NOVELTY - A water activity/concentration measuring sensor comprises a probe with a body defining a cavity; a membrane able to permit water vapor transmission while blocking the passage of aqueous liquids, wherein the membrane is located between the cavity and the liquid; a water vapor sensing component adjacent the membrane and within the cavity; and a temperature sensor located within probe body in close proximity or connected to the water vapor sensor.    USE - For measuring water activity/concentration of aqueous solutions containing non-volatile solute(s) or for measuring water activity/concentration in the cells of a lead-acid battery (claimed).    ADVANTAGE - The invention provides a water concentration sensor that has rapid response characteristics and that is stable in operation and simple to operate. The water concentration sensor is able to continuously and automatically measure water concentration in a range of aqueous solutions. The invention provides a sensor that is able to measure water concentration in a lead acid battery, particularly as charging or discharging takes place.    DETAILED DESCRIPTION - A water activity/concentration measuring sensor comprises a probe, with a body defining a cavity, which can be partially immersed in an aqueous solution containing dissolved non-volatile species; a membrane able to permit water vapor transmission while blocking the passage of aqueous liquids, wherein the membrane is located between the cavity of the probe and the liquid whose water activity/concentration is to be determined; a water vapor sensing component adjacent the membrane and located within the cavity of the body of the probe; a temperature sensor located within the body of the probe in close proximity or connected to the water vapor sensor; and mechanism for measuring the electrical response of the sensing components and for deducing the water activity/concentration of the solution in contact with the probe.    INDEPENDENT CLAIMS are also included for:    (1) an installation for measuring water activity/concentration of aqueous solutions containing non-volatile solute(s) comprising water activity/concentration measuring sensor(s);    (2) an installation for measuring water activity/concentration in the cells of a lead-acid battery comprising sensor(s) which may be indicative of the state-of-charge of the lead-acid battery;    (3) methods of computation that may be stored in ROM or RAM associated with the microprocessor allowing the electrical output to be converted to specific gravity; and    (4) a visual display device electrically or without wires connected to the microprocessor.    DESCRIPTION OF DRAWING(S) - The figure shows an overall axial section with exploded perspective through the sensor of the invention.