Coffee bulb physics?

A few weeks ago some friends near my school invited me to lunch. I went. I ate. I saw. Specifically, I saw another of the nifty glass-and-metal coffee makers I discovered when I came to Taiwan. The contraption, if memory serves, consists of a lower bulb (mini-coffee pot) for boiling water; a metal base Bunsen burner for the boiler bulb to rest on, with a small upright metal pole for holding up a metal hoop (about mini-coffee-pot high); a long glass vase with a tapered bottom (to rest in the metal hoop), joined to a glass tube by a rubber stopper; and, finally, a metal ring wrapped in nylon to filter the coffee grounds as the coffee percolates into the bulb below. The nozzle of the tube sticks into the boiler bulb.

Set the water to boil for a few minutes and it suddenly "slurps" its way up into the vase, at which point you drop in the coffee grounds. You must make sure the ring filter is secure at the base of the vase. Once you're satisfied the grounds have boiled enough, kill the Bunsen flame and the coffee drains into the bulb for serving, leaving the grounds on the nylon filter in the vase. Elegant and efficient.

What really caught my eye were two things. First, I noted how quickly and suddenly the water moved up from the bulb into the vase, and, second, how if the flame were lowered or removed for even a few moments, the coffee would slide back down into the bulb. (As long as the grounds were on the vase-side of the filter, though, you could just re-apply the heat, and the fluid would re-enter the vase.)

This whole operation -- not to mention the outrageous buzz from a laarrge midday coffee -- had me transfixed. "How did it work?" I asked, my teeth chattering on the mug. Which got me to wondering, and then to blogging...

INSCITIA:

How does this sort of coffee maker work?

COGITATIO:

My brain staggered back almost ten years to Mr. Ritter's senior physics class. Temperature... volume... pressure... Boyle's Law! Boyle's law, as far as I understand it, describes the directly proportional relationship between pressure (p), volume (V), for an ideal gas (I dream of gases!) at a constant temperature (C), to wit, p*V = C. In other words, for example, as pressure increases in a uniformly hot volume of gas, the gas's volume increases (think of a plastic bottle swelling in the sun and then hissing when you open it). Conversely, as volume increases in a constant temperature gas environment, the gas's pressure increases (think of a water balloon filling to bursting).

The interesting thing about Boyle's law, though, is that theoretically (or, operationally speaking), C need not be the constant. Any of the equation's elements could be posited as constant, though this would change some proportions from direct to inverse. To wit, at constant pressure, volume is inversely proportional to volume (p = C/V), meaning, the higher the volume, the lower the temperature. And so forth for other scenarios and operations.

I realize now that the coffee maker not only has no constant temperature as such, but also concerns fluids, not agses, so Boyle's primary law is not technically apposite in this case. But the basic idea of the law got me thinking of an explanation: in the bulb, as the water, which has a constant volume (ignoring evaporation and small ejected droplets), gets hotter, its pressure will increase. Since there is only one way out of the increasingly pressurized bulb (namely, up, up and away!), the boiling water will shoot up the tube to a lower volume. (The vase has no flame in it or on it to pressurize its contents.) As the fluid cools in the vase away from the flame, its pressure "gives in" to the air pressure in the vase, and it tries to sink back into the bulb. The constant Bunsen flame, though, prevents it from doing so, since that high temperature always "kicks" sinking fluid back to the lower pressure vase-system above.

I believe the mechanism is that heat energy raises the water's pressure in the bulb higher than the combined force of gravity and air pressure in the vase. This greater thermal, kinetic energy, according to Boyle's law, will propel the water up. Once the temperature decreases, however, the fluid succumbs to air pressure and gravity, and finds itself back in the bulb.

Voila, ou no?

RESPONSUM: