a 35 or 40 second extraction @9bar is reasonably acceptable... In which case you're now just trying to get your grind/tamp to come close to that 25-30 second extraction mark.

[HB]

If I am following this correctly, then the 9bar setting is more important than achieving that through grind/tamp... you're now just trying to get your grind/tamp to come close to that 25-30 second extraction mark.

Exact-o-mondo.

[buzzmc]

Well then, until I get somehow ultra interested in understanding all of this in detail, I'll take this for what it is, and make my life easier.

Then, seemingly if I figure out if I'm overdosing or not, I'll be in pretty darn good shape with the new machine.

Thx!

[HB]

To close on this subject...

In a nutshell, the over-pressure valve on a vibratory pump espresso machine's raison d'être is the elimination of maximum brew pressure variance due to the flow rate. Twiddling with the grind to produce the desired pressure doesn't really make sense and is physically impossible for espresso machines equipped with rotary pumps.

That said, you can adjust the maximum brew pressure to produce different flavor and texture profiles. Most recommend around 9 bar, but I've set the brew pressure as low as 7.5 with good results, depending on the blend. And then there are those who methodically manipulate the brew pressure within an extraction, such as Greg's Experiments in programmable, variable brew pressure profiling, part 2, and part 3.

[buzzmc]

We'll always need folks like Greg around, but that's definitely not my intent, nor will I probably ever possess that much intelligence on this subject.

I did set the OPV to about 9.6, and I'll probably take the cover off again and lower it a tad more... I got a bit impatient.

Now I'll be off to see if people have devised ways to quiet their machines down.. I'm sure they have... This one's decent, but more 'vibratory' than the Andreja.. If there's a nifty way to isolate a bit of that it'd be nice... Since my espresso cups need to be held down so they don't slide away

[HB]

If there's a nifty way to isolate a bit of that it'd be nice... Since my espresso cups need to be held down so they don't slide away

Yes, there's several threads on the subject, e.g., Eliminating noise from vibration (excerpted below) and Vibiemme Domobar Decibel Level.

I added this to the top of of the Vibiemme to stop the rattle of the cups and top cover.
It has little 'feet' to raise the mat above the cover so ventilation is not impeded.

It's an InterDesign SinkWorks small clear sink protector.
Available at Bed Bath & Beyond, Linens n Things and sundry other kitchen retailers.

[dsc]

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.

[buzzmccowan]

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

[cannonfodder]

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