The Importance of Measuring Moisture Content in Wood
by David Welch
Improper moisture levels in wood can affect the quality and durability of a product as much as, if not more than, product design or the actual production process itself. With the rising costs of raw materials and the decline in lumber quality, it is important that the wood being received is checked immediately for proper moisture content levels. A wood product manufacturer’s reputation, not to mention bottom line, depends upon it.
As long as manufacturers are dealing in wood, they are responsible for moisture control. "There is always moisture in wood," said Fred Lamb, ‘Drying and Rough Mill’ editor of Modern Woodworking and professor and extension specialist, Department of Wood Science and Forest Products at Virginia Tech. "We do not use oven dry or zero moisture content wood. The real issue is whether or not the moisture content of the wood is in balance with the equilibrium moisture content of the environment. Is the wood stable or is it shrinking or swelling? Shrinking is usually worse than swelling. Shrinkage generally brings splits, cracks and warp. However, swelling can cause warp and issues such as sticking door and drawers.
"Furniture stock kiln dried to 7 percent moisture content means that the water in a volume of that wood weighs 7 percent of the weight of that volume of wood oven dried (oven dry meaning zero moisture content). Whether we use a moisture meter or the oven dry method (which is the standard), moisture content is measured in percent."
Wood should be dried to the moisture content (MC) percentage of the environment it is intended to occupy. "The relative humidity in most homes and offices in the U.S. averages 30 to 40 percent. This equals 6 to 7 percent equilibrium moisture content, which means that wood in interior locations will average 6 to 7 percent MC. Therefore, lumber intended for interior use should be dried to 6 to 7 percent MC and should be kept at this MC prior to and during manufacturing. Wood is constantly adjusting to its environment. It is constantly losing moisture to or gaining moisture from the air that surrounds it. Therefore, even after wood has been dried to the proper MC, its MC can change during storage, manufacturing or use.
How Moisture Affects Wood
"Any kind of extreme moisture content (very high or very low) can affect wood in a variety of ways," said Michael Boyce, director of creative services, Wagner Electronics. "Wood that is too dry can shrink, split or crack. Wood that is too wet will warp, swell or have adhesive and finish failure. Because wood will always have some moisture, it becomes necessary to find the equilibrium moisture content in the wood. You can determine the equilibrium moisture content (EMC) in your part of the country by hanging thin, small samples of a wood species in your shop or plant and taking daily readings. Once the samples’ moisture content remains constant, you have reached the equilibrium level. This level will change from season to season, but you’ll know the range to work within."
"Moisture affects at least eight wood characteristics," according to Bob Little, Ph.D., technical director and wood technologist, SII Dry Kilns. "It affects size in that wood shrinks as it dries and swells as it takes on moisture, and strength because dry wood is stronger but less flexible than green wood. Finishing properties are affected in that moisture escaping from wood of too high MC may create finishing imperfections, and finish applied to too dry wood may not flow on the surface properly. Gluing properties are affected because some adhesives do not bond well with wood of high MC. Wood of too high MC may cause arching when RF cured. If wood MC is too low, the adhesive may dry before the parts are assembled.
"Moisture affects the weight of wood, which affects cost. Costs are higher when shipping the excess water in incompletely dried wood. Machinability is affected when the surface of too wet wood ‘fuzzes’ when planed; the surface of too dry wood may chip out.
"Lumber dried to the proper moisture content, under controlled conditions, will have a lower incidence of drying-related degrade. So the degrade-level of wood is affected by moisture as well. For example, wide boards may cup during drying. Over-drying leads to excessive cupping. Excessively cupped boards may split when run through the feed rolls on a planer. As wood is dried below 5 percent MC, the level of drying-related defects will increase markedly. Finally, wood moisture affects decay and insects. Wood above 19 percent MC is much more susceptible to attack by insects and decay."
Moisture Content Measuring Technology in Wood Product Manufacturing
There are ways to guard against receiving wood at improper moisture levels. "Put in a good quality control program in the front end of the rough mill (or at the receiving dock for operations without a rough mill) to monitor the MC of all incoming lumber," said Lamb. "In the future there will be more use of in-line moisture meters. Sampling today is inadequate for good monitoring and control because there are too few samples and too few consistent procedures."
Boyce agrees, "In the new millennium, the first in-line continuous MC technology designed specifically for wood products manufacturing will be available. This new technology measures each piece of wood that passes through its sensors automatically and completely. Solving the MC problems in a wood products manufacturing operation requires MC technology at the receiving point.
"You must prevent any piece of wood having an average MC outside your target MC range (usually 6-8%) from entering your operation. To do this, you should begin by setting that MC standard with your suppliers for all wood that you buy. Put it right on the purchase order. To ensure that your standard is being maintained at all times, you should install a continuous in-line MC monitoring system at your receiving point to identify and drop out any piece that violates the target range. Your system also should have the capability of generating supplier performance reports for each delivery. This combination is unbeatable, and can ensure that no piece of wood with an average MC outside your target range gets into your operation in the first place."
Until the in-line systems are widely available, however, the hand held moisture meters can be an effective way to monitor incoming materials. "Although weight is by far the most accurate and reliable means of determining wood MC, wood’s electrical properties may also be an indicator of moisture content," said Little.
"Commonly used, hand-held moisture meters estimate moisture content by measuring an electrical property of wood. Electrical MC determination should be considered accurate only within the range of 5 to 25 percent MC. Corrections for wood species and temperature must be applied to the raw meter reading to insure accuracy. Since most secondary wood manufacturers would wish the MC of their wood to be between 5 and 25 percent, an electrical MC meter can be used to check the MC of several samples of incoming wood."
Economics of Improper Moisture Control
Moisture in wood can affect the manufacturing process as well as the finished product. "When you work with wood, you must be cognizant not only of the characteristics of MC in wood, but also of the effects MC has on your work in progress and your finished products," said Boyce. "Obviously, shaping and sizing a piece of wood that is subject to drying beyond its current MC can change both its finish size and shape, which will directly affect the work you have done and the product you are creating. Moreover, joining two pieces of wood brings in an entirely new set of circumstances. If you create a panel by joining two or more boards that have different MCs, there is a three-to-one chance that you will have problems at the glue joints and when sanding. If you then apply a laminate to that panel, there is an equally high probability that you will have problems with delamination."
"Changes during, or improper monitoring of moisture content in, the manufacturing process cause at least a dozen different specific manufacturing problems like end splits, cupped and curved panels, splitting, chipped grain, fuzzy grain, sunken joints, uneven surfaces, glue failure, delamination and a number of problems with finishes," Boyce adds. "The secondary wood manufacturing and distribution segments of the industry are not as familiar with MC, or as understanding of MC, as the primary segment is. Part of that reason is a lag in technology. While MC technology has spawned MC monitoring, measuring and management systems in the primary industry sector, they were designed specifically for that application and are scarce in the rest of the wood products manufacturing arena.
"The results of improper MC in the manufacturing process, depending upon where it is identified, end up either as waste or a customer claim. The costs of waste, 75 percent of which we can relate directly to improper MC, are hard to get a handle on because, believe it or not, not a lot of statistics or study on the subject exist. Many industry experts estimate that waste costs an average of five percent of sales. I have heard individual operators say that they are losing $50,000 and as much as $250,000 a year to waste. The long-term effect to your business from a customer claim is incalculable. Many studies have shown that for every one complaint you hear, there are up to twenty-six more complaints existing that are not voiced - business just is taken elsewhere."
And if a manufacturer receives a shipment of improperly measured lumber? "Send it back," says Lamb. "You can not manufacture a quality product from lumber that is not properly dried to the correct MC. It is cheaper and easier to dry it correctly than it is to attempt to correct improper MC in the manufacturing plant. Lumber should be dried correctly where it is most efficient and effective to do so - in the drying operation, not in the manufacturing plant (see Lamb’s column on drying in the October 1999 issue of Modern Woodworking). You need to include a Temperature - Relative Humidity - EMC chart. This is the real heart of the moisture problem in manufacturing. Wood changes MC (and therefore shape and size) with changing environmental EMC, which is affected by temperature and relative humidity. Too many operations think that because they have water spray in the plant, their moisture problems are solved. Unfortunately, it is more complicated than that. There still can be varying EMC’s in the plant due to varying temperatures. Air flows and make-up air complicate the situation further."
The agreement between the supplier and the manufacturer should be detailed. "The moisture content of purchased wood should be clearly stated in the contract between the supplier and the manufacturer as should the technique used to check moisture content," said Little. "If a shipment is determined to have a moisture content outside of the specified range, the shipper should be contacted and some remedy negotiated. Few remedies are available if a shipment is too dry; defects associated with over-drying are permanent. Over-dry lumber should be carefully inspected. If the defect level is too high for the intended use, the shipment should be refused or a lower price negotiated. Lumber can be re-dried in a kiln if the MC is too high. In the case of international shipments, it likely will be cheaper to negotiate a lower sales price to compensate the purchaser for local re-drying. In domestic shipments, a shipper may elect to have the lumber returned for re-drying."
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