Water vapour sorption and humidity - a survey on measuring methods and standards

Under environmental conditions water exists in all three classical states of matter: solid, liquid and gas. The water molecule is non-linear and therefore polar. In comparison with other liq-uids water has anomalous features; about 63 exceptional properties are recorded. This article starts with reviewing properties of water, typical occurrences and definitions such as relative and absolute humidity and moisture content. Water is present everywhere in nature and engineering; it may be helpful or harmful. The survey concerns both: atmospheric hygrometry and usual measuring methods of the mois-ture content of solids and liquids as well as water sorption. The determination of the atmos-pheric humidity is among the more difficult problems in metrology. In contrast, humidity de-termination of materials is simple; however the definition of the dry state is difficult. Because water is bound at and in solids and liquids in many different ways it turns out that the humidi-ty content of materials is difficult to define and to measure accurately. We provide a survey on the measuring methods, describe the most important ones and discuss advantages and ac-curacy. In the search for extraterrestrial water special remote measuring methods have been developed analysing the spectrum of electromagnetic radiation either of natural sources or produced by a probe and reflected. Spacious deposits have been detected photographically. In situ investigations are made using conventional methods. With regard to the problems of measurements, standardisation of measuring methods and procedures is required. There exist many institutions, which are engaged in investigating the use of water and standardising measuring methods. In tabular form, we give a survey on existing standards