I have recently bought and repaired a second hand Isomac Brio (same as Super Giada but with built-in grinder) which I have been failing to get tasty coffe from. Initially I found the coffee to have a strong acid feeling in the throat in the aftertaste, and just tasted worse with milk. I tried some different coffees with same result. The pressure for a 25 sec extraction was about 14.5 bars & 60 ml.

After googling it seems like the standard pressure for a extraction is 9 bar, but this was impossible to get on my machine, if lower pressure - too high flow. So I decided to put an extra OPV that regulates the pressure from the pump. The built-in OPV is strangely placed *after* the boiler, so using that to regulate pressure would result in a very unstable boiler temperature.

I have now added the OPV and tried at 9, 10 and 11 bars. The acid taste is gone, much smoother in taste but a bit weak, less and blonder crema.

Most machines that do pressure regulation at 9 bar seems to be more professional machines, with E61 groups and larger diameter PF's. Is there any consensus on how and if the PF diameter affects the optimal extraction pressure. Using same amount of coffee (i use 14g) the larger diameter will result in a thinner coffepuck, which could indicate that a higher pressure is suitable.

Wisdom on this matter appreciated

Isomac Brio Modficiation:

A lot of the 53mm machines are smaller lever machines that get much less than 9bar of pressure. And you will find that a lot of the owners think that the espresso that they drink is at least as good or better than average, if not in some ways superior to pump machines.

So there must be some other operative device than pure pressure going on.

I advocate a wider search in this case.

-Cecil

My next step will be a PID controller for the temperature, which seems to be a common modification that many people are experiencing significant improvements from.

But still, there is quite a lot of discussion of the 9-bar rule, and after some browsing I've seen that some professional machines uses 53 mm PF as well, found no figure at what pressure these are bewing at.