Images of the experiments were recorded using a digital CCD
camera at 2 frames per second. The frame rate was chosen to
“freeze” the motion of the tube, like when using a strobe light to
measure the rotation speed of a motor. Filming against the light
of a halogen 450W lamp, allowed us to obtain grayscale images
where the granular material could be easily isolated using standard
binarization algorithms. Then, the average height of the granular
material in each branch of the tube was measured to get h as a
function of time.