The second half of the quote (taken from the EP website; let's give credit where due) makes more sense to me than the "off center" hypothesis:

The other benefit it that with not all the pressure being directed straight down, the curved base allows part of the force to push out towards the edge of the basket helping to seal the sides and prevent water from bypassing the puck and not be infused with espresso before leaving the portafilter.

Convex pistons appear to be slightly more forgiving; a perplexing observation, since a level, even puck surface should result in an optimal pour. But water under pressure will seek the path of least resistance to flow. When the puck has been tapped or overdosed, this path may be the side seal between the puck and basket. A convex piston forces coffee away from the center towards the sides, helping to correct such flaws.

Again, this is a subtle effect. I prefer to concentrate on grind, dose and distribution. Get those right and any tamp should work.

Sorry, but all force is directed straight down. Assuming a level distribution, the center of a convex tamper will contact the coffee first. Coffee is not 'slippery.' As the piston continues into the coffee, the center compresses more than the sides. Whether and how much the coffee displaces from the center or just compresses more needs investigation.

It's possible to measure the difference in force applied to the side of the basket with a convex tamper vs a flat one. It's my guess that it's in the realm of inconsequential.

Seems like a project for Ken and Jim.

cafeIKE wrote:Sorry, but all force is directed straight down. Assuming a level distribution, the center of a convex tamper will contact the coffee first. Coffee is not 'slippery.' As the piston continues into the coffee, the center compresses more than the sides. Whether and how much the coffee displaces from the center or just compresses more needs investigation.

If so (greater compression in the center), I am truly perplexed. This would imply that the chances of a donut extraction are enhanced with a convex temper, which runs counter to my experience. Ditto for the infamous nutating tamp, which by all rights should create a small mound of coffee in the center of the puck prior to compaction.

I think coffee may indeed be somewhat slippery.

Methinks we don't know our arse from a hole in the ground when it comes to what happens in the basket.

In nearly 3 e61 years and flat vs several convex tampers, other than the aforementioned clearance on updoses*, no advantage accrues. Ditto ultra close tolerance.

* IMO, the e61 is next to impervious to shower screen contact.

E61 may be easy going but I have a couple different machines that are very picky about dose, and one in particular that is very picky about tamp. That machine had a noticeable improvement based on the tamper shape used, and it was an original Faema with a solenoid actuated group. So on a rare occasion, yes the tamper shape will make a difference, but I believe that 99.5% of the time, it is all window dressing. Personally, I tend to use a flat and convex interchangeably although I usually stay with a tamper that matches the shower screen. Convex screen, convex tamper, flat screen flat tamper but I see no marked difference between the two on my current kit. No amount of tamping or a 0.001mm basket to piston tolerance will do any good if your grind/dose/distribution is off.

The only purpose tamping has is to ensure an evenly dense puck of coffee so the water flows through it evenly. So start with an even distribution and you could tamp with anything that is level and reasonably sized to the basket.

Sarah, do you now see that you are over thinking this? The tamper is probably the least important aspect of espresso making. However, I think they look great. Find one that looks good and fits the basket and you will be good to go. I really want a Pullman tamper because they look great. I have no thoughts that one will improve my shots.

*Oh, you want to know what has improved my shots the most? Dave Stephens when he came over to work with the MVP Blend! I suggest buying a Dave Stephens or someone similar. I think EPNW carries them.

cannonfodder wrote:The only purpose tamping has is to ensure an evenly dense puck of coffee so the water flows through it evenly. So start with an even distribution and you could tamp with anything that is level and reasonably sized to the basket.

Or nothing at all

Poor Sarah, she just wanted advice on buying a tamper, and gets sucked into a life and death struggle between the forces of flat and convex, all over the existence or not or fluid like particle flow at the micro level and the dynamics of irregular shaped particles under pressure. Where's a micro-geologist when you need one.

I read the same tamping quote about convex/flat while trying to decide what to buy. I don't have the privilege right now of buying several tampers, but in the future I hope to buy more and want to own a flat, but for my first I bought the convex. Not that I expect any magic, but I had to choose! I bought the espresso lab design one from Australia for a good price that included shipping to Canada. Can't wait to try it!

Oy! What a "hobby" this is! It seems hard to see the end of the tunnel where I will finally have all the equipment I need for a great shot and great microfoamed milk....

Someone mentioned which vendor the quote came from. I purposely had left that information off in case there was dissent and I didn't want that to negatively reflect on my vendor....