It's been two months of playing with flow control, added by dimmer mod to my Classic in this thread, and I'm beginning to wonder if there's a better way to achieve the desired result.

I've been using the dimmer mod to hold the machine under 1 bar for about 7 seconds from pump charging up, roughly the time it takes to push the first drops of liquid through the puck and into the cup, at that pressure. I actually suspect something less than 1 bar would be even better, but there's really just no way to reliably hold the machine at any lower number with a dimmer switch and reading off a 15 bar gauge. Even targeting 1 bar, my pressure can vary ±50% (ie 0.5 to 1.5 bar), depending on how quick the machine charges up and how quick my response is with the dimmer. Not ideal.

I'm thinking a second OPV set at the lower pressure and solenoid valve are a better solution. Tee this assembly into the line between pump and 9 bar OPV, and switch it in/out at the beginning of the brew, for a more repeatable pre-infusion. Has anyone done this? Are there any machines that use this operating principle?

bean74 wrote:It's been two months of playing with flow control, added by dimmer mod to my Classic in this thread, and I'm beginning to wonder if there's a better way to achieve the desired result.

I've been using the dimmer mod to hold the machine under 1 bar for about 7 seconds from pump charging up, roughly the time it takes to push the first drops of liquid through the puck and into the cup, at that pressure. I actually suspect something less than 1 bar would be even better, but there's really just no way to reliably hold the machine at any lower number with a dimmer switch and reading off a 15 bar gauge. Even targeting 1 bar, my pressure can vary ±50% (ie 0.5 to 1.5 bar), depending on how quick the machine charges up and how quick my response is with the dimmer. Not ideal.

I'm thinking a second OPV set at the lower pressure and solenoid valve are a better solution. Tee this assembly into the line between pump and 9 bar OPV, and switch it in/out at the beginning of the brew, for a more repeatable pre-infusion. Has anyone done this? Are there any machines that use this operating principle?

Have you tried bleeding water from the steam wand during the early stages of the extraction? I can't tell from my own experience whether it is feasible/reproducible or not, but it has been suggested here and there from time to time.

Thank you for the recommendation, I should give that another try. I used to use that method, flying blind and just opening it 1/4 turn until I'd see first liquid in the cup, prior to installing the pressure gauge and dimmer mod. Since installing the dimmer and pressure gauge, on the same day, I hadn't really thought to go back to playing with the steam knob.

Since the timing for the pump to charge up is a big variable, and very dependent on dimmer setting, this may improve the process. However, I can't help but think a binary switch from ~0.5 bar to 9 bars might still be preferable to any manual variable control, on a machine where things can happen quickly.

Could you not just wire the existing solenoid to a manual switch? If you are looking to pre-infuse at a low pressure, why not use the steam pressure in the boiler?

That's an interesting idea. However, the steam pressure in the boiler appears to be highly variable, it reads anywhere from 0 to 4 bar, depending on whether machine is coming up from a cold start or recovering from a prior shot. I actually usually relieve it by cracking open the steam wand for a second and re-closing before I pull a shot, if I see any reading on the pressure gauge, never thought of trying to use that pressure for pre-infusion.

Muscle memory is a funny thing. I've pulled more than a half dozen shots for myself, since DeGaulle suggested trying to play the steam valve, and each time I kick myself half way thru the shot for forgetting to try this. I had been entertaining over the weekend, and had to pull many shots for others, so it wasn't the time to experiment with new technique.

As for the variability, the steam pressure should be constant at steady state for a given temperature. It could be reading low from a cold start due to air in the boiler.

It actually reads high when the machine has come up from a cold start, due to pressure building with temperature increasing from 23C to 93C.

I wasn't even aware that existed. Looks very cool. Will investigate further.