Moisture Content Determination

EXERCISE NO. 1: MOISTURE CONTENT DETERMINATION

Ramos, James Edelbert

FPPS 121 – Wood Physics I

1. INTRODUCTION

Internal Water Content of a Tree

Over 50% of the total fresh weight of a tree consists of water but the water concentration varies widely in different parts of a tree and also with species, age, site, season and even the time of the day. Although the heartwood usually has the lowest water content, it often contains over 100% on a dry-weight basis, and the sapwood of some species may contain up to 250% (Gibbs 1935). Once the tree is cut, the wood immediately starts to lose some of its water through diffusion and evaporation to the surrounding atmosphere (Wood Handbook, 2010).

Wood Moisture Content

Wood is a hygroscopic material. It has natural affinity for water in both liquid and vapor form due to the presence of hydroxyl (OH) groups that exist throughout its structure, particularly cellulosic and hemicellulosic portions of the wood.

Wood gains or loses moisture to equilibrate with its immediate environment. The amount of water in wood is indicated by its moisture content. It is a steady-state level that wood achieves when exposed to a particular relative humidity and temperature.

In wood technology, it is frequently necessary to determine the amount of moisture present in wood and wood products, because it has a profound effect on its physical properties including its weight, strength, dimensional stability, and resistance to attack of decay fungi and insects. Thus, when using wood as a raw material, it is important to evaluate its water content, and to know where these moisture is held in the wood material (Hoadley, 1980).

There are several ways of determining the wood moisture content of wood, and the exercise explored two of the most common method: ovendry method and the use of electrical moisture meter.

Further, the exercise aimed to familiarize the students with methods to determine and calculate the moisture content of wood, particularly the ovendry method and moisture meter method.

1. MATERIALS AND METHODOLOGY

1. Moisture Meter Method

The students were provided with 5 different wood samples of unknown species, then each sample were labeled accordingly with sample numbers: 1, 2, 3, 4, and 5. Using a moisture meter, the moisture content of each samples was obtained. Each sample were given two readings, with each measurement done across the sample’s grain. Moisture meter readings were recorded, and the average from the two MC readings of each samples were calculated using the formula:

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1. Ovendry Method

The same 5 wood samples used in the moisture meter method were soaked in water for three days to ensure that the green weight of each sample was achieved. Samples were removed from the water and the green weight of each sample was determined using an analytical balance. The wood samples were air dried for a week, and the wood samples were placed inside the oven. Every approximately 24 hours, the oven dry weight of each sample was determined. All readings of the green weight and the ovendry weights of each samples were tabulated accordingly.

Moisture content of each sample were calculated using the formula below:

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1. RESULTS AND DISCUSSION

1. Moisture Meter Method

The results are shown in Table 1.

Table 1. Moisture content determination using resistance type moisture meter.

The average moisture content of the five different wood samples using the resistance type moisture meter yielded different results. Sample 3 obtained the lowest average MC (8.25%), while sample 5 obtained the highest average MC (14.50%). The obtained MC values of all the samples implies that they are in airdry condition (wood at green condition has an MC of at least 21%, whereas wood at airdry condition has an MC of at most 21%).  

1. Ovendry Method

        The results are shown in Table 2.

Table 2. Moisture content determination by ovendry method.

The five wood samples also yielded different values of percent moisture content. Similar with the trend of MC readings in the resistance type moisture meter, sample 3 obtained the lowest MC (42.66%) using the ovendry method, while sample 5 obtained the highest MC (69.40%).