Hi all,

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).

Damn! That's why I'm on this forum. I was wondering and there it is.

Thanks, Sam!

It's great to have the theory of it all. I have often wondered what the extraction pressure has been for the magic shots that my Creminas and Caravel make.

I don't think that I press the lever down with anywhere near 30lbs of force with a domestic lever machine. I have a feeling that the optimum extraction pressure for one of these little guys is less than the 9 bar that we have all been told is the Holy Grail for espresso. I say this as I have a Rossa manual device that extracts pure gold with a peak of around 6.5bar.

Maybe the key to non-spring lever magic is the pressure profiling that you can do with varying amounts of force on the lever.

thanks for doing the maths on this one- v interesting. I have made good shots putting a great deal of pressure on the Caravel Lever- to the point of worrying a little for the machine... But I have found that the best shots of all were easier to pull- the same with La Pavoni. These machines do not need to be stressed.

Thanks Sam,
I did a similar calculation a while back for the Pavoni and came up with about the same result. I think it's useful to have a ballpark idea of how hard a 'plenty hard' pull is, and with this calc and a bathroom scale under the machine one can do that.

I've never used a Caravel and have always been curious about how that design deals with the pocket of air above the puck after you raise the lever up. Is the pull very spongy feeling? Is the cylinder somehow vented so that the air escapes from the top of the cylinder as the unpressurized water flows beneath it? I know on the Pavoni the air is trapped, and the theory goes that it's forced out through the puck because the initial superheated water vaporizes and drives it out. Of course, you don't have that on a Caravel.

(BTW, Sam, I'm really loving your old ECM Giotto - is running strong and getting lots of use.)

Pat

I always calculate the force for the spring lever machines using the piston chamber diameter, isn't that right?

That is correct. It is the cylinder diameter that defines the area generating pressure.

Interesting. I thought of the cylinder, all the way down to the coffee puck, as a whole and closed system. Since it's a closed (sorta, easier to think this way) system, the pressure throughout the system will, or at least try to, reach equalization. So, if the pressure at top(cylinder) is higher than the bottom, the excess in pressure will be dispersed immediately into the bottom(basket) to reach equilibrium before allowing the pressure on top to rise(All because the basket diameter is larger than the cylinder chamber). Thus the limiting factor of the pressure will be due to the weakest point (or the widest diameter) in the system, which is at the basket. And what matters eventually is the pressure at the basket, which is dictated by the basket's diameter. That's my logic and it is possible it may be flawed. That's why the forum is great for these discussion.

And the whole math exercise was to search for the top limit of needed pull pressure, it's another reason to err on the high side by choosing basket's diameter.

homeburrero wrote: I've never used a Caravel and have always been curious about how that design deals with the pocket of air above the puck after you raise the lever up. Is the pull very spongy feeling? Is the cylinder somehow vented so that the air escapes from the top of the cylinder as the unpressurized water flows beneath it? I know on the Pavoni the air is trapped, and the theory goes that it's forced out through the puck because the initial superheated water vaporizes and drives it out. Of course, you don't have that on a Caravel.

(BTW, Sam, I'm really loving your old ECM Giotto - is running strong and getting lots of use.)

Pat

Hi Pat! Great to hear that the machine is getting a lot of love! The ECM was one of my most beloved baby when it was in my possesion. It's the single one machine that gives the most dense chocolatey & full-bodied shots. Really glad it got the right owner that deserves it.

As for your question about the air pocket, in the most of the Caravels, there is a small hole with O-ring at the bottom of the piston(so yeah, it's just what you've guessed). The water fills in the cylinder chamber with the piston above the water inlet. So I suspect most air would be eliminated through the hole when the chamber is filled with water.



Image from Why does my Caravel o-ring keep breaking?

Thanks Sam,
That picture explains a lot. In addition to the hole in the piston I see that these have very widely spaced seals (and I think gigantic inlet ports.) Makes it pretty clear that gravity fed designs are radically different.

P.S.
I agree with others that to calculate force at the lever you want to use the cylinder area, not the basket. One way to think about this is that the pressure on the inside surfaces of the brew chamber is the same everywhere - at 9 bar, you will have appx 10 kgf/cm2 everywhere. The diameter of the basket can be used to calculate the downward force on the basket and the diameter of the cylinder (the piston seal) can be used to calculate the upward force on the piston, and that's the force that is opposed by the lever.

homeburrero wrote:I agree with others that to calculate force at the lever you want to use the cylinder area, not the basket. One way to think about this is that the pressure on the inside surfaces of the brew chamber is the same everywhere - at 9 bar, you will have appx 10 kgf/cm2 everywhere. The diameter of the basket can be used to calculate the downward force on the basket and the diameter of the cylinder (the piston seal) can be used to calculate the upward force on the piston, and that's the force that is opposed by the lever.

I went back to refresh what I'd learned in high school and it seems you guys are probably right on the calculation by using the cylinder chamber. But one thing I don't get is: if it really works that way, why basket size is such an important consideration when designing a lever? We could've just designed a 58mm basket lever with a really narrow cylinder chamber. Something is not so right there..