When filling lever machine cylinder, where does the air go?

[hbuchtel]

When the piston is raised up all the way water rushes into the cylinder. Where does the air that was filling the cylinder go?

I've looked at pictures of many lever machine groups but have never seen an extra valve to release the air, does that mean it is forced through the coffee puck? No . . .couldn't be . . .

A bit confused, I am.

Thanks for any thoughts,

Henry

[srobinson]

Through the coffee. If it is porous enough to let water through, then it will let air through.

[hbuchtel]

after a brief respite, I'm back to being confused.

As the water rushes in to the cylinder, it would start to pool at the bottom and air would be trapped above the water. At this point, how does the water escape through the coffee? Could it be forced through the water?

I suppose if it was rushing all about in the cylinder (while it was filling) some air could escape, but once it settled the air would just get compressed above the water.

(I recognize that knowing what the water does probably won't help me or anyone else make better espresso . . . just find it interesting!)

Henry

[another_jim]

I think in most cases, the cylinder is always filled with water.

When the lever goes up, it travels through the water of the filled cylinder, as it goes down, expelling water through the coffee, the cylinder refills over the piston, with steam pressure forcing the water in. This causes the boiler pressure to drop; so the water boils a little, the new steam takes up the extra space at the top of the boiler, and the pressure returns to normal.

[hbuchtel]

I think in most cases, the cylinder is always filled with water.

When the lever goes up, it travels through the water of the filled cylinder, as it goes down, expelling water through the coffee, the cylinder refills over the piston, with steam pressure forcing the water in. This causes the boiler pressure to drop; so the water boils a little, the new steam takes up the extra space at the top of the boiler, and the pressure returns to normal.

Ok, I'm with you now. I was thinking that the piston was long enough to block the water inlet tube until the piston was all the way up. Nope!

So Lever Machines have saturated group heads!

The Olympia Cremina diagrams are awesome: this one in particular shows the internals of the group- Hydraulics

There appears to be a passage to release the water above the piston which I've never seen in a Pavoni group diagram. But la Pavoni must have something like this, otherwise it would be impossible to raise the lever.

It is still not clear to me how the air under the piston is displaced. I know from using the Presso (no laughing please) that air trapped above water will simply be compressed rather then forced out.

The only thing I can think of is that once the piston is in 'pre-infuse' position the water comes in at a terrific speed and there is enough agitation to allow most of the air to escape through the bed of coffee.

If there was a small layer of air still trapped above the water it would presumedly be forced back into the water inlet tube when bringing the piston down (before the 'valve' was closed off).

Where oh where does the air go? There must be something obvious . . .

Henry

[naznar]

some spring/lever groups are saturated, and some are not i think.

What about- Where does all the air come from?
You raise the piston- pulling air into the chamber

and then on my machine the cam pushes the water valve open.


so if youre pulling air into the chamber what happens to our grounds that we worked so
hard to pack nicely..

-joel

[hbuchtel]

so if youre pulling air into the chamber what happens to our grounds that we worked so
hard to pack nicely..

Yea, that seems strange, doesn't it? But it doesn't seem to cause much trouble. Air moves through the puck very easily, I think you'd have to really jerk the lever up to disturb the grounds.

some spring/lever groups are saturated, and some are not i think.

Which ones are you thinking of? From the pictures on srobinson's Elektra/Pavoni comparison it looks like the Elektra piston might be long enough to block the water even when down all the way.

Being able to fill the cylinder with water could be useful for warming up before the first shot I guess.

Henry

[naznar]

The saturation thing im trying to figure out

All i know is that I was told some spring/lever san marcos had saturated groups, where hot water would be circulated around the head. Now Ive never quite understood this. Below I quote off Kees site ( check out his gorgeous spring lever hex ). The quoted I found as an option on the mirage, but i wonder if it applies to the idrocompresso?????? I mean restrictors on groups might be an entirely different thing, but if it could be applied to a spring lever group id like to know.

Secondly the coffeewater temperature reaching the ground coffee can be manipulated by altering the basic working temperature of the group. This basic group temperature can be adjusted by installing restrictors with different orifices at the rear of the group.
With different restrictors installed it is quite possible to operate one machine with temperatures adjusted differently from group to group to accommodate for the use of lighter and darker roasted coffees.

http://www.keesvanderwesten.com/index2.html


-joel

[hbuchtel]

( . . . ) i wonder if it applies to the idrocompresso? ( . . . )
http://www.keesvanderwesten.com/index2.html


-joel

Wow . . . I didn't realize he made a lever machine . . . beautiful!


I think that spring lever machines would be 'unsaturated' to keep the spring dry. But I have very little evidence for that (just the Elektra piston picture).

Henry

[naznar]

no I agree. My san marco has exposed springs, unlike Kees design.
The springs might rust if they are in water. What are springs high in again?
carbon?

[Dogshot]

Something that I don't understand about the brew process on a lever machine is what is in the chamber (between the shower screen and the piston) as the piston is raised but before the piston passes the port that allows boiler water to enter? Does air get pulled into the chamber, and if so, where does it come from (I hope not through the puck)?

Mark

EDIT: Thanks for finding a home for this question. ps - the autosubject feature let me down.

[HB]

I merged your question with a similar imponderable.

You're right, the space created by the retreating piston has to be filled with something. In the case of your typical lever, that's air and it must be drawn through the puck. One of Steve's advanced lever techniques to avoid disturbing the puck is to defer the final twist of the lock-in until the piston is raised a bit. This allows the air to enter along the basket edges instead of through the puck.

Interestingly the Gaggia Achille avoids this with a nifty double-action pump; as you pull up, water above the piston flows through a one-way valve to beneath the piston, eliminating the risk of an air vacuum. Dave explains the mechanism in Gaggia Achille - Materials and Workmanship.

[Dogshot]

I merged your question with a similar imponderable.

You're right, the space created by the retreating piston has to be filled with something. In the case of your typical lever, that's air and it must be drawn through the puck. One of Steve's advanced lever techniques to avoid disturbing the puck is to defer the final twist of the lock-in until the piston is raised a bit. This allows the air to enter along the basket edges instead of through the puck.

I'm not sure how correct the assessment is that air is pulled through the puck. Jim's comment that there is always water above and/or below the piston explains the presence of the 3 small holes in the Elektra column that can be seen in steve's photo:



There is a small hole at the bottom of the column and another at the top. Presumably water is exchanged between the space above the piston and below as the piston travels through its range of motion. It looks like there is no need for air to be involved in the displacement, which makes sense given that none of the lever videos show the machine sighing from the escape of air at the end of the pull.

Thanks for finding this very informative thread Dan,

Mark

[HB]

There is a small hole at the bottom of the column and another at the top. Presumably water is exchanged between the space above the piston and below as the piston travels through its range of motion.

Thanks for the correction, the channel in the bridge does help with pressure equalization. Steve explained how it can also help with heating the group in Olympia Cremina 2002: The evolution of design:

Now with regards to heating the machine... The next step that I do is a set of 6 short half pulls to quickly heat the group head. First time I showed this to Dan he had his head scratching as to how this worked, but I am simply using the basic physics of the machine to pull this off.

Let's take a look at the hydraulic diagram for the machine:



You will notice that there are two inlets into the grouphead. One port when the handle is down which allows water to come in on top of the piston and the second one which allows water to enter the grouphead when the handle is raised. Thus the half pulls allow you to circulate hot boiler water on top of the piston to quickly heat the head. I have been doing 4-6 until the group rim is hot to the touch then do a flush shot which I use to also heat my cup and then go into my routine for the first shot. This technique will give you a first shot good enough for caps and you second shot will be right on the money.

As you noted, the suction isn't through the entire upstroke, just the beginning.

[timo888]

Experiment: unscrew the spouts from a Cremina 67 portafilter, dose the basket, lock the PF in place in the ready-to-go machine, press the palp of your index finger against the spout hole in the bottom of PF, and lift the lever halfway. Now release your index finger. Pffffft!

Regards
Timo

P.S. BTW, in the diagram, the Cremina's boiler pressure is set to 0.9 bar (not to suggest that it relates to the vacuum).

[HB]

Pffffft!

Now where's the fun in that? It's more amusing to debate the point based on diagrams and specifications than actually using the machine!

[peacecup]

The Ponte Vecchio group has a small hole on the top on the boiler side. When the lever is pulled down one hears air escaping. In fact I use this sound to tell when the piston is full of water before releasing the lever.

PC

[hbuchtel]

Does air get pulled into the chamber, and if so, where does it come from (I hope not through the puck)?

It seems that air moves very easily through the puck- I just tried raising and lowering (quite quickly) the Presso arms with no water and could feel no difference in resistance between filled basket vs. no basket.

Pulled a shot with the same filled basket immediately afterwards and there was no channeling...actually it was one of the best looking shots I've pulled in a while ... maybe getting some air in the puck has a beneficial effect?

Henry

ps. anybody have a manufacturers cross-section of the Pavoni or PV groups?

[timo888]

It seems that air moves very easily through the puck- I just tried raising and lowering (quite quickly) the Presso arms with no water and could feel no difference in resistance between filled basket vs. no basket.

Henry, you're on the cusp. We can call it the BAT (Buchtel Aeration Technique).

Regards
Timo

P.S. The farther down you move from the tamped surface towards the filter, the greater the porosity of the puck.

[timo888]

Now where's the fun in that? It's more amusing to debate the point based on diagrams and specifications than actually using the machine!

Making the sound was so much fun, my wife and son had to drag me physically away from the machine. If they hadn't, I would still be at it, without a doubt, yelling Eureka! after each Pfffft!

Regards
Timo