We Caravel-ians always wonder about the strength needed to pull an optimal shot: some suggested it's almost like cutting hard-butter, some said full body strength. And there's no quick and easy way to check the brew pressure. So I'm putting this out there so y'all could check the calculation. From there we could get a rough estimation of the ballpark range and to adapt from there to your preference. Caravel's Theoretical Needed Pull Pressure
Caravel's basket diameter : 43.5mm =~44mm = 4.4cm
Radius (r)= Diameter/2 = 4.4cm/2 = 2.2 cm
Assuming Caravel's basket is cylinder-shaped throughout:
Area: Pi x (r)^2 = 3.142 x 2.2 cm x 2.2 cm = 15.21 cm^2
Let's say 9 bar is the maximum brew pressure we ever need. So we can calculate it with 10 bar of static pressure(since espresso is a dynamic/flowing pressure, not static):
1 bar = 1.02 kg/cm^2
10 bar = 10 x 1.02 kg/cm^2 = 10.2 kg/cm^2
10.2kg/1cm^2 = x kg/15.21cm^2 (x is the pull pressure we're looking for)
By arranging the equation above:
x kg = 10.2kg/1cm^2 x 15.21 cm^2 = 155.2 kg
Mechanical Ratio = 3.5cm : 40cm (I measured a full travel of piston to be about 3.5cm and lever about 40cm)
Mechanical advantage: 11.5
Pull Pressure needed: 155.2/11.5 =13.5 kg = ~30lb
Theoretically, we only need about 30lb max pull pressure to get 9 bar brew pressure. I am being conservative by rounding up some of the numbers. So it's pretty safe to conclude that to get a good espresso extraction, there's no need to exceed 30lb pull pressure.
30lb is as far as we go and most levers are brewing well below 9 bar.
Let me know if the numbers or the calculation needs correction.
p/s: Using the similar concept, preinfusion of 1.5 bar is about 2kg(4.4lb) pull pressure and 7 bars about 10kg(22lb).