That would give you a measurement of mass, but mass will not correlate to volume (in this case).
We agree that using a naked portafilter results in "fluffier" espresso (bigger bubbles) than using spouted portafilter. Volume is a measurement of SPACE (not mass). I guess the bottom line is that the density is different...
Jon, that's exactly my point. Problem is (IMHO), espresso people are extraordinarily sloppy in their use of the word "volume." It has caused much confusion, and it continues to cause much confusion.
Here's some text from a post I made on alt.coffee in Oct 2004 (link
There are TWO distinct espresso "volumes":
1. "Apparent volume," aka "gross volume"
2. "Actual volume," aka "net volume"
"Apparent volume" is what you see in the cup as you pull the shot. It is the volumetric measurement of liquid and crema at the moment the pump cuts out. Apparent volume gradually decreases as the foam collapses.
"Actual volume" is the amount of LIQUID espresso that you extract. Since espresso is whipped up into a foam as it exits the PF, it's harder to directly measure actual volume until all the foam collapses. Paradoxically, weighing the shots is the quickest method of measuring actual volume. As you know, automatic espresso machine can conveniently keep actual volume consistent by metering the amount of water fed into the portafilter.
Very fresh beans, robusta beans, and bottomless portafilters all increase the proportion of crema delivered to your cup. This increases the apparent volume when actual volume is kept constant, and decreases the actual volume when apparent volume is kept constant.
Baristas who understand the difference between actual and apparent volume are less easily fooled when going from conventional to bottomless portafilters. They understand that the bottomless PF shot looks bigger, even though the actual volume is the same. They understand that robusta shots look bigger, even though the actual volume is the same. They understand that shots with older beans look smaller, even though the actual volume is the same.
When the bottomless PF phenomenon spread like wildfire across the net, some folks were fooled into thinking that the new portafilters magically allowed them to pull much "longer" shots. Others thought that their regular shots were higher in dissolved solids. Neither of these ideas were correct; the folks simply made the mistake of confusing actual and apparent volume.