Hard tamping and channeling

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#1: Post by Chase106G »

Hi, I have some no name 58 tamper with a lighter spring, and also Normcore 58.5 v4 with the 30lb spring.

When I use the no name tamper, the extraction is good without channeling and the flow is consistent all the way, and when I use the Normcore I do have channeling and the the extraction start slow and after few seconds the flow move faster.

except the tamper, everything else is the same.

Why I have channeling when pressing the puck harder (with the Normcore)?

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#2: Post by Nunas »

I don't know why, either; but your observation matches mine. Years ago, I tamped to 30#, mostly because that's what nearly everyone said to do, and I was always battling channeling and trying out various gadgets. Then I saw a video recommending not to tamp, just level and lightly press (sorry can't recall who posted it, when or where). Now, I give the portafilter a couple of good raps to spread out the cone of grinds, lightly nutate the tamper to eliminate voids, and end with no more than about 5# of tamp. Now, I rarely have channeling. Also, I no longer use any gadgets. More recently, I started using sintered metal puck screens, not to reduce channeling, but because they seem to keep the group much cleaner. I think they might also help mitigate channeling in some circumstances, but the jury is out on that.

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#3: Post by H2c4 »

Denser puck = higher extraction yield, more likely to channel

Matters more if you are grinding finer. If you're grinding less fine, coarse, tamping pressure has less impact

Better puck prep if you are going 30lb or more

For reference
https://www.caffeinated.science/posts/e ... -evenness/

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#4: Post by kye »

I thought that the water pressure compacted the puck a lot more than we do, so the eventual density of the grind wasn't dependent on how hard we tamp?
IIRC Lance interviewed someone who said that 9 bar over a 58mm basket translated to about 250kg (550lbs). That video with the transparent portafilter showed that the puck was hugely compressed by the water too.

Of course, how hard we tamp is potentially still going to matter in terms of how resistant the top surface of the puck is to the water ingress when the pump is first turned on etc, so I'm not saying it has no impact, but maybe not the impact we might think it has.

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#5: Post by Mike-R »

Here is my take...

Applying a tamping force causes the bottom of the basket to flex, and removing the tamping force allows the basket to spring back to its original shape. I expect that even with perfect tamping technique, this flex/spring can lead to cracks (and therefore channeling). A greater tamping force introduces more flex/spring and therefore there should be a greater chance of introducing cracks.

Also, a strong tamping results in a more compressed puck, which may be more brittle than a less compressed puck.

During pre-infusion, the water pressure causes the basket to flex a second time, which may cause further opening of any cracks in the dry portion of the puck.

As Kye mentioned, the water pressure also compresses the puck with a stronger force than tamping. This makes a strong tamp unnecessary. More about this below...

Only the dry portion of the puck that gets compressed by the water pressure. Once any portion of the puck becomes saturated, it will no longer compress because the water in the pores is incompressible. (Caveat: There is some compression force due to friction as water travels through the puck, but I think that's a relatively small force.)

Some quick math... 1 bar of pressure is about 1 kg/cm2 and a 58 mm basket is about 32 cm2, so each bar of pressure pushes with an equivalent tamping force of about 32 kg (70 lb).

It can be seen in the transparent portafilter YouTube videos that the water covers the top of the puck quite quickly and then it slowly travels through the puck during pre-infusion. As long as the puck is compressed enough during tamping that the water travels slowly through the puck, the result is that most of the puck will be compressed by the pre-infusion pressure.

If pre-infusion is 2 bar, this means the equivalent "tamping" force of the water pressure is about 140 lbs. If pre-infusion is 6 bar, the force is about 420 lb. This makes pre-infusion pressure a very important variable in terms of how compressed the dry portions of the puck become before water is introduced. More compression means smaller pores which means more flow resistance.

In the past, some have proposed that "fines migration" is the cause of higher puck resistance when pre-infusing at higher pressures. I don't think this is the main cause, I think puck compression is the main driver.

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#6: Post by kye »

Very interesting post Mike-R!

I'm reminded of Lance saying that he found that tamping twice had some sort of benefit. I haven't watched his video on that, but I should go back and watch it - perhaps what he found was related to this behaviour of the basket flexing? I knew the basket flexed but hadn't thought it through like this before.

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#7: Post by Mike-R »

Thanks, Kye.

I'm interested to know what Lance was saying about double tamping. If you can find it, please share a link.

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#8: Post by Mike-R »

Ok, I found it. Lance didn't say much about it, but he referenced an article on Quantitative Cafe blog. It seems that the article is saying that repeat and/or longer tamps result in more compaction. But the blog author notes that he hasn't looked into what, if any effect this has on extraction. He also gives an update that someone else reported that tamping force greater than 20 lb does not affect extraction.

That would fit my understanding that as long as the initial tamp and grind setting prevent the water from passing through the puck too quickly, then the pre-infusion pressure is what will dictate the final compaction of the grounds before the water passes through.

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#9: Post by Mike-R »

And one more thing about the article relating to multiple tamps. The author's graphs show that tamp force has much more impact on density than repeated tamping. He measured a 6% increase in puck density just by increasing the tamping force from 20 lb to 30 lb. One can only imagine how much increased density might be achieved with just a 2 bar pre-infusion pressure, equivalent to 140 lb of force. So surely a double tamp will have no impact on the extraction.

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#10: Post by Jeff »

It could have an impact if it improves uniformity of wetting of the puck. I am not aware of any studies to support or undermine that idea.