Study on Measurement Methods for Moisture Content Inside Wood

Abstract

There has been growing interest in constructing mid- and high-rise wooden buildings in recent years. To ensure the feasibility of these structures, it is necessary to provide evidence that their long-term reliability can be guaranteed. While long-term testing is typically necessary, a continuous monitoring system for the moisture content of wood materials used in buildings has been proposed as an alternative. The proposed method measures the change in the local moisture content using the equilibrium moisture content calculated from the temperature and humidity measured using temperature and humidity sensors. The study used Japanese cypress specimens with dimensions of 50 mm, 75 mm, and 100 mm cubes and Douglas fir specimens of 50 mm cubes. The moisture content was measured under various external environments. Results showed that this system effectively captured changes in local moisture content, reflecting fluctuations in temperature and humidity in a controlled thermo-hygrostat over a three-day moisture absorption environment (20 °C, 95% humidity). Additionally, it was observed that higher moisture content levels yielded correspondingly higher local moisture content measurements compared to those obtained using the oven-drying method.

1. Introduction

In recent years, the growing global focus on environmental issues, highlighted by initiatives such as the Conference of the Parties (COP) [1], has spurred discussions on promoting the use of wood as a sustainable resource. In Japan, there is a growing interest in mid- and high-rise wooden buildings, as seen in the enforcement of the “Act on the Promotion of Wood in Buildings to Contribute to the Realization of a Decarbonized Society” [2] to promote the utilization of wood in buildings in general. To construct mid- and high-rise wooden buildings that will be used in the long term, it is necessary to provide evidence that their reliability can be guaranteed in the long term, as with other structures. All materials deteriorate, but the mechanical properties of wood-based materials, in particular, change significantly depending on their moisture content [3,4,5], and it is known that an ongoing high moisture content causes various types of deterioration [6,7,8]. The continuous monitoring of changes in the moisture content of wood and wood-based materials used in buildings can lead to the early detection of damage and reduced inspection costs [9,10]. However, it is often difficult to measure the moisture content change in mass timber used in mid- and high-rise wooden structures from the surface owing to the external fireproof materials. Therefore, it is necessary to measure the local moisture content near the point of change. Currently, various methods have been established for measuring the moisture content in specific areas of wood, including using the oven-drying method, measuring it with an electrical resistance-type wood moisture meter, such as that used in the study by Grönquis [11,12,13], deriving the equilibrium moisture content of wood from the local temperature and relative humidity of the component [14,15], and measuring it using X-rays [16,17]. However, destructive measurements are not suitable for continuous monitoring. Considering the cost of the measuring equipment, ease of operation in the field, and frequency of measurements, the electrical resistance method and the method using equilibrium moisture content are more suitable for practical use. These two methods are easy to apply and exhibit high measurement accuracies [18]. However, the disadvantage of the electrical resistance method is that it has been suggested that it may affect readings when there is a gap between the electrode and wood owing to repeated drying and wetting, or when there is chemical treatment or glue [19]. The method of determining the humidity from the equilibrium moisture content requires installing temperature and humidity sensors inside the members, and the creation of measurement holes causes damage to the members. Therefore, each method has its limitations. However, the method of determining humidity from the equilibrium moisture content can be applied to wood treated with chemicals or adhesive layers, which are expected to be used in mid- and high-rise wooden structures. It is possible to reduce damage to members with the development of wireless devices and the miniaturization of sensors. Therefore, this method will be highly effective for monitoring building materials in the future. In this study, we investigated a method to measure the change in local moisture content using the equilibrium moisture content calculated from the temperature and humidity measured using temperature and humidity sensors [20]. This paper presents the results of our investigation into the measurement accuracy of this method using small wood specimens.

2. Measuring Methods for the Moisture Content Inside Wood

In this method, temperature and humidity sensors (Hygrochron: DS1923-F5#, Analog Devices, Wilmington, MA, USA, T&H sensor) were installed inside the wood, and the immediate vicinity was assumed to be in an equilibrium state. The change in local moisture content was measured using Kollmann’s equilibrium moisture content curve [21]. The equilibrium moisture content was calculated using the approximation Formula (1) proposed by Saito et al. [22]:

$${EMC}_1={\displaystyle \frac{a+bx+c{x}^2+dy+e{y}^2+{fy}^3}{1+gx+hy+i{y}^2+{jy}^3}},$$

(1)

$x$: Temperature ($℃$) ($1\le x\le 90$);

$y$: Relative humidity (%) ($5\le y\le 99.5$);

$a=1.33;$

$b=8.27\times {10}^{-3};$

$c=-6.40\times {10}^{-5};$

$d=1.10\times {10}^{-1};$

$e=-2.28\times {10}^3;$

$f=1.47\times {10}^{-5};$

$g=-1.16\times {10}^{-5};$

$h=-2.42\times {10}^{-2};$

$i=2.54\times {10}^{-4};$

$j=-9.85\times {10}^{-7}.$

3. Testing Method

To compare the local moisture content calculated using this method (M_l) with that calculated using the oven-drying method (M_od), the specimens shown in Section 3.1 were placed in a thermo-hygrostat (LHU114: ESPEC) and recorded under different external environments, as described in Section 3.2. This experiment was conducted in the laboratory only.

3.1. Specimens and Measurement Method

Figure 1 presents an overview of the specimens, and

Table 1. Number of specimens measured in each environment.

Specimens	50 mm Cube	75 mm Cube	100 mm Cube
Japanese cypress	3	3	2
Douglas fir	3	0	0

lists the number of specimens measured in each environment. The specimens included Japanese cypress (Chamaecyparis obtusa) cubes measuring 50 mm, 75 mm, and 100 mm and Douglas fir (Pseudotsuga menziesii) cubes measuring 50 mm. The same size specimens were cut from different woods. The average oven-dry density of cypress was 386 kg/m³, and that of Douglas fir was 347 kg/m³. The timbers were cut to avoid large knots or splits within the specimens. A hole with a diameter of 22 mm was drilled in the center of the specimens to insert the T&H sensor. The sensor was a button battery type and was attached to a cover made by 3D printing to protect the sensor unit. Figure 2 shows the T&H sensor, and

Table 2. Details about the T&H sensor.

Dimensions/weight	Diameter: 17 mm×Thickness: 6 mm/about 3.3 g
Cover dimensions	Diameter: 19 mm × Height: 13 mm
Measurement range	Temperature: −20 °C to +80 °C Relative humidity: 0% to 95%RH
Measurement accuracy	Temperature: ±0.8 °C Relative humidity: ±5%RH
Measurement resolution	Temperature: 0.5 °C Relative humidity: 0.6%RH
Measurement interval	1 s to 3 h
Lifespan of battery	About 2 years (When measured every 10 min in an environment of −20 °C to +25 °C)

lists the specifications of the sensor and dimensions of the cover. To prevent moisture from entering the measurement hole after installing the T&H sensor, the hole was plugged using a urethane rod and putty for plumbing (AP-1000-I). The specimens checked during the measurements were replaced with new specimens for subsequent measurements. If more than half of the specimens were examined, the specimens were replaced. The temperature and humidity were recorded at hourly intervals using T&H sensors. The weight of the specimens was measured four times a day, and the end split of the specimens was observed once a day. The specimens were placed within a thermo-hygrostat, as shown in Figure 3.

3.2. Environment of the Testing Space

The internal environment of the thermo-hygrostat, that is, the external environment of the specimens, consisted of four types, as listed in

Table 3. External environment of the specimens.

	Day 1	Day 2	Day 3	Day 4
Environment 1	20 °C/95%	⇒	⇒	20 °C/65%
Environment 2	20 °C/40%	⇒	⇒	20 °C/65%
Environment 3	20 °C/90%	20 °C/65%	20 °C/40%	20 °C/65%
Environment 4	40 °C/90%	20 °C/65%	0 °C	20 °C/65%

. Environment 1 was characterized by moisture absorption, Environment 2 by moisture desorption, Environment 3 by a change in humidity only, and Environment 4 by a repetitive dry and wet state with both temperature and humidity changes. Measurements were taken over four days, with the specimens undergoing a two-day curing period at 20 °C and 65% humidity, prior to the commencement of measurements.

4. Measurement Results of the T&H Sensors

4.1. Change in the Local Temperature and Humidity

Figure 4 shows the temperature and humidity measured under the moisture absorption and desorption states (Environments 1 and 2). The relative humidity in the local area of the specimens fluctuated with a delay relative to the relative humidity inside the thermo-hygrostat, as recorded by a cord-type temperature and humidity sensor. Figure 5 shows the temperature and humidity recorded under repeated dry and wet conditions (Environment 4). The relative humidity in the local area changed later than that in the thermo-hygrostat, as in Environments 1 and 2; however, the temperatures in the local area fluctuated similarly to those in the thermo-hygrostat. When the temperature and humidity were simultaneously varied, the relative humidity in the local area fluctuated rapidly, as indicated by the red circles in the graph. For example, on the second day, the relative humidity changed after the temperature first dropped to 20 °C, according to the specifications of the thermo-hygrostat, so the relative humidity in the local area should have fluctuated in the direction of an increase, but the measured values decreased rapidly. Although the exact reason is unknown, this may be due to the characteristics of the button-battery-type T&H sensor.

4.2. Changes of M_l

Figure 6 shows the changes in temperature and humidity measured under the moisture absorption and desorption states. These changes are influenced by relative humidity, as determined by the properties outlined in Equation (1). Figure 7 shows the relationship between the moisture content (M₃) at the end of three days in the moisture absorption and desorption states, the moisture content (M₀) at the start of the measurement, and the distance from the end grain to the measurement point. In both the moisture absorption and desorption states, the value of M₃–M₀ increased as the distance between the measurement point and the end grain decreased. This suggests that the measurement method may be utilized to measure the distribution of moisture content in the fiber direction.

5. Comparison of M_l and M_od

5.1. Moisture Absorption and Desorption Environment (Environments 1 and 2)

The M_l of the 50 mm cubes in Environments 1 and 2 and the M_od are shown in Figure 8 as an example for one specimen from each species. M_od was calculated using Formula (2).

$$M_{od}={\displaystyle \frac{W-W_{od}}{W_{od}}}\times 100,$$

(2)

$M_{od}$: Moisture content of the oven-drying method (%);

$W$: Weight at the time of measurement (g);

$W_{od}$: Weight at the time of oven-drying (g).

In the moisture absorption state (Environment 1), a difference was observed between M_l and M_od. In particular, the higher the moisture content, the larger M_l tended to be. In the moisture desorption state, M_l and M_od fluctuated almost identically for all the species. The moisture content measured using this method is a local value. Therefore, it was lower than the average value of M_od in the moisture desorption state. However, the results showed the opposite trend. This may be because the amount of moisture that passed through the tracheid and directly into the air filling the measurement hole was higher than the amount of moisture adsorbed on the wood during the moisture absorption process. Therefore, it is important that the measurement point is not too close to the end grain.

Figure 9 shows the difference between M_l and M_od in Environment 1. The difference between these two moisture contents was largest on the third day of measurement. In Douglas fir, the differences varied, even for the same specimens. Even within the same species, the amount of water passing through the tracheid may vary depending on factors such as fiber arrangement. The oven-dried densities of Douglas fir used in this experiment were 0.37, 0.38, and 0.38, respectively, showing minimal variation. Figure 10 shows the relationship between M_l and M_od in Environments 1 and 2. A linear approximation with a y-intercept of zero demonstrated a strong correlation for each species and dimension.

5.2. Dry and Wet Repetitive Environments (Environment 3, 4)

Figure 11 shows the change in M_l and M_od of 50 mm cubes in Environments 3 and 4, representing one specimen from each species. In Environments 3 and 4, the two moisture contents were almost identical, regardless of the species. It was found that this method could be utilized to measure changes in the external environment at one-day intervals starting from 20 °C and 65%, as observed in this experiment. In Environment 4, a check was conducted after day 2, but M_l was identical to M_od. Checks as small as 1 mm did not affect measured values. However, if a check occurs up to the measurement hole, moisture migration from the check hole may dominate, and the values may differ greatly from the actual environment.

6. Conclusions

In this study, we investigated a method to measure the changes in the local moisture content using the equilibrium moisture content calculated from the temperature and humidity measured using temperature and humidity sensors. Furthermore, the local moisture content calculated using this method (M_l) was compared to that calculated using the oven-drying method (M_od). As a result, it was possible to measure the change in the M_l of the wood in the thermo-hygrostat in a three-day moisture absorption environment. Therefore, this method can be used to measure changes in the local wood area. In addition, M_l showed a tendency to be higher than M_od at higher moisture contents. The results showed that M_l and M_od were in close agreement in the moisture desorption environment for a three-day period and in dry and wet repetitive environments. Additionally, a comparison of the difference between the moisture content after three days and the initial moisture content, relative to the distance from the end grain in Environments 1 and 2, revealed that smaller specimen dimensions exhibited greater variations in moisture content. Now that it has become clear that this measurement method can be used with some degree of error, it is necessary to accumulate data on large specimens and in colder environments. For this reason, we have begun measuring large specimens outdoors.