It's been erroneously stated that OPV flow is inconsequential for a vibe pump on a 'normal double' at 'normal pressure.'
Some machines forego an OPV altogether.

I had the Scace out today, so measured the OPV flow for a 60ml 'shot'
Fully 40ml, or 66% of the shot volume, is returned to the tank via the OPV.



Brew pressure on the Scace @ about 60ml / 30 sec.

What is an inconsequential flow or proper pressure?

Your test doesn't really speak to this. Getting a proper pressure shot with an OPV that is dumping lots of water does not imply that shots without an OPV will be at improper pressure. That will depend complexly on the pump curve of the naked pump versus that of the pump curve with OPV and brewing circuit in line.

More importantly, it will depend on your definition of proper pressure.

Everybody agrees that if you want to regulate pressure at all accurately, you need an OPV or some other pump controller. The argument about the need for an OPV on a vibe pump is how high the pressure has to get before it spoils shots.

Italian practice is that 9 bar is for rotary pumps and vibes are fine up to 11 bar. Quite possibly this is because of the slower ramp of vibes. Many vibe machines do not exceed 11 bar when making doubles, so by this standard do not require an OPV.

My own experience and that of others has been different. Many machines profit from lower pressures on vibes, while some machines seem relatively immune. My Tea ran best at around 8.5 bar, on my Elektra, shots at 8 or 12 bar are pretty much indistinguishable. On Ken's twin Cimbalis, blind tasters preferred the rotary until the Vibe was adjusted down from 12 to 9 bar, at which point the tests were a toss up.

Back in 2002, I was one of the first people on-line to demand that vibe machines should have adjustable OPVs. But recently I've tested several machines where the vibe ran quieter and more consistently, and shots tasted better, without an OPV than with one. So I'm less certain now about the range of proper pressure and the unconditional need for OPVs.

The other missing variable is pump voltage. At 110 volts a 2oz double will run just below 10bar, while at 120v it will be above 11 bar. 108 volts gets it to around 9 bar. Fairly sensitive. This with no flow from the OPV, tests run on my Isomac Venus. YMMV.