Development and validation of an open-source four-pole electrical conductivity, temperature, depth sensor for in situ...

Most recent implementations of low-cost electrical conductivity (EC) sensors intended for water quality measurements are based on simple two-pole designs. However, in marine settings, EC often exceeds the range where two-pole sensors provide reliable results. We have developed a simple four-pole EC sensor that relies exclusively on analog-to-digital measurements made using readily available circuit boards (pyboard v.1.1 or Raspberry Pi Pico 2040) programmed using MicroPython. Other than resistors and graphite or wire electrodes, no other electronic components are required for the EC sensor. When combined with a pressure/temperature sensor (MS5803-05), an optional NTC thermistor, batteries, and a waterproof housing constructed using a PVC pipe and a 3-D-printed cap, the device becomes a working conductivity-temperature-depth sensor capable of extended field deployments. Construction is sufficiently simple that undergraduate science students can construct one during three 3-h lab periods. Lab calibrations performed on several prototypes at ECs between 0.18 and 45 mS/cm show that confidence limits as good as about ±3% of EC are possible. Re-calibration of several prototypes 1 year after initial calibration shows that long-term calibration drift is modest. Data collected by the prototypes over several tidal cycles in the Duwamish River, Washington, USA, are in agreement with data from a co-located commercial YSI-EX03 conductivity probe. When distributed across a constructed off-channel wetland in the Duwamish system, the sensors documented large amounts of spatial and temporal variability in EC, highlighting the importance of such wetlands for providing unique temperature/salinity environments potentially valuable for outmigrating juvenile salmon.