A small "In situ" conductivity instrument

An instrument for measuring the conductivity (specific conductance) of seawater in situ is described. Basically the instrument consists of a three-terminal, platinum electrode flow-through cell which acts as the variable resistance in a Wien bridge oscillator. The oscillator section is well suited to drive the cell, since the ac signal amplitude is so small that polarization effects are minimized. The results of field and laboratory tests are given. Noise level and resolution are about 0.0005 mmho/cm. Short-term absolute accuracy is on the order of
\pm 0.01
mmho/cm over a conductivity range of 26 to 36 mmho/cm. Data on long-term stability, although not known at this time, will become available with further testing and field use during the coming years. The instrument weighs 26 ounces in air and 12 ounces in water.