30lb tamp pressure?

[quink]

I've been wondering about this for some time, does any one know where it comes from On my joke of a machine at the moment I grind tamp and brew, but since I've been looing for a new macine this figure comes up a lot. I think I may be missing something here but pressure is measured in psi and a 58mm protafilter has an area of about 4.5 inches square (apologies for the mixed measurements) so to get 30psi I'm going to have to apply 4.5x30 pounds or a force of 135 pounds. Everybody following me? Or is it 30lbs of force and not pressure which give about 6psi The other reason I'm wondering is with different sized filters this 30 pound thing would have to change to be a constant tamp pressure, if its the later case above but I've never seen any other figure mentioned, say a 53mm filter may only need 25 lbs to get the 6 psi and the 30lbs would tamp too much.
A little help, anyone

[stevenzaat]

Quin k,

I can not follow you cpmpletely, but it is 30 lb of force and not pressure, so just take your scale and force it to 30 lb. This is exactly the pressure which is needed to tamp. So a bigger or smaller size tamper will have no impact on the force used..

Did this help? If i was not clear enough, please let me now and I will loke in Sommers book and will quote word for word.

Best regards,

Steven

[kingseven]

The scales is a good way - I've tamped with the pf sat on some bathroom scales before - and around 30 pounds/15 kilos (I think that conversion is right!) of force/weight/pushing is good. To be honest anywhere between around 20 and 40 is ok - just as long as you keep it constant...

[alans]

Forget the numbers, constant is what matters. But then I grind fine and tamp light compared to the numbers you're quoting and I think of them as "percieved truth" rather than any real advice

Get your ground right so that your shots are good at 30lbs, then throw that out the windows and ajust your grinder till you get good shots with half that tamp.

[quink]

Thanks for the help guys, I'll work out something when I eventually get to buying this new machine, as long as it tastes good that will do me.

[kingseven]

Too light a tamp is going to cause problems though as the cake is more likely to split, opening up channels.

But then - you forget that if you've hardly tamped then you lock the pf into the machine the shower head will probably compress things further.

[wlodek]

[...] around 30 pounds/15 kilos (I think that conversion is right!).

No, it's not! If you are supposed to keep water temperature within 1% then you need to keep other parameters exact too! 30 pounds is 13.608 kg, alright, you'll get away with 13.6 .... and don't even think about making it 14kg.

Seriously though, has anyone done a tamping force vs. cake permeability study?

W.