I am still deciding between convex or flat and the below quote is from a vendor's website. I am not sure it makes sense to me. Is it really true the convex "corrects" the problem of being off center? I mean, if the whole puck is off center, isn't it just as bad with the curve as with a flat surface?

Convex means that the surface bows out, not in. The idea with convex an how it works it at if ever you are off center with your tamp then the surface will remain the same because the curve left behind will be uniformly the same every time. In extreme instances this is not the case, but for the most part consistency and shot quality will be improved. 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.

Hog wash

I have half a dozen tampers : Flat, C-Flat, European and American Curves. The shape has only minimal effect when updosing on machines that must have clearance between the puck and the shower screen

EspressoGirl wrote: I mean, if the whole puck is off center, isn't it just as bad with the curve as with a flat surface?

the idea is that a curve represents the same surface to the puck even if it isn't centered. Tilting a curved tamper a few degrees either way will present (theoretically, at least) the same profile to the surface of the puck, whereas doing the same thing with a flat tamp will have one side a few millimeters lower than the other, showing much less resistance to the water pressure and generating a poor extraction on the high side of the puck, while over-extracting the low side.
Neither tamp will correct a bad distribution, but a curved tamp will not create a bad distribution from a small perpendicularity (word?) error in the same way that a flat tamp will.

<scary midget medium exorcist voice>
This Hog is clean!
</scary midget medium exorcist voice>

That's interesting. I have not had extensive experience with both types though I have used both. I tend to go with flat more often, believing that I'll wind up with fewer grinds trapped along the wall around the grouphead and the sides of shower screen that way. This leads to easier cleaning/maintenance, and a feeling the grouphead and screen will stay cleaner, and just be "better off".

I know this doesn't address the issue of lopsided tamping, if we can call it that, but as for this concern, is it valid, or more hogwash?

"In theory, there is no difference between theory and practice. But, in practice, there is." - Yogi Berra

A tamper with 1mm rocker has a 421mm face radius. The face circumference is 2.645 METERS.

If you tilt a 58mm tamper 3.945° you will raise one side 1mm and lower the other 1mm about the center of the face, for a 2mm differential in your puck. It makes no difference whether the tamper is flat or concave. A BALL tamper with a 58mm diameter could be rotated any which way for a constant, probably crappy, tamp.

Additionally, if the force is directly on the handle axis, and assuming an even initial distribution, the low side will compress / displace the coffee more.

ok, but what about whether convex gives the puck "higher" sides, leading to more grounds left on the side of the shower screen?

pavman wrote:ok, but what about whether convex gives the puck "higher" sides, leading to more grounds left on the side of the shower screen?

If you updose, the puck is more solid and leaves less mess on the screen. If you 'normal' dose, the puck explodes into a goopy mess.

Personally, I dose for taste and don't give a rat's patoot about sanitation.