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

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buzzmc

#1: Post by 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?

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another_jim
Team HB

#2: Post by 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.
Jim Schulman

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HB
Admin

#3: Post by HB »

buzzmc wrote: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:

Image
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?
Dan Kehn

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HB
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#4: Post by HB »

buzzmc wrote: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.
Dan Kehn

buzzmc

#5: Post by buzzmc »

HB wrote: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.

:?:

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HB
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#6: Post by HB »

buzzmc wrote: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.
Dan Kehn

buzzmc

#7: Post by 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!

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HB
Admin

#8: Post by 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.
Dan Kehn

buzzmc

#9: Post by 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.

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dsc

#10: Post by 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.