An excerpt from Drying Oak Lumber
by Eugene M. Wengert
Department of Forestry
University of Wisconsin
Madison, WI
Part 1 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
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.
Units
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.)
Part 2
Moisture Content Measurement With Electric Meters
All electrical moisture meters for wood measure an electrical property of the wood and then convert the value of this property to MC by a previously established relationship. There are four basic types of electric moisture meters classified by the property that they measure:
a) resistance,
b) capacitance,
Only the resistance type meter is in widespread use for oak lumber; rarely, a noncontact capacitance admittance meter will be used in-line to monitor the incoming MC of every piece of dried lumber at a furniture or cabinet plant rough mill. More widespread use of this or other in-line meters is warranted, as over three-fourths of the manufacturing problems in the furniture or cabinet plant are related to improper MC of the lumber.
Characteristics of the Electrical Resistance Meter (12)
The electrical resistance meter is an ohmmeter, measuring the resistance between two needles that are driven into the wood. The following are important characteristics and operating procedures of the pin- or needle-type electrical resistance moisture meter.
MC range. The working range is 7 to 25% MC, with most measured MC values being within 1% MC of the true MC. Below 7% MC, the resistance is too high to measure easily; above 25% MC, the resistance change with changing MC is too small to be accurately measured, and also is affected by other variables, in addition to MC. In other words, above 30% MC, meter readings are subject to large variations (10% MC or more) from the true MC. The meter itself is usually quite accurate in measuring resistance; the inaccuracy or uncertainty comes from the variation of the resistance of wood as a function of MC.
Species. The original, standard calibration is based on Douglas-fir; this calibration agrees well with oak (Table 8) and most other North American hardwoods. With the advent of the microprocessor, moisture meters now have several different species calibrations built-in.
Temperature. There is a very large effect of temperature. For the highest accuracy, use the correction table that comes with the meter. A rule-of-thumb is to subtract 1% MC from the meter reading for every 20o F that the wood is above 70o F. Conversely, add 1% MC for every 20o F below 70o F. Again, the microprocessor has resulted in built-in temperature calibrations in some meters.
What temperature should be used when measuring hot lumber in a kiln? For wetter lumber, use a temperature between the dry-bulb and the wet-bulb temperatures; and for drier lumber, use the dry-bulb temperature. However, experience with oak lumber in the kiln has shown some uncertainty in the converted MC values. Of course, the meter can be used confidently only for MCs below 30% MC.
Grain angle. The needles must be parallel to grain at MCs above 15% MC.
MC gradient. To estimate the average MC when there is a moisture gradient in the lumber when the shell is drier than the core, the needles must be driven one-fourth of the lumber thickness for rough lumber and one-fifth for planed lumber.
Preservatives and glue. The effect of preservatives and glue lines are usually insignificant, especially below 15% MC, but this should be verified for the chemicals being used by running an oven-dry test to be certain.
Surface moisture. Liquid moisture on the surface of the lumber can be wicked down the probe and thereby give an incorrect (too high) reading. Do not use the meter when there is surface moisture present.
Condensation on meter and/or probe. If the meter or probe is brought from a cold into a warm environment and the meter is colder than the dew point temperature of the warm air, then moisture will condense on the meter and probe. The condensation may give an extremely high reading or may just give a reading of 10% MC. Low MCs cannot be measured until the moisture is evaporated; it may take many hours for this moisture to evaporate. An operating guideline is "Don't take the meter into a hot kiln unless the meter has been thoroughly warmed." Excessive heating, above 120o F, can damage the meter and shorten battery life.
Static electricity. In a very dry environment (under 30% RH especially) or when very dry lumber is planed, a static charge can develop on the lumber. This static will result in erroneous readings by the electric meter. Often the meter will exhibit erratic behavior of the MC readout. The meter may also begin to indicate a MC value before the needles even touch the lumber. In extreme cases, it may be necessary to take the MC readings on a grounded metal table to dissipate the static charge. The meter operator should not wear static prone clothes, such as a wool sweater.
Part 3
Shrinkage
When the drying of a green cell begins, the free water from a cell evaporates first. As the free water is removed, the MC of the wood is decreased from green to approximately 28% MC. At 28% MC in a cell, all the free water has been removed, but the cell wall is still fully saturated. The point at which all free water is gone, but no bound water has been removed is called the fiber saturation point (FSP). Note that the FSP of 28% MC refers to the MC of a cell and not to the average MC of a larger piece of wood. No shrinkage has occurred up to this point (that is, in drying a cell from green to FSP), unless there is collapse, which is rare in oak lumber. However, any drying below 28% MC results in removal of the bound water from the cell wall; with this removal, shrinkage occurs.
As drying continues below FSP, more and more water is removed until there is no appreciable amount of water left in the wall. This level is called 0% MC or, if done at 215o F (as detailed above), oven-dry. Shrinkage continues from FSP to 0% MC in a direct, linear proportion to the MC loss.
The cell wall always has an affinity for water. This characteristic, called hygroscopicity, means that dry wood will not stay dry if the wood is exposed to a higher RH. Therefore, if the cell wall has lost moisture and then is exposed to high relative humidities, the wall will absorb water until equilibrium between the air and the wall is obtained. So wood not only dries and shrinks when exposed to low humidities, but it also regains moisture and swells when exposed to higher humidities.
Temperature does not make the cell shrink or swell appreciably; the only factor of importance that causes shrinkage is moisture loss and the only factor that causes swelling is moisture gain. In turn, the only factor that causes the MC to change is the relative humidity (RH) of the environment. So, changes in RH cause shrinkage or swelling.
In general, the denser the wood, the more it will shrink and swell. As the amount of shrinkage is directly related to checking, the denser woods are harder to dry. Therefore, as mentioned in Section 2, density (or specific gravity) is a good predictor of drying behavior.
Wood shrinks the greatest amount in the tangential direction, as mentioned earlier. Radial shrinkage is about half of the tangential.
Longitudinal shrinkage is usually negligible. However, there can be appreciable longitudinal shrinkage in wood in the juvenile core or in tension wood. This longitudinal shrinkage will cause bow and crook (or side bend), and may contribute to twist. As an example of why longitudinal shrinkage causes warp, consider a quartersawn piece of lumber with one edge having juvenile wood and the remainder of the piece having mature wood. The juvenile wood edge will shrink lengthwise, while the rest of the piece will shrink very little lengthwise. This difference results in crook toward the pith. The same scenario, but with the shrinkage differences being between faces rather than edges, is a major reason why lumber bows.
With the basic information on shrinkage presented in the preceding paragraphs, the shrinkage behavior, should be expected as wood dries from green to 7% MC. The following explanations should help in understanding the illustrated behavior:
The round piece has become oval shaped. This is because there is more shrinkage tangentially than radially.
The square piece on the right has become diamond shaped. With the grain pattern resulting in the tangential direction being from corner to corner and with more tangential shrinkage than radial, the diamond shape results. The other square has become rectangular with the tangential dimension being slightly smaller than the radial direction as a result of the shrinkage difference.
The quartersawn lumber pieces have stayed flat, but have decreased size in width less, percentwise, than in thickness. Quartersawn lumber, when used where the MC will fluctuate, will hold paint, varnish, and other finishes better with less finish cracking than flatsawn lumber because a quartersawn surface will not move so much as a flatsawn surface would. Careful examination of the quartersawn piece with the pith shows that it is a little thicker in the center than at the edges. This results because thickness shrinkage near the pith is radial, while at the edge the piece shrinks tangentially in thickness.
The flatsawn lumber has cupped toward the bark. This is a natural tendency, because the bark face of the lumber is more tangential than the pith side. As a result the bark side will shrink more than the pith side. The difference in shrinkage will be greater if the lumber is sawn from an area closer to the pith. (Combine this information with the fact that logs are getting smaller, and the result is a greater tendency for today's lumber to cup than lumber had in the past. Further, because most lower grade lumber is from the central sections of the tree (that is, near the pith), there is a tendency for lower grade lumber to cup more than upper grade.)
The mixed grain lumber piece exhibits the same cupping tendency as flat sawn lumber, for the same reasons. However, the amount of cup is less for mixed grain lumber.
Shrinkage Exercises
Several shrinkage problems are presented in Appendix C to assist in achieving a better understanding of shrinkage. Remember, probably 80 percent or more of the drying and processing problems of wood are shrinkage related (which in turn means MC related), so a good understanding of shrinkage is essential for a good understanding of lumber drying. The reader is encouraged to do the shrinkage exercises before preceding further.
Table 7. MC calculation procedure (for most calculators).
1. Clear machine, if necessary
2. Punch in green weight
3. Push divide key
4. Punch in oven-dry weight
5. Push minus key
6. Punch in 1
7. Push multiply key
8. Punch in 100
9. Push = and then read % MC
Note: For some calculators, it may be necessary to push = between steps 4 and 5, and between steps 6 and 7.
Table 8. Suggested species corrections for the pin-type electrical resistance moisture meter (13).
Uncorrected Meter Reading, %
8 | 10 | 12 | 14 | 16 | 18 | 20 | 22 | 24 | |
| Red oak | 0 | 0 | -.1 | +.1 | 0 | 0 | 0 | -.4 | -.2 |
| White oak | +.2 | +.5 | +.6 | +.7 | +1.0 | +1.3 | +1.8 | +2.0 | +1.2 |
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison
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