For many applications, it is essential that timber be dried to the correct moisture content.
There are three common methods of measuring moisture content:
- The oven-dry method is the most accurate means of measuring moisture content. The sample piece of timber is weighed on an accurate balance, oven dried at 103± 2° C until it reaches constant weight, and then weighed again. The moisture content as a percentage is the loss in weight (water removed by drying) divided by the oven-dry weight (dry wood) multiplied by 100. The complete procedure is specified in Standard AS/NZS 1080.1:1997 "Timber - Methods of Test 1 - Moisture Content". Also shown are procedures for determination of moisture distribution;
- Electrical resistance moisture meters measure the resistance of timber to electrical flow. They provide a convenient and non-destructive means of measuring moisture content, but are not as accurate as the oven-dry method. They are useful for measurement of moisture contents between 8% and 25%. Insulated electrodes are available for use on timber that may have a wet surface. Note that the moisture content at a particular depth cannot be measured by inserting the electrodes to that depth. Use of these meters is covered by AS/NZS 1080.1:1997, which also contains tables of correction factors for timber species and temperature.
- Capacitance type moisture meters measure the capacitance of timber, usually between a pair of plates inserted in a stack of timber. They are finding increasing use in industry as they do not mark the timber and may be easier to use in a production environment. They are sensitive to the timber density and hence are less suited to mills processing species with a range of densities. Their use is not included in AS/NZS 1080.1:1997.
References: Australian/New Zealand Standard AS/NZS 1080.1:1997 "Timber - Methods of Test 1 - Moisture Content"