I wanted to discuss prefinsuion but I think a stepping stone in working up to that topic is to address time vs volume and one methodology I use in evaluating espresso.
I have made a lot of empirical observations in espresso, troublesome finicky espresso at that, which have led me to value timing over volume. If I had not been using such an 'all or nothing' espresso, I might have a different perspective so don't take this as gospel but rather a set of observations to be tested on your own.
When dialing in an espresso, I often would establish a dose, volume, and time, sometimes new temp for the blend. In essence, I would set all the variables down to timing and leave volume as the last variable which I would control by adjusting the grind. This means, the timing of the shot would be set. This somewhat contradicts a lot of people out there so let me explain.
I noticed shots at the 'ideal volume' and and 'ideal time', let's say 28 seconds, were good. If at the same volume but plus or minus 3 seconds, the shot quality deteriorated immensely.
Conversely, shots that hit that 28 second extraction time but had a plus or minus of .25 ounce change in volume from the 'ideal volume' were actually quite drinkable. In fact, they were either a bit more intense or a bit thin but not as bad as the time variance shots.
Why volume is a tricky subject!
I surmised timing plays a very important role on extraction and was therefore more important as a perceived constant than volume. The problem is that most semi autos don't let you adjust the volume unless you go through a lot of trouble. This means that anyone using the semi autos would have a hard time adjusting for roast aging or variances in the cup while keeping the shot timing as a constant. They would have to rely on the flow meter or go for free pour thereby eliminating the need for a flow meter.
It occurs to us that a volumetric setup is indeed a problem on a machine. It is often the source of temperature fluctuations and is a relic of the super auto focus.
Why not put delay timers on machines that counted down from a certain time and killed the shot leaving you to adjust the volume as your only variable, constantly tweaking it towards your 'ideal volume'?
I propose for a volume cafe, everything on the machine should be set and programmed leaving your one external variable, the grind, to be tweaked.