Golly: Did you forget Boyles and Bernoulli's law's or do they not come into play?
KDM

Actually Charles' and Boyle's laws are combined into the ideal gas equation, PV=nRT. In the La Pavoni lever systems the volume above the piston when it is down is very close to 50 mL. When it is up, the volume below it is essentially the same. If one use the ideal gas equation with 100 kPa pressure, V=50 mL, and T=373K=100C, and R=8.314 J/K/mol, and appropriately convert units, one finds that at the temperature of boiling water, 1.6mmole or only about 30 milligrams of water vapour is all that is required to sweep all the air out of the group. That is less than 1 drop!!!! Gases are dramatically less dense than liquids!

I find that it is convenient to purge air both from the group and from the boiler at once. When the pressure light goes off, I put the porta filter without the filter basket in place, put a ramekin under the porta filter to catch water, and a cup under the steamer to catch steam and the water that comes from it. I open the steamer valve, let it run a moment, and then let it continue to run whilst I lift the handle all the way to the top to release a bit of water into the ramekin. Then I close the steam valve and wait until the system comes back up to pressure. I usually wait about a minute more for the group to be heated and then proceed.

rpavlis wrote:If the piston skirt or cylinder wall be too cold not enough air will leave before this occurs and the pull will be spongy.

Trying to get around my head why this is the case. Not sinking in! I do notice the initial burst of vapor and steam when the grouphead is on the cooler side when my lever is fully depressed. I find I have to clear this steam/vapor otherwise my puck can be pretty much destroyed. When the grouphead is up to temp however it seems to be less of an issue.

Interesting thread this. Thanks for sharing it.

Very interesting thread, and thank you so much, Robert, for supplying this technical understanding.

I'll sum up what I interpret as the "how to" element as a La Pavoni Europiccola owner wanting to achieve consistent shots with appropriate temperature and volume. Please correct me if I'm missing anything:

1. Purging of air is necessary for ensuring the machine is up to the intended pressure and temperature before other parameters are controlled for a successful shot (i.e., cooling the group by cooling the portafilter and inserting it, varying dose and grind, pressure profiling, multiple pumps, and so on).

2. If the group isn't thoroughly heated before pulling the first shot, one or more of the following problems is likely to occur: a) a sour shot because the brew water has been insufficiently heated due to premature heating element shut-off (by PSTAT or human error) because of a false pressure reading that includes air as well as water vapor, where air carries less heat than vaporized water; b) undersaturation of the puck, because air that hasn't been purged can prevent a full infusion, which can be sensed as a "spongy" feel when pressing the lever.

3. Properly pre-heating the group can be achieved by briefly and simultaneously venting the steam wand and running a small amount of brew water through the group by lifting the lever.

I expect that this procedure will change my temperature management regime for the initial shot and make that more consistent with subsequent shots.

Perhaps routines that others have described accomplish similar ends and can work well if consistently applied. For example, Christopher Cara advised me to vent the steam wand before the first shot, but he didn't recommend purging the group. I can now see why venting the steam wand isn't necessary with subsequent shots. Dave Stephens shared his routine in one of these threads of lifting the lever for five seconds, then pressing it down until droplets appear in the cup, then raising it for another five seconds before pulling through.

Others have experimented with multiple pumps of the lever. I've also done multiple pumps if I choke the machine and want to try and achieve a pull. It's understandable given Robert's explanation that this would purge air through the coffee cake and perhaps infuse sufficient steam that water can then enter the group and have a better chance to move through.

I had been dialing in a Europiccola without a PSTAT by purging the steam wand, then lifting the lever until the first droplets appear before pulling through. I've noted the need for a coarse grind for this to succeed and at times a spongy feel on the pull unless I allow enough time for pre-infusion.

I'm also interested to see that crema isn't just the venting of carbon dioxide from fresh beans but also consists of air saturated into the brew water. It was already apparent by taste and oily texture that crema is more than offgassing but also contains solubles that differ from the rest of the shot. I wonder if crema is also formed by water vapor, which would be at a higher temperature than the brew water and thus extract a different variety of solubles?

Very interesting thread! Since we have a board of persons here with a knowledge in physics that clearly exceeds my own, I would like to adress another related question, in regard of levermachines and laws of physics. In several threads I have seen comments like "and extra pull in a spring powered" (Fellini move) will increase the pressure (ramp up) delivered to the puck. I have difficulties with this assumption, in my head the spring can only deliver a certain max pressure, I think priming the puck with longer preinfusion could affect the resistance against the spring driven pressure but never the maximal pressure that we achieve?
Maybe I am all wrong but would like to hear your opinion as well
Thank you!
Ulrik

I don't think it was meant that it increases the maximum pressure but rather it increases the duration of the maximum pressure by bringing the spring back to compressed and also introduces more water for larger volume.

It would be interesting to see if there have been any experiments done to determine the gas composition of crema. Although the solubility of oxygen and nitrogen in water is slight, it is approximately proportional to pressure, and the pressure of the air trapped in the group during a shot is about 1000 kPa. (Oxygen is about twice as soluble as nitrogen.) Under these conditions it is ten times as soluble as under atmospheric pressure. When the pressure is released bubbles will form.

I have often heard that crema comes from CO2 in the beans, but if this were true why is there no crema from a stove top espresso system? There are compounds that can thermally decompose to produce CO2, but any that were in the original beans should have been decomposed by roasting. Gases diffuse rapidly and free CO2 produced from the roasting process should diffuse out very quickly. CO2 is dramatically more soluble in water, by the way, than oxygen and nitrogen. Basic compounds in the beans could absorb CO2 that might release CO2 when the interact with water, but again if that were the source of crema why do we not see crema from stove top espresso machines?

Helium is inert and has very low solubility in water. It would be interesting to make espresso by filling the space below the piston with helium and determine if crema forms or not! Some day if I have a chance I may run the experiment!

rpavlis wrote:I have often heard that crema comes from CO2 in the beans, but if this were true why is there no crema from a stove top espresso system?

Robert,

I'm out of my league here. But I do notice crema forming in the cylinder when pouring hot water into an AeroPress. It also forms when cupping, i.e., pouring hot water into ground coffee in a cup. I wonder if stovetop coffee makers somehow eliminate crema?

A little "light reading" for anyone interested in the chemistry (and science) of coffee.



http://www.sweetmarias.com/sweetmarias/ ... vaini.html

no crema from a stovetop?



and even when you cant see the crema- it seems it is still there (nice one Lucio):

http://caffettiere.blogspot.com/2011/12 ... le-il.html

I have had a few internet forum fights with folks over this issue. Some people stick firmly to a rule: only pump and lever machines make 'true' crema. I have argued that the crema you get from a stovetop is the same thing: albeit it usually thinner. Some people seem to want to maintain that it isn't- well perhaps it is 'fools crema'.