It is generally accepted that the ideal head space is slightly less than the thickness of a nickle coin. Too big a space is thought to lead to channeling. I am curious about the physics behind this. Is it because excess room can allow for too much turbulence on initial water injection and disturb the top surface of the puck? I am thinking that during the pull, water flows through the shower screen very slowly. Thus, the thickness of the water layer between the puck and the screen should not matter.

With a La Pavoni, it is possible to lift the lever such that water is injected into the chamber very gently. The top surface of the puck can be further protected by a piece of filter paper. Does this mean that manual lever machine can be safely dosed to leave a larger head space? What is your actual experience?

Thanks for your feedback.

This video shows a La Pavoni with a transparent portafilter. This might answer some of your questions.

https://www.youtube.com/watch?v=8DB_9wXv0XI

Thanks, kofi. The video shows a very big head space. The only time a lower shower screen could have had an effect is when he did Fellini raises. If the screen was closer to the puck, it might have restricted the expansion.

Is it possible that the nickle test is actually indirectly measuring the thickness to the puck?

wkmok1 wrote:... Is it possible that the nickle test is actually indirectly measuring the thickness to the puck?

The nickel test is a direct measurement of the clearance between the screen and tamped puck so it also determines the thickness of the puck.

Having a spring lever I'm puzzled as to why there would be much difference between all types of machine, with the faemina you can also inject water reasonably gentle but I'm thinking that for all machines the starting point will be similar: a puck with an air filled space above it.
The inrush of water, no matter by what means (other than perhaps a slow running controlled or volumetric pump) will be relatively violent and that is imo one of the purposes of the shower screen; to act as a resistance. Once part of the air is replaced by water the influx of water will have different dynamics.
To me the practicalities of making espresso are encountered in current machine, you need a certain amount of grounds for certain amount of espresso and the shape and size of the PF basket and group are designed to work under those conditions.

Or better said, why would you want to severely underdose?

One trick I have found helpful with both manual and spring levers is to pull to the point where flow into the group is only partially open, rather than cocking the lever all the way at once and letting the water rush in under full boiler pressure. I hold it there for a second or two until the puck is saturated, and then open the flow all the way to finish pre-infusing under boiler pressure. This makes it less likely that the initial spray of water will disturb the structure of the puck. I find it most useful when I am pulling unusually small doses in a double basket.

that is one of the nice control options that you get with the La Pavonis. You can just crack into the position the piston allows water in, and control the flow that way. So it is not an "all or nothing" situation.

Not sure if it works the same for all levers but on mine I can try slow flow inlet without the PF in place, that way you can easily see the difference between full blast and slow flow...beware of the steamy hot water gushing out before you try though!

Too much head space increases the amount of air between the bottom of the piston and the top of the coffee. The more air in this space, the more air there is that needs to driven from the space by evaporation when the first water begins to enter the chamber. The more air that remains, the "spongier" will be the pull. The screen will also likely be cool enough to condense the water vapour and thus reduce air loss even more. (We tend not to notice the sponginess problem as much with spring levers as with direct piston operated machines like the La Pavoni, but it is still there.)

(As has been mentioned in several posts here before, by several participants, raising the handle of machines like the La Pavonis, and allowing a bit of water to be expelled prior to attaching the portafilter, and then leaving the handle most of the way up whilst attaching the portafilter, tends to leave the space filled with water vapour rather than air, and thus eliminates almost all sponginess. One would expect it to eliminate it less efficiently with a lot of head space. This also eliminates cracking the puck as the handle is raised from the upward rush of air. Similar things can be done with spring levers, but it is even more awkward!)

Yes. I use no head space. Final tamp being done by the head. If there is no room for the coffee to move, how can it be disturbed when lifting the lever?