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Moisture Content of Timber The amount of water in a piece of wood is known as its moisture content. Because this is expressed as a percentage of the dry weight of the piece, not of the total weight, it is possible to have moisture contents of well over 100%.
The moisture content of green wood varies greatly from one species to another. Moisture content can vary between apparently similar pieces of the same species and in addition there may be differences, between and within species, in the rates at which moisture is lost from timber during drying.
These inherent differences in timber mean that it is important during the drying process to be able to monitor moisture content and check that the drying process is proceeding correctly. The following sections describe the methods which can be used to determine timber moisture content.
- Moisture Content Determination by The Oven Drying Method The oven drying method is the standard way of determining wood moisture content. With this method a piece of wood is initially weighed and then dried in an oven at 103°C ± 2°C. Drying is continued until the piece is completely dry (when no further weight loss occurs) and this oven dry weight recorded. The loss in weight during drying indicates how much water was originally present in the piece and the moisture content can be calculated simply, as follows:
|Initial moisture content (%) ||= Initial (wet) weight - Dry weight |
|x 100 |
For example, if the initial weight of the piece was 30.51g and its dry weight 22.60g, then the difference of 7.91g is the weight of moisture initially in the piece and its initial moisture content would be:
(30.51 - 22.60)/22.60 x 100 = 7.91/22.60 x 100 = 35.0%
Alternatively the formula can be written:
Moisture content (%) = [(Initial weight/Dry weight) - 1 ] x 100
So that only the division sum needs to be carried out:
[(30.51/22.60) - 1] x 100 = 0.35 x 100 = 35.0 %
When a particular piece of wood is oven dried, the moisture content value obtained (as above) is an average moisture content for that piece and it is important to emphasise that actual moisture contents at different locations within this piece may vary considerably from the average value.
Pieces used for oven drying will normally be unsuitable for further use and therefore with this method it would be wasteful to use large pieces. Instead small pieces of wood are removed from selected planks or boards and used as follows to estimate moisture content. First, cut off and reject a length of at least 230mm from the end of the board or plank; the more rapid drying through the end grain will usually cause this portion to be drier than the rest of the piece. A full cross-section piece (test section) about 15mm wide is then cut from the newly sawn end and used for moisture content determination.
This should be free from knots as these tend to falsify the result.
The initial weighing of the test section should be made as quickly as possible after it has been cut, after removal of sawdust and any loose slivers of wood. Errors, due to unavoidable delay before weighing, can be minimised by placing the sections in polythene bags immediately after cutting. The sections should be placed in an oven until they cease to lose weight, weighings being made at intervals until drying is complete. Because wood will absorb moisture from the atmosphere, weighings must be made immediately after removal from the oven. With any particular oven and test section, experience will soon indicate the approximate time taken to complete drying and so periodic weighing becomes unnecessary. The time will actually depend on the moisture content, the size and the species of the piece, and on the number of other sections packed into the oven. It may vary from about 6 to 18 hours, but results to within 1 or 2% of the true moisture content can be obtained in much shorter times. If a quick result is required on relatively dry wood a close approximation can be obtained by cutting a number of 3mm thick sections, immediately weighing them all together and spacing them out for about half an hour in an oven containing no other test sections, and then reweighing.
With oven drying, fresh sections should not be loaded into an oven shortly before taking others out for final weighing, since the drier pieces will temporarily gain weight by absorbing moisture released from the wetter ones.
Oven drying requires a well ventilated oven which can control the temperature to between 101 and 105°C and a balance for weighing the test samples. The balance should have a capacity of about 200g and be capable of detecting differences of 0.005g, an automatic type is recommended as these give an instantaneous reading.
Infrared ovens are available for rapid drying. In some of these the heating lamps are directed on to the test section on the pan of a balance (incorporated in the equipment). Drying takes from about 3 to 10 minutes according to species and moisture content. However only one piece can be dried at a time and experience is needed to avoid overheating which can cause inaccurate results.
- Interpretation of Moisture Content Determinations Using the Oven Drying Method
The oven drying method is usually an accurate way of estimating the moisture content of a test section of wood. Indeed recommended moisture contents for most uses are based on values obtained by the oven drying of timber which has been allowed to equilibrate to particular service conditions.
However, the moisture content of one test section may not be a typical value for the whole load of timber being dried because moisture content will vary within a piece and between different parts of the same piece and between different pieces of the same species. To obtain a more accurate estimate of the average moisture content for a kiln load, it is necessary to take test sections from a series of sample pieces incorporated in the load. If the moisture contents from all these test sections are then averaged, this should give an accurate estimate of the overall average moisture content of the load and an indication of the range of moisture content in the load, provided the samples have been selected correctly.
- Moisture Content Determination be Electrical Moisture Meters
Moisture meters are available which can give an instant indication of the moisture content of a piece of wood by measuring one of its electrical properties. The electrical resistance of wood increases rapidly with decreasing moisture content once this is below about 25 to 30 %, whilst the capacitance and dielectric loss both decrease with a fall in moisture content at all levels.
Most commercial moisture meters measure the electrical resistance between two electrodes which are driven into the wood. The electrical resistance of the wood is then converted into percentage moisture content and displayed on the meter. The following sections are concerned with the use of this type of meter.
- Good electrical contact with the timber.
To avoid artificially high moisture content readings, it is important to maintain good electrode contact with the wood as the measurements are taken. With short needle or blade electrodes this can usually be achieved by applying a controlled pressure to the electrode holder; with long insulated electrodes, the hammer action, by which they are driven into the wood, ensures good electrode contact.
- Measurable range of moisture content.
With electrical resistance meters physical limitations restrict the range of moisture content which can be measured. The fibre saturation point (usually between 25 and 30 % moisture content) is a practical upper limit because above this the differences in electrical resistance are small and cannot be accurately interpreted. A lower measurable limit of 7% moisture content is imposed by the high electrical resistance of drier wood. Capacitance and dielectric loss type meters can operate at all levels but the density of the test pieces has to be known to obtain an accurate estimation of their moisture content.
During drying the average and core moisture contents in thick pieces will be significantly higher than those in the outer layers. Meters with short electrodes may therefore consistently underestimate the overall moisture content - they may also give inaccurately high results if, for example, the surfaces of the timber under test have been exposed to an unusually damp atmosphere or to rain. With equipment having long insulated probes these problems are largely overcome, and the electrodes can be driven in to provide moisture content readings at considerable depths. It is also possible to obtain a rough indication of the moisture gradient within a piece because the resistance between the exposed electrode tips can be recorded at various depths as the electrode is driven into the piece.
- Variation in electrical resistance.
The electrical resistance at any given moisture content can vary considerably between pieces of the same species and between pieces of different species. Manufacturers can provide meters with multiple scales which provide a corrected reading for the more common commercial species. Alternatively correction factors can be applied to account for this source of error. The resistance differences between pieces of the same species are more difficult to account for and it is normally accepted that moisture content estimations using resistance meters can be inaccurate by up to ± 2%.
- The effect of temperature.
The electrical resistance of wood at any given moisture content decreases as the temperature increases and the effect of temperature is greater the higher the moisture content. When testing timber at moisture contents below or around 15% and when the the temperature of the wood is known, an approximate correction can be made by subtracting 1% for each 8°C below 20°C.
Because it is not easy to measure the temperature of the wood between the tips of the moisture meter electrodes, the accurate estimation of moisture contents is not normally possible during kiln drying when temperatures are high. The moisture contents of samples which have been removed from the kiln and allowed to cool can be determined using meters in the usual way. However, as with the oven-drying method, a number of samples would be needed to take account of variability between pieces in the load.
The presence of certain chemicals in timber, particularly salts from preservative or flame retardant treatments, can cause a marked decrease in electrical resistance. Meter readings from treated wood or wood accidentally wetted by sea water will therefore give moisture content readings which are artificially high. This effect will vary considerably depending on the amount and type of chemical used and its distribution in the wood. At 20% moisture content, for example, the actual moisture content of treated or contaminated wood may be between 1 and 5% lower than the value indicated by the meter; consequently it is rarely possible to provide reliable correction factors for this effect.
With certain wood-based panel products, meter moisture content determinations may be artificially high; here a lower electrical resistance will often be caused by the adhesive present. The effect is particularly noticeable with plywoods bonded with phenolic adhesives, where meter measurements of moisture content may be up to twice the real value. Again it is not possible to allow for this effect accurately because the error varies according not only to the type of board but also the type of bond and its history and location within the board. Some meter manufacturers supply general correction tables for use with wood particleboards.
- The manufacturer's instructions.
It is always important to follow precisely the manufacturer's instructions supplied with the meter. For example, electrodes inserted into end grain will not yield accurate moisture content values.
The estimation of timber moisture content is a straightforward operation whether oven drying or a moisture meter is used, provided the limitations of each method are appreciated. In both cases repeated measurement and careful interpretation of values obtained are required to obtain an accurate estimate of moisture content. A useful practical approach is to utilise the advantages of both methods.
- Advantages of the Oven Drying Method.
It can be used to estimate the complete range of moisture content (electrical resistance meters can only measure moisture content accurately between 7 and 25/30 %).
It normally gives a direct and definitive moisture content value for the piece (meter readings are liable to inaccuracies depending on for example, variable resistance and effects of electrolytic contaminants).
It can be used to obtain a definitive measurement of moisture gradient with depth of the piece (meter readings are liable to inaccuracies caused by variation of resistance within the timber). It can be used to estimate moisture content or moisture gradient with depth of a piece during a kiln run. (Moisture meters cannot normally be used to determine moisture content accurately at kiln-operating temperatures).
- Advantages of Moisture Meters.
For many uses, frequent moisture content readings can be taken without loss in value of the timber (oven drying results in a small loss in value).
Many readings can be obtained quickly and with minimal effort (oven drying is a slower process and more labour intensive).
Recommended practice is to monitor moisture content and moisture distribution during drying with the oven method. For each load several samples of wood should be monitored and care is needed to select and position these so that the progression of drying can be monitored and controlled accurately. Once the wood is below fibre saturation point, moisture meters can be used to give estimates of the moisture gradients along a piece and to confirm that the pieces being monitored by the oven method are indeed representative of the load.