physics lab has a Cartesian Diver on the shelf to demonstrate the Ideal
Gas Law and Archimedes Principle. The Cartesian Diver is assembled from
an inverted glass cup, attached to a counterweight. Dropped into a tall
container of liquid, the glass traps air within the diver. By adjusting
the mass of the counterweight, so that the average density of the weight,
glass and trapped air matches that of the surrounding liquid, the diver
is made neutrally buoyant. Then, if the pressure outside the container
changes (e.g. rising barometric pressure, or by closing the top of the
container with a rubber sheet and pushing on the sheet), the pressure
on the liquid follows in lock-step, and is communicated through the liquid
to the air within the diver. So, if the air outside the container increases
in pressure, then the pressure within the diver increases.
on air, and it compresses and increases in pressure. It will keep compressing
until the pressure within the diver is the same as the pressure in the
liquid just outside the diver. Water replaces the space previously occupied
by the uncompressed air, making the average density of the diver higher
than the surrounding liquid. The diver then sinks to the bottom of the
container. Conversely, reducing the air pressure above the liquid causes
the diver to rise. By pushing and pulling on the rubber sheet, the diver
bobs down and up the height of the container.
The Solar Cartesian Diver is sealed off from outside air
pressure changes, and relies on temperature changes within the diver to
create pressure changes. A thin black metal plate is mounted inside the
diver. Light, from the sun or a lamp, goes through the liquid and clear
diver wall, until it is absorbed on the plate. Sunlight can easily raise
the internal temperature of the diver by 25 to 30 C. This resulting temperature
increase, increases the pressure in the diver proportionately (by the
Ideal Gas Law), forcing some of the water out of the diver. More air volume
means a lower density, and once the density drops below that of the surrounding
liquid, the diver rises.
the diver bobs to the surface, where it is hidden by a light shield. The
diver cools, the air inside the diver contracts in volume, and it once
again sinks to the bottom. And so the cycle repeats. Silently, smoothly,
practice, mineral oil is a convenient liquid. Mineral oil absorbs 10 times
less solar energy than water, which prevents thermal gradients and temperature
increases in the liquid which might cause the diver to bob uselessly at
the surface (see simulation for more
detail). Also, while a glass diver is practical in a laboratory environment,
it cannot be shipped as a toy since the air will leak out of the diver
when the package is inverted. Thus, a manufacturable device consists of
a sealed clear plastic bag, surrounding a black absorbing sheet. One open
issue with this design is longevity- if the plastic absorbs oil it will
increase in density, or if air diffuses out from the bag, it will decrease
in volume. Careful selection of the diver plastic is critical.
if the diver is designed for solar operation, the side of the diver facing
the light needs is preferably a flat plastic window. Otherwise, total
internal reflection will prevent light from entering the diver except
from directly perpendicular to the column.
you are interested in constructing a Solar Cartesian Diver, the simulation
will give you a feel for the challenge. I have never successfully prototyped
a diver using solar energy and an unsealed column- however, with a strong
(50 watt halogen), broad area light, it is possible for the diver to run
continuously in an open column.
see a movie of an actual Solar Diver in action (900 Kbytes, MP4 format),
experiment with a Java simulator embodying the diver physics, click HERE.
build a conventional Cartesian Diver from a soda bottle, try this SITE.