Diver directions:

      For a simple device, the physics behind the diver can be quite convoluted. Roughly speaking, the diver density should be matched to the liquid- in other words, neutrally buoyant so a slight change in volume causes it to float or sink. The liquid has a density of 1 in the simulation- thus the mass/volume should also be about 1. Or slightly greater, so it sinks until hit by the light.

     However, if the diver is in a viscous liquid (with a small terminal velocity), it will rise very slowly, until the top of the diver is just hidden by the shield. This part of the diver cools, and quickly reaches an equilibrium where the heat input is matched by the heat output, until the net diver density matches the liquid. Where upon the diver stalls, partly obscured by the shield. At lower viscosities, the diver kinetic motion drives it past the shield, and stalling is avoided.

     Similarly, if heat transfer to the liquid is fast, then the part of the diver hidden by the shield will cool too quickly, and again stall. And, of course, more light will be needed to raise the temperature of the diver, if heat transfer is rapid. In the real diver, heat transfer is controlled by the thickness and material of the diver wall.

     The diver column is sealed from outside air pressure. The diver is so sensitive (particularly if designed for solar illumination) that barometric air pressure changes could prevent its operation. More importantly, even though the liquid is chosen to be very transparent, the liquid does rise above room temperature. The hotter liquid causes the air within the diver to expand, and unless compensated, causes the diver to bob to the surface. Fortunately, the rising temperature cause the liquid to expand, reducing its density. So the diver tends to sink. And, since the liquid now takes up more space in the container, it squeezes on the air above the liquid, causing the pressure to increase in the column AND in the diver. Again tending to increase diver density. (The simulation actually models a temperature gradient from the light to the top of the column, but the basic idea is the same).

     Column height also matters. As the diver dives, the air within is compressed. This compression must eventually be overcome by a pressure increase due to a temperature rise within the diver. But, the lamp is only so bright; the maximum temperature rise is limited (in the real diver, to about 30 degrees Celsius), so the depth is limited as well.

     See if you can uncover these and other effects within the simulator. The sliders and text boxes can be changed as the simulator runs. The yellow oval is the light source, and the short vertical line the shield. The rapidly varying numbers next to the diver and at the top of the column is the temperature (roughly in Kelvin) inside the diver, and in the air above the liquid. There is a narrow operating range for the diver- sometimes it will work for a while, and then stop. Good Luck!

Paul Falstad was kind enough to convert my iBasic program to Java for posting on the web.