Sensors for monitoring the salinity of water are useful tools in both environmental sciences and chemical engineering. Sensors based on a large variety of principles have been developed. Here we describe the design and testing of two different, noninvasive measurement methods for monitoring the salt concentration of water. The first method is based measurements of the refractive index using a folded-path optical refractometer, while the second utilizes an electromagnetic resonator, consisting of a magnetic coil and a capacitor, which is brought to resonance by an external magnetic field source. It is demonstrated that the folded-path optical refractometer allows one to monitor changes in salt concentration down to 1 mM in the range between 0 and 0.7 M, whereas the electromagnetic resonator has the capability of monitoring concentration changes as small as 0.3 mM over the more limited range between 0 and 5 mM.
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