When filling lever machine cylinder, where does the air go? - Page 2

by **Dogshot** on Fri Feb 02, 2007 11:47 pm

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.

by HB on Sat Feb 03, 2007 12:33 am

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.

by **Dogshot** on Sat Feb 03, 2007 8:29 am

HB wrote: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

by HB on Sat Feb 03, 2007 9:46 am

Dogshot wrote: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:

srobinson wrote: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.

by **timo888** on Sat Feb 03, 2007 10:13 am

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).

by HB on Sun Feb 04, 2007 12:33 am

timo888 wrote: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! :lol:

by **peacecup** on Sun Feb 04, 2007 1:22 am

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

by **hbuchtel** on Sun Feb 04, 2007 9:24 am

Dogshot wrote: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?

by **timo888** on Sun Feb 04, 2007 9:44 am

hbuchtel wrote: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.

by **timo888** on Sun Feb 04, 2007 9:56 am

HB wrote: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


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