Convex vs. Flat Tampers: an open-ended theoretical question

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#1: Post by harrisonpatm »

I saw a convex tamper on buy/sell a few weeks ago, and I had never seen one prior to that. The concept makes sense, to promote flow towards the center and reduce channeling. I was wondering though, if anybody had any insight: if it's too convex, and theres essentially a graded divot at the top of the puck, is it possible that it's directing flow too much towards the center, and runs the risk of underextracting the bottom outer perimeter of the puck? Loaded question, I know, since a dozen other variables are at play, namely preinfusion, flow rate, temp... I was just wondering if anybody had any experience on whether there are pockets of coffee that don't get extraction. My search on these forums so far, seems to come up with a 50/50 preference-based division on whether flat or convex tampers are better, with the takeaway being that there's no substitute for good technique, a good grinder, and puck prep.

So then I have part 2 of this theoretical question. If the goal is to direct flow towards the center and avoid channeling, why not make the basket concave (as viewed from above) instead of the tamper? Again, a precursory search suggests that old baskets become convex anyway, with wear and tear distorting the filter holes, indicating they should be replaced. But what if a basket was engineered from new to be convex in the first place, while still having correctly-sized filter holes? Would that promote proper flow?

Anyway, it's just a theory that I had with my morning shot. I'd love to hear if there are any other thoughts on the matter.

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#2: Post by cafeIKE »

There is no direction of flow. Pressure is same everywhere. Theoretically, thinner center should extract faster, promoting channeling. Within reason, IMO, radius is irrelevant.

About 15 years ago, H-B has the Great Tamper Shootout. I broke the rules and tried them all. Only difference I found was the Radical Pro severely pointed conical spritzed like the dickens.
cafeIKE wrote:Parameters I would like to investigate in a controlled test, for a standard grind, dose, tamp and pump pressure:
- Optimum basket to tamper clearance. [5 tampers in .1mm increments]
- Optimum tamper corner profile. [5 tampers each in round and chamfer profile in .05mm increments]
- Optimum profile of tamper face to shower screen face. [5 tampers from flat to same profile to more convex]
- Optimum of the above optimums. [This Golden Tamper is 1 of 625]
Then start again, varying grind, dose, tamp, pump pressure and brew temp.
Then start again, varying CBLF [Carbon Base Life Form]
from HB Roadshow - Espresso Tamper Reviews #38

Full Thread FYI:
HB Roadshow - Espresso Tamper Reviews


#3: Post by erik82 »

I tried different tampers over the last 15 years and while I thought a convex tamper was better, in the end it all came down to technique. Once dialed in everything I came to the conclusion that flat tampers do the best job. I did also like the C-ripple from Reg Barber but it doesn't do a better job then a flat base once your technique is good enough.

Maybe with subpar techique or grinder it does seem to do a better job but in the end it's just a patch fix of some other bottleneck in your setup. For me it's kind of the same as tamping pressure. Once you start this hobby you think it does make a big difference but the more experience you have the more you come to senses and realise that it doesn't do anything except ruining your tamp when your apply to much pressure and can't control it well anymore.

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#4: Post by Jeff »

There have been some studies of radial variation in extraction. I don't recall any evidence that flat vs. convex had a "clear winner." In some circles of advanced users with lighter roasts, modern flats, highly repeatable machines, and refractometers, people are intentionally placing more grinds in the center to increase EY and perceived quality in the cup.

Erik hit my point head on while I was composing:
erik82 wrote:Maybe with subpar techique or grinder it does seem to do a better job but in the end it's just a patch fix of some other bottleneck in your setup.
I do not recommend the mounding technique as general practice without already high skill levels. Even "good" puck prep is likely to degrade the in-cup results more than this has the potential to improve them. It does not "fix" anything.

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#5: Post by RapidCoffee »

There are certainly more important factors in espresso, but flat vs convex tamper base does make a difference. First, convex tampers generally produce prettier pours, with less chance of spritzing, channeling, and donut extractions. This is likely due to direction of flow through the (slightly thinner) center of the puck.

Second, several highly respectable individuals (including Andy Schecter, typically a decade ahead of the rest of us) have noted a significant increase in extraction yield with a flat tamper base. Recent work by Stephane Ribes (posted on the Decent Diaspora forum) suggests this may be due to increased radial extraction uniformity of the puck when tamped with a flat base. Apparently creating a small mound of coffee grinds in the center of the puck prior to tamping increases extraction yield still further (credit Dan Calabro on Decent Diaspora for this idea).* Convex tampers do the opposite, by pushing grinds outwards, away from the center of the puck.

These days, with all the "my EY is bigger than your EY" bragging, the consensus seems to be in favor of flat tamper bases. :wink:

* I see Jeff beat me to it.
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#6: Post by erik82 »

We have just unveiled the tip of the iceberg when it comes to coffee. For me there are still some "flaws" in the research done when translating it to the average home-barista. This is what we tend to do as there are some HB-ers that have professional gear but we are the minority. Almost all research is done on professional equipment whereas the average HB-er has something like a Silvia and Rocky at home. Most of us just aren't able to spend $5.000,- or more on coffee gear to get to that level.

When struggling to get decent shots from such typical HB equipment it could be the case that a convex tamper does improve quality significantly. It could aid in overcoming some of the downsides of such equipment. We as a community do a lot of research also but that's also limited to the more fortunate under as, as users with a silvia and rocky don't tend to own equipment like refractometers. So again most results found here are also on professional gear.

It don't think we'll ever come to a pont where all of this is researched as it takes way too much time and money. It may give us some very different insights from what we know now.


#7: Post by cgibsong002 »

RapidCoffee wrote:First, convex tampers generally produce prettier pours, with less chance of spritzing, channeling, and donut extractions.
On the contrary, I've been using the supplied convex tamper with my Synchronika, and just got my bottomless pf. I'm very much getting reverse donut pulls (is there a name for that?) with my VST. Knocking out the puck it's clear the edges are heavily under extracted.

A lot of people right now are going through great lengths to get even extraction all the way to the edges (mesh filter underneath puck), so to me it's confusing why you would want to push more coffee to the sides, and makes sense that you'd get lower EY with convex.

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#8: Post by yertchuk »

My Linea Mini came with a convex tamper. I get nice results with a bottomless portafilter; holes fill evenly at the start of the pull, and rapidly coalesce into a single central stream. I've tried a flat tamper, and that produces the infamous 'donut pattern'.

This is with the standard LaMarzocco basket, again as supplied with the machine. The bottom of the basket is slighly convex, and becomes more so under pressure, so maybe effective puck depth is more even than you would think from looking at the tamper alone.

-- Peter

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#9: Post by cafeIKE »

IMO, it's more complicated than flat or curved.
  • face curve
  • headspace
  • screen curve
  • basket shape
  • grinder
  • grind
  • coffee
  • brew pressure
  • ramp
  • hand
With a maximum of 10 samples for each of the incomplete list above offers TEN BILLION possibilities.
[Edit math error]


#10: Post by boren »

To all those who are in favor of flat tampers, are you using flat baskets (doubles)? If so, it makes sense that such tampers would be optimal, as they apply equal pressure throughout the basket. With single baskets (usually with slanted sides) flat tampers would probably not be as optimal, and curved tampers may more closely match the profile of the basket.