ira wrote:How does the air reduce the pressure? What's the difference between air at 9 bar and water at 9 bar?

If the piston column has an infinity length that wouldn't be a problem. The problem is it takes a lot more piston travel distance to generate the 9 bar from air.

When the column is filled with incompressible fluid, the piston only has to travel minimal distance (say 0.1cm) to fully translate that force into 9 bar. However, when it's full of compressible air, your piston now may have to travel 15cm to compress the air into 9 bar. The column itself may only be 10cm long and that's when (too much) air can be a problem.

Air also creates a disconnected feeling/lag from the moment of applying force to the generated pressure.

ira wrote:How does the air reduce the pressure? What's the difference between air at 9 bar and water at 9 bar?

Ira

My understanding is that it's the force that is absorbed by air and released over time. Water will transmit force pretty much immediately.
We are not directly creating a system of 9 bar at a snapshot in time, we are using movement to create dynamic pressure and other factors count into that, hence we have engineers!

aecletec wrote: hence we have engineers!

Yes we do, I are one. Air might effect the ability to hold 9 bar to the end and or at least cause a declining pressure profile, possibly a very fast declining profile and it might slightly change how fast you reach 9 bar, but if you press with the same weight as with no air, those are the only effects it can have.

Ira

ira wrote:Yes we do, I are one.

Excellent!
I'm not arguing with this point, but perhaps you can help explain the problem Sam's raises about piston travel... how far does the piston need to travel to compress to 9 bar?
Then, what's happening at the puck considering flow varies depending on pressure (Illy notes 7 bar is slower than 5 bar) do we run out of piston movement before 9 bar is reached? It seems like the problem is not so simple.

You'll need a considerably longer piston. That is all.

Due to the ideal gas law. PV=nRT. This isn't adiabatic system, so air in the piston will release and absorb heat from the environment ands its volume will change to compensate for you attempt to squeeze it.

Pushing more and more and more eventually will get you to 9 bar. Obviously.

That is why as drivers we don't like air in our hydraulic brakes. You get a squishy feeling and even more annoying is not being able to stop (need a longer piston in the brake paddle).

Definitely don't see this thing making much better coffee than the Minipresso does, and is probably a farcry from espresso. But I'm sure it still makes a tasty cup that's great for when you're traveling or camping.

AssafL wrote:You'll need a considerably longer piston.

This is what is raising questions about the constraints of the device, right? "Is it long enough" is the question I interpret from Sam.

It was originally intended to answer Ira's question how air can reduce pressure.

I am afraid the discussion is going too far/technical..but the bottomline is the piston cylinder should be long enough to answer that question.

I think many other readers would be more interested how the Compresso tastes instead.... And similar to above, I think it will be more similar to the Minipresso than a pump espresso machine (they're also in the same price range). Minipresso does actually brew at a much higher pressure from my understanding but with a different pressurizing method.


Here's my quick crude calculation in excel if anyone's interested (if there's a major mistake let me know).



p/s: Also counterintuitively to me, I've played around with the numbers...the air doesn't seem to affect THAT much (just a little)...so it's probably a false assumption to begin with Air does affect max pressure on spring lever, because of how spring works (weaker as it extends) but probably not so much on manual levers.

Nice work Sam!
So... our hive mind has gone from "it won't work" to "it'll probably work"?

Only if you ignore the video's instructions to "gently press piston."