Maria Fredriksson

Small resistive wood moisture sensors: a method for moisture content determination in wood structures

The wood moisture content influences the service life of wood structures since wood is susceptible to decay by rot fungi if it is exposed to high moisture contents during long periods of time. In rain exposed structures, the moisture content close to end grain surfaces and joints can be significantly higher than the average moisture content, but moisture content determinations at such locations require small moisture content sensors. This paper presents small resistive moisture content sensors fastened by electrically conductive adhesive. The relationship between moisture content and electrical resistance was determined for Norway spruce (Piceaabies (L.) Karst.) for a wide range of moisture conditions achieved both by equilibrating specimens over saturated salt solutions and by the pressure plate method. The error, i.e. the difference between the gravimetric moisture content and the moisture content from the regression equation, increased with increasing moisture content. Neither the wood type (heartwood/sapwood) nor the growth rate (southern or northern Sweden) influenced the resistance-moisture content relationship.ZusammenfassungDie Holzfeuchte hat einen Einfluss auf die Nutzungsdauer von Holzkonstruktionen, da Holz anfällig für Pilzbefall ist, wenn es über einen längeren Zeitraum hoher Feuchte ausgesetzt ist. Bei Konstruktionen, die Regen ausgesetzt sind, kann der Feuchtegehalt im Hirnholzbereich und im Bereich von Verbindungen deutlich höher als der mittlere Feuchtegehalt sein. Zur Bestimmung des Feuchtegehalts an diesen Stellen sind kleine Feuchtesensoren erforderlich. In diesem Artikel werden kleine widerstandsbasierte Feuchtesensoren vorgestellt, die mit elektrisch leitfähigem Klebstoff befestigt werden. Der Zusammenhang zwischen Feuchtegehalt und elektrischem Widerstand wurde am Fichtenholz (Picea abies (L.) Karst.) in einem weiten Feuchtebereich bestimmt, wobei die Feuchten sowohl durch Lagerung über gesättigten Salzlösungen als auch mittels der Drucktopfmethode eingestellt wurden. Der Fehler, d.h. die Differenz zwischen dem gravimetrischem und dem über die Regressionsgleichung ermittelten Feuchtegehalt nahm mit steigendem Feuchtegehalt zu. Weder die Art des Holzes (Kernholz/Splintholz) noch die Wuchsbedingungen (Süd- oder Nordschweden) hatten einen Einfluss auf die Beziehung zwischen Widerstand und Holzfeuchte.

The states of water in Norway spruce (Picea abies (L.) Karst.) studied by low-field nuclear magnetic resonance (LFNMR) relaxometry: assignment of free-water populations based on quantitative wood anatomy

Abstract Low-field nuclear magnetic resonance (LFNMR) relaxometry was applied to determine the spin-spin relaxation time (T2) of water-saturated Norway spruce (Picea abies (L.) Karst.) specimens cut from mature sapwood (sW) and mature and juvenile heartwood (hW), where earlywood (EW) and latewood (LW) were separated. In combination with quantitative wood anatomy data focusing on the void volumes in various morphological regions, the NMR data served for a more reliable assignment of free-water populations found in water-saturated solid wood. Two free-water populations were identified within most sample types. One was assigned to water in the tracheid lumen and the other to water inside bordered pits. Whether water in the ray cell lumina was included in one or the other of these two populations depends on the curve-fit method applied (continuous or discrete). In addition, T2 differences between the different tissue types were studied and, for comparison, sorption isotherms were measured by means of a sorption balance. There was a significant difference between EW and LW as well as between juvenile wood and mature wood in terms of T2 related to the cell wall water. However, no differences were seen between the sorption isotherms, which indicates that the observed T2 differences were not due to differences in cell wall moisture content (MC).

Moisture sorption and swelling of Norway spruce [Picea abies (L.) Karst.] impregnated with linseed oil.

Abstract The moisture sorption and swelling of spruce specimens impregnated with linseed oil and unimpregnated controls were studied in two sorption experiments: from 0 to 55% relative humidity (RH) and from 55% RH to water saturation. Sorption isotherms were also determined using a sorption balance. The impregnated specimens had lower rates of both moisture sorption and swelling than the unimpregnated controls; however, there was no significant difference in final moisture contents. In addition, the linseed oil impregnation did not improve the dimensional stability of the wood; in most cases the specimens impregnated with linseed oil swelled more than the unimpregnated controls. For the impregnated specimens, reductions in swelling rates were in most cases seen without corresponding reductions in final swelling. The moisture sorption rate was reduced for the impregnated specimens but there were generally no reductions in equilibrium moisture content.

Microclimate and moisture content profile measurements in rain exposed Norway spruce (Picea abies (L.) Karst.) joints

Abstract In order to perform service life predictions of rain exposed wood structures, the moisture and temperature conditions in the structure need to be known as well as which degradation that occurs under those exposure conditions. The microclimate (the moisture conditions at the surface) is the boundary condition for moisture transport into the wood and depends on the detail design; joints between two pieces of wood can act as a water trap which give long durations of surface moisture after rain events and hinders drying. This study presents moisture content and microclimate measurements in three types of Norway spruce joints exposed to artificial rain in the laboratory. Both the microclimate (the duration of water on surfaces and in gaps) and the moisture content profiles were monitored. The microclimate was changed by changing the size of the gap between the two boards. The duration of water in the gap depended both on the gap size and on the permeability of the wood (sapwood/heartwood, end grain surface/side grain surface). In many cases, a larger gap width gave shorter durations of high moisture contents since a larger gap gave more favourable drying conditions, but the magnitude of this reduction varied between joint types.

Methods for determination of duration of surface moisture and presence of water in gaps in wood joints

If water stays on wood surfaces or is trapped in gaps, the wood is supplied with water for a long period of time and high local moisture contents are reached. This can lead to decay by rot fungi, and it is therefore important to avoid such water traps in order to limit the decay rate. This paper presents two methods: one for determination of duration of surface moisture on wood surfaces and one for determination of duration of water trapped in gaps in wood joints. Both these methods are based on electrical conductance measurements that are commonly used to measure moisture content of wood. Both sensor types consist of insulating tubing, electrically conductive adhesive and copper wire. The sensors monitor the presence of moisture on the actual material surface, and because of their small size, they do not influence the amount and duration of moisture that stays on the surface or in the gap.

A method for determination of absorption isotherms at high relative humidity levels: measurements on lime-silica brick and Norway spruce (Picea abies (L.) Karst.)

ABSTRACT The relationship between the relative humidity and the moisture content of a material (for relative humidity levels close to 100%) can be studied using the pressure plate technique. However, this method is designed for desorption experiments and modifications are necessary in order to perform absorption experiments. This article presents a method to condition samples to a uniform moisture content by absorption with the pressure plate technique. The specimens were suspended in magnetic clamps until the ceramic plate had equilibrated with the applied pressure and modifications were made so that water could circulate through the pressure plate cell.

The Influence of Specimen Size and Distance to a Surface on Resistive Moisture Content Measurements in Wood

The moisture content of wood is commonly determined by measuring the electrical resistance between two electrodes inserted in the wood. However, problems using this method close to wood surfaces were reported in a previous study. In the present study, the effect of the distance to a surface and the specimen size on the measured electrical resistance was studied analytically as follows. The two electrodes create an electrical potential in the wood specimen. The boundary condition for the electrical potential is that the electrical current across all specimen surfaces is zero, which is achieved by using a suitable array of mirror sources. The analytical solution for the electrical potential was used to analyse the influence of the distance from the electrodes to the specimen surface as well as the size of the specimen. In addition, the error in moisture content was evaluated. The effect of the distance to a surface and the specimen size depended on the equivalent radius of the electrodes; if large electrodes are used in small specimens or close to surfaces, there is a risk that a higher resistance is measured which results in slightly lower measured moisture content than the actual moisture content of the specimen.

The cell wall composition of Norway spruce earlywood and latewood revisited

ABSTRACT The xylem of gymnosperm trees consists mainly of tracheids, which generally have larger radial diameters and thinner walls in earlywood than in latewood. Information on possible chemical differences is limited, but according to a few studies, earlywood normally contains more lignin relative to cellulose than latewood. However, for methods giving results per gram dry matter, this result is expected since a larger part of the earlywood cell wall material consists of lignin-rich middle lamella. In this study, we used Raman microspectroscopy to assess relative biopolymer composition of earlywood and latewood secondary cell walls from mature sapwood of Norway spruce (Picea abies (L.) Karst.). In addition, infrared spectroscopy of isolated earlywood and latewood from the same growth rings as well as additional growth rings was performed. Both methods showed significantly higher relative lignin content in the earlywood which confirms results from earlier studies.

Scanning or desorption isotherms?: Characterising sorption hysteresis of wood

Sorption isotherms describe the relation between the equilibrium moisture content of a material and the ambient relative humidity. Most materials exhibits sorption hysteresis, that is, desorption give higher equilibrium moisture contents than absorption at equal ambient climate conditions. Sorption hysteresis is commonly evaluated by determination of an absorption isotherm followed by desorption starting from the highest relative humidity used in the absorption measurement (typically 95%). The latter is often interpreted as the desorption isotherm but is in fact a scanning isotherm, i.e. an isotherm obtained from neither dry nor water-saturated state. In the present study, we investigated the difference between desorption isotherms and scanning isotherms determined by desorption from different high relative humidity levels reached by absorption and how this difference influenced the evaluation of sorption hysteresis. The measurements were performed on Norway spruce (Picea abies (L.) Karst.) using automated sorption balances. Hysteresis evaluated from desorption isotherms gave linear absolute sorption hysteresis for the studied relative humidity range (0–96%), whereas hysteresis evaluated from scanning isotherms gave non-linear curves with a peak between 50 and 80% relative humidity. The position of this peak depended on the relative humidity from which desorption was initiated. Consequently, understanding and evaluation of sorption hysteresis might be challenging if scanning isotherms are used instead of desorption isotherms, hereby increasing the risk of misinterpreting the results.Graphical Abstract