The pages I posted links to (quoted above) provide links to several possible gauges, one of which displays bars. But it's easy enough to convert psi to bars using online calculators or Google, so I wouldn't get too hung up on units.
This is the youtube video I was referring to in regard to the 10 bar setting to get an effective 9 bar at the portafilter.
Posted Mon Mar 5, 2012, 8:31pm Subject: Re: Gaggia Classic.....Adjusting the OPV
I've been thinking about this a lot. I had forgotten that the OPV is a spring powered device. When you are not flowing any simulated shot, the spring in the OPV is being compressed further (and the spring increases in tension the further you compress it). When you are flowing a simulated shot, the OPV spring is being less compressed so the tension is less. This would explain why you would want to set the pressure higher when measuring static than when measuring with a simulated shot.
In my testing, I don't see 2 bars difference (and I don't agree with the math of the guy in the video where 12 bars minus 2 bars equals 9 bars). I think you would see the difference between brew and static being a variable depending on the rate of flow and probably different for every pump/OPV combination. I pull ristrettos and I don't see any discernible change in my PF gauge reading between static and a small flow simulating a 1.5-2oz shot. But others report seeing .5 bar or .7 bar or 1 bar or even 2 bar difference between static and brew.
In the end --- if it tastes good to you, then that's all that matters. If it tastes better when you set the pressure to 10 or 11 bars then let that be your guide.
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