Microclimate measurements in the built environment
Sammanfattning: Surface moisture plays an important role in thedeterioration of building surfaces. The extent and duration ofsurface moisture is generally impossible to predictfrommeteorological data and consequently direct measurement ofthis quantity is essential,e.g. using the WETCORR method. Thismethod has been developed in Scandinaviaduring the past 25years. From the beginning it was intended for measurementsofinstantaneous atmospheric corrosion rates and TOW (time ofwetness) using corrodingelectrolytic cells. Over the past 15years the method has been extended tomeasurements of surfacemoisture and TOW on building materials in general. To thatend amodified measuring concept has gradually been developed,including an inertelectrolytic cell with electrodes of gold(Au). More recently, the method has also been applied tomeasurements of moisture content (MC) in various materialsusingmodifications of the traditional pin-type electrodes.This thesis summarises various measurement projects thathave involved theWETCORR method during the past 10 years. Someprojects are entirely focused on the method as such, some aremore concerned with the interaction between themoisture sensorand the environment. In some cases attempts are made tocorrelate TOW with corrosion.The limitations of the ISO 9223 standard for estimating TOW(RH>80%, T>0°C) isclearly illustrated. Theshortcomings of the ISO standard become evident in climateswith sub-zero temperatures, in environments with significantdeposition of pollutantsand salt, and in situations where theexchange of radiation between building surfaces and thesurrounding environment creates large temperature differenceswhich in turnmay either promote or inhibit condensation.A generalised definition of TOW based on the conductivity ofthe surface electrolyterather than the thickness of themoisture film is proposed. The modified TOW is called time ofconduction or time of corrosion, (TOC). Strict measurement ofTOC requiresthe use of an inert electrolytic sensor andexcitation by AC or pulsed DC withreversing of the polarity.This is different from the present WETCORR technique.Theadoption of the TOC concept opens up the possibility ofdividing time into "wet" and"dry" periods. This is believed tofacilitate for the development of dose-responsefunctions basedon the real physical/chemical processes occurring on materialsurfacesrather than on a parametric approach.The WETCORR technique has proven to be very useful also formeasurements of MCin wood, a measurement concept called INWOOD.The general principles andtheoretical considerations for woodmoisture measurements are reviewed, includingthe derivation ofsemi-empirical relationships describing the dependence ofresistivity on MC, temperature and dry density of wood. Thesame technique should be possible to use with almost any porousbuilding material.
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