Osmolarity sensor

An osmolarity sensor continuously senses an osmolarity of a fluid. A principle of operation is based on achieving an equilibrium of a vapor pressure of a specimen solution with a vapor pressure of a standard solution. In an embodiment, the standard solution is contained in a cavity defined by a vapor-permeable membrane which is immersed in a specimen solution. The standard solution and the specimen solution are allowed to come into equilibrium by a movement of water vapor across the vapor-permeable membrane, which is permeable only to water vapor and not to any solutes or water itself. The movement of the water vapor between the standard solution and the specimen is measured by sensing a volume of water transferred between the standard solution and the specimen solution. A measurement of a volume change of the standard solution is made by sensing a change a conductivity of the standard solution. Alternatively, the volume change is determined by measuring a change in a photo-absorbance of the standard solution with a chromophore added therein. In yet another embodiment the volume change is determined by a direct visual measurement of the volume of the standard solution.