i have a PID Alexia from ChrisCoffee, and i find for my purposes a fantastic machine, not being a Milk person for espresso.

Ive adjusted the OPV to 9.5 bar.

when I pull a shot, its my understanding that any pressure generated over 9.5 bar releases water back into the boiler.

now when I do a back flush w cleaner, Im also told that when the pressure reaches 9.5 water from the group head (which now contains cleaner) does not go back to the boiler.

therefore these two areas are some how separate in turns of overflow.

is this correct?

many thanks!

When you are running a backflush.. The water brought into the group, and the cleaner, go ONLY through the 3-way valve, and out the bottom of the grouphead. The OPV has no part in this.

The overflow from the OPV valve, runs back to the reservoir. That is why there is two tubes in the tank. Next time you are pulling a shot, or doing a backflush, lift the shorter tube above the water.. You will see water flowing back into the tank.

Since a vibe pump, in itself, can't be controlled to put out a certain pressure.. meaning it only puts out the pressure it puts out.. The OPV valve is used to control the flowrate of the water, getting the pressure where you want it. Say the pump pushes out X amount of water at its normal operation(say 15BAR).. Once the water leaves the pump, the OPV valve takes off the right amount of water to give you only 9.5BAR at the group.

Here is a good description of how an OPV does what it does. The page is from Randy Glass's website.. Which is a wealth of information as well.

many thanks for this information, and the link to Randy Glass' web site

cheers!

Here's a photo of the Alexia's over-pressure valve (OPV):


From Buyer's Guide to the Quick Mill Alexia

The FAQs contains a few interesting threads on OPV trivia, e.g., The importance of brew pressure, purpose of adjusting OPV? and I still don't get it: Why adjust the OPV?

thanks for the pic. I actually found it previously. of note in the PID model, its turned around 180 degrees to allow the PID to fit. the tube going up would be in the way.

I initially started to adjust the wrong thread. chriscoffee set me right.

thanks again!