Zooplankton measurements using a conductance cell

The fact that plankton are near insulators relative to the ocean allows them to be counted and their volume and volume-to-length ratio determined by the use of a tubular conductance cell with ring electrodes spaced along it. The system described employs DC current in a tubular cell with three equally spaced potential electrodes, an I.D. of 3 to 5 mm. and a measuring zone length of 1 to 2 cm. Displacement of the potential of the central electrode relative to the other two by a plankton gives a characteristic shaped signal which can be distinguished from all spurious ones, and which contains information on size and length of the plankton and on towing speed. This information is transmitted to the deck readout system by a combination of time interval and digital coding.