A non-contact planar microwave sensor for detection of high-salinity water containing NaCl, KCl, CaCl2, MgCl2 and Na2CO3

•Non-contact detection.•Simple operational principle.•Non-destructive and non-invasive.•Real time for the determination of saltwater concentration. In this paper, the concentrations of chloride salts (sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl2), calcium chloride (CaCl2)) and a carbonate salt (sodium carbonate (Na2CO3)) were measured using a non-contact planar microwave sensor. The design of the sensor was based on a combination of a microstrip line and a split ring resonator (SRR) with a circular hole in the middle of the SRR. The sensor was fabricated using dry film photoresist lithography technique and its performance was validated through simulation and experiment. Chloride and carbonate salt solutions at concentrations from 40 mg/mL – 200 mg/mL were measured in the frequency range 0.5 GHz – 2.2 GHz with four resonance frequencies applied in sensing. The various salt concentrations were measured under ambient conditions (room temperature (T): 25 ± 1 ๐C and relative humidity (RH): 40 ± 1 %). The reflection coefficient (S11) and the resonance frequency (Fr) were recorded and analyzed by logarithmic regression and principal component analysis (PCA) methods to find correlations between salt concentration and classification group. The sensor was able to detect and classify water containing NaCl, KCl, CaCl2, MgCl2 and Na2CO3. The planar microwave sensor is suitable to be used in non-contact detection applications of inorganics.