coyote-1 wrote:What if you hold both time and bean dose approximately constant? And the only thing varying is yield in the cup (ratio)?

From a non-theoretical perspective, I just did exactly this. Varied only the flow, to reduce the output. And I got what I expected: the same flavor profile as yesterday's shot, just more concentrated.

I think the problem is that what you describe can happen, but it's not guaranteed to happen. There are a couple of big variables:

First, when you grind finer, you slow down the shot, but how much it slows down will depend on your beans and grinder. Flow rate depends not just on nominal particle size, but also on fines production, which will change depending on beans, grinder, and grind setting.

Second, time isn't a very well-behaved variable in espresso extraction. Even when we talk about something seemingly simple like keeping time constant, what this means will depend very much on when you start the clock--if you start the clock when you turn the pump on, holding time constant is going to give a different result than if you start the clock when the first drops come out.

Holding brew ratio constant solves both these problems. First, we can define where we are in the brewing control chart much better, since we know we must be on a paritcular "spoke"; and second, we have a much better behaved variable, since brew ratio is directly tied to things like how much water you've pushed through the coffee.

coyote-1 wrote:What if you hold both time and bean dose approximately constant? And the only thing varying is yield in the cup (ratio)?

From a non-theoretical perspective, I just did exactly this. Varied only the flow, to reduce the output. And I got what I expected: the same flavor profile as yesterday's shot, just more concentrated.

I would think that if you brewed two shots with the same dose and time, and one came out to say 36g and the other 42g, the latter is going to be more extracted. Maybe it is due to channeling, or maybe the grinder is producing variable results, or pressure/profiling... something has changed the flow between shots. Thus the 42 would taste a bit bitter and the 36 may be just right. Or the 42 would be just right and the 36 would be a bit sour. I have had coffees though where they don't change that much with varying output.

The only way I can wrap my head around being able to consistently do what you describe is to take a shot and add water after the fact to change strength without messing with the extraction.

I did not grind finer. I used a dimmer to control the flow, which also has the effect of controlling brew temperature. In fact, temperature is serving as a guide to controlling the flow.

Flow control is cheating.

But seriously, when you put crwper's charts into practice and introduce the ability to navigate the extraction space where all you do stretch out the time it takes to reach a specific yield by limiting the available flow through the puck, you have reached easy mode.

Without this ability, you are forced to down-dose when you grind finer if you want to hold the extraction time constant, which then means if you want to stay on a constant brew ratio line on the control chart, you have to lower your yield, as well. The opposite is true when you grind coarser. Coarser grind, higher dose (if your basket will allow it), higher yield.

Flow control, pressure control, dimmer mod, gear pump, needle valve, manual lever, whatever - they all get you the ability to hold your dose, ratio and time relatively constant and adjust your grind to find the spot that gets you what you want. You can also adjust dose without messing with grind, or shorten your yield but leave everything else the same. You're breaking the rules, and that is why manual control over the rate of flow into the cup is such a powerful addition to the home barista experience.

Unless it makes these products significantly more difficult to certify for safety or something, I'm having trouble understanding why flow control is not a default feature on espresso machines. Especially on "entry level" gear. It would make it doable instead of frustrating for folks who got started with canned espresso and pressurized portafilters, and now want to take that second big step.

Flow control is not on entry level machines because it is still vanguard technology and therefore expensive due to economies of scale in production and value to marketing difference.

Love all the detail and thought this has brought up. Yet, to digest the excellent info, especially crwpr's charts, and to the OP question:
/ brew ratio is the bigger determinant of flavor and cup quality
/ tenths of grams are discernible in taste even to non-connoisseurs (especially side by side)
/ achieving a ratio in a defined time range is general guidance to *coarse vs fine grinding, which is a secondary tuning of the taste (more impactful and controllable than using time as a control)
/ *or its correlate, rate of flow - I'd like to understand these relations better
/ following the above, one might use a brew recipe to asses whether they want to start with a high flow basket or not.
/ With a little confidence and maturity in technique, one finds what is repeated in these forums constantly: educate your palate and learn to suit your own tastes.