A simple method for controlling the fluid level in a small experimental chamber during slow and rapid fluid exchange.

A simple and cheap method is devised for controlling the fluid level in an open chamber for physiological or other experiments. Fluid is pumped into the chamber at variable flow rate using a conventional pump. Autoregulated drainage is achieved through a capillary tube whose bevelled opening is covered with a net with fine meshes. Part of the opening dips into the fluid surface. The correct amount of fluid is sucked out of the chamber by a constant negative pressure at the back of the tube. The 1 mm depth of a 2 microliters volume stays within +/- 2% when the normal flow of 1 microliters/s is doubled and halved, respectively. A 100-fold increase of flow, as needed for flushing the chamber, causes a rise of the surface level by 0.5 mm only.