Capacitive Detection of Very Small Aquatic Animals

Abstract

The detection and recording of aquatic organisms was reported for animals of size about 7 mm by means of a thermistor and for bacteria, a few microns in size, by means of the Coulter counter which relies upon a salt solution of the bacteria to detect changes in resistance as they pass through a very small orifice. However, the detection of free moving organisms between these 2 size ranges, particularly between 100 and 1000 [mu], without disturbing their environment in such a way as to modify their behavior, necessitates another approach. Resistance measurements in fresh water subject the organisms to unacceptably large electric fields and the somewhat uncertain constitution of the aquatic medium makes repeatable measurements difficult. Preliminary measurements indicated that for an orifice diameter of about 1 mm and organisms 500 to 800 [mu] in size, capacitance variations of a least 0.01 pF could be expected. In view of this, an apparatus with a maximum sensitivity of about 0.001 pF and a correspondingly smaller orifice would suffice to detect 100 m organisms. The capacitance fluctuations are converted to a voltage output by a sensing oscillator followed by a digital discriminator.