Very interesting. One can (quite imprecisely) explore these ideas on dipper lever groups. For the commercial group one needs the brass to be quite warm - 83 C or more (lest the temperature fall too much during the dwell or no flow phase - but the boiler temperature dialed down below 1 bar boiler pressure. When the group fills with water. Lift the lever enough to stop inflow from the boiler. Vary this 0 pressure phase around the ~ 20 sec phase. Then pressure profile by retarding lever return from there. After I source some light roast espresso and SO coffee this weekend I'll have some experimenting to enjoy. Thank you Nicholas.

Cool that this topic has reached this level. I was the one at La Marzocco that found that the 0.6mm orifice was the best balance of pre-infusion flow rate and overall extraction flow rate. A balance of low flow rate for the beginning of the process and a larger capability for the remaining part of the process. Additionally there was some scale build up consideration for the hole size.

Most I have spoken to use the 2 terms, pressure and flow rate, interchangeable. They are proportional to each other but vary depending upon where in the system you are measuring. During the first "1/3" of the brew process you are effectively flow profiling (if at a low flow rate) since you are displacing air and filling the empty space with water. Once full you can begin to pressure profile. Pressure has 2 effects upon the process, 1) it has the ability to change the flow rate of the water and 2) it applies a greater stress (or force) on the top of the coffee cake. Also there is a pressure gradient in the coffee cake (from pump pressure to atmospheric pressure) so not all of the coffee is subject to the same pressure.

Once full however (the second "1/3") the coffee itself becomes such a great restrictor that it is difficult to have an orifice small enough to actually change the pressure enough to actually have an effect on the flow rate. Therefore the change in pressure really is changing the stress on the coffee cake and changing the pressure gradient in the coffee cake. Once some solids are removed, the coffee the cake becomes a bit more porous (last "1/3") and again the flow can be controlled a bit more.

Aside: Pressure is chosen to control because you are able to measure accurately the static pressure of a system over a wide range easily with the sensors available today. Flow rate (or mass flow rate) is more difficult to measure and is very rate dependent i.e. you must have a sensor calibrated in a range that is very close to your estimated flow rate. This is of course with the expectation that we are going to use sensors that would be affordable to mankind.

Using a Gicar flow meter for example has an error of about 3-5% during the first "1/3" but can have up to 30% error during the brew process. They do have a fairly consistent error so they are acceptable for dosing a quantity but not for control.

BillC

Billc wrote:Using a Gicar flow meter for example has an error of about 3-5% during the first "1/3" but can have up to 30% error during the brew process. They do have a fairly consistent error so they are acceptable for dosing a quantity but not for control.

That's interesting and unsurprising. Are there flow-meters that are fast and accurate enough to provide effective feedback for reliable dynamic flow control? I assume so; I guess the real question in that case would be "how much does such a meter cost?"

Sean Lennon is building an extremely advanced espresso machine incorporating an elliptical gear flow meter that measures 30 pulses/ml flowmeter here. Custom espresso machine build

It's the AW CAPM-3 and goes for about $350.

This combined with servo actuated needle valves appears to be a perfect test bed for this under-explored flow profiling in the first third of the shot. But it will probably take another few months to complete!

this is interesting; so the idea on the slayer is the adjust the flow rate lower to match a finer grind; the lower flow rate allows you to grind finer to extract a sweeter shot

why do overly fine grind shots that take too long on a regular machine taste bad

what might be the overall time for one of these slayer shots; I know its hard to generalize and would seem to depend on how fine you grind, but it doesn't sound like 25-30 seconds

where is the flow rate control on the new single group slayer and what is its range of variation; is there some way to keep track of where the flow control is set so you can get back to something that you liked after trying various flow rates

what effect does moving the handle on the brew group of the slayer away from the "full on" position toward the end of the shot have, or is this something that is not done with a slayer; if done, would this reduce the flow a bit and thereby reduce the pressure at that point or am I still confused

michael wrote:why do overly fine grind shots that take too long on a regular machine taste bad

If you grind too fine, you get over-extraction (more on that here). The point is that the rate of the pressure ramp at the beginning of a shot determines the level of flow resistance the puck will give later in the shot. If you ramp pressure too quickly (i.e., your initial flow rate is too high) for your dose and grind, you will choke the machine, causing the water to be in contact with the coffee for too long and yielding an overextracted shot.

michael wrote:what might be the overall time for one of these slayer shots; I know its hard to generalize and would seem to depend on how fine you grind, but it doesn't sound like 25-30 seconds

My original post addressed this by way of example: ~20s of no or very low pressure, 20-40s extraction (which includes a ramp to 9 bars). The total time, then, is anything in the 40-60s range. I wouldn't hold fast to this range; I have occasionally had nice results with even longer shot times. Note though that this isn't on a Slayer, but on my Shot Brewer with a gear pump.

michael wrote:where is the flow rate control on the new single group slayer and what is its range of variation; is there some way to keep track of where the flow control is set so you can get back to something that you liked after trying various flow rates

The flow rate control is on the knob pictured in this post. From the images I have seen, it is under the cup tray right behind the grouphead. Theoretically you could mark it, but I am not sure. I don't have any firsthand experience with the V3 group. If all else fails, you can adjust it until you get the desired water debit, which you can measure easily. I don't know the range of adjustment on the Slayer's needle valve, but I would guess it's between 0.0 and 0.7mm at a minimum.

michael wrote:what effect does moving the handle on the brew group of the slayer away from the "full on" position toward the end of the shot have, or is this something that is not done with a slayer; if done, would this reduce the flow a bit and thereby reduce the pressure at that point or am I still confused

I haven't used one, but this is how it was described to me. It definitely takes you back to the restricted flow, which in most cases should cause some pressure drop.

The point is that the rate of the pressure ramp at the beginning of a shot determines the level of flow resistance the puck will give later in the shot.

I might be repeating points you made earlier in other discussions, but isn't this a result of how much the puck can expand?

A more gradual pressure rise allows the puck volume to expand more, therefore less resistance. But a more rapid pressure increase keeps the puck volume smaller and particles tighter, hence greater resistance.

michael wrote:...what might be the overall time for one of these slayer shots; I know its hard to generalize and would seem to depend on how fine you grind, but it doesn't sound like 25-30 seconds

Dustin (dustin360) uses the Slayer daily in his cafe. The last I saw him post on FB about the fun stuff he was doing on the Slayer was super long lungos of Esmeralda Geisha, well over a minute per extraction I believe. And I think he was saying they were the best thing he's had off an espresso machine ever. I think he ground super fine, and just trickled in the flow like what folks are describing here (in a roundabout way).

Chert wrote:I might be repeating points you made earlier in other discussions, but isn't this a result of how much the puck can expand?

A more gradual pressure rise allows the puck volume to expand more, therefore less resistance. But a more rapid pressure increase keeps the puck volume smaller and particles tighter, hence greater resistance.

The puck in the transparent portafilter didn't appear to expand until the shot was finished.

shadowfax wrote: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.

Nicholas, how did you reduce line pressure to 1 BAR? Did you dial down your regulator or did you add an in-line needle valve?

shadowfax wrote:This is potentially important, because having a highly restricted valve for the entire shot can prevent you from ever attaining the pressure required to produce an actual espresso.

In a related question, have you been able to determine the minimum line pressure required by your gear pump to attain maximum shot pressure?