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An Oscillator Circuit for Automated Salinity Sensor Measurements¹

ABSTRACT

An oscillator circuit is described that enables automatic reading of salinity sensors and other a.c. conductivity cells by measuring frequency of oscillation. The oscillator frequency varies between 0.13 to 30 KHz and changes linearly with conductance in the range of 0.03 to 10 mmho/cm. The temperature sensitivity of the frequency of the oscillator is negligible between 8 and 31C. The calibration curves between frequency and conductance obtained with the oscillator circuit for both the electrical conductivity and thermistor elements of the salinity sensor are linear and readings can be obtained with lead lengths of several hundred meters.

¹ Contribution from the US Salinity Laboratory, Soil and Water Conservation Res. Div., ARS, USDA, Riverside. California.

² Electronics Technician and Soil Scientist, respectively. US Salinity Laboratory.

Received for publication July 31, 1972. Accepted for publication September 26, 1972.