If the difference in density is great enough, fluid motion results. It is important that the lower boundary have a much larger thermal conductivity coefficient than the medium, so that the boundary maintains a uniform temperature. The upper boundary is made of sapphire, used because it has a high thermal conductivity and is transparent. A constant flow of cool water is maintained over the sapphire boundary to ensure that there are no horizontal temperature gradients along the upper boundary and that convection is generated from vertical temperature gradients alone. The reason for using a single crystal as the upper boundary, and for replacing the typical bead setup with a more regular design, is the measurement technique for determining the convection pattern.
Laser light is used for pattern visualization.
If there is convective heat flow within the material that is not parallel to the angle of incidence of the laser, the light will be diffracted. Therefore, as light travels through the porous medium, it will constantly be reoriented according to the magnitude of the density gradient of the fluid. The light reflects off the lower metal boundary, passes back through the medium, and reflects on a photosensitive plate, which calculates the total deflection of the incident ray. The intensity of the returning laser light is used to determine the convection pattern in the porous medium. When beads are used within the porous medium, laser light is refracted considerably by the walls of the marbles, creating much difficulty in analyzing the total deflection of laser beams as a function of convective heat flow.