Measuring moisture content of wood

Any property of wood that varies in some known and predictable way with moisture content could in theory be used to measure moisture content. According to Skaar, there are as many as fifteen methods that have been used (Skaar, 1984). Methods of moisture measurement commonly used or potentially useful in conservation are based on one or other of the following: changes in mass; changes in electrical resistance: change in the dielectric constant.

In the gravimetric method, wood is weighed then dried to constant weight in a convection oven at 103 ± 2 °C. The MC is then calculated from the formula given earlier. This method assumes that the oven is completely dry and that only water is lost from the sample. It provides an absolute measure of moisture content at one point in time.

The mass of an applied load can be measured as an electrical signal by a device called a load cell. Typically these devices measure 20 mm X 10 mm in size. A wide range of load cells is available that can be used to provide a continuous record of changes in mass of large or small furniture objects as water is absorbed or desorbed from the atmosphere. Although they can be extremely sensitive, load cells cannot readily provide information about the location of moisture in an object or the absolute amount present.

The Karl Fischer reagent, consisting of pyridine, sulphur dioxide, iodine and methanol, can be used to measure the moisture content of wood, and a wide variety of other materials, by titration. It gives the best results of any of the standard methods but is not practical for large samples (Kollman and Hockele, 1962).

Two types of moisture meter are routinely used to determine moisture in wood. They are the electrical resistance type and the dielectric type.

Dry wood is an effective electrical insulator but water is a conductor of electricity. As the moisture content of wood increases its electrical resistance decreases (approximately halving for each 1% increase in mC between 6% and the FSP). The electrical resistance meter, measuring the flow of direct current parallel to the grain between two electrodes inserted into the wood, is commonly used to measure moisture content over this range. Below about 6% MC, resistance metres are not reliable because resistance is too high (> 1011 Ohms). Above about 24%, MC readings become less reliable, partly due to loss of sensitivity as resistance falls and partly due to polarization and heating effects. AC metres and those using short repetitive pulses rather than continuous current are able to overcome these shortcomings to some extent.

As electricity takes the path of least resistance, the electrical resistance meter tends to measure the highest MC in the area between the electrodes. However, by insulating the probes along their length except for the penetrating tips that are to serve as the electrodes, resistance metres can be used to measure the magnitude of moisture gradients in wood by measuring the MC at different depths from the surface. Resistance metres require pin electrodes to be inserted into the wood so their use on presentation surfaces of furniture is not acceptable. However, a possible way round this is to take measurements on wood of identical size, species and finish that is kept with the object and allowed to reach the same equilibrium conditions of RH and temperature. Given a suitable meter, electrodes can be left in situ and readings taken as required. The English firm of Hutton and Rostron have developed a small, cheap resistance electrode that can be inserted in multiples into structural timbers and used to monitor moisture content remotely by PC. This system is employed in the Royal Pavilion, Brighton, where it is used to monitor moisture levels in the fabric of the building as part of the decay prevention strategy.

The resistance of wood increases with decrease in temperature, and correction for temperature is therefore required. Some meters can achieve this automatically through a temperature probe connected to the meter. The change in resistance of wood is also to some extent dependent on species. However, variation between species is not as marked with the electrical resistance meter as with the dielectric type of moisture meter.

Dielectric moisture metres use alternating current (AC), usually at radio frequencies and are of two types, the capacitance type and the power loss type. The capacitance type measures the dielectric constant of wood. The more common power loss meter measures the rate of energy absorption as the product of dielectric constant and loss factor. At a given frequency, the dielectric constant increases with increasing moisture content, increasing density and increasing temperature. Power loss generally increases with wood moisture content and with temperature. Usually a concentric arrangement of electrodes placed on one surface is used by both types of dielectric meter. This is normally fixed for a particular meter and the field generated penetrates to a standard depth so that the reading obtained is more or less an average value. Dielectric metres are normally calibrated to read between

0 and 25% moisture content. They have the distinct advantage of being able to take readings without damaging wood surfaces but the disadvantage of operating at a fixed depth of field.

To get the best out of either type, familiarity with the nature of wood-moisture relations and with the meter is necessary and it is a good idea to experiment with different methods and compare the results. Other instrumental methods that have been used to measure moisture content include the neutron moisture meter and nuclear magnetic resonance (Skaar, 1984).

The curve in Figure 2.14 showing equilibrium moisture content as a function of relative humidity (or relative vapour pressure) at constant temperature is called a moisture sorption isotherm. After initial seasoning, a sample of wood taken through repetitive cycles of RH exposure between 0 and 100% tends to follow the same adsorption and desorption curve repetitively. Therefore, an indirect method of estimating moisture content is to place the wooden object inside a well-sealed container with a hygrometer and to measure the RH produced.

that takes place in a given direction from the green to the oven-dry condition, expressed as a percentage of the green dimension. Thus, the total shrinkage percentage is calculated as follows:

where S = total shrinkage, expressed as a percentage (St = tangential shrinkage, Sr = radial shrinkage, Sl = longitudinal shrinkage) and D = change in dimension (Dg = green dimension, Dod = oven-dry dimension). Figure 2.15 illustrates the application of the formula in the determination of tangential shrinkage based on green and oven-dry measurements of a tan-gentially sawn strip of wood.

Total of shrinkage of wood along the grain (i.e. longitudinal shrinkage) is normally in the range of 0.1-0.2%. In practical situations involving typical moisture content changes over a moderate range, only a portion of this small quantity would be effected and the resulting dimensional change becomes insignificant. It is reasonable to assume that wood is stable along its grain direction, and for most purposes longitudinal shrinkage and swelling are ignored. In fact, longitudinal shrinkage data

2.4.3 Dimensional change

Of all the properties affected by moisture content, dimensional stability commands the greatest attention. Not only is wood hygroscopic, it is also anisotropic. That is, it exhibits different properties when tested along axes in different directions. Because of this, dimensional change in wood is usually considered separately in the three principal linear directions: longitudinal, radial and tangential. In previous sections the general effects of bound water sorption were reviewed relative to the cellulose structure within the cell wall. Discussion here will concentrate on the quantitative effects of moisture content on the anisotropic dimensional behaviour of wood tissue. In the initial drying of wood, there is no dimensional response to the loss of free water. Only when a portion of wood tissue has reached the fibre saturation point and begins to lose bound water does shrinkage begin.

The common basis for indicating the relative dimensional instability of a given wood is to measure the total amount of linear shrinkage

Figure 2.15 The calculation for percentage of tangential shrinkage (St) based on green and oven-dry measurements of a tangentially sawn strip of wood. AD = the change in dimension, Dg = the dimension of the wood when green, Dod = the dimension of the wood when oven-dried. Note that this formula is only applicable to shrinkage starting from the green condition. For dimensional change of partly seasoned wood this formula will introduce an average error of about 5% of the calculated change in dimension. Calculation of the dimensional change of partly seasoned wood is described in section 2.4.4

Figure 2.15 The calculation for percentage of tangential shrinkage (St) based on green and oven-dry measurements of a tangentially sawn strip of wood. AD = the change in dimension, Dg = the dimension of the wood when green, Dod = the dimension of the wood when oven-dried. Note that this formula is only applicable to shrinkage starting from the green condition. For dimensional change of partly seasoned wood this formula will introduce an average error of about 5% of the calculated change in dimension. Calculation of the dimensional change of partly seasoned wood is described in section 2.4.4

MTTl

(c)

Shrinkage across the grain (transverse shrinkage), however, is significant, and tangential shrinkage is always greater than radial shrinkage. Average tangential shrinkage varies between species over the range of about 4 to 12%, with an overall average of about 8%. Average radial shrinkage values range from about 2 to 8%, averaging slightly over 4%. The result of differences in radial and tangential shrinkage is illustrated in Figure 2.16.

Wood Working for Amateur Craftsman

Wood Working for Amateur Craftsman

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