To determine the moisture content of timber by oven dry method
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Apparatus
Oven, weighing device, timber specimen.
Reference:
D2016-(Reaffirmed in 1983)
Scope
This method covers the determination of the moisture content of the wood.
Background
The moisture content of timber is the quantity of moisture or water contained in it, expressed as a percentage of dry weight of timber.
(m.c. = ((w1=w2)/w2) x 100
Where
W1 = Weight of saturated specimen of timber
W2 = Oven dried Weight of Timber
m.c. = Moisture Content Measured in Percentage (%)
In new cut tree, the amount of moisture is about 50%. We require reducing it to about 10-12%. Moisture Content in wood is in tow form, moisture which is present in cell walls called as “Bound Moisture” and the moisture which is present in wall cavities called as “Free Moisture”. The moisture content at the critical stage when cell walls are saturated but no free moisture is present in cell cavities is known as “fiber saturated point” which for most wood vary from about 25% – 30%. The moisture content for four characteristic conditions are:-
(i) In Green Timber 30% – 25%
(ii) In Air Dried Wood 15%-20%
(iii) In Kiln Dried Wood 15%-10%
(iv) In oven dried wood 0%
This has been the most universally accepted method for determining moisture contents in wood-working factories.
Hygroscopic Timber
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Timber has the ability to absorb water from the atmosphere. All timber being hygroscopic, attempt to achieve on equilibrium stage with its environment i.e. there is no interchange of water.
Effect of moisture on properties of wood
Moisture presents in the cell walls and cell cavities, may be removed up to fiber saturation point, without any effect of other than reduction in bulk density but any other reduction in moisture content results in the shrinkage of timber. The shrinkage of a kiln dired wood and oven dried wood is about “One Half” and “Three fourth” respectively. The wrapping of timber is due to unequal shrinkage of various portion of the timber. Also the wood cells vary in thickness; therefore, unequal shrinkage take place, resulting in wraping the timber.
Properties
Take the specimen and weigh its initial weight accurately. Then put it in the oven at a temperature of 103+2 degree Celsius for 24 hours to evaporate the moisture. After this, take it out from the oven and again weigh it. Now calculate the moisture content of the specimen by the formula given above.
Conditions of Oven Specimen
Oven should be maintained at a temperature of 103 + 2 degree Celsius throughout the drying chamber for the time required to dry the specimen to constant weight. It should be thermostatically controlled and vapors should be allowed to escape out. Specimen should be a piece of wood having more moisture. It should be selected from a sample that represents the weight full of cross-section not less than 25 mm.
Weighing Device
A scale or balance that will weigh a specimen within an accuracy of +0.2%. Harvard trip balance, triple beam balance and direct reading balance are some of the examples.
Precautions
Don’t heat the specimen longer than 24 hours. Vent the oven so that the evaporated water may escape out. Take the initial and final readings accurately. No chemical will be present in the wood.
Limitations of Oven Drying Method
It is a destructive method in which the sample representing the lot must be cut to produce such specimen. It is a time consuming method. Its accuracy is limited if the wood has been treated with some preservation.
Observation and Calculation
Dimensions of Specimen: __________1.932 x 1.056 x 1.080____
|
S/No
|
Saturated Spciment Weight (W1) g
|
Oven Dried Weight (W2) (g)
|
Moisture Content (%)
|
|
1.
|
16
|
15
|
6.67
|
|
2.
|
20
|
19
|
5.26
|
|
3.
|
18
|
16.5
|
9.09
|
|
Mean Value
|
7.00
|
