Combining Forchheimer Air Permeameter, Oxygen Diffusion Apparatus, and Dry Cup Method for Characterising Bio-based Building Materials
Authors
Larsen, Simon Niels ; Rasmussen, Nicklas Wærnstrøm Harding
Term
4. term
Publication year
2026
Submitted on
2026-06-04
Abstract
This thesis investigates whether a Forchheimer Air Permeameter (FAP) can replace or complement the conventional dry cup method and an oxygen diffusion apparatus (ODA) for characterising moisture transport in building materials. The motivation is to develop faster and more efficient measurement techniques, particularly for bio-based materials, which are increasingly important in a building sector with high CO2 emissions. Eleven materials – including bio-based, mineral, and plastic types – are tested, and three transport coefficients are determined: water vapour permeability (WVP), relative oxygen diffusivity, and air permeability. The FAP is developed through two prototypes, evolving from a simple single-flow-sensor setup into a fully automated multi-sensor system capable of measuring air flows up to 200 L/min, and is validated using test dummies with known geometries and Reynolds number analysis to define the valid operating range of the Forchheimer approach. The influence of sample height is assessed at four heights (2.55, 5.1, 10.2, and 20.4 cm), showing that height has negligible impact on the measured transport properties for samples of 5.1 cm and above, while the smallest height of 2.55 cm produces unstable and unreliable results; thus, a sample height of 5.1 cm is recommended for future measurements, balancing accuracy and test duration. The study also finds that silica gel used in dry cup experiments loses performance significantly after gaining about 9% of its initial weight, highlighting the need for regular replacement to maintain reliable WVP measurements. A strong linear correlation between WVP and relative oxygen diffusivity is confirmed across all material types, consistent with previous work, whereas the relationship between air permeability and relative oxygen diffusivity follows a semi-logarithmic trend. This correlation is most convincing for dense mineral materials, but data for highly permeable bio-based materials show considerable scatter, preventing a robust general relationship. Analysis using the pore-network index P suggests that a unique correlation between air permeability and diffusivity can only be expected for materials with similar pore structures, indicating that further research is required, especially for bio-based building materials.
Denne afhandling undersøger, om en Forchheimer Air Permeameter (FAP) kan erstatte eller supplere den traditionelle dry cup-metode og et oxygen-diffusionsapparat (ODA) til karakterisering af fugttransport i byggematerialer. Baggrunden er behovet for hurtigere og mere effektive målemetoder, særligt for bio-baserede materialer, der får stigende betydning i en bygningerhverv med høje CO2-udledninger. I studiet testes 11 forskellige materialer – bio-baserede, mineralske og plastbaserede – hvor tre transportkoefficienter bestemmes: vanddamppermeabilitet (WVP), relativ oxygen-diffusivitet og luftpermeabilitet. FAP’en udvikles i to prototyper fra en enkel opsætning med én flowsensor til et automatiseret multisensorsystem, der kan måle op til 200 L/min, og kalibreres ved hjælp af testdummies med kendt geometri samt Reynolds-talsanalyse for at fastlægge et gyldigt anvendelsesområde for Forchheimer-tilnærmelsen. Effekten af prøvehøjde undersøges ved fire højder (2,55; 5,1; 10,2; 20,4 cm), hvor resultaterne viser, at højde har minimal betydning for målte transportegenskaber fra 5,1 cm og opefter, mens 2,55 cm giver ustabile og upålidelige målinger; på den baggrund anbefales 5,1 cm som standardhøjde af hensyn til både nøjagtighed og måletid. Undersøgelsen fremhæver også, at silica-gel i dry cup-forsøgene mister sin effektivitet markant efter ca. 9 % vægtøgning, hvilket kræver regelmæssig udskiftning for at sikre valide WVP-målinger. En klar lineær sammenhæng mellem WVP og relativ oxygen-diffusivitet bekræftes på tværs af alle materialetyper, i tråd med tidligere studier, mens relationen mellem luftpermeabilitet og relativ oxygen-diffusivitet følger en semilogaritmisk trend. Denne korrelation er mest tydelig for tætte mineralske materialer, men for meget permeable bio-baserede materialer er dataspredningen så stor, at der ikke kan etableres en robust sammenhæng. Analyse med et porenetværksindeks P indikerer, at en entydig kobling mellem luftpermeabilitet og diffusivitet kun kan forventes for materialer med sammenlignelig porestruktur, og at yderligere forskning er nødvendig for især bio-baserede materialer.
[This abstract has been generated with the help of AI directly from the project full text]