Salt Content Detection Using a Microwave Sensor

A microwave interdigital-electrode (IDE) sensor is used to detect the salt content in saline solutions. As there is electrode polarization when measuring a liquid that contains lots of ions, and considering its dispersion property at microwaves, frequency dependent complex conductivity is investigated in the present study. A reasonable equivalent circuit of the liquid loaded IDE sensor is provided, which is further used to deduce the characterization theory. The theory demonstrates that the measured scattering (S-)parameters can be accurately translated to the complex conductivity of the liquid under detection. The characterization theory is validated with finite-element simulation method. The measurements on some saline solutions demonstrate that the IDE method has the capability of detecting the salt content in liquids in a non-invasive and contactless way. This technique is targeted for a fast pollution detection or characterization of sea water.