Choosing a quality tamper is often overlooked as an important consideration of brewing good espresso. I like to think that brewing espresso is a lot like a science experiment. You have many variables that need to be adjusted or altered so that you are able to brew the best shot possible. Tamping is a key part of making that great shot of espresso, and a quality tamper can contribute to your success.
Too much tamping pressure can result in a bitter cup and too little pressure can result in a watery shot. There won't be enough resistance in the grounds to impede the flow of the water. You will need to find the pressure that works well for you based on your grind setting. Experienced baristas know that once the combination of tamping pressure and grind fineness are crucial elements that must remain constant every time you brew so that the end result does not vary from shot to shot.
You'll notice that there are two main varieties of tampers offered, convex and flat. Convex tampers have a slightly rounded surface that leaves a mild indentation in the coffee puck after tamping. Flat portafilters (as their name suggests) have flat surfaces that compacts the coffee evenly. An argument in favor of convex portafilters is that they help to eliminate side-channeling in the portafilter while brewing although which variety is truly superior is subject to debate.
Channeling is a phenomenon that occurs when water flows through areas of lower back pressure in your espresso puck like cracks or unevenly tamped grinds. When channeling occurs, the extraction becomes uneven as the water follows the path of least resistance. Ultimately, you should choose the tamper that feels most comfortable in your hand and that can produce the most consistent results.
I like to use the Espro calibrated tamper because it gives you feedback by clicking once you have reached 30 PSI of pressure. 30 PSI is often considered the optimal tamping pressure, although this may vary based on the fineness of your grind. Having a consistent tamping pressure is beneficial because you will be able to eliminate one variable when trying to pull perfect shot. Alternatively, you can use a bathroom scale to gauge how hard you're tamping. Just cover the scale with a towel and push down a few times to get a basic idea of how hard you're tamping.
Remeber that consistent results are more important than exact variables. In any case, I am confident that a little more attention to the important element of tamping pressure will make a wonderful difference in your espresso brewing, helping you yield more consistent and more flavorful shots of espresso.
Sometimes too, your choice of tamper may be purely cosmetic. Many brands like Profitec and ECM have designed their own tampers for use with their espresso machines. Still other brands like [Joe Frex] allow you to custom build your own tamper based on your preferred materials, portafilter diameter, and aesthetic preferences.