Posted Tue Nov 30, 2010, 2:18pm Subject: Gaggia Espresso machines w/out OPV - how is pressure regulated?
AndyPanda Said:
I have a Carezza with the "spring and mushroom" group valve. I had read many posts where people claimed to use softer springs in the group valve because the group valve was restricting the pressure to the brew head. I have read posts from several people in various forums that have tested and found that the spring loaded, mushroom valve reduces pressure to the brew head.
<snip>
So I set up the test exactly as described in this thread. I mounted a portafilter gauge and removed the spring and mushroom valve from the group valve and I measured 16 bars on the PF gauge. I put the spring and mushroom valve back in and measured 9.5 bars.
I also tried not tightening the valve so the spring wasn't compressed as tightly and got higher pressure at the PF (about 10.5 bars). Then I tightened the valve up, compressing the spring tighter, and the pressure dropped back down again.
I have decided to start a new thread on this subject hoping that we can have a discussion about how these machines actually work, and maybe also to discuss the differences in machines with and without an adjustable OPV.
Hopefully we can stay ontopic, and as a forwarning I'll just say that offtopic posts regarding the gaggia user's group will be deleted. Let's talk about the machine and the espresso, OK?
cappuccinoboy Senior Member Joined: 27 Jun 2009 Posts: 789 Location: MILANO Expertise: Professional
Espresso: Milano pod, Milano fully... Grinder: grind on demand
Posted Tue Nov 30, 2010, 2:59pm Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
JonR10 Said:
I have decided to start a new thread on this subject hoping that we can have a discussion about how these machines actually work, and maybe also to discuss the differences in machines with and without an adjustable OPV.
Hopefully we can stay ontopic, and as a forwarning I'll just say that offtopic posts regarding the gaggia user's group will be deleted. Let's talk about the machine and the espresso, OK?
may I join, if I reckon I can contribute ?? or do you still believe that : "And I am saying that some of your "scientific" statements are grossly misleading. (see above)" By the way, I never pretend to be making "scientific statements...... ciao, Pietro
Posted Tue Nov 30, 2010, 3:15pm Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
It would be best to run these pressure tests at a 'typical double' flow rate (using a needle valve on the portafilter to adjust flow). Some of the test context here was lost, but the pump only reaches 16 Bar at no flow. The more data points, the better. The valve can be built with different springs to get different rates/pressures. When the valve leaks badly, it can effect shot quality as well.
Posted Tue Nov 30, 2010, 4:02pm Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
cappuccinoboy Said:
may I join, if I reckon I can contribute ?? or do you still believe that : "And I am saying that some of your "scientific" statements are grossly misleading. (see above)"
Please do join Pietro. And on the subject of the check valve reducing the brewing pressure...it seems that you were right and I was wrong ;-)
It seems to me like maybe (MAYBE) the Gaggia brew groups have been equipped with different types of check valves (on machines without OPV). Some people have claimed to show the valve opens at 1.5-2 bar and others have shown 4-5 bar.
Likewise, it seems that some folks have been able to show deadhead brewing pressure at the group of 9 bar and others have shown it running around 11-12 bar.
Posted Wed Dec 1, 2010, 8:15am Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
I read back over the old thread and a few others and noticed something. To at least some of us, when we thought about it, we expected that the group valve might require some bit of pressure to open but then once it had opened, the pressure would be the same on both sides. And so we were surprised (or maybe even didn't believe) that the pressure could be lower on the PF side than it was on the boiler side.
But remember that this is not a simple one-way check valve - the spring is a pretty heavy one and also remember that the further you compress a spring, the more force it takes. It take more force to push the valve open 50% than it does to open it 40%. So it may well open just a crack (enough to let out a sneeze of steam) at 1.5 bars and it may require considerably more pressure to open the valve fully and hold it open against the spring trying to force it back closed.
I was trying to come up with a thought experiment and at first I thought of a door with a crowd of people trying to get though it. If the door were simply on hinges and once you pushed hard enough to open it the door simply swung open and stayed open - then the door would have no effect on the pressure of the people coming through - but if the door had a strong spring pushing it closed then you can easily see how much more effort would be required by the people coming through and how much slower and fewer they would be able to move through the doorway.
OK ... so people and doors aren't a good metaphor. How about a U shaped glass tube. You could size it so a cross section of the tube is exactly 1 square inch for easy math. Fill it with water and put a piston with a 150 pound weight on one side and a pressure gauge on the other side - and let's make this imaginary piston frictionless. I would expect that 150 pounds pressing down on a piston of 1 square inch area should give 150 psi (about 10.3 bars) - am I correct in thinking that?
And the next step would be to remove the gauge and place a second piston on the other side of the U-shaped tube (also 1 square inch in area) and see how much weight it would lift. I would expect that 150 pounds placed on the first piston would lift 150 pounds on the second piston to an equal height.
If I haven't gone terribly wrong with this so far, the next step would be to put a strong spring loaded valve at the bottom of the U so that the first piston has to do work to push against both the spring and the weight on the other side. When I look at it this way, it seems more clear that the work required to push against the spring would show up by the second piston not being able to raise as much weight as it did without the spring valve.
I'm just guessing now, but I would expect that if it takes 2 bars (29 psi) to hold the spring valve open then 150 pounds on the first piston would only be able to lift 121 pounds after using up 29 pounds against the spring.
As far as real world experiments - I could certainly make a video of a PF gauge on a Carezza with and without the spring valve installed. Would that be of any value to anyone?
cappuccinoboy Senior Member Joined: 27 Jun 2009 Posts: 789 Location: MILANO Expertise: Professional
Espresso: Milano pod, Milano fully... Grinder: grind on demand
Posted Wed Dec 1, 2010, 1:48pm Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
JonR10 Said:
Please do join Pietro. And on the subject of the check valve reducing the brewing pressure...it seems that you were right and I was wrong ;-)
It seems to me like maybe (MAYBE) the Gaggia brew groups have been equipped with different types of check valves (on machines without OPV). Some people have claimed to show the valve opens at 1.5-2 bar and others have shown 4-5 bar.
Likewise, it seems that some folks have been able to show deadhead brewing pressure at the group of 9 bar and others have shown it running around 11-12 bar.
when you take things for granted you do not consider carefully every aspect of them: now I am trying to find a simple answer to a simple situation: check valve is set at 5.5bar, pump is holding boiler at max (constant) pressure of 15bar (zero debit..), portafilter is brewing at 9.5bar (with right debit, because no water no brew..) = WHERE IS water coming from if at max pressure the pump debit is basically zero ?? Now common sense says that in order to keep boiler pressure at 15 bar pump must work at max. pressure, still replenishing water beeing evacuated from check valve to portafilter, but we all know that vibe pumps debit diminishes as pressure increases......and becomes zero at max pressure.... That shows how little scientific I am ...HELP...... Ciao, Pietro
Posted Sun Dec 5, 2010, 2:33pm Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
OK ... I don't know if anyone is that interested ... but I took a video of the pressure test on a Gaggia Carezza. Now I need to figure out how to post the thing. I could just post it to YouTube and then put the link here - is that the best way?
I used a PF gauge with no flow and let the pump lock up - I did this with the spring removed (14.5 bars) with the spring tightened all the way (10 bars) and with the cap loose so the spring isn't compressed so much (11 bars).
Then I let water flow enough for a single (1 oz in 25 sec) with the spring installed (9.3 bars). Then let it flow for a double (it ended up a little large at 2.7 oz) and I got (6.5-7 bars).
I'll try to get the videos converted to a small enough format to post (they are rather large HD files at the moment) and put up a link to them in the next day or two.
Posted Mon Dec 6, 2010, 11:54am Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
Thanks for posting the video! It looks like one can readily adjust the deadhead pressure at the group in the 9-10 bar neighborhood that is desired for espresso.
Intersting also that you saw such a high pressure drop for espresso-type flow rates. I guess my next question is : with the deadhead pressure adjusted to max (11 bar?), does that drop to under 7 bar when flowing 2 ounces in 30 seconds?
My typical double shot flows just under 2 ounces (volume) in roughly 30 seconds.
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