it's interesting to me that we're starting to see a difference between measured results and observed results.

in other words - a lot of people are saying "I haven't noticed any difference between light and heavy tamp" while some are saying "I've measured a difference in results."

kind of a good illustration to me....

I think this might depend on the distribution of coffee in basked before tamp.

If distribution was good and even, then just light tamping will do the job. However, if distribution was not even heavier tamp will create better distribution through pressure and slow down the pour because now everything is more evenly distributed. Lighter tamp keeps uneven distribution and thus faster pour...

With good distribution then the tamp pressure does not matter...

Just a theory...

Jepy wrote: I did notice though, the shot forms slower and much more even with the 40 lb., but somehow it passes up the lighter tamp in weight.

Well, we often observe that a gentle preinfusion results in faster flow compared to a sudden pressure rampup. Why? Presumably a slow preinfusion allows coffee particles to hydrate, swell, and form an interlocking matrix, and the matrix makes it harder for fines to migrate towards the bottom where they create a compact, restricting layer.

Perhaps a hard tamp can under certain circumstances produce its own kind of interlocking matrix, again restricting fines migration.

Key word is "perhaps."

The "hard tamp produces slower flow" line of reasoning was always a little suspicious to me because no one ever tamps as hard as the 500+ lbs we get with 9 bar pressure.

No one except you, John!

AndyS wrote:Well, we often observe that a gentle preinfusion results in faster flow compared to a sudden pressure rampup. Why? Presumably a slow preinfusion allows coffee particles to hydrate, swell, and form an interlocking matrix, and the matrix makes it harder for fines to migrate towards the bottom where they create a compact, restricting layer.

Interesting. I've observed the opposite on my La Spaz S1V1. With the mechanical preinfusion cylinder installed, a coarser grind was required. I assumed that the prewetting caused hydration and swelling of coffee particles, which restricted the flow (as opposed to restricting fines migration).

Be nice if there was more concrete evidence for fines migration, and the role it plays in regulating flow...

RapidCoffee wrote:Interesting. I've observed the opposite on my La Spaz S1V1.With the mechanical preinfusion cylinder installed, a coarser grind was required. I assumed that the prewetting caused hydration and swelling of coffee particles, which restricted the flow (as opposed to restricting fines migration).

Yup that is interesting. Perhaps it is dose dependent, but I've never noticed that. I'm certain that a "gentle preinfusion" on one machine may be not-so-gentle on another machine. I've seen dozens of times that preinfusing a shot with line pressure (~3 bar) until the first drips show, and only then turning on the pump, will result in a faster flow rate than if the pump came on from the start.

AndyS wrote:But hey, that's espresso. It's very hard to duplicate someone else's results on your equipment.

The role of fines migration may depend strongly on basket geometry. The Spaz has a tall 53mm double basket, kinda like a mini triple, and might respond differently than a 58mm double. One of these days I'll have to hook up the P/I cylinder again and try timing some pours.

Hi guys,

the grinder might play a big role in this, if it spits out a lot of fines at the end of the grind (perhaps when per shot grinding?) the exctraction will behave differently to what happens with a grind that has fines spread evenly throughout the puck.

Regards,
dsc.

I did a test this morning, two shots back to back, same dose, same distribution, barely tamped one (maybe 5lbs) and really crushed the other one (probably 50 lbs+). Extractions looked pretty much the same, they both started to drip at about 7 seconds, and both got cut at about 24 seconds, just as they started to go blond, and ended up with about the same volume. All of which lines up with my observations up until this point, no noticeable difference in pours between light and heavy tamps.

But - I weighed both after, and there was a significant difference. The light tamp was about 13g, and the heavy tamp was 18g. Which was really surprising. The difference also showed up in the cup, with the heavy tamp tasting thicker; although the flavors were pretty much the same, and this wasn't blind, so I don't know if I would've noticed the extra body if I wasn't paying attention to it.

A couple notes, the coffee was getting towards the end of its roast window which may have been a factor, and I was grinding finer than I usually would. Also, the heavy tamp cup ended up with a few coffee grounds in the bottom of the cup, I assume I pushed some fines through the portafilter holes (but it wasn't 5g worth)

I don't really have any good ideas on where the extra weight came from, especially since I didn't notice a corresponding change in the flavor. I'll try this again to see if I get repeatable results.

In the "not sure what it means" column ...

Abe Carmeli is visiting, and choked a shot using the same coffee, grinder, grind setting, weight, and machine when it flowed fine for me. I nutated then did a pro forma 2 pound tamp, whereas he nutated then did a heavy 30 pound tamp.

I'm too bored with tamping to experiment, but it may be a lead for anyone who's so inclined.

dsc wrote:the grinder might play a big role in this, if it spits out a lot of fines at the end of the grind (perhaps when per shot grinding?) the exctraction will behave differently to what happens with a grind that has fines spread evenly throughout the puck.

Or any other differences in the grind that any particular grinder might induce.
Thus:

RapidCoffee wrote:AndyS wrote:
But hey, that's espresso. It's very hard to duplicate someone else's results on your equipment.