by drdna on Sat May 30, 2009 3:49 pm
The convex piston specifically is probably good. Coffee grounds are essentially incompressible beyond a certain point. The convex piston directs the greatest pressure toward the center of the basket, resulting in a uniform distribution of pressure in the surrounding area.
Here is a little experiment for you to do. Get a tennis ball. place it on the table. Okay, the contact point is in the center of the bottom, right? Now rotate the tennis ball by 30 degrees. OMG! The contact point is still in the center of the bottom! Now, try it with something flat, like a dinner plate. Notice how even a slight tilt to the dinner plate results in asymmetric pressure application?
Okay, that is kind of an extreme example, but basically this is part of what is going on. The goal of tamping is to press down very uniformly with an even pressure distribution. The convexity allows us to compensate for errors in our technique, to some extent. If you have weaknesses in your technique that this can compensate for, then you will prefer the convex piston. If your technique is stellar in this regard, you will probably not notice a difference.
And honestly it has been my experience that the difference that tamper bottom shape makes is subtle and for the most part not directly on the extraction. Maybe if you had an automatic tamping machine with a flat and a convex piston, you could assess this, but there are so many other variables that come into play, this detail is just minimal.
What I think is important is how the shape of the tamper piston (or in fact the shape of the tamper in general) will affect your technique for tamping. It is my experience that the most important aspect is to be able manipulate the tamper very easily, as if it were simply and extension of your hand. If you have absolute control over the tool, you can be more accurate, and your tamp will be more consistent. THAT is really the key: consistency.
So pick whatever tamper feels best for you in terms of weight, shape, size, etc. and learn to use it.
Adrian