Moist Air. Basics and measurement

The air that surrounds us is humid air, containing water vapor (water in gaseous form). The thermodynamic properties of this gas mixture are strongly influenced by the presence of water vapor, which can also condense as liquid water or frost under certain temperature and pressure conditions. Knowing the thermodynamic state of moist air is useful in many applications: ventilation, air-conditioning, drying, industrial manufacturing processes in the health and electronics sectors, and so on.

In the case of a pure fluid, the thermodynamic state is entirely characterized by the knowledge of two quantities, e.g. temperature and pressure, or pressure and volume, etc. On the other hand, in the case of a mixture, the presence of several constituents requires the knowledge of a third quantity to characterize the thermodynamic state. In the case of a mixture, on the other hand, the presence of several constituents requires knowledge of a third quantity to characterize the thermodynamic state. In the case of humid air, this third quantity can be one of the many commonly used to define "humidity": mixing ratio, relative humidity, dew point temperature, wet bulb temperature, etc.

Once the temperature, pressure and any of these parameters are known, it is then possible to determine all the others, as well as the various quantities such as density, mass volume, enthalpy, etc.

Air humidity is measured using a hygrometer. Different models are available, each based on different physical principles. As with any measurement, obtaining a correct result depends first and foremost on the choice of a suitable sensor, then on its appropriate use, and finally on the analysis of the result obtained.