How low can you set OPV pressure?

[henri]

My Casa V manual says the OPV should be set between 8.5 and 12 bars. It's unclear to me whether these numbers somehow represent the physical extremes of the valve or if it's just a recommendation born of the tradition of 9 bars.

Question (possibly a stupid one): How low can an OPV (over-pressure valve) be set without harming the machine?

Rationale: inexplicable lingering bitterness in shots, especially with non-comfort coffees, when using a single basket. I'm thinking the higher pressure (currently at about 8.5 bars according to the gauge) may be disturbing the little puck too much, and would like to try something lower, around 6 bar perhaps. As a point of comparison, shots I pull using the Flair Classic (same coffee, same grinder, same grind size and same dose) are much nicer. (I don't have the pressure gauge for the Flair but I think the pulls must be well under 8 bars.)

[DaveC]

For most expansion valves, the lower limit is around 7 bars, some can't go below 8, after that you are off the spring and there is no pressure to prevent the valve leaking all the time.

[HB]

Rationale: inexplicable lingering bitterness in shots, especially with non-comfort coffees, when using a single basket ...and I would like to try something lower, around 6 bar perhaps.

With single boilers like the ECM Casa V, opening the steam valve / water wand during the extraction should temporarily lower the brew pressure.

[henri (original poster)]

For most expansion valves, the lower limit is around 7 bars, some can't go below 8, after that you are off the spring and there is no pressure to prevent the valve leaking all the time.

Thanks! You're right indeed - I tried opening it as far as it goes, and the gauge then shows about 7.5 bars with the blind basket. This is good to know, and I think I now understand better how those valves work.

With single boilers like the ECM Casa V, opening the steam valve / water wand during the extraction should temporarily lower the brew pressure.

That's clever! I've just given it a try, using the steam valve to regulate pressure down to 7 bars at the gauge (6ish at the puck?) during a pull. The shot was better (= less bitter), but I'll have to keep experimenting - with a sample size of one I can't rule out this having been a lucky accident. I'm thinking this could also be a temperature thing, as I've noticed the machine tends to overheat especially if left on for too long.

... which leads me to my next question: is there a way (a tried and true procedure, one that doesn't require specialized thermometry at the group) to adjust the boiler pressurestat (?) so that temperatures are a bit lower?

Though I suppose eventually I'll just have to bite the bullet and invest in a PID'd machine.