Change in surface conductance caused by a sudden change in humidity

As part of a programme aimed at the development of a sensor with a rapid response to changes in relative humidity the changes in capacitance and surface conductance fellowing a sudden change in ambient humidity have been recorded for specimens of non-porous aluminium oxide. As the equilibrium water uptake and the consequent changes in capacitance and conductance depend on the relative water vapour pressure and are independent of the total gas pressure, the measurements were made by subjecting the specimens to increasing and decreasing step changes in water vapour pressure in a vacuum system. Capacitive and conductive currents were separated using phase sensitive detection. The resulting responses were not simple exponentials. They suggest that the overall response is determined by the time required for the formation of the adsorbed film and by the time required for equilibrium dissociation to be established.