Bleeding pressurestat lever machines. (Especially 2nd gen La Pavonis) - Page 2

#11: Post by **OldNuc** » April 29th, 2016, 8:03 am

The practice of releasing water/steam intro the group space before locking in portafilter does not purge all noncondensables from the group but it will changes its internal temperature. Raising the lever to the point just before water release does reduce the amount of air drawn through the puck.

Do not ignore the non condensible gasses trapped in the fresh dose of coffee you just locked into the group either. The preinfusion process will purge these noncondensibles into the group. To displace all of those noncondensibles multiple micro pumps, less than a 2-3cm total stroke measured from full up measured at the end of the handle. This works just fine unless the pressure in the group drops below the saturation pressure for the contained water temperature. IF the group piston seals are properly lubed you can both feel and hear the noncondensibles being forced into the boiler. You will also feel the handle contact a very solid resistance to any further down stroke when the group is completely purged.

The above process functions fine as described on a 96 La Pavoni at least 6 times/day with boiler level variations from full glass to almost empty glass. There is no technical reason that I can conceive that would prevent this from functioning with any other La Pavoni version.

#12: Post by **drgary** » April 29th, 2016, 10:44 am

Great post, Rich. I will try the micropumps on the 3rd gen machine.

#13: Post by **jonr** » April 29th, 2016, 10:56 am

If you feel a sponginess that continues all the way through the shot, that would be consistent with an air bubble under the piston. But after a few micro-pumps (and the noise of air flowing back into the boiler), I only feel an initial sponginess that goes away as the piston begins to descend. This is consistent with pushing air out of the grounds. I don't consider this a problem, but a higher boiler pressure (ie, stronger pre-infusion pressure) or coarser grounds should eliminate it.

My limited measurements of boiler temp indicate that there is no temp change after doing any additional air purging of my automatic air/steam venting, two-switch Europiccola. Vigorous venting continues and soon vents all of the air. There is nothing to be gained by manual air bleeding my Europiccola. edit: I see that the first post has been updated to reflect this.

#14: Post by **drgary** » April 29th, 2016, 11:05 am

jonr wrote:But after a few micro-pumps (and the noise of air flowing back into the boiler), I only feel an initial sponginess that goes away as the piston begins to descend. This is consistent with pushing air out of the grounds. I don't consider this a problem, but a higher boiler pressure (ie, stronger pre-infusion pressure) should eliminate it.

That is my experience and suggests a need to increase the PSTAT setting. The 1st gen machine I was using was at a higher setting than the 3rd gen machine.

#15: Post by **jonr** » April 29th, 2016, 11:11 am

As a test, one could try coarser grounds - this would allow even a low boiler pressure to push all of the air out of the grounds.

#16: Post by **wkmok1** » April 29th, 2016, 5:00 pm

For the longest time, I couldn't figure why the micro-pumps work.

Bad reasoning:
Initially, the lever is at the top and there is a bubble of air sitting just below the piston. Assuming the late-locking is effective, the bubble should be reasonable small. Pushing down pushes the air into the dipper tube. The tube may be able to accommodate much, if not, the entire bubble. When one lift the lever up for the micro-pump, why wouldn't the air just come back in? There should be a bubble with a volume equal to the capacity of the dipper tube that is impossible to get rid of.

Better reasoning:
Once the piston is pushed down far enough, the filler hole in the sleeve is exposed to the top of the piston. Air trapped in the dipper tube will escape to the top of the group. We have transferred the air from below the piston to be atop the piston. LIfting the lever lets boiler water (not air) into the cavity and voila, no sponey-ness. It's like an air-lock. It may take several transfers to move all the air out, especially when the out-gassing from the puck adds "air" over time.

Perhaps the more mechanically inclined among you have figured this out long ago

#17: Post by **drgary** » April 30th, 2016, 10:32 am

After a cold start this morning, I tried small pumps at the very top of the lever travel. This eliminated some but not all of the sponginess. For the next cup I tried reducing grounds and did the small pumps. That almost entirely eliminated sponginess. This machine has done spongy pulls for subsequent shots, so that seemed to be an improvement. It suggests that one possible cause of spongy pulls is overfilling the basket. I will try again from a cold start with reduced dose and will post about that.

#18: Post by **OldNuc** » April 30th, 2016, 11:18 am

As I mentioned some time in the past and successfully upset at least 1 poster I will try again to pass on the clue to this odd behavior. The La Pavoni is operating as a saturated system. This basic thermodynamics. As the temperature of the fluid in the group approaches the saturation temperature for the existing pressure the vapor pressure increases, that would be water vapor. You can not eject this vapor out of the group but you can condense it back to a liquid by increasing the pressure it sees. This is the cause of that initial slight sponginess at the beginning of the stroke. The lower the system pressure and higher the in group fluid temperature the higher the vapor pressure and the more "space" it appears to occupy. Increasing system pressure and lowering group temperature is the easy fix. Group temperature increase is directly influenced by the boiler level, level at top of the glass will result in a higher group temperature and a more rapid heat-up rate. Starting with an initial level at 1/2-2/3 full sight glass will take this rapid heat-up out of the picture in most cases. It helps to only vent-purge non-condensibles after the p-stat has switched off on the initial heat-up. You do end up venting-purging twice doing this but the time spent on the initial heatup does not markedly increase group temperature above ambient and as you will not completely purge on the initial vent the rate of increase is less during the return to full pressure. Once returned to full pressure monitor for desired group temperature for the pull.

In a properly lubed machine you can feel the micro pumps purging the non-condensibles that were trapped in the puck and the collapse of the vapor bubble. In a quit setting you can hear the bubbles being pushed into the boiler so you will know when to stop with the micro pump stroke.

#19: Post by **drgary** » April 30th, 2016, 11:37 am

Thanks, Rich, for that clear explanation.

On cold startup I reduced the dose of coffee so it tamped several cm below the basket rim. I used half pumps, the sponginess disappeared and my demitasse cup filled as much as with my 1st gen machine.

#20: Post by **OldNuc** » April 30th, 2016, 1:28 pm

The tamp force is not a big deal but having that 10.5-11mm of head space has proven to be important. I just built a fixed depth tamper for 10.5mm and adjust does and grind to make it work in an Elektra double basket.