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Gaggia Espresso machines w/out OPV - how is pressure regulated?
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Discussions > Espresso > Machines > Gaggia Espresso...  
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AndyPanda
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Posted Mon Dec 6, 2010, 2:17pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

I'm not familiar with the term "deadhead" other than as pertains to Grateful Dead - haha.   Is "deadhead" when there is no flow and you allow the pump to lock up?

I have a couple of Carezzas - and I think the pump on the other one puts out a little more pressure when the spring is removed - I'll have to test it.  But I do know that if I put a softer spring in, the pressure at the brew head will go up from that with a stiffer spring.  

On the particular machine I tested in that video - 11 bars is about what it would reach if I didn't tighten up the cap all the way - so it was just hanging by a few threads.  That is how I used to run that machine before I got my Classic and I got decent shots with it.  I expected the cap would vibrate itself loose - but I usually pull the showerscreen and dispersion block every few weeks to clean it and that cap had never vibrated itself off.

There are a few ways to soften the spring a little - though the softer it gets, the more drips you will get at the brew head when the heater hits the top of its heat cycle or when steaming.  I don't mind but I see plenty of posts where people complain about the drips and are advised to replace that valve or clean it.

Anyway, I would guess that I could soften the spring a little or use a softer spring or leave the cap even a little looser still and be able to get closer to 9 bars while flowing 2 oz.    You could see from the video that I didn't have a precise way of adjusting the flow - I was simply leaving the pipe threads a little loose to leak approximately the right amount.  With a little fiddling I should be able to get 2oz exactly. Though my doubles are typically a little less than 2 oz.   I'll try to get 2oz in 25-30 and report the pressure reading.

I was thinking about it - I suspect the size of the hole, where the rubber plug seals against, would affect the pressure as well.  I would expect that if the area of the circle (where the rubber seals against the hole) were larger, it would take less force to open it - and if it were smaller it would take more force.  So perhaps this is another way to tune it?
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Frost
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Posted Tue Dec 7, 2010, 2:19pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

Thanks for doing those tests and posting your results Andy. I am a bit surprised your 2.7 oz 25 sec.  pressure is that low. If you are in there again for more tests, (if you can), check the voltage to the pump during the test. They are fairly voltage sensitive, (that's how I tuned my pressure/flow.)

If your double is near 8 bar, I would call it good to go, but would like to see 7 bar or more. 5- 6 bar is a good cafe crema pressure though......
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cappuccinoboy
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Posted Tue Dec 7, 2010, 3:26pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

AndyPanda Said:

I'm not familiar with the term "deadhead" other than as pertains to Grateful Dead - haha.   Is "deadhead" when there is no flow and you allow the pump to lock up?

.................................

I was thinking about it - I suspect the size of the hole, where the rubber plug seals against, would affect the pressure as well.  I would expect that if the area of the circle (where the rubber seals against the hole) were larger, it would take less force to open it - and if it were smaller it would take more force.  So perhaps this is another way to tune it?

Posted December 6, 2010 link

Of course the size of hole affects pressure, but it is just the other way round : bigger hole (bigger surface..)=More force to keep sealed : is the spring that needs to be calibrated to exert wanted BAR force against the hole. Otherwise you are right because bigger hole will require lower pressure to open and viceversa with same spring....
It is not by chance that seals are normally calibrated to 5.5 bar, and that is to give 9.5bar to espresso brewing...
It just puzzles me where the flow comes from, since at 15 bar pump has basically zero output...
Ciao, Pietro
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Frost
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Frost
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Posted Wed Dec 8, 2010, 9:56am
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

cappuccinoboy Said:

........................
It just puzzles me where the flow comes from, since at 15 bar pump has basically zero output...
Ciao, Pietro

Posted December 7, 2010 link


Pietro, How the pump works may help to explain the pressure flow behavior.

The vibe pump is a piston oscillating in a cylinder  with a check valve at both ends (water supply and delivery 'ends').  Under low pressure, the piston is free to oscillate at maximum stroke, producing maximum volume.  As back pressure rises to 'stall point',  the energy required to overcome this resistance goes up, the piston can't move as far (and eventually not at all), Thus the lower flow at higher pressures.

This pressure/flow curve is the 'characteristic curve' of the pump, Here's an old thread with the graph and discussion.......

"Re: Brew Pressure and Salvatore"
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cappuccinoboy
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Posted Wed Dec 8, 2010, 3:44pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

Frost Said:

Pietro, How the pump works may help to explain the pressure flow behavior.

The vibe pump is a piston oscillating in a cylinder  with a check valve at both ends (water supply and delivery 'ends').  Under low pressure, the piston is free to oscillate at maximum stroke, producing maximum volume.  As back pressure rises to 'stall point',  the energy required to overcome this resistance goes up, the piston can't move as far (and eventually not at all), Thus the lower flow at higher pressures.

This pressure/flow curve is the 'characteristic curve' of the pump, Here's an old thread with the graph and discussion.......

"Re: Brew Pressure and Salvatore"

Posted December 8, 2010 link

hi Frost, I am aware of how a vibe pump works, and that machines should always be fitted with OPV because at 9-9.5bar debit is double of required, that's about 200cc/min, when required debit is 60cc /min (see a diagram of Ulka E5 pump..)(golden rule is 1cc/sec): Point is that in test meter on boiler shows max pressure 15bar (0 debit) and at (blind) portafilter 9.5. Now if you were bewing coffee correctly you would have a flow of 30cc in 30 seconds, and if pump is at max pressure of 15 bar it would be unable to pump any water.....OK, good sense says that I have at portafilter residual pressure and regular brew and that to keep max pressure pump MUST replenish water that is evacuated : I just cannot find scientific answer...(and JonR is making sure to leave me with this question unanswered.....)
Ciao, Pietro
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gks
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Posted Wed Feb 22, 2012, 10:30am
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

Hello to all! This is my first post, but i have been reading coffegeek for years. I made some measuments with a non-solenoid saeco, but i think that the results are the same for all non-solenoid machines. Here is the link for the video:
http://www.youtube.com/watch?v=jG6NEJYCaSk
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Espressoliver
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Posted Sat Jun 23, 2012, 12:46pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

That video is awesome. I had no idea about the pressure drop during flow.
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Espressoliver
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Posted Sat Jun 23, 2012, 1:05pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

I'm guessing, but I think "deadhead pressure" must be the pressure under zero-flow, since it's a well-defined standard procedure, such as technicians and spec sheets rely on, and your video shows that your machine's pressure is sensitive to the rate of flow. Of course, better to look it up in a dictionary.

I'm a little confused about the theory of the spring. In terms of force, high school physics teaches that an ideal spring will resist or push back no more after you've  compressed it than when it's almost completely relaxed or at full length. F=kx is the rule, or Force = x [distance compressed]. Likewise, the work to compress it a given distance is I think the same whether you're reducing it from 75% to 50% or from 50% to 25% of its uncompressed length. That's the same constant times x squared. So to the extent the group valve spring is ideal, then from the observation that it matters how far you thread in the spring, I think we have to conclude the ball is being displaced less when the assembly is less deeply screwed in--not that the force of the ball against the hole is less.
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AndyPanda
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AndyPanda
Joined: 12 Jul 2010
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Location: San Francisco Bay Area
Expertise: I love coffee

Espresso: Olympia Cremina, Various...
Grinder: Mazzer Major, Fiorenzato,...
Vac Pot: vintage Corey
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Roaster: BreadMachine/HeatGun
Posted Sat Jun 23, 2012, 2:01pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

Espressoliver Said:

I think "deadhead pressure" must be the pressure under zero-flow, since it's a well-defined standard procedure, such as technicians and spec sheets rely on

Posted June 23, 2012 link

For machines equipped with a 3-way solenoid valve, yes, this is a valid technique.  For machines without a 3-way solenoid valve this is not a valid technique and the workings of the spring loaded group valve is generally misunderstood by most forums posts/user groups IMO.

Espressoliver Said:

I'm a little confused about the theory of the spring. In terms of force, high school physics teaches that an ideal spring will resist or push back no more after you've  compressed it than when it's almost completely relaxed or at full length.

Posted June 23, 2012 link

In real life, a coil spring definitely pushes back harder the more you compress it.  Just sit on a car and notice that your added weight (pressure) causes the springs in the car to compress more than they were compressed without your added weight. If it took the same force to compress a spring 75% as it does to compress it 25% -- why doesn't the car sink all the way to the ground?  The weight of the car compresses the spring until the spring is pushing back with equal force to the force of the car's weight on the spring - when the forces are in equilibrium the spring stops compressing.

Here is where I think many people are getting thrown off... most people think of the spring as once it has been pushed back by the pressure of the pump, the valve is now open and the spring has no more effect after that.

But instead think of this as an opening with a restriction in the opening and a force (the spring) pushing the restriction back into the opening.   If it helps, imagine a building on fire and all the people trying to exit out a single door while people are outside pushing the door closed again.  Fewer people would be able to get out of the door while it is has back pressure trying to close it -- and more people would be able to get out if there was no pressure and the door could simply swing open and stay open.  

The doorway and people metaphor only works so far ... a spring pushes back with more force the further you compress it.  You could replace the people outside the door with a spring pushing the door back.  You could even say that four people can fit into the doorway and if the door were wide open four people per second could pass through - the spring is strong enough that it requires two of those people standing in the doorway to push the door open while two people per second go out the door.  This is closer to a working metaphor.

Back to espresso machines and water trying to force its way through a spring loaded group valve - it may only take 3 bars of pressure to push the spring far enough to open a crack and let a trickle of water through the group valve ... but it might take 5-6 bars to push the spring back far enough to allow 2-3 oz of water to pass through the groupvalve in 25 seconds and only 4 bars of pressure to push it far enough open to allow 1oz through in 25 seconds. The further you open the valve the more pressure will be used up holding it open.

Your steam valve (or a water faucet) has a restriction being pushing into the opening - but it is not a spring holding the restriction - so you can adjust the valve to the desired opening and you get more or less flow depending on how far the restriction is pushed against the opening.  The group valve is just like that except a spring is pushing the restriction into the opening -- the further back the restriction is pushed, the wider the opening and the more flow through the opening.

My testing showed that with a stock groupvalve spring tension ... I was able to pull about 1oz (a single or a ristretto) while keeping 9.5 bars of pressure ... and when flowing 2-3oz in the same amount of time the pressure dropped to 6.5 bars.

I then tested again by relaxing the spring tension and was then able to get 9.5 bars while flowing the larger volume shots.  The drawback to less tension on the spring is that now the normal boiler pressure (when the T-Stat is swinging the temps up and down) at the hotter temps will push the valve open and sneeze out steam. (this can happen anyway - but it will happen a lot more with the softer spring tension).  And if you then want to pull a single or ristretto, you would end up at 11-12 bars (and this would be one case where an OPV might be useful on the non-solenoid machine).

Myself - I generally pull 25-30ml doubles, that's how I like them - so the failings of the group valve spring seem to work out just about perfect for my preferred shot ... no OPV required.
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Espressoliver
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Posted Sat Jun 23, 2012, 4:41pm
Subject: Re: Gaggia Espresso machines w/out OPV - how is pressure regulated?
 

On the Carezza, there's a second spring-less ball valve that branches off from the top of the boiler and conducts flow back to the reservoir. Seemingly low pressure in the boiler would suck the ball into its seat and seal the aperture, while under high pressure the ball would move out of the way--and I have indeed seen hot water flowing back to the reservoir on my machine. Someone online wrote that the Carezza manual describes its function as having to do with preventing dripping, and I can see how it could help in that way, but as an outflow route it would also seem to function as a fixed-resistance/always-open over-pressure valve. So I think in a way you don't really have deadhead conditions in your video when you have the brew head closed off with the manometer. There will be flow to the reservoir through this tiny side port. Unless your is so crudded up its clogged, which I doubt, the port must be sized small enough to let the vibe pump to build up the high pressure you observe with no flow past the manometer.  Maybe it's to compensate for pump variability, ensuring that whether it's capable of 15 or 12, that's still over and above what this port will allow, which some lower fixed pressure (or range of pressures?) suitable for espresso.  Some people advocate sealing it off, which seems wrong-headed to me, no pun intended. I'm not sure if it counters in any way the fiddling you can do with the spring valve.
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