A critical analysis of one standard and five methods to monitor surface wetness and time-of-wetness

Surface wetness is a synergistic factor to determine atmospheric corrosion, monument weathering, mould growth, sick buildings etc. However, its detection and monitoring are neither easy nor homogeneous, for a number of factors that may affect readings. Various types of methods and sensors, either commercial or prototypes built in the lab, have been investigated and compared, i.e.: the international standard ISO 9223 to evaluate corrosivity after wetness and time-of-wetness; indirect evaluation of wetness, based on the dew point calculated after the output of temperature and relative humidity sensors; direct measurements by means of capacitive wetness sensors, safety sensors, rain sensors (also known as leaf wetness sensors), infrared reflection sensors, fibre optic sensors. A comparison between the different methods is presented, specifying physical principle, forms of wetting to which they are respondent (i.e. condensation, ice melting, splashing drops, percolation, and capillary rise); critical factors, use and cost.