Detecting phase transitions in a CaCl₂-H₂O system at low temperatures using a fiber-optic Fresnel reflection sensor.

Temperature-induced crystallization events in an aqueous calcium chloride solution in the concentration range of 15-40 mass% are monitored using an optical fiber Fresnel reflection sensor in the temperature range of 30°C to -200°C. The deviation of the phase boundary from equilibrium and the formation of an eutectic mixture followed by its densification during rapid cooling are inferred from the distinct signatures of the optical fiber sensor via the changes in refractive index. During the natural heating at laboratory ambient conditions, the optical signals impart the completion of dissolution of ice and CaCl₂·6H₂O. The corresponding temperatures have been used in Linke's equations to obtain the salinities, which are in good agreement with the intended solution concentrations. The sensor signal imparts simultaneous melting of the constituents of the eutectic mixture of a 29.7 mass% solution during the natural heating phase. The persistence of the metastable liquid phase at -200°C for tens of minutes followed by solidification is observed at all the concentrations studied. Finally, the feasibility of monitoring phase transitions in a NaCl-CaCl₂-H₂O system has been demonstrated.