How do the Salinity Sensors measure soil salinity?
The sensors measure EC in dS/m directly. The measurement process is as follows: There is an AC impedance bridge in the sensor interface which necessarily generates a capacitance signal, this is cancelled out to give a resistance value. Then this resistance value has the sensor calibration applied to it for the individual sensor and the value in dS/m is stored and displayed. It is not possible to measure the salinity of the soil accurately using Direct Current (DC) resistance. If you apply a direct DC voltage you would polarise the chemicals in the soil solution and an incorrect reading would result. In the process of connecting an ohm meter you would see a drift in the DC resistance as you are making the measurement. It’s for this reason that an AC impedance bridge is used and then the capacitance correction is made to get the true resistance value. This is the reason these sensors cannot be connected to a common, analogue data logger.
