I did the OPV mod on my Classic a while ago and found the results satisfactory with my shots producing fairly good crema, however there was a lot of inconsistency which I put down to having a cheap grinder. I recently bought my first proper grinder and wanted to re-do the mod using a pressure gauge, which I put together using an air gauge from my local tool shop. I adjusted the OPV to produce roughly 9 bar and immediately saw an improvement in my shots with nice syrupy shots, however they were being pulled a lot quicker than 25 seconds. As a result, I decided to try and adjust the OPV this time using the blind filter method once again and found that I was getting a lot more than 100ml in 30sec, so I adjusted the OPV to get the 100ml and then tried the pressure gauge again. The result was way over 9 bar.

So in a nutshell, Im confused as to why the popular method on coffeesnobs using the blind filter and return pipe is giving me a pressure reading way above the magic 9 bar?

Which method is regarded as more accurate?