I'm not sure, but I think the Saeco Aroma doesn't have a 3-way solenoid valve ... so what do you expect to measure with a pressure gauge between the boiler and the pump? It will most likely be much higher than the pressure that hits the coffee puck ... at least that is what I would expect due to the spring loaded resistance group valve in non 3-way machines.
If you have the means to measure with a PF mounted gauge and want to do an experiment, see what a PF mounted gauge reads compared to what your internal gauge between pump and boiler reads. And compare with and without allowing a shots worth of water to flow from the PF.
My prediction is that if you have a PF mounted gauge and don't allow any flow from the PF, you will see 15-17 bars with your internal gauge and 11-13 bars at the PF ... and then allowing 2oz to flow in 25 seconds i expect you'll see 8-9 bars at the PF gauge.
I have not seen any explanation for exactly how this machine works, but have watched it through shots to see where water is flowing.
From what I can tell, the opv is built in to the steam wand assembly. There is a T with one end going to the steam wand and the other end plumbed back into the pump intake tube. Hot water flows from what I believe to be the OPV during every shot.
My guess is this. The spring valve in the group head is set at some pressure less than 9 bar, so water starts flowing here first. The OPV is probably at 11 bar (its also a pod machine) and controls overal system pressure. So, as the pressure ramps up, first the group head valve releases, then the OPV.
What I want to know is what the actual boiler system pressure is during the main part of the shot. This in turn would determine the presure at the group head much like in any other espresso machine. I'm expecting something around 11 bar.
I was originally thinking that I could introduce another OPV, set at 9 bar, between the pump and boiler. This would do two things: lower system pressure to a range more optimal to espresso,by regulating pressure before the boiler reduce the amount of cold water introduced to the boiler during a shot reducing the temperature drop during a shot.
I'm not sure I will be able to physically put an OPV where I want. I'm still explorint that one.
Interesting. Looking at the parts diagram on the parts-guru site, I can't tell what's going on int he boiler but you're right that the OPV also serves as the path to the steam wand and of course back to the pump.
There's a small chance that it's adjustable in a normal sense, meaning it's built in a way that it's intended to be adjustable. You could try cuttting spring coils and reducing the force it has on the seat (thus lowering the pressure required to release it) but it's a 1-way trip.
It would seam that inserting another OPV somewhere in that system should work - the path from the pump to the boiler, the inside of the boiler, and all paths from the boiler that are open are all connected to each other and under the same pressure as each other. Or at least could be if any other restrictions in the system were removed (a spring valve in the group head if there is one).
I haven't done a thorough search but I've checked out a number of OPV's in all sorts of parts diagrams and haven't yet seen one that was truly inline and adaptable to any machine. Only OPV's that mounted to boilers or pumps directly. I guess any OPV that had fittings all of the way around it should be able to be adapted but it may be an interesting set of fittings and connections you'd have to make and take a chance on leaks, etc. with.
Not to discourage you. From what I can tell your plan makes sense, it's the execution that could be tough.
tracerbullet. I'm not a big fan of one way changes. DIY is full of surprises, so its always a good idea to be able to back out. The stock OPV is definitely not adjustable.
I found the following bypass/OPV a while back. For that price, there is not much risk if it doesn't work out.
From what I can tell your plan makes sense, it's the execution that could be tough.
Its always about the execution. Having grown up on a farm, I'm no stranger to hacks, mods and fixes. The first step is to see what kind of pressure I'm dealing with. Plumbing parts for 4/6mm hose seem to be fairly standard, so the big question is if I physically have enough room for both the T for the pressure gauge and an OPV
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