TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Dimensions: The membrane has a thickness of 0.02-0.5 mm with a porosity of 20-60% and pore sizes of 0.6-10 mum. Preferred Components: The membrane is surface-treated on the non-liquid exposed side to render it capable of being bonded to the body of the probe. The membrane is bonded to the body of the probe. The sensor has a tubular shaped body. The water vapor sensitive component produces a change in electrical resistance, impedance, capacitance, resonance frequency, or digital encoded signal. The temperature sensor produces a change in electrical resistance, impedance, capacitance, voltage, or digital encoded signal. A barrier is placed over the membrane and in contact with the aqueous liquid, providing a mechanism of physical protection for the membrane while enabling passage of liquid from the bulk of the liquid to the membrane surface. The mechanism for measuring the electrical response of the sensors comprises an electrical circuit for processing the signal from both a water vapor sensitive component and a temperature sensor and provides an electrical output, e.g. voltage, digital encoded signal or frequency, which can be used to measure water activity/concentration. The mechanism for deducing the water activity/concentration of the solution in contact with the probe includes electronic mechanism for storing water concentration isotherm curves associated with the solutes present in the solution. It includes a microprocessor component that uses the signals from vapor and temperature sensors as an electrical input and provides an electrical output which can be used to display a quantitative value of the water activity/concentration of the aqueous solution in which it is immersed.    TECHNOLOGY FOCUS - POLYMERS - Preferred Materials: The membrane is manufactured from a composite mixture of polytetrafluoroethylene and a polyolefin, e.g. polypropylene or polyethylene. The sensor may be constructed from chemically and thermally resistant material including polyethylene, polypropylene, polycarbonate, or polyvinylchloride. The cavity is lined with a non-water vapor-absorbing, non-water vapor-permeable or non-water vapor-releasing material, e.g. fluoropolymer.    TECHNOLOGY FOCUS - CERAMICS AND GLASS - Preferred Materials: The sensor may be constructed from ceramic or glass.