TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Mixture. The fluid mixture can be a liquid or a gas mixture.Preferred Process: The dielectric response of the test cell is measured in terms of the dielectric constant of the fluid mixture in the cell.The dielectric response of the fluid mixture is tested in a linear and monotonic relation to the ratio of methanol to water in the mixture at a given temperature of the fluid.The electrodes of the test cell are totally immersed in the liquid mixture.The process further includes providing a reference cell established by opposed electrodes having operational characteristics equivalent to those of the electrodes of the test cell and containing a fluid mixture having a known ratio of methanol to water; maintaining the fluid mixture of the reference cell at the same temperature as the fluid mixture being tested; measuring the dielectric response of the reference cell; and comparing the dielectric response measured in the reference cell to that measured in the test cell so as to determine the ratio of the methanol to water in the fluid mixture in the test cell.One of the opposed electrodes of the test cell and one of the opposed electrodes of the reference cell are respectively connected to opposite poles of a source of the alternating voltage and the other electrodes of the opposed electrodes of each of the cell are connected so that the electrodes of the opposed electrodes of each of the cells are connected so that the electrodes of the opposed electrodes of each of the cells are connected so that the electrodes of the cells are in series.The impedance of the test cell is determined in relation to that of the reference cell at a given voltage and frequency of the voltage source so as to compare the respective dielectric responses of the cells.The voltage drop across the electrodes of the test cell is determined in relation to the voltage drop across the electrodes of the reference cell so as to determine the respective impedances of the cells.The capacitive components of the impedances being correlated with the dielectric constants of the fluid mixture are tested and that of the reference cell.The process also includes providing a reference element of given impedance which is equivalent to that of a reference cell having equivalent operating characteristics to those of the test cell.The reference cell having a fluid mixture of methanol and water of a known ratio and of known temperature, is operated at the same frequency as that of the test cell.The process also includes measuring the temperature of the fluid mixture being tested, measuring the impedance of the fluid mixture in the test cell, comparing the impedance of the test cell to that of the reference element so as to determine the capacitance of the test cell, and determining the ratio of methanol to water in the fluid mixture of the test cell on the basis of the capacitance and temperature of the mixture in the test cell.The fluid mixture being tested is supplied to a reformer reactor for producing hydrogen.The dielectric response of the fluid mixture is measured continuously.Preferred Components: The electrical circuit acts as a capacitance divider circuit to determine the capacitances of the respective cells.The sensor further includes compartment walls defining a compartment for the test cell.The compartment walls include an opening communicating with the interior of an enclosure (7) containing the fluid is tested.The walls further define an adjacent compartment for the reference cell, i.e., a compartment not having an opening communicating with the interior of the enclosure containing the fluid is tested, thus there is a common wall separating the compartment.The common wall is provided by a common electrode for both compartments, each of the opposed electrodes of the test cell and reference cell, respectively, including the common electrode, and the other electrode of each of the pair of opposed electrodes defining the reference cell having an exposed outer surface in contact with the fluid in the enclosure.The exposed surface provides heat exchange with the fluid mixture being tested. The opposed electrodes are parallel plates or coaxial cylinders.