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.

This is an interesting thought, and I don't have an answer. As mentioned above, the transparent portafilter videos somewhat dispel the notion of the puck expanding until after the shot, but they show a conventional extraction (perhaps even an unusually fast ramp, by the look of it) rather than what I described above. I suspect that changes in the nature of the so-called "fines migration" are at the heart of why this increased resistance happens with a faster pressure ramp, but I don't know-and I don't have the means to establish a causal link here. The main goal, to me, is to investigate how to leverage the dynamic rather than to explain why exactly it happens.

Peppersass wrote: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?

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

I used a pressure regulator. It's set at about 20 psi (1.4 bar), the lowest pressure the regulator can deliver to the pump without causing cavitation (or something similar that sounds awful) at high flow rates, like when flushing the group. That's not to say that a different regulator than mine would have the same minimum pressure.

When the pump is off at the beginning of the shot, it appears to give enough flow resistance by itself that the pressure I see on the puck eventually reaches just below 1 bar. I haven't experimented with a needle valve yet, although I have played with using the paddle as a choke valve at the beginning of the shot by setting it in the middle position. This works to curtail the initial flow even more, except it's very difficult to exactly reproduce shot-shot.

Does anyone how much water is involved in preinfusions fast and slow?

If a slow preinfusion soaks up more water than a fast one; more of the fines will be dissolved before the flow even starts, and the flow will go faster.

What would you need to do to determine that?

I guess a flow accumulator read at the point the dwell time ends might work

It also occurs to me that one can test these ideas with a Clever dripper. If the Clever's draw down time is also faster after steeping longer; differences in puck expansion or water volume cannot be the explanation there. That would make these explanations much more unlikely for the same phenomena in an espresso shot.

The past couple of mornings I experimented as best I could with the procedure described by Nicholas.

I was able to do this because about 10 months ago I replaced the stock pump in my GS/3 with a manually-controlled gear pump. The setup is such that I have very fine control over the motor speed, and thus the ramp-up and ramp-down time.

I pulled Verve Kochere. I don't have the expertise to judge roast levels visually, but my understanding is that Verve tends to roast their SOs light, and they behave similarly to other light-roasted coffees I've pulled. But just to make sure, this weekend I'll repeat the experiments with some Terroir Matalapa that I have in the freezer. That one is light-roasted for sure.

I was able to confirm that the shots run faster after a lengthy, low-pressure preinfusion, which allowed me to tighten the grind on my K10 by 3-4 marks. This, in turn, caused the the flavor to pop considerably more than with either constant pressure at 9 BARs (i.e., stock GS/3 operation) or my usual profile of allowing a stock ramp up to 9 BAR, which takes maybe 3-4 seconds, followed by a slow ramp down during which I try to keep a constant flow rate.

However, the shots were somewhat under-extracted, at least to my taste buds. They weren't unacceptably sour, but they definitely leaned in that direction. I'll have to confirm with refractometer measurements, but I'm pretty sure my tongue was telling the truth: more tingle on the back of the tongue than the front. I didn't try increasing temperature (it was set to 201.5), which might help.

I should point out that there are some significant limitations in my system:

First, the line pressure was slightly over 2 BAR and I didn't try to lower it further. The regulator is in the basement, and I didn't have the time to run up and down the stairs to find the lowest line pressure that would allow the gear pump to reach maximum pressure without cavitation. I'll try to do that this weekend, when I switch to the Matalapa. With line pressure at around 2 BAR, it took around 5-7 seconds to pre-infuse (i.e., to reach the point where pressure began to rise.) This may not be long enough to get the full effect.

Once pressure started to rise, I slowly increased pump speed until the pressure reached 9 BAR (actually about 1.5 BAR higher than that at the boiler, which translates to 9 BAR at the group head.) So max pressure was reached in about 20 seconds.

Nicholas, you said that you pre-infused at roughly 1 BAR for 20 seconds, then ramped up to 9 BAR. How long did it take to do the ramp-up?

The second, and more problematic limitation in my system is that the GS/3 won't allow a shot to run more than 50 seconds. When I tightened the grind down four notches, the shot weight just barely reached the target in 50 seconds (it was actually 1 gram less than the target.) The color and consistency of the stream wasn't right for ending the shot, either. This means the grind was too fine, likely because the 0 BAR preinfusion time was too short. I ended up loosening the grind by one notch to compensate (still three notches less than nominal).

Getting the line pressure down to 1 BAR or less might allow a finer grind, but if the shot needs more than 50 seconds to complete, then I'm out of luck. I might be able to rig a switchable manual trigger to extend the AC signal to the brew solenoid, but there many be some undesirable side effects. For example, the CPU allows autofill to proceed if it thinks the shot is done, and that could lower temperature of the water in contact with the HX. I don't know if the CPU does any other fancy stuff during a shot. Bill, can you comment on that?

another_jim wrote:I guess a flow accumulator read at the point the dwell time ends might work

I believe I'll have this capability eventually. I've designed and partially built an opto-isolator interface board that monitors most of the GS/3 signals, both AC and DC, including the pulses from the flow meter. It's a key component in my project to add Arduino control to my pressure profiling system, an effort that's been greatly delayed by work demands (I hate it when reality interferes with my hobbies!)

I decided to monitor the flow meter in the hopes that I could use it to maintain a constant flow rate after the shot reaches peak pressure (i.e., to automate a theory of profiling you advanced last year.) Some preliminary investigation with a frequency counter suggested this might work, though there have been a number of discussions here that indicate the meter simply isn't accurate enough. But it was easy enough to add another optoisolator, so I thought it was worth a try to verify.

That said, considering that the GS/3 uses the flow meter to do volumetric dosing, it's probably accurate enough to measure the difference in water volume between fast and slow preinfusion.

Dick,

Thanks for looking into this. I appreciate your feedback.

Peppersass wrote:However, the shots were somewhat under-extracted, at least to my taste buds. They weren't unacceptably sour, but they definitely leaned in that direction. I'll have to confirm with refractometer measurements, but I'm pretty sure my tongue was telling the truth: more tingle on the back of the tongue than the front. I didn't try increasing temperature (it was set to 201.5), which might help.

On my shot brewer, I have found myself often on the high side of temperature since fiddling with this gentle pressure ramp. I would definitely up the temperature to 202-203 if you're getting too much sourness.

Peppersass wrote:Nicholas, you said that you pre-infused at roughly 1 BAR for 20 seconds, then ramped up to 9 BAR. How long did it take to do the ramp-up?

Usually it takes around 8-10 seconds to start reading pressure on the gauge above the group, and I typically let it sit there for another few seconds. 20 seconds total time is just an example. Think of this as a spectrum that you can tweak-don't get TOO held up on exact values.

I do think cutting your line pressure a little bit will let you grind even finer. This may let you extend the time with no pressure, but speed up the ramp to 9 bars after you do start registering pressure. Maybe try 15 seconds of low pressure and a ~5 second ramp to full pressure. The 50 second total time limitation does limit your exploration, but I think you'll still have ample room to see what I am talking about without extending it somehow.

Taking a much lower tech approach with my Strega I pulled several shots yesterday using the 2011 Ethiopia Dara Kebado that I had roasted to City+ for syphon brew. With only a 1:30 stretch after 1C started it wouldn't normally be an ideal choice for espresso. I used a much finer grind setting then normal & dialed in the dimmer switch which controls the pump for a 1 bar preinfusion pressure as suggested by Nicholas. Once the group had pressurized I lifted the lever just enough to cut off the pump & let the shot proceed at 1 -1.5b until the bottom of the cup was covered. I then engaged the lever retarding its movement until cutting the shot at about the 11:00 position. Total shot time was well over a minute producing about 28-30ml of liquid & crema. The shots were deliciously fruity with a mild acidity & probably would have made a phenomenal Americano.

shadowfax wrote:I do think cutting your line pressure a little bit will let you grind even finer. This may let you extend the time with no pressure, but speed up the ramp to 9 bars after you do start registering pressure. Maybe try 15 seconds of low pressure and a ~5 second ramp to full pressure. The 50 second total time limitation does limit your exploration, but I think you'll still have ample room to see what I am talking about without extending it somehow.

Couldn't get my pressure regulator to go much below 2 BAR, maybe 20-25 psi. It's an inexpensive plastic 0-160 model that I got from CC about five years ago. I just ordered a 0-25 psi brass Watts regulator that hopefully will go down to zero psi or close to it.

Tried pulling Terroir Matalapa this morning. The slightly lower regulator setting extended the pump-off preinfusion time a little, and I was able to grind a full four notches lower on my K10 than the usual setting for this coffee. I ramped a little faster to 9 BAR and the shot completed in about 45 seconds.

Once again, the flavor popped much more with this technique. It was even evident in a skinny latte, allowing the brightness of the coffee to push through the milk quite a bit more than usual.

The shot wasn't as under-extracted as the Verve Kochere, but I didn't have time to do refractometer tests this morning. Hope to get to that tomorrow.

I forgot to mention that I bumped the temperature up to 203F for the Matalapa pulls, which might have contributed to the more balanced shots.