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:


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?

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.

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.

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 wrote: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.

cafeIKE wrote: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.