I'll have to look at the link some other time. Eating lunch with one hand and surfing with another right now and need to get back to work.
Yes, the OPV has a spring, and the more you push against the spring (the more it gets compressed) the more it fights back. F = kx (Force = spring constant x deflection).
Changing the flow (brewing an espresso) changes the amount of water going through the OPV, changing the exact distance the OPV opens (the distance the spring deflects) to allow that different amount of water through, changing the force exerted by the spring, changing the pressure in the system.
The pressure difference between brew & static should then vary from one machine design to another, especially the diameter of the OPV at the point it passes water through and the spring that is used there. I could see a long weak spring and a short stiff spring ultimately exerting the same force when the OPV is closed, but acting totally different when the water flows (different k values even at the same x distance will give different Forces).
Makes sense to me. Doesn't guarantee it's correct, but it makes sense.
Sounds agreed and settled. I found similar with a T fitting off of the portafilter. Gauge on one end and bleed valve on the other. Static pressure was greater than when I cracked the valve to simulate a double shot pull.
If you don't have the 3-way valve on that model, you won't be reading brew pressure, because the non 3-way machines have a group valve that reduces the pressure to the PF by anywhere from 3-5bars depending on a number of things.
Would you be willing to expound upon that statement a bit? I'm exploring some issues with checking pressure on my Gaggia Color and it sounds as though your working knowledge of the variables surrounding pressure is pretty good. The color does not have a three way valve but I'm having difficulty getting a reading much over 9 bar with my homemade pressure gauge.
I'm not sure what the threshold for 'off-topic' is here so perhaps you can email me personally if this feels 'off topicky' at all.
In a non 3-way machine like the cheaper Gaggias, if you remove the spring loaded valve, then you get the same pressure at the PF gauge as you would expect from the pump. But when you put the valve back in, the spring pressure in the valve is pushing back against the pump and effectively eats up a lot of the energy the pump is putting out. The pump has to push the spring open to allow the water from the boiler to the coffee puck and the spring keeps pushing back trying to close the opening - so that energy is eaten up and subtracted from the pumps output.
I found that I could stretch the spring to make it stronger and I would reduce even more pressure to the puck. Or I could soften the spring by compressing it with pliers until it was shorter and that would increase the pressure to the puck. Or you could cut a little bit off the end of the spring to make it shorter. Or you can just not tighten the cap on the valve all the way - so it doesn't compress the spring all the way.
However ... making the spring softer to get the pressure higher will also result in more drips when heating to steam temps since that spring pressure is what holds back the pressure in the boiler.
I also found that for any given spring tension, the pressure will still go up and down dramatically based on how much you allow to flow. For example, if I set the spring tension so that I get exactly 9 bars when flowing a 1oz shot in 25 seconds I would get much lower pressure (like 7 bars) when flowing 2oz in 25 seconds. So in these machines without a 3-way there will be one specific shot volume that they can produce at 9 bars and pulling a larger volume will give lower pressure and pulling a smaller volume will give higher pressure (pulling small volume shots would be the only case I can see where an OPV would be useful on these machines - they usually can barely or not quite reach 9 bars when pulling a normal double).
Doing the same sort of test on a classic with 3-way and OPV ... you get very close to the same pressure (what you set the OPV to) regardless of whether pulling a half ounce or a three ounce shot.
Thank you so much for that explanation. It makes a lot of sense in light of what I've been experiencing.It appears that if what you say is true then it isn't at all unlikely that I'm currently getting a brew pressure level somewhere near 9 Bar. I've checked it now with two gauges which gave identical readings of 130 psi. So the question then is why would Gaggia go to the trouble of advertising a machine which produces 14-15 Bar of pressure if owing to the group valve the machine brews at a much lower pressure? This would be a bit of a Godsend for me really since conventional wisdom states that approx. 9 bar of brew pressure is near ideal. I've been considering a modification of the machine which would lower the brew pressure from 14 to 9 bar but it seems that perhaps that is quite unnecessary. Would be a relief for me ....as a newbie I'm already a little overwhelmed by the number of variables at play in the production of an espresso shot and not having to modify the machine's pressure would be a welcome thing. Your response was super fun to read....I knew you'd have a thorough answer...and you did! Thank you.
Pump pressures and pump flow rates are intertwined, so a pump needs to be capable of higher static pressures in order to still maintain at least 9 with things flowing. Also as the pump ages it'll be less likely to make as much pressure so some initial overhead is nice there as well.
Agreed though it's a silly thing to advertise once you know how it all works.
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