Water and Wood

Some options for measuring moisture in lumber during the kiln-drying process. 1998.

An excerpt from Drying Oak Lumber

by Eugene M. Wengert

Department of Forestry
University of Wisconsin
Madison, WI

Part 1 (of 3) from Section 3:
Interaction of Wood and Water

Main Points
How water exists in wood
Moisture content measurement--oven-dry method
Moisture content measurement--electrical resistance meter
Shrinkage in the three directions

Water In Wood

The living tree uses water as the medium to convey food between the roots and the leaves. The living tree, therefore, contains large amounts of water. For every 1 pound of wet, living wood, about 55 to 60 percent is dry wood (that is, cells) and 40 to 45 percent is water. Some of this water is contained in the cell lumen and some is contained within the cell wall itself. Water in the lumen is called free water while water in the walls is called bound water. These terms arise because the free water is not held chemically within the cell, while the bound water is held by hydrogen bonding. Free water evaporates as easily as water from a tea kettle; however, bound water requires a little extra energy for evaporation. Further, and most importantly, when the bound water leaves the cell walls, the cells shrink. When all the free water and all the bound water has been removed from a cell (at temperatures no greater than 217o F), the cell is at 0% MC; this is also called bone-dry or oven-dry.

Measuring Moisture Content--Oven-Drying Method
In order to avoid confusion when discussing MC and to prevent any variations in measured MC values attributed to the measuring techniques used, standard methods of measuring the MC of wood have been established. These methods are described in detail by the American Society for Testing and Materials (ASTM), Standard D-4442. The important features of the Standard that apply to lumber drying are summarized here.

In lumber drying, the MC of wood is expressed as a ratio of the amount of water in a piece wood compared to the oven-dry weight of wood. This is called the "moisture content on the oven-dry basis." In lumber drying, MC is always expressed in percent. Specifically,

(Amount of water in the wood)
% MC = ------------------------------- x 100
(Oven-dry weight of wood)

(Wet weight - Oven-dry weight)
= -------------------------------- x 100
(Oven-dry weight of wood)

The wet weight in the above formula is the weight of the piece at the unknown MC. It could be the weight when green, partly-dried, air-dried, kiln-dried, or in use. The wet weight is also called the green weight, the current weight, or the present weight.

The measuring units used for weighing--pounds, grams, kilograms, or ounces--are not critical, so long as all weights are in the same units. Do not mix measuring units. However, because the decimal system of grams or kilograms lends itself to calculations, these units are commonly used.

When an electronic calculator will be used, the following version of the formula is suggested, as it will save a little time when entering numbers and may reduce entry errors.

(Wet weight)
% MC = {[-------------------] - 1 } x 100
(Oven-dry weight)

The calculation procedure when using an electronic calculator is summarized in Table 7.

Moisture Content Determination
The oven-drying method of MC determination is the fundamental MC measuring method when drying oak lumber. All decisions on how to dry oak successfully depend on the knowledge of the correct MC of the lumber. Therefore, it is essential to be intimately familiar with this procedure. Variations cannot be permitted if quality is the #1 concern in drying.

First, a sample of wood is prepared. For small pieces, the entire piece may be used as the sample; for lumber and large pieces of wood, a smaller piece, called a moisture section, full thickness and full width but only an inch along the grain (or along the length of the lumber) would be used. The moisture section chosen for oven-drying usually is free of knots, edge splinters or loose slivers, and bark.

The moisture section is weighed, immediately after cutting, to the nearest 0.01 grams, if under 100 grams total weight, or 0.01%, if heavier. This wet weight is written on the section with a permanent marker.

The section is then placed in an oven at 215o to 217o F and left there until it stops losing weight (about 24-hours, but it can be longer; it seldom is shorter). To determine if the section is oven-dry, the section is weighed, put back in the oven for another hour, and then weighed again. If the two weights are the same, then the section is oven-dried. The oven-dried weight is then written on the section with a marker.

It should be noted that an oven with internal forced circulation (that is, one with an internal fan) is much better than a natural convection oven. When the value of the lumber in a kiln is considered, it does not pay to cut corners by using a noncirculating oven when measuring the critical variable of MC.

Alternative Method of Oven-Drying: Using a Kitchen Microwave
An alternate method of oven-drying is available using a kitchen-type microwave oven. The method is fully tested and accurate if and only if the following criteria are met: the microwave has a carousel tray; the sections must be placed on the outer edge of the tray; the sections must not touch each other; and the oven must be run on medium low (360 watts) to low power (200 watts). Medium power (490 watts) can possibly be used when four or more sections are in the oven. If the sections begin to smoke, then the power level is too high. Other techniques and equipment will not be satisfactory.

Typical microwave oven-drying time is 20 to 30 minutes for green pieces and 10 to 15 minutes for dry pieces. After 20 minutes (or less if the initial MC is lower), the section is weighed, dried for another minute, and reweighed. If the two weights are the same, then the section is oven-dried. The calculation procedures for MC are the same as with the hot air oven.

Exercises: Moisture Content Calculations
As MC is the most critical measurement made during drying, several problems are presented (Appendix B) to assure that the reader can properly calculate MC. These exercises should be completed before proceeding further. Note: For kiln drying, MCs are calculated to the closest tenth of a percent, such as 34.7% MC, rounding the answer. (Answers are rounded up when the hundredth is greater than 5; that is, 34.752 would be rounded up to 34.8, while 34.748 would be 34.7.)

Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison

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