Has anyone experienced their espressos getting worse after adjusting the OPV down to 9bar?
So I have an isomac millenium and used a gauged PF to check the brew pressure. It was sitting at around 11 bar, making nice thick rich espresso, but a little harsh on the after taste. I adjusted the OPV to bring it down to 9 bar using the PF gauge, but the espresso was not as good as before.
Over the last couple of months I've then played around with higher and lower pressures trying to get back to what I was getting before, but it seems with every adjustment the taste gets worse. I marked where 9bar is on the OPV as a reference so I'm not adjusting blind, but it's just not producing the goods.
I have also played with giottos, a technika and a Alex duetto, and on all I can make better espresso, so I don't think it's technique. I also using a super jolly with 6 month old burrs so I don't think it's that either.
Has anyone had anything like this before?

I suspect it is really hard to measure the correct bar. I also made a gauge and tried to get it to sip through the water amount of an espresso in the time i measured, but still it seems far from an ideal way to get a scientific measure of the actual brew pressure it will be when making an espresso.

I don't know about your situation, but from my experience it could be that the measure is wrong. Do you know where the 11bar mark is and have you tried to set it back to its starting position?

I had an IM for ten years, so I know the machine well (sitting in storage now). I think jb is right, it is difficult to measure since it is dynamic, not static. One of the problems is that the little vibe pump barely has the capacity to keep up. As flow increases pressure drops. This doesn't happen with larger, vane pumps. If you really want to deal with brew pressure you will need to change out that pump.

I think the first mistake was changing it that much. if it was pretty good at 11 then setting it at 10 would have been a better move. The dynamic nature of the system is the other problem as the above responses state. A PF manometer is a good place to get an idea as to what to set the OPV at, but particularly with a vibe pump, that should be just the starting point.

"Doctor. It hurts when I do this."
"Don't do that!"

If the espresso gets worse after an adjustment, then it was the wrong adjustment (the universal theory of "Don't do that"). If you can taste the differences in the cup when changing the OPV, the obvious solution is to use your palate to judge what setting is best. Where you are now is too low, where it was before beginning was too high. Look for something in between.

You are also assuming that the gauge you are using for the manometer is accurate and able to accurately judge the pressure from the vibe pump which pulses 60 times per second. Even oil-filled gauges can have difficulties with that. Forget the numbers. Think of the gauge as being lettered. If "D" is too high and "B" is too low, try "C" and don't worry about anything else other than the taste of the espresso.

I had an Isomac TEA, very similar to the IM. Adjusted with blind @ 10.5 which gave me around 10 bar on the gauge with good crema, and dark syrupy espresso shots. At 9 bar i would get less crema, and shots were not as syrupy. I would mark the OPV barrel to adjust it but placing it on the old mark would not guarantee the same brew pressure, use the mark as a guide and tweak from there. By the way i think the IM and TEA Isomac machines make great espresso.

WS

The Tea and IM are nearly identical internally.

Because of water delivery concerns, I retrofitted my IM with a rotary pump installed where the water tank used to be after I converted it to plumbed. I set the OPV at about 12 bar (using it as a pressure relief valve) and then used the pump's bypass valve to adjust brew pressure. I could reach into the machine and adjust it on the fly using a gauge inserted between the pump and grouphead.

I wonder if putting two vibe pumps in parallel would solve the problem? You'd have twice the volume, which might get you past the volume-versus-pressure problem threshold.

Thanks all,
Yeah vibe pumps are much weaker compared to rotaries, but I come back to the facts that it used to be better, and also that I've used newer giottos and technikas - both vibe pump - that performed much better and tasted much nicer. Maybe that comes down to those machines being built better than the IM? I dunno.

I was kind of trying to see if anyone had a similar experience of changing their OPV setting and then having problems? Sounds like sort of - but not really.