I wish I knew how to created animated drawings. Not because I have all the answers, but because it would be so much easier to communicate.

http://pavoni.varnelis.net/repairs

wkmok1 wrote:Thanks, Rich.

A couple of questions, if you don't mind.

Why is the group temp affected by water level? Being pressurized, wouldn't the water and water vapor have the same temperature? Because water is a much better conductor of heat energy when compared to air or steam(water vapor).

Does the following sound right?

When the lever is first lifted to the top, some amount of gas, mix of air and water vapor sits at the top of the cavity. Yes ...

Pushing down on the lever initially pushes some of the gas into the dipper tube. The pressure remains at boiler level because the inlet hole is open. No condensing effects yet. Pressure is constant or slightly increased in the group due to lever stroke and the group bulk temperature is BELOW boiler temperature due to normal heat loss to atmosphere. This is operating as a saturated system, any vapor transitions to liquid until both saturation temperature and pressure are reached.

The piston descends sufficiently to close off the inlet. Further lowering of the lever increases pressure. Water vapor begins to condense. The idea is to QUIT lowering the piston at the instant the water port is closed. Continued pumping is highly counter productive. This is why the machine MUST be properly cleaned, polished, and lubed.

After some more lowering of the lever, the user stops and begins to reverse direction. On the way up, pressure is reduced and we get some evaporation. This is a saturated system and when pressure is lowered below saturation the liquid flashes to steam cooling the group.

When the piston rises enough to uncover the inlet, only water, no gas, comes in. Any gas previously expelled into the tube has escaped to the space on top of the piston. You are forgetting the noncondensible gas in and around the coffee puck. It takes multiple strokes to get all of the noncondensibles displaced into the boiler.

Repetition of the process exponentially reduces the proportion of air in the gas mixture. This only works if you are very precise in lever manipulation and are aware of the group temperature differential with the liquid temperature at the liquid-gas interface in the boiler.

This process will work if it is done as described. Keep in mind you allow the machine to heat up to p-stat cut-off with no system venting and at that point the system is vented carefully until the pitch of the escaping gas sounds hollow and soft. This means do not wing the stream valve far open or you will never know when the system is vented completely. Close valve and allow p-stat to again reach cut-off. A 2nd short vent will verify that the 1st vent was effective and if not this vent will be. Now latch in portafilter, raise lever full up and hold for 5 seconds or so. Very small slow lever lowering until the slightest resistance is felt, stop and lift lever, 5 sec. and repeat. If all works well the system will go solid and instead of feeling a soft resistance it will be solid --now vented. If the machine is not free moving you will be in a fight with the basic thermodynamic laws and will loose. If you want a higher group temperature just hold the lever pu and allow it to increase in temperature.

Saturated steam is much better at transferring heat than water. But the point remains - if you want consistency, don't switch between them.

http://www.engineeringtoolbox.com/overa ... d_284.html

Steam is better at moving/transporting heat energy, it is considerably worse than water at the same temperature at conducting heat energy from one medium to another. There are a very large number of common heating boilers that use this little bit of thermal info to control water level automatically. It is these misconceptions that make this process so difficult to understand. It is easy enough to demonstrate. Fill boiler to top of glass at ambient and then heat up and when P-stat cuts off measure group temp at back of bell. Depressurize and lower level to 1/2 glass then repeat a heat-up from ambient. Going through the venting steps after reaching the 1st shut-off will produce the same results with an even greater temperature at the bell with a full boiler as compared to 1/2 full. Keep in mind that this is vapor to group or water to group there is no other medium involved and both the vapor and liquid in this case is almost static, there is very little agitation in the vapor section compared to the liquid region.

Or here if you prefer. http://www.tlv.com/global/TI/steam-theo ... anism.html

The proposed test ignores the significant effect of more water in the boiler taking longer to heat up. More time to heat up -> more time for heat to move into the group (with steam or water).

By actual time test the time difference is not that great, do not have numbers at hand. It does have an impact though. You could time the 1/2 full and them run the full test for identical time. Check for temp difference. What I find is from 1/2 it takes many minutes for the group to fail the 5 second index finger test before venting and the group temperature tends to closely follow the boiler temperature starting with a full sight glass and is quite warm even before venting. From a practical operating point of view the group is at a higher initial temperature when the vent process initiates if the sight glass is full at the start of the heat-up. All of the various variables involved and which is dominant really do not impact the end result which can easily end in an over heated, over extracted, spongy pull. You have lots of fiddle around time if the boiler heat-up was initiated from 1/2 and next to none if it was initiated from full.

Once a space is filled only with vapours of a condensible liquid areas of this space that become cooled will have massive condensation occurring on the surface, and this transfers massive amount of heat energy to it, practically instantly when the surface is made of a conducting material. When non condensibles are present, however, the non condensible molecules get in the way of the condensible molecules, and energy transport is dramatically slower. Conductance of heat through gases is rather slow, but transfer of energy by evaporation and then condensation is extremely fast!

If you take an unbled 2nd gen La Pavoni and bring it up to pressure, the group will be hardly above room temperature, because of the air molecules getting in the way of the water vapour ones. Once the machine is thoroughly bled, the top part of the group will become almost instantly boiler temperature. Unfortunately one cannot keep constant temperatures by partial bleeding, because there is no way to control the quantity of trapped air accurately. (As far as I can see a 2nd generation machine requires about half an hour to have the group warmed up enough to produce espresso by conductance through the brass from the boiler, more if the level of water in the boiler be low.)

Once the boiler is thoroughly bled overheating can be a problem quite soon, one should be ready to pull the shot as soon as the boiler is up to temperature after the bleed. One can make this an advantage. This makes it possible to have a good shot in only about six or seven minutes from connexion to the power! Sadly, it also makes it possible to have extremely bitter espresso from elution of bitter compounds that elute readily when the puck is too hot.