IntroductionTop

The moisture content (MC) influences most of the physical, mechanical and technological properties of wood, and also affects its quality and service life, especially in constructions. The MC in service can vary widely depending on the surrounding climatic conditions and its hygroscopic history.

The time to initiation and subsequent rate of wood decay is affected by a series of concomitant factors which make up the so-called ‘‘material climate’’ (the temperature and moisture content of the wood), which in turn has a direct impact on the service life of the wood products and constructions (Brischke et al., 2007). For this reason, it is vitally important to have moisture and temperature measurement devices which are reliable, safe and precise in order to control building decay, especially where these are highly valued historical buildings.

Different studies have been carried out (Brischke et al., 2008; Dyken & Kepp, 2010; Tannert et al., 2010 and 2011; Dietsch et al., 2015b; Li et al., 2018; Niklewski et al., 2018) in which both environmental and wood temperature as well as moisture content are monitored, mainly on in-service wood structures (bridges, buildings, etc.) exposed to different climatic conditions and with different design details.

Moreover, the introduction of the Building Products Regulation has made compulsory for European manufacturers of solid wood structural products to measure the moisture content with an accuracy of at least ±3% (for at least 95% of the measurements) and to ensure the maintenance of this accuracy over time by checking it (at least annually) with a set of at least three resistances from 1 MOhm to 100 GOhm in the case of resistance-type moisture meters (EN 14081-1:2016+A1:2020).

Accuracy of moisture content measurement is also important to reduce the percentage of wood declassified due to excessive or reduced moisture content. The presence of a high declassification percentage is a factor that seriously affects the profitability of the transformation process.

To determine the MC, two general approaches can be distinguished: a) In-laboratory measurements, the MC is determined by oven-drying according to international standards (EN 13183-1) or water extraction, and b) In-service measurements, the MC is usually estimated by means of indirect measurement methods that use physical properties of wood which are correlated to the MC. Indirect and non-destructive measurement methods include the capacitive, electrical resistance, microwave, radiometric, spectrometric, sorption and color reaction methods (Dietsch et al., 2015). A complete review of methods for measuring MC can be read in Dietsch et al. (2015) and Engelund Thybring et al. (2018).

The resistance-type method is widely used in the monitoring the MC in practice, forming an integrated part of the control and regulation equipment of the sawmill wood-drying process, and for assessing the conformity of the final MC of sawn timber and other kinds of wood products. Dielectric, radiometric and spectrometric-type moisture meters are mainly used in production lines to provide on-line and non-contact measurements.

In order to monitor the MC of in-service timber structures and to assess the conformity of MC in the reception process and/or in the inspection procedures for wood products, the most used and reliable method is the resistance-type. Among the reasons for arriving at this conclusion are: its acceptable accuracy, the possibility of localized measuring (at exactly the worst point of the wood element), the durability and fast response of moisture-pin sensors under changing environmental conditions, the ease of connection of the measuring devices to data-logging systems and the limited damage to the structure, although it is necessary to deal with the problem of maintaining close contact over time between the pin-sensors and the wood in certain places where the dimensional changes in the wood are extreme.

Since Stamm (1927) is known that the resistance of wood decreases with increasing moisture content. The main matter of concern about this technology is its accuracy. One of the main factors affecting its accuracy is the in-built calibration curves that relate the non-destructive measurements (electrical resistance) to real MC. The effect of other factors related to the methodology of measurements can be easily overcome by following the recommendations included in the EN 13183-2 standard.

Forsén & Tarvainen (2000) carried out an extensive study on the accuracy of the hand-held wood moisture content meters working not only according to the resistance-type method but also to the capacitance-type method. They concluded that most of the resistance meters showed a systematic deviation from the oven-dry determined MC because of incorrect MC-resistance curves. According to these authors, the accuracy of the resistance-type meters (95% confidence interval) for in-field measurements is about ±2.0 to ±5.0 MC percentage degrees (%) and about ±3.0% to ±5.0 MC percentage degrees (%) for the capacitance-type meters. In this sense, the most important source of quantitative error in resistance-type meters is associated with the different effect of wood species followed by the effect of material temperature. In accordance with this, resistance-type MC measurements must be compensated for variation caused by wood species and wood temperature. These authors provided clear evidenced of the fact that only meters that can offer sufficient accuracy (which they considered to be over 55% of readings within 1% of oven-dry measurements) should be used for contractual quality control purposes. For this reason, the wood sector industry as a whole, as well as its users, demand improved measurement accuracy of these devices.

All the resistance-type meters have their own builtin resistance curves for different species, although these curves differ for different species depending on the electrical properties of the wood. Due to the different builtin resistance curves and the fact that many commercial devices use a single internal calibration curve, along with the multiplicity of methods for correcting the effect of species, instruments produced by different manufacturers also show different readings when used on the same wood material (Forsén & Tarvainen, 2000).

Vermaas (1982) reported that most of the moisture meters did not provide the required accuracy when used on Monterrey (Pinus radiata) and maritime pines (P. pinaster) grown in South Africa due to inadequate correction of the effect of wood species. Only by using the specific calibration functions, or by means of appropriate species correction systems, can the accuracy of the MC estimation attain values better than the ±3% demanded by the EN 14081-1 standard.

The literature contains different expressions describing the relationship between the electrical resistance and MC (see Vermaas, 1982) but most of them are on non-European species. One of the last and most extensive studies carried out on native European species (softwoods, hardwoods and modified woods) is that of Brischke & Lampen (2014), who proposed a functional relation between measured electrical resistance and gravimetrical MC in a limited range between 15% and 50 % MC. By using the proposed specific calibration functions, the precision of MC estimation of the measurements was in the range between ±2.5 % and ±5 %, far from the objective of ±1% established by Forsén & Tarvainen (2000) and that of better than ±3% considered by the EN 14081-1:2020 standard.

In the present work, the function initially proposed by Samuelsson (1990) has been used not only because of its demonstrated viability in predicting the value of the ovendry determined moisture content but also for the purpose of comparing our results with those of the study conducted by Forsén & Tarvainen (2000) in which common species were sampled (Scots pine and maritime pine), albeit of different origin. The use of the same fitting function and measurement methodology as the ones used in the studies carried out by Forsen & Tarvainen (2000) and Fernández-Golfín et al. (2012, 2014) has the advantage of being able to increase the number of species and origins for which the Samuelsson fitting coefficients (a and b) are known. Another added advantage is to be able to compare our fitting coefficients with those obtained by Forsen & Tarvainen (2000) in those species also sampled by us (Scots pine and maritime pine), increasing the number of origins, as well as to build up comparable databases.

The aim of the present work was to improve the estimation accuracy of the oven-dry determined MC for the most important softwood species available on Spanish markets (and also present on European markets), below the lower threshold of 2% revealed in the study by Forsén & Tarvainen (2000).

It is not within the scope of this work to compare the accuracy of the different mathematical models dealt with in the literature, although the database created is likely to be used for this purpose in future studies.

Materials and methodsTop