Excerpted from Section 4.3 of this post:
WBC Procedure for Measurement of Brewing Water Temperature
Water absorption by the coffee cake: A series of measurements were performed, in which a dry portafilter was dosed with coffee and weighed. The portafilter was reweighed after espresso was brewed from the coffee. The measurements show that a dry coffee cake absorbs approximately its own weight in water during the brewing process, about 18ml for a double espresso shot.
Just for the heck of it, I measured the weight of the originally dry 15 gram "used" puck this AM and it tracked very closely with the above - 15.5 ml. That's about 0.5 ounces of water.
In this post:
Can anyone explain preinfusion on the E61?
There were conflicting (harmless) reports of exactly what happens when the brew lever is moved to the mid position - a very pronounced detent. A short perusal of lots of posts on the subject would show, I believe, that the brew valve is nudged slightly open when the lever is in this position HOWEVER I would tend to agree with Ian when he responded that all valves are closed when the lever is in the mid position - it seems natural. In any event, whether the valve is nudged open or remains shut could rather easily be changed by the end-user by swapping a few parts or adding/removing a shim in the right place. I, for one, don't make any use of this mid-position (although my Anita is one of the nudgers) as the inherent pre-infusion of the group seems to work OK.
But back to your 1.75 ounces - that's a lot of water and, exactly how much of it is used to saturate or partially saturate your puck can be seen/measured simply by removing your portafilter after "X" seconds of having the lever in the mid-position and "catching" the water that escapes. Keep in mind, of course, that when you start the pump, that 1.75 ounces has to be made up before any flow through the puck takes place. In addition, that 1.75 ounces was a VERY RUDE interruption to the thermosyphon flow that was happily moving along.