On the Design of Low-Cost Inductive Conductivity Sensors for Salinity Measurement in Oceangoing IoT Applications

In this paper, a low-cost transformer type inductive conductivity sensor for the use of in situ measurement of seawater on oceangoing sensor buoys is designed, developed, and tested. The sensor design uses DAC and ADC peripherals of a SAME51 microcontroller. A single PCB holds the MCU, the coils of the transformer type sensor, and circuits for measurement and power supply. The sensor was tested in an ionic solution of increasing conductivity in the range of seawater. The processed readouts from the MCUs ADC sampling at 40 kHz are compared to those sampled by an external oscilloscope of 40 kHz sampling rate, connected to the ADCs inputs. The processed ADC data show a significantly higher sensor gain and sensor gain slope compared to the oscilloscope data. The influence of high-frequency noise in combination with the lower sampling rate is considered a possible cause. The measurement results show that a revision of the amplifier circuit, the target offset voltage, and the amplifier loop gain is necessary.