How to get rid of the trapped air inside the chamber

#1: Post by **Carneiro** » March 4th, 2013, 7:26 am

Hello!

The restoration of my Gaggia Tell took me to this common problem of trapped air inside the chamber when pulling the lever, but I think in a bigger difference in that case:

Gaggia Tell restoration

I found this old thread around:

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

As far as I remember, I had little problem with the Elektra MCAL. But in general I filled the basket almost to max (16 g) and it slightly touched the screen. And the piston touches the screen too...

The lower pre-infusion pressure (1 bar) from the boiler doesn't help much to push more water inside the chamber. The tests I've done with the Velox and 4 bar of line pressure gave me almost no problem.

Does anybody has such problems with the lever machines? I see a lot of videos where the lever goes up a lot before it slows down (pressure goes up), probably something similar to the Gaggia group. I don't care much about water volume (I like short ristrettos), but it would be nice to reach the 8-9 bar, and I'm stuck to 7 bar 'cause this problem.

Márcio.

March 4th, 2013, 7:33 am

BTW, I know the Fellini move should help, but in this case I couldn't improve the results... And the machine in the movie is a Gaggia! But the gasket setup I'm using is different from the original...

March 4th, 2013, 9:13 am

I've found the solution in this case, at least it was the only thing that worked: lock the porta-filter only after few seconds of pre-infusion, to let the water fill the chamber. On the Gaggia I can close the valve at the neck (group to boiler) a little to reduce the flow and avoid a mess.

Doing that, I could have the shot starting with the lever down, I estimate it went from at least 9 bar to 6.5-7 bar (I removed the cup before the end of the shot, the volume in that case is very large). It was slow at first but the flow raised and the taste was great. Very different from the 7 bar shot (good too, but more mild). Well, this can be turned in a feature instead of a bug...

The lever is heavy and my right arm is sore...

#4: Post by **drgary** » March 4th, 2013, 9:26 am

Hi Marcio:

With my Conti Prestina commercial lever I've been doing pretty much what Robert Pavlis suggested in his thread on Dalton's Law. I flush the group for a couple of seconds before first locking in. I can see and hear air purge before the water shower starts. Then I lock in. This seems to work well. BTW I'm using both arms to pull the lever into the down position and often use a Fellini move. (Two arms aren't necessary but it seems natural with the machine in front of me. Lever resistance is about the same as MCALs with a booster spring.)

A bit off topic but on your Velox and Gaggia are you able to push the lever to add pressure?

#5: Post by **cafebmw** » March 4th, 2013, 10:17 am

fellini moves or not, i didn't notice any difference. the gaggias are design for commercial operation. fellini moves wouldn't be very practical...
how far up the lever will go before pressurizing depends on your ground, tamper, pre-infusion time, etc. the 'trapped air' is nearly irrelevant (11cubic cm before compression). most the time the lever goes up less then 45 degrees=ca. 10mm piston travel (of 25mm total).
again, the problem will NOT be the trapped air, but blow-by either past the piston seals (they are not gaskets, moving parts!) or through the one way valve, when not absolut sealing, in the piston bottom.

March 4th, 2013, 11:54 am

Well, the air is compressible and if it can't leave through the coffee puck, lower volume of water will enter the chamber and the piston will travel to compress the air, then rise the pressure of the water.

I think I'm doing what Gary described too, let the water enter then lock the portafilter, this seems to work. I believe with a different ground and tamp maybe the puck would let the air out too, but I imagine it would be a coarse grind and/or very light tamp.

Gary, yes, on the Gaggia and Faema group you can help the lever, as they are coupled to the piston (Gaggia has the pinion and shaft, Famea has one piece that holds the bearings and the pin that pull the piston rod).

Márcio.

#7: Post by **yakster** » March 4th, 2013, 2:04 pm

I had a related problem this morning with my La Peppina. Instead of pumping out an ounce or so of water before my shot to warm the cup, I left the cup on top to warm it and just let the La Peppina idle at 94°C for a bit so that everything was warmed up. When I went to pull my shot, there was no water in the group and I ended up having to pump several times to get any water into the cylinder after sitting dry over the weekend.

I finally got the water flowing and pre-infused the puck for a very nice shot using 17 grams of the Chromatic Coffee Ethiopia Guji Shakiso. I think I'll go back to pumping out some water to avoid this in the future, just did this to minimize water usage.

#8: Post by **bmb** » March 4th, 2013, 4:31 pm

There was a post by Jim Schulmann in the Strega forum, about this aspect, I remember something that the Strega's pump would compress the air and get something like 75cc into the chamber, and that with mains pressure it would only half fill to about 45cc.

March 4th, 2013, 5:40 pm

cafebmw wrote:fellini moves or not, i didn't notice any difference. the gaggias are design for commercial operation. fellini moves wouldn't be very practical...
how far up the lever will go before pressurizing depends on your ground, tamper, pre-infusion time, etc. the 'trapped air' is nearly irrelevant (11cubic cm before compression). most the time the lever goes up less then 45 degrees=ca. 10mm piston travel (of 25mm total).
again, the problem will NOT be the trapped air, but blow-by either past the piston seals (they are not gaskets, moving parts!) or through the one way valve, when not absolut sealing, in the piston bottom.

exactly!

i.e. as the piston rises, but before the bottom of the piston clears the inlet port, if air is not able to be drawn through the puck a vacuum will form

if the vacuum becomes too great &/or the seals on the piston will allow air to be drawn down past them from above the piston to eliminate the vacuum.

as you already know the presence of this air prevents water fully occupying the brew chamber and results in a short measure

#10: Post by **drgary** » March 4th, 2013, 5:51 pm

So you are both saying that the issue may be leaky seals? Thus if the vacuum remains there won't be air to obstruct water filling the brew chamber. Correct?