I was doing some thinking a while back(always dangerous) about the inner workings of my Pavoni. I was actually wondering about how much more volume I could get out of my shots if I set my piston to just touch the group screen at the bottom of the stroke, with the jam nuts on top. Turns out the difference between snugging the nut all the way down and kissing the screen with the piston was a little less than 2 turns, not enough to take the trouble(~.15oz).

While I was in there I did a little poking around. I've heard lots of different opinions around the internets as to how water flows in these, including such absurdities like - the water flows between an inner and outer wall before entering the group. Just to be clear about what's actually there I quickly sketched up the grouphead on lunch a while back.
The reason to not overfill the boiler past the top of the sight glass is because water will spill through the vent onto the top of the piston. Not a huge deal but I have had water drip past because of it. Piston seal design is such that it works better the more pressure there is. Depending on the freshness of your seals YMMV.

As you lift the handle the air above the piston is pushed into the boiler through the vent, air under the piston is drawn through the groupsceen and/or coffee depending on how you lock and load. As the piston passes the lower hole, pressure from the boiler forces water into the group, hopefully your portafilter is locked at this point.

This is actually a critical point, for two reasons. First, if you lift past the opening quickly water can rush in towards the front of the group with enough force to appreciably disrupt your carefully formed coffee puck. Second, any space left between the top of the hole and the bottom of the piston at full lift will trap air, hence the need for the fellini. In proper use the fellini technique isn't so much to add additional volume to the shot but to ensure a full group and consistent water charge, with the Pavoni at least.

What I've done to solve both these issues is actually quite simple. Start the machine up to get just a little pressure in the boiler, grab a fine tip permanent marker, make sure the portafilter is out of the group, lift the handle to the point where water just barely starts to drip, draw a line across the piston rod and top of the group. Kind of hard to see the marks in the photo, I circled them.



Now when loading I can keep the portafilter loose and lift quickly until I'm about a line width below. Lock it in and slowly lift just through the line and hold for a few seconds to let water fill the top of the puck, then lift the rest of the way. Now to clear air just give 2 or 3 quick pumps between the top of the stroke and the line. Staying above the line keeps the hydraulic system open to the boiler and movement in this range won't disrupt the puck. As soon as the piston seal goes below the fill hole any upwards travel will disrupt the puck.

I've experienced great success with this. I can lock and load quickly and clear air quickly without any disruption to the coffee. My shots are better because there is no disruption to cause channeling and my water volumes are much more consistent.

Interesting. Thanks for the write-up and the pictures. I'll have to give it a try sometime.

meastway wrote:I can lock and load quickly and clear air quickly without any disruption to the coffee. My shots are better because there is no disruption to cause channeling and my water volumes are much more consistent.

I've said (here and elsewhere) that the easiest way to NOT disturb the puck is to leave no headroom.

I asked a question awhile ago (to which I received no response) on why Richard Penney's tampers had 5mm sidewalls, when clearly my LaPav needed a clearance of 7mm. I was under the impression that I didn't want the grinds to touch the grouphead dispersion screen (leaving headroom), when in fact it is complete opposite. With no headroom, that final 2mm will be "tamped" once the portafilter is locked in place. I find the only way you can disturb the puck is if you leave headroom. No headroom, no disturbance.

I agree that not leaving headroom is an adequate way to avoid most disruption. And the draw up and initial wetting are not hard to do by feel, I just find that having the marks makes it quicker and more repeatable.

The real advantage I've found is with the felliniesque pumps. While most aspects of the process can be done by feel it can be very difficult to feel when the piston passes back below the fill hole. Pavoni users seem to be split about whether or not it's a useful move. I have a feeling that people who have success with it are doing shorter pulls than those who aren't. I'd tried to make it work before but it was inconsistent and I would often get channeling when I used it. If the piston is moved below the fill hole it IS possible to pull back up without disruption but only if you don't have any headroom, and the coffee was given a fairly light tamp. It's physically impossible to draw back up at that point and not reverse flow in the puck. Interestingly the light tamp advice given to many Pavoni users may have much to do with this.

I'm not suggesting that marking the piston rod is necessary to get a great shot out of the machine. I am suggesting that it takes a variable out of the equation and gives the ability for an exact, consistently repeatable technique. This opens up more variables to be played with: Playing with doses and tamps that do leave headroom, playing with pre-infusion times from a couple seconds and up as opposed the 10+ needed for a slow fellini. For example the coffee I'm currently using tastes best, to me, with a dose that does leave headroom, and about a 5sec pre-infusion before ramping to full pressure.

If your current method is working then by all means continue; if it ain't broke don't fix it. However if you're still dealing with inconsistencies, or are having trouble with short multiple pulls, or have given up on the technique in the past I offer it as something to try.

meastway wrote:...I've heard lots of different opinions around the internets as to how water flows in these, including such absurdities like - the water flows between an inner and outer wall before entering the group. Just to be clear about what's actually there

i'm not exactly sure what you're referring to,
but there are two models of la pavoni that do feature a sleeve inside the group:
the current production millenium models with a plastic insert in the group head,
and the first design early europiccolas which have a brass inner sleeve of similar design..

in this arrangement the water flows from the boiler and gathers in the space between the inner sleeve and the outer wall of the group head before it can fill up and flow into the space on top of the *coffee

(that said, i don't see why your technique wouldn't work similarly on these machines...
just wanted to be clear that there are some design differences among years)

If that's the case I stand corrected, and I apologize. I remember reading that tidbit as a generalization to all of the Europiccolas and here I am making the same error in the other direction. Obviously my machine is a pre-millennium model and I have not had an opportunity to dig into a model with the sleeve. I have yet to find an accurate cut-away of one as well, other than the standard parts blow-ups. I was under the impression that sleeve served only as thermal separation and I have not found any pictures that show an obvious inlet hole or slot in the sleeve.

On that note, David Jenkins' drawing (found here -http://pavoni.varnelis.net/repairs) of the group head is much more complete than my basic iso view sketch. The only thing I see missing in his drawing are the vent holes above the inlet.

I would assume my proposed methodology would have the same effect on both pre and post millennium models but we can see where that approach has gotten me so far.

i;m not sure how the inner sleeve affects water flow in the group head
i suppose they are designed there for the thermal properties

my experience is specifically with the early machines
you can see an inlet hole in one of these photos
it's actually lower on the sleeve than i recalled from the last time i was in there
i imagine the millenium machines are pretty similar

I can see how water could flow around the sleeve first, then into the group. I wonder if the design has an effect on how air displaces in the system?

I agree that having some water on top of the puck does buffer from disturbance from rapid inflow. Having the markings helps me rapidly get to the point where I can trickle initial water in slowly before filling the group. This can also be done by feel, but until one builds sufficient muscle memory it's very easy to blow right past that point.

From an engineering point of view water is not compressible, it follows strictly hydraulic principles. If you're doing short pulls above the inlet hole, and for the sake of example there's no air in the system, you're just oscillating water back and forth between the group and the boiler. The puck of coffee is offering much more resistance to flow than the .9ish bar of pressure in the boiler. The coffee is also absorbing water during this time and if one waits until a drip before moving the piston back below the inlet hole you will get a bit more volume but not because you're somehow jamming more water in there.

There's always air in the group on my first shot, I can feel it bubbling. Air is compressible and it can cause a pressure buffer. If there is still air above the water after the piston has passed the inlet hole the air will have to be compressed to whatever the opposing resistance of the coffee is before the water will move. If the piston is pulled back up at this point there may be enough buffer in the air to avoid disruption.

If one pulls up after flow has been established and there is some liquid in the cup then yes, re cocking will refill the group and give more volume, although this will cause disruption and I've never had any use for it as I can't possibly pack enough coffee in the basket to necessitate that much water.

The reason for increased shot volume with this move is the replacement of the air with water. When I apply pressure between the top of the stroke and the inlet hole, and that pressure is greater than the boiler pressure the air will travel down the standpipe and into the boiler, this is the bubbling that is felt, as I lift again that air gets replaced with water. A few pumps can cause a vapor lock effect on any air still in the standpipe and it's possible to displace almost all the air.

If I don't do the move my first shot has appreciably less volume than the second shot, which I rarely feel any air in. If I do the move my first and second shot get identical shot volumes. This has been a big aid in making adjustments as I almost never pull more than two shots at a time.

Basically if I feel the air bubbling, I do the move. If I don't feel air I don't need to do any short pulls, the result in the cup is the same.
If you're not feeling any air then there's no need for the move. However the markings still aid in trickle-in.

I do pull lightly at the beginning of the final stroke as a type of active pre-infusion but I don't re-cock afterwards, I just ramp straight into full pressure.

I don't mean to ruffle any feathers, just trying to approach the situation with a healthy dose of science and sound physics.
If I ever have enough spare time on my hands I might try to build the whole thing in SoildWorks and do a flow simulation. Although I have a feeling that mechanical properties of a coffee puck would be a mighty headache to model.