another_jim wrote:Imagine you are using a hydraulic pump to push the water through the puck. You'd be pressing down a long distance, at less force, to create 9 bar over a narrower crosssection of water. So if the lever travels 22.5 cm, what is the crossection of the tube to get 45mL of water? 2cm-square. How much pressure is required to get 9 bar on this? 18 KG. The mechanical advantage is being provided by a lever, rather than a U or flare shaped tube of water, with one side having a small area, the other large, however, the same basic physic must apply. So it's easier to think about pushing a narrow column of water a long way rather than worrying about piston and puck sizes

Heh-heh... reminds me of this one-



his homepage
the 2nd version

Henry

Staffan wrote:The problem is that i expect that all lever machines are slightly different when it comes to the pressure needed on the lever..

Indeed they are different in the amount of force one has to apply to the lever with one's arm because their pistons vary in diameter and their levers are not all identical in terms of mechanical advantage (a function of the work-arm length and the distance between the pins).


Brew Pressure= Force On Piston divided by Piston Surface Area
(ignoring loss of pressure from flow through the filter)


To attain a specified brew pressure on a 45mm piston requires considerably less force than is required to attain the same brew pressure on a 58mm piston.

If the lever design on the two machines were identical, then the 45mm machine would be the easier on the arm.

It is possible to attain 9 bar on the little Caravel, for example, despite its rather meek lever, because the diameter of the Caravel's piston is so small. Of course, the piston has to travel a greater distance in order to produce the desired volume.

Generally speaking, if the barista's arm strength is treated as the constant in the equation, the greater the piston surface area, the greater the mechanical advantage required of the lever design; i.e. the pins must be closer together and/or the arm of the lever must be extended.

Regards
Timo

P.S. Basket diameter and piston diameter are often different.