Dynamics of Pressure Profiling: a New Rule of Thirds?
A year or two ago, I had the honor of spending some time with Scott Guglielmino of La Marzocco. We discussed pressure profiling espresso machines, their potential value, and what pressure profile(s) work best. One of the things Scott mentioned during our discussion was that they had observed that the biggest factor in determinining how a shot would flow in the last 2/3 of the shot was the pressure profile of the first 1/3 of the shot (roughly). Perhaps counter-intuitively, a gentle pressure ramp tends to yield a faster flow rate later in the shot, whereas a swift pressure ramp tends to yield a tightly restricted flow during the last 2/3 of the shot.
The practical upshot of this "rule of thirds," as it were, is that changing the shape of the initial pressure ramp can allow you to vary grind size dramatically while maintaining a desired brew ratio (and the texture/strength that accompany it), rather than using manipulations of grind size, dose, and shot time to achieve your desired brew ratio. With an exceptionally slow initial pressure ramp, you can grind significantly finer, all other things being equal, than you would with a conventional espresso machine with a fixed pump and flow restrictor. The idea, in this case, would be to make it simpler to dial in very light-roasted coffees as espresso—potentially opening up coffees to be extracted as espresso that really wouldn't be viable at all on a conventional machine.
Before I get into that though, how does flow enter these dynamics?
Pressure vs. Flow profiling: Pressure Profiling at 0 Bars
Some of you have no doubt read about Slayer espresso machines, and Jason Prefontaine's contention that flow profiling is critically different from pressure profiling (if not, check the thread above for Jason's very approachable explanation). In that thread, the OP (John) hit the nail on the head with the question:
bostonbuzz wrote:I'm missing something, otherwise flow profiling and pressure profiling are two ways of talking about the same thing... (except for the speed of simply filling the chamber with water at 0 bar ((once it's full it's simply slowly raising in pressure))).
Once a shot has stabilized to a positive pressure, pressure profiling and flow profiling are the same process with different variables held steady, sort of like the difference between shutter priority and aperture priority in photography. But in light of the "new rule of thirds," which demonstrates the impact of the initial pressure ramp, what about the flow rate before there's any appreciable pressure to ramp? That is, the part of the shot when water is trickling onto the puck but pressure isn't yet being applied at all because the space in the tube and grouphead above the puck and downstream of the 3-way valve hasn't been filled with water. At this stage, there is water flow, but no pressure. In this context, pressure profiling has no meaning, but flow profiling certainly does.
In all espresso machines I've seen that support pressure profiling by pump delivery variation (namely, the La Marzocco Strada EP), a fixed flow restrictor is used. In the case of La Marzoccos, this is 0.6 mm. Now, consider Jason's fire hose vs. garden hose analogy. You can put 3 bars or 9 bars of upstream pressure against such a flow restrictor, and it doesn't change the flow-rate against 0 bars of downstream pressure too substantially. I would say by not much more than 50-70%. With a variable flow restrictor, you can tweak flow rate in a much more dramatic way, even without manipulating brew pressure.
Given that the first third of the shot is so critical to the way a shot develops later on, is it possible or likely that the ability to "flow profile" at the earliest stages of extraction is much more important than being able to manipulate pressure or flow later in the shot? Is it possible that the ability to change the flow at the beginning stages of the shot is considerably more important than shaping pressure and flow later on?
As I mentioned earlier this year, I have pre-ordered a Slayer 1-group. I have reasons far outside the scope of the discussion at hand for buying one. If I wanted to duplicate its abilities, or even expand on them, adding a needle valve to my current machine would be an easy (or not) option.
But what's the real difference?
So this is the topic that has occupied my interest for some months. I've observed with my own equipment that dramatically reducing line pressure (to around 1 bar, vs. 2-3 bars typical line pressure) and turning my pump entirely off for the first 20s of brewing (and then ramping to 9 bars and extracting for another 20-40s), I can pull a shot with the same brew ratio that I'd get with a conventional pressure profile while grinding 3-4 notches (on a K10) finer for the same coffee. As you can imagine, this makes a huge difference in the shot's flavor. I've found so far that this works very well for dialing in very light roasted coffees to get maximum sweetness, balanced acidity, and thicker/heavier body; but does not lend itself to darker roasts where a fine grind is not desirable.
When I first played with pressure profiling, I focused almost entirely on the profile from when I achieved line pressure (at the time, around 3 bars) onwards, and in particular in declining the brew pressure later in the shot, like a lever machine. The differences I observed in blind testing this method were noticeable (and generally an improvement), but subtle. I'd like to revisit such rigorous testing again soon, focusing on profiling the beginning stages of the espresso extraction, rather than the end.
That may be some ways off, but I wanted to begin a public discussion of the topic with an eye toward collecting observations and suggestions from other forum members. I know a number of you on this forum also have access to equipment that lets you explore these variables, and I'd love to read your observations around this question. I'd especially love to read about your experiences in dialing in espresso in this fashion, if possible.