I have a question that I can't find addressed in the archives but believe has been discussed, so please excuse the repetition... It relates to the flow of the water to saturate the puck during the preinfusion cycle.

Here is my scenario of how water comes to saturate the puck...

Hot water comes into the group head and into the space (headroom) between the puck and the screen. Depending upon the preinfusion pressure and the glicuer (gliceur?) characteristics, the water comes in at a trickle or it comes in like a firehose and starts to fill the gap between the screen and the puck. There is no vent above the screen, so either the water mixes with the air to form a froth that is forced through the puck or bubbles form in the puck headroom. As the pressure builds to say 9 ATM, the volume of the bubbles decrease proportionately to a very small proportion of the headroom volume at maximum pressure.

As pressure starts to decline, any bubbles would reexpand.

If the air in the bubble started at room temp, then is surrounded by the hot water, its temperature increases so when the shot is finished being pulled, and the pressure drops, the bubble(s) expand to a greater volume than initially present.

If you tamp firmly, there is greater headroom between the puck and the screen, so more air volume to consider. If you don't tamp, the puck is more porous, and perhaps more able to accept froth.

If the air volume in the headroom is relatively large at the end of the shot, it displaces the remaining water in the puck and a "dry puck" is the result.

If the bubble is small (due to the original froth being carried through the puck initially, or venting around the seal of the portafilter) then the puck remains saturated at the end of the shot, and a "wet puck" results.

Is this what is going on? If not, then does air travel back up the puck at the end of the shot to allow the puck and headroom to drain?

Thanks
Joe

How would the air get pushed through the shot, given that it floats?

pocojoe wrote:Here is my scenario of how water comes to saturate the puck...

Interesting topic. Some initial reactions:

* Water does not come out of the shower screen "like a firehose", but more like a trickle. It may take 8-10 seconds on a preinfusing grouphead before the first drops appear on the bottom of the basket. During this time, some (perhaps most) of the air will escape through the puck, before the grinds get saturated with water.

* Tamping hard may slow down the infusion process, but it does not change either the volume of the basket or the mass of the grinds. So the amount of air will not change with tamp pressure. Air may exit a lightly tamped puck more easily during infusion, but this is pure speculation.

* At the end of the shot, the air is released either gradually or explosively (3-way solenoid valve). In both cases, updosed pucks tend to be drier, and downdosed pucks tend to be soupier. This is at odds with your claim that drier pucks result from greater headroom. Perhaps the greater mass of grinds soaks up the remaining water better.

Where oh where is that mythical plexiglass portafilter when you need it?

Put a balloon over the bottom of your filter basket (maybe you'd better keep the heat off) and see if it fills with air before you pull the shot.

Maybe you could hold a candle or light a match and hold it under the group to detect any airflow out of the filter.

Final thought, you could connect a cylinder around the filter basket that narrows to a smaller opening (like a funnel) that is placed in water which should show air bubbling through the water if there is airflow through the filter during pre-infusion.

Thanks for the suggestions.

I like the idea of a plexiglass portafilter... but to cast one in pyrex would provide all sorts of excuses to buy new toys! Seriously, the balloon is on target, but it might be more informative to hook a short length of Tygon to the bottom of the portafilter (onto the threaded spout) and run it into an inverted graduated cylinder to measure the air volume vs time directly.

I was able to find the reference that I recalled regarding air displacement. The discussion was not in the H-B forums but in the oft-cited article on "Espresso Machine Design" found on the Espresso Resource website: http://www.espressoresource.com/index.p ... Itemid=110

The pertinent section is in the discussion on preinfusion. One method of generating a "softer" preinfusion pressure, in a design without a gliceur (the "firehose" scenario that I originally posed) is to create a volume of air above the puck that cannot vent. As the water rushes in to the portafilter, the air pressure builds in this chamber. The article claims a linear pressure curve would result- however, theory would predict that an exponential pressure rise would occur. The pressure head is akin to voltage; the lack of a gliceur like a low input impedance; the pressure chamber like a big electrolytic capacitor; and the puck a time-varying (as extraction changes puck characteristics) load. I do think that if the air in the chamber gets hot during the shot, the volume of that air at atmospheric pressure will increase, and in the air chamber design, at the end of the shot the air will get forced through the puck.

For me, the question is not purely a Gedankenexperiment- it goes to where, and how deep, to drill the hole in my group head for mounting the thermocouple to measure temperature. I'm building up a HX Lever machine and instrumenting it as I go:

Playa Espresso Cart 4 - Where to Measure Pressure and Temperature?

My concern is that if there is indeed a bubble at the top of the group, the relatively poor thermal conductivity of the air would suggest that I should stop short of drilling all the way through the group in the hopes of getting the thermocouple in the water for direct measurement. Instead, maybe it would be better to drill and tap just short of entry into the group, and fill the void with thermal grease before inserting the thermocouple. It would certainly decrease the risk of a later leak developing.

From a process control point of view, knowing the group head temperature before the shot is pulled on a HX machine is more likely to be useful than knowing the water temperature in the group as the shot is pulled. So, bubble or not, it seems to me that I should instrument the mass of brass as close to the screen as possible, but not try and measure the actual water (or perhaps air bubble) temperature above the puck.