Preinfusion -> pressure profiling mechanism or switch

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harrisonpatm

#1: Post by harrisonpatm »

I'm looking for a favor from anybody with access to a machine that offers pressure profiling via a paddle mechanism. That is to say, one where moving the paddle a little starts preinfusion, then moving it more starts at low pressure, and high pressure the rest of the way. The LM Strada comes to mind, I know there's more out there.

What I'm wondering is the actual switching mechanism, what does it look like? I'm sure there's more than one way to do it, which is why I'm curious about pictures under the hood. Presumably there's an on/off switch to open the brew valve, followed by a potentiometer. Anybody have pics of it?

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Peppersass
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#2: Post by Peppersass »

The current version of the La Marzocco GS/3 MP does what you describe. There's a microswitch in the group head that turns on the pump motor. The microswitch is activated by moving the paddle a small distance to the left. There's also a "conical" valve in the group head that opens wider and wider as the paddle is moved to the left, allowing more and more water to flow into the puck. When the valve isn't fully opened, and the flow is restricted, the excess water being pushed by the pump is diverted by the conical valve into the drain box.

For example, you can do a shot with long, slow preinfusion by moving the paddle to the left enough to turn on the pump and allow whatever flow rate you desire for preinfusion. This is called "flow profiling" rather than "pressure profiling" because until the basket fills completely there's zero pressure against the puck. When the basket fills and full pressure is reached, you can move the paddle further to the far left to allow the full rate of flow generated by the pump. At this point any adjustments you make with the paddle change the pressure and flow together, so we call that "pressure profiling". For example, after full pressure and flow are reached (paddle all the way to the left), the puck will normally become more permeable as the shot progresses and the flow rate will increase. You can move the paddle slowly to the right to decrease the pressure and reduce the flow rate or maintain a constant flow rate. This sometimes helps to "tame" lighter roasts.

Below is a diagram of the GS/3 MP group head with the microswitch and part of the conical valve:



Here's the rest of the conical valve, showing the water input port and the exhaust port that diverts excess flow to the drain box:



Item #13 is the 0.6mm "gicleur" or flow restrictor, which all GS/3 models have. It reduces the flow regardless of the position of the paddle, providing a gentle ramp to full pressure when you do a "standard" shot without flow or pressure profiling by quickly moving the paddle all the way to the left.
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harrisonpatm (original poster)

#3: Post by harrisonpatm (original poster) »

Thank you for that, this is helpful. And while I know that flow profiling and pressure profiling are similar, related, but separate, I guess I was unaware that the "pressure profiling" provided by the GS3 is controlled by flow profiling first, which then affects pressure. Did I understand your explanation correctly? If so, why is the choice to provide profiling done via the flow of water rather than the speed or power of the pump/motor? I appreciate you bearing with me as I'm understanding these concepts.

harrisonpatm (original poster)

#4: Post by harrisonpatm (original poster) »

Wait wait, my bad. I realized i asked a stupid question after I posted. Since flow and pressure are related, pressure profiling is always controlled by flow, whether by a gicleur and valve at the grouphead, or by restricting the speed of the pump, which reduces the water that it pushes into that direction. So then let me rephrase my question: why did LM decide the cheapest, easiest, or simplest way to control flow was at the grouphead rather than at the pump?

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Jake_G
Team HB

#5: Post by Jake_G »

The MP uses an elegant (albeit wasteful) approach where a common port in the conical valve shaft moves between a pressure port and a waste port in the valve body. This is elegant because it eliminates the need for an electric 3-way solenoid valve and provides for a means to directly control the brew pressure in the process.

It is wasteful because it controls the pressure by providing overlap between the pressure and exhaust ports as the common port in the conical valve shaft passes by them. A little pressure port and a lot of waste port yields low brew pressure and lots of fresh water out the waste port. A little waste port and a lot of pressure port results in higher brew pressure and lower fresh water out the waste port. All the positions in between offer more or less pressure and varying degrees of waste water. When the paddle is all the way to the left, there is no waste and full pump pressure (and flow) is applied to the group.

All the electronics do in an MP is regulate brew temperature and start and stop the pump when the paddle is moved (in addition to managing the steam boiler and providing other UI functions like power scheduling, etc...)

Cheers!

- Jake

alunare

#6: Post by alunare »

So If I understand correctly, there is necessarily a limited preinfusion time on the MP because the basket is going to reach full water stage at one point and then the pressure will build on the puck and water will start to get pushed down ?

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Jake_G
Team HB

#7: Post by Jake_G »

Nope.

The waste port will just bypass more water to the drain box. You can pull a 2 minute shot at 2 bar if you like, but you'll be wasting a good deal of water to do so.

Cheers!

- Jake

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harrisonpatm (original poster)

#8: Post by harrisonpatm (original poster) »

Jake_G wrote:The MP uses an elegant (albeit wasteful) approach where a common port in the conical valve shaft moves between a pressure port and a waste port in the valve body. This is elegant because it eliminates the need for an electric 3-way solenoid valve and provides for a means to directly control the brew pressure in the process.

It is wasteful because it controls the pressure by providing overlap between the pressure and exhaust ports as the common port in the conical valve shaft passes by them. A little pressure port and a lot of waste port yields low brew pressure and lots of fresh water out the waste port. A little waste port and a lot of pressure port results in higher brew pressure and lower fresh water out the waste port. All the positions in between offer more or less pressure and varying degrees of waste water. When the paddle is all the way to the left, there is no waste and full pump pressure (and flow) is applied to the group.

All the electronics do in an MP is regulate brew temperature and start and stop the pump when the paddle is moved (in addition to managing the steam boiler and providing other UI functions like power scheduling, etc...)

Cheers!

- Jake
What an excellent description, thank you so much, I greatly appreciate it. My head was about 80% wrapped around how the mechanics work, you brought it home the rest of the way.

Not to sound rude, but it doesn't answer the "why." As you said, this solution is elegant in that it is mechanically controlled, though wasteful. The pump has an on/off and a set pressure setting. Why, in your opinion, was that the chosen route, rather than restrict flow at the pump and the amount of water sent to the grouphead at the first place? I've seen that done in this forum, using a variable speed motor at the pump. Wondering why that wasn't chosen.

Or perhaps this question is unanswerable. Who's to say why a large company chooses one route over another?

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Peppersass
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#9: Post by Peppersass »

harrisonpatm wrote:So then let me rephrase my question: why did LM decide the cheapest, easiest, or simplest way to control flow was at the grouphead rather than at the pump?
I can take a swing at that one.

The GS/3 MP uses a rotary pump with a bypass valve. The pump is driven by a constant speed AC motor. It's possible to vary the speed of an AC motor, but it's not easy and not cheap. See this post: 'Pressure Profiling' With The Fluid-O-Tech TMFR Pump - Or, Wholesale Copying Greg Scace's Ideas about an attempt by our member shadowfax to do so. Also note that even though the motor speed could be varied, the pump's bypass valve presented some significant problems.

It's simpler to use a gear pump, which can be driven by a relatively small variable-speed DC motor. La Marzocco uses that approach in their Strada EP. The Strada can do flow and pressure profiling by varying the speed of the gear pump, but there's a major limitation: even if you set the pump speed to zero, the preinfusion flow rate can't be lowered below the input line pressure, which usually needs to be about 3 BAR to keep the pump from cavitation. So you can't achieve the very long, slow preinfusion that's sometimes needed to fully extract light and very light roasts. Thus, the Strada is really a pressure profiling machine that's not capable of flow profiling.

Slayer took a different approach that gets around this limitation: regulate the flow with a needle valve that can be switched in during preinfusion (and optionally after peak pressure is reached if the flow gets too fast.) The Slayer happens to use a gear pump, but the speed can't be varied during a shot. That sort of eliminates the main reason for using a gear pump. The pump speed can be fine tuned for a range of grind settings, but I doubt many Slayer owners do that. Since the gear pump doesn't have a bypass valve, the pressure naturally drops as the puck becomes more permeable and the flow increases during the back end of the shot, a kind of automatic pressure profiling that can be helpful with light roasts. But it's a minor "feature". I think the company that acquired Slayer finally realized that a gear pump isn't necessary, and they had had some reliability problems with them. So Slayer recently replaced the gear pump with a rotary pump.

Years ago, I replaced the rotary pump in my GS/3 with a gear pump, and ran into the line pressure problem. To get around it, I installed a high-quality Swagelok pressure regulator that allowed me to lower the line pressure to 1-2 BAR. But that wasn't low enough for a lot of the light roasts I pull, and I couldn't lower pressure more because the pump would cavitate. I finally installed a "Jake Valve" for Simple Profiling on a La Marzocco GS/3 AV (yeah, same Jake who replied to your query), which is a needle valve that allows me to drop the flow rate to zero during preinfusion, even with the gear pump running at full brew speed. I installed a solenoid-driven bypass valve to switch the needle valve in and out of the circuit, just like the Slayer. Since I can freely vary pump speed, I can do both flow profiling (long, slow preinfusion set by the needle valve), followed by pressure profiling, usually lowering the gear pump speed as the flow increases after max pressure is reached in order to keep the flow rate constant.

Although I haven't had problems with mine, gear pumps are less rugged than rotary pumps. Also, unlike a rotary pump, they don't have a bypass valve to set a constant operational pressure (that would defeat the purpose of the variable speed). They do have an optional safety valve to protect the pump, but AFAIK, neither Slayer nor LM gear pumps have that feature. So care must be taken not to run the gear pump speed too high against a blind puck or closed needle valve without providing an external expansion or bypass valve.

With that background, let's get back to your question. LM designed the latest version of the GS/3 MP to provide the best of both worlds. You can do both flow and pressure profiling, but with a more rugged and better-protected rotary pump. There are two trade offs:

1) The profiling is entirely manual. So you have to remember where the paddle needs to be set for preinfusion, any pressure profiling you do, etc., and you have to practice to be able to repeat the settings consistently. From what I hear, most MP owners learn to do that and don't see it as a problem.

2) As Jake points out, reducing the flow rate can produce a lot of waste water. In the grand scheme of things it's not a lot of water (tiny by dishwasher and laundry machine standards), and if you're plumbed in you probably won't notice it.

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Jake_G
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#10: Post by Jake_G »

harrisonpatm wrote:Why, in your opinion, was that the chosen route, rather than restrict flow at the pump and the amount of water sent to the grouphead at the first place?
Well, in order to explain my opinion, you have to go back to the helix valve MP, which predates the Strada. While not "the original" MP, it was the first modern paddle group and the M stands for "mechanical", thus replacing the electric solenoid 3-way valve.

This group started off with a simple premise; move the paddle halfway and expose the group to line pressure, move it a little bit further and turn the pump on. Again, all mechanical.

Then with the development of the Strada, they bored the center of the helix shaft all the way to the top and mounted a periscope gauge measuring group pressure and kicked the pump on as soon as the common port began to approach the pressure port in the valve body.

Boom. Pressure profiling was born.

Boom. O-rings started failing much more quickly than before.

So, the Helix valve shaft moves straight up and down while the paddle sweeps left and right. At the lower position, the common port is sandwiched between two O-rings and aligned with the exhaust port. When you begin to move the paddle to the left, the valve shaft is lifted past the upper O-ring and exposes the pressure port, as well. Move the paddle further to the left and the common port comes to rest again between two O-rings, this time aligned with the pressure port in the valve body and sealed off from the exhaust port. There was precious little overlap between the exhaust and pressure ports as the common port crossed over the center O-ring separating them, but it worked! Our own Pat Campbell was the first to implement the so-called "Strada Mod" to a GS/3 a small handful of years ago.

So I bring up all of this because in this context, the conical valve is a natural progression. Using a Ryton valve body and a stainless steel valve shaft with a conical sealing surface, LM was able to turn the up and down periscope motion of the helix valve into a smooth rotating motion that requires no seals between the ports. Gone are the O-ring failures! Yes, there are still a small handful of O-rings in the group, but no longer does the common port in the shaft have to scrape past an O-ring (twice!) every single time you pull a shot. The conical valve also allowed LM to tweak the geometry to provide a better transition from exhaust position with zero pressure to full pressure. Where the Strada mod machines had a small range of motion where the port crossed the O-ring, the conical valve has a much wider (although still somewhat small) window where you can dial in different brew pressures at will.

So there. That is my opinion on why the MP is the way it is.

Now the EP uses a switch and a fan-cooled contact potentiometer to track the paddle position and translate that into a pressure setpoint in a closed loop PID controller that spins a gear pump at varying speeds to hit the target. So, LM is not afraid of that approach.

As for restricted flows, that's Slayer's thing, and I think LM is content to let Slayer play in their space. Yeah, Lelit brought flow control to market with Bianca and ECM/Profitec followed suit, but as far as exposed group, saturated boiler machines go, Slayer has the flow restriction market under their belt and some fairly broad-ranging IP on the subject, to boot!

Cheers!

- Jake

P.S.

See what Dick said