On the concept of relative humidity

Standard definitions of relative humidity psi of a gas at temperature t and pressure P involve the ratio of some humidity quantity to the same quantity when the gas is saturated at the same temperature and pressure. These definitions do not apply under certain conditions, notably when the gas is pure unsaturated water vapour, and more generally when the theoretical saturation vapour pressure is greater than the actual pressure. The use of impedance-based relative humidity sensors to measure humidity under such conditions is increasingly common and consequently other less well defined definitions may be used over which there is some debate and confusion in the metrological community. In this paper we identify the source of the problem, explore possible definitions and champion a simple and useful definition of relative humidity which extends the traditional definition to cover the whole range.