The Technique of FURNITURE MAKING

Measurement of moisture

The most accurate method of measuring the moisture content is the oven method, in which thin slices of the wood to be tested are accurately weighed, heated in an oven or drying-chamber, repeatedly reweighed until it has been established beyond question that all the moisture has been driven off, and the

moisture loss then calculated in accordance with the formula—net weight minus dry weight over dry weight and multiplied by 100, to yield the percentage moisture content. Thus, if the timber sample weighed initially 5 gm, and after reweighing only 4 gm, then the calculation would be:

5 gm 4 gm x 100 = 25 percent moisture content 4gm

Care must be taken not to scorch the wood, and the weighing must be done on an accurate chemical balance, while knots and other defects should not be included in the sample.

A simple oven using one or more 40 watt electric light bulbs is illustrated in Figure 12.

While the results obtained by the oven

method are extremely reliable, they are more applicable to scientific research, and for field­work the electric moisture-meter will give instantaneous readings accurate enough for most practical purposes. These meters work on the principle that, as wood itself is a bad conductor of electricity, any moisture present will facilitate the passage of an electrical current passing between two electrodes spaced apart and inserted into the wood, and the measure of resistance to the passage of the current can be expressed in terms of moisture present as a percentage of the total bulk. One such meter uses a transistorized sensing-plate to record the effect of the material being tested in a high - frequency electrostatic field; but most common forms of meter employ contact, clamp or drive - in pin electrodes attached to cable leads, with the meter actuated either direct from the mains. or with standard dry-cell high - and low-tension batteries.

The range of measurement varies according to the type of meter, with a normal coverage of from 6 to 30 per cent, and a margin of error of from 1 to 2 per cent. It should be borne in mind. however, that the presence of mineral salts in the wood may affect the readings, while some woods give large errors, especially in the upper moisture range; but most manufacturers give a table of adjustments covering many of the commonly used woods.

In taking any measurement, whether by oven method or moisture-meter, the readings should not be taken from the end of the plank, but from the thickness of a cut section at least 9 in •228 mm) in from the end and immediately the cut has been made, otherwise surface evaporation may falsify the test. If an approximate moisture content only is required, then the electrodes should be pushed into the surface at several points throughout the length of the plank not less than 3/16 in (4.5 mm) deep, and then the average of the reading used.