jmatt wrote:OK  I hear about 30 pounds of pressure when tamping. Some tampers even have a "click" or other pressure mechanism set for 30 pounds.
However, I did some math:
A 49mm tamper covers 2.92 square inches. 30 pounds of pressure equals 10.3 pounds per square inch.
A 53mm tamper covers 3.42 square inches. 30 pounds of pressure equals 8.8 pounds per square inch.
A 58mm tamper covers 4.10 square inches. 30 pounds of pressure equals 7.3 pounds per square inch.
Looked at another way, to get 8.8 pounds of pressure with a 58mm tamper requires 36 pounds of force.
To get only 7.3 pounds of pressure with a 53mm tamper requires only 25 pounds of force.
So what's the rule? 30 pounds with a 58mm tamper, but only 25 with a 53mm? 30 pounds with a 53mm, but 36 pounds with a 58mm?
Or is the pounds of pressure irrelevant (within reason) so long as the tamp is consistent, and pull time is adjusted with grind fineness?
Assuming a cylindrical shape for the basket and a flat tamper, the compression of grains diminshes with distance from tamping surface:
TAMPER
\/
 most compressed grains

 middling compressed grains

 least compressed grains
Two baskets: one broad and shallow the other tall and narrow. The taller requires greater pressure to reach the same depth and degree of compaction as the broader shallower basket. Thus, although the pressure varies by surface area (force remaining constant) the compaction could average out to be much the same degree, the inequality in pressure notwithstanding.
Which isn't to say I'm wedded to the 30# idea. I sometimes do little more than level. Sometimes tamp ~ 30#.
Regards
Timo