|Really cool tampers|
Perhaps the most iconic image of the working barista is the tamper in hand. Barista techniques have centered around this tool for a long time.
I like to discuss techniques and develop standards, here are my thoughts on this one:
Tamping pressure is not an a good way to adjust espresso flow rate. Contrary to popular belief, the difference in flow resistance caused by lighter or harder tamping is negligible. Furthermore, tamping pressure is relieved as water saturates the coffee grounds.
Barista's folklore typically teaches to aim between 30 and 50 pounds of tamping. Often measured on a bathroom scale during trainings. 30 to 50 pounds of force, measured over the total surface of the coffee, would result in 7-12 pounds per square inch (psi).
This force is easily overwhelmed by the water pressure from the group head.
For example, if the grouphead is dispensing 9 bar:
9 bar = 130.5psi
And a 58mm (2.28in) espresso basket has a surface area of 4.08 square inches, or:
It follows then that:
(130.5psi)x(4.08in^2) = 533.7psi
That's much more than a barista's 7.5-12psi.
All that combined with the risk of repetitive stress injury leads me to conclude that a lighter tamping technique is both more consistent and safer.
The primary goal of tamping is consistency. Other advantages include; keeping the grouphead cleaner and reducing waste (less channeling).
I propose that a smooth even tamping technique is more productive then a forceful and level one. Further, it has been my observation that attempting to correct an unlevel tamp causes more problems than it solves.
Push it in there, pull it out, brew and serve.