kr'di f* lift C.f ELECTRICAL MCISTLRE METERS ECI> WOOD Revised, July 1944 / r~ of f DRY U No. Pii4e UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation vitb the University of Wisconsin ELECTRICAL MOISTURE METERS FOR T70OL By M. E, LU1JLAF, Senior Engineer The measurement of moisture in wood by rapid electrical methods is now fairly well established in the United States; about seven or eight years have elapsed since electrical moisture meters for wood first made their appearance in a practical form. The Forest Froducts Labor?- :as developed several types of eleci.ric.al moisture meters, including the "blinker," which has been on the market for a number of years; a caracity- type machine, which has never been commercialized; and a recently patented resistance type employing a novel Wheatstone bridge balancing circuit (see list of references at end of article). Electrical moisture meters have an advantage over other methods for determining the moisture content of wood because of their convenience and speed, the time required to determine the amount of moisture in any piece of wood being only a few seconds, They are therefore adaptable f sorting lumber and veneers on the basis of moisture content. Furthermore, they offer the only practical means thus far developed of determining the moisture content of finished woodwork in place without serious injury to the wood. Two general types of instrument are available, one evaluating the moisture content by measuring the electrical resistance of the wood and the oth,r by measuring the electrical capacity of the wood (see list of makers at end of article). Resistance Meters Below the fiber-saturation point (about 25 percent moisture con- tent) the electrical resistance of wood varies widely with changes in moisture content, the resistance increasing as the moisture content de- creases. This enables one to evaluate the moisture content ty measuring the electrical resistance of wood. The resistance increases with a fall- ing temperature and decreases as the temperature rises. Furthermore, there are variations among different species that must also be taken into account. Fortunately, reasonably accurate corrections for teopera- ture and spteoies variations can be rather easily made. Makers usually furnish correction data with their instruments. Electrical contact i6 generally made by driving four needle points into the wood to be tesl 10 that the flew .of «U is parallo'l to the grain* The two points of like poladty are ukurXIv "-raced or.c-h inch apart, and the points of unlike polarity about 1-i/U inches apart in RllU6 the direction of current flow. All the points are usually mounted in a block of insulating material and' arranged so that they can be readily driven into the v/ood and withdrawn again after testing. Several different designs are available. Contact may also be made by clamping surface plates to the opposite faces of lumber, veneer, etc. The needle-point type of electrode is thought to be preferable to the surface contact type in making resistance measurements where a drying moisture gradient is present, which is usually the case in lumber passing through industrial processes* A study of moisture gradients in drying boards and planks has shown thatj after the entire piece has passed the fiber-saturation point> the moisture content in a plane located at one-fifth of the thickness of the material from its surface is usually very near the average moisture of the piece, tfhen using a needle-point type of electrode in wood contain- ing a drying gradient the moisture meter indicates the moisture content at or near the points of the needles, since the wood becomes a better con- ductor as the moisture content increases. Because of these facts it is possible to estimate the average moisture content by driving the needles to a depth of one-fifth the thickness of the piece. Further, it is possible, by driving electrodes to any desired depth, to evaluate the moisture content at that particular distance from the surface. This is of great importance in the treatment of timber with wood preservatives, in the use of heavy timbers for many structural purposes, and, in fact* wher- ever moisture content is a vital factor. Nails may be used for this purpose to advantage, in place of the regular electrode. Surface plate electrodes, which make contact with opposite sur- faces of a board, indicate the moisture content of the surface of the board under and near the plates and are specially suitable for thin material, like veneers, and for material that has practically no moisture gradient, such as thoroughly seasoned lumber. The moisture content of veneer ranging from about .>5 or 4 percent up to lb or 17 percent may be also measured by the resistance method by merely increasing the electrode area. This has been accomplished by setting a large number of contact points in the wood simultaneously. Points of positive polarity are driven from one side and those of negative polarity from the opposite side. The range of most re si stance- type moisture meters for lumber lies between 7 and 25 percent moisture content. Two instruments may be had which are calibrated from 7 to 60 percent moisture. In the 7-25 percent moisture range the accuracy of resistance- type instruments, when properly calibrated and used for testing relatively thin samples or on heavier mate- rial that is known to be of uniform mdisture content, should be with - 1 percent of moisture content in the majority of -ca^es. It is not to be ex- pected that readings of moisture content above 25 percent will be as accurate as those in the lower range; nor do they ordinarily need to be. Resistance- type meters will not give satisfactory readings on lumber wet by rain or fog, since only the surface moisture content will be HllUb _2- shown. Further, when used In wet weather, the surfaces of the instrument itself may become damp and prevent readings at low moisture content values. In the meters measuring moisture content by resistance methods, the actual measurement is made by balancing tne resistance between the electrodes by known resistances, or, in the case of the blinker type, by adjusting the rate of flash of a neon tube to that of anotner, flashing at a standard rate, by introducing into the electrode circuit calibrated condensers having a greater or less capacity. Both types of instruments are easy to use and the choice in the selection of a meter rests largely in cost and personal preference. Capaci ty Meters Ihe electiical capacity of wood varies directly with tne amount of moisture in it, throughout the entire range of moisture content irom green to oven dry. Therefore, the moisture content may be evaluated by measuring tne electrical capacity. Temperature effects are so small as to be negligible for ordinary use, and theie are no errors introduced la the readings by variations in properties inherent amon t different species. The capacity method is, in principle, an excellent means for evaluating tne quantity, by weight, of water in wood. However, it is :. possible to convert this weignt into a percentage witnout knowing the weight or specific gravity of the wood. This property cannot be determined quickly enough by any method now available, and it is present practice to assume the specific gravity of the individual piece when dry to be the same as the average for the species and to calibiate tne moisture meter accordingly* Each readin £ on a meter so calibrated cariies an error proportional to the actual eiror in the assumed specific gravity of the piece. oeveral specific forms of condenser plates for 0Dtamin t measure- ments on wood nave Deen developed. One fonn consists of two suitably i. sulated plates tnat are placed on opposite faces ol the piece under test. In another form, four ^uadrant- shaped plates aie assembled In the shape of a flat circular disk several inches in diameter. In use this assembly of plates is pressed a fc ain'st one su.face of the piece under test. Meters using condensers with plates on opposite sides of the piece under tests evaluate tne total quantity of water in tne wood, irre- spective of moisture gradient, and the readings are therefore inherently trus average 8 4 Moisture fe iadients may well affect the readin e s of meters ISing condense! plates on one side only of the piece under test. Howev . it is not feasible to actually determine moisture gradients or to measure % moisture contents at various depths with any capacity-type meter. i : capacity- type meter now on the American market uses the ^rant-shaped condenser plates and is calibrated to rend from to about 25 percent moisture content. hi. -j- Electrical Moisture Meter Makers and Dealers Makers and dealers Colloid Equipment Co. , Inc. 50 Church St . , New York City- Hart Moisture Gauges, Inc f 126 Liberty St. , New York City Raymond S. Hart 1950 Grand Central Terminal New York City Industrial Instruments, Inc. 156 Culver Ave. Jersey City, N. J. Measurements, Inc. Boonton, P. J. Moisture Register Co. , 133 N. Garfield Ave. Alhambra, Calif. Moore Dry Kiln Co., Jacksonville, Ela. also at ^orth Portland, Oreg. C. M. Lovsted & Co. Seattle, Wash. Standard Dry Kiln Co. , Indianapolis, Ind. C. J. Tagliabue Mfg. Co. , Park & Nostrand Aves. , Brooklyn, N. Y. Wilbur Instrument Co. , 1123 N. ''/. Gils on St., Portland, Oreg. National Engineering Co. F. 0. Box 1U75 Indianapolis, Ind. "Trade name Delmhorst Moisture Detector Hart Moisture Gauge Moisture Meter Megohm Bridge Delmhorst Moisture Met er Moisture Register Tag-Heppens t all Moisture Meter Moisture Register Moisture Register Tag-PIeppenstall T ag-Hepp ens t al 1 Moisture Meter Wilbur Moisture Indicator Tag-Heppens tall Moisture Meter Type Resistance Resistance Resistance Resistance Resistance Radio frequency power loss Resistance Radio frequency power loss Radio frequency power loss Resistance Resistance Capacity Resistance R11U6 -U_ References to Published Articles on Electrical Moisture Meters by the Forest Products Laboratory The Electrical Resistance of Wood as a Measure of Its Moisture Content, by A. J. Stamm, p. 1021, Indus. & Eng. Chem. , V.19, No. 9, Sept. 1927. An Electrical Conductivity Method for Determining the Moisture Content of Wood, By A. J". Stamm. p. 2Uo, Analytical Ed. Indus. S> Eng. Chem. , V.2. July 15, 1930. Lessons in Kiln Drying (Nos. 18 to 2h and 30) , by H. D. Tiemann. Pub- lished in the Southern Lumberman in 1936 , 1937. and 1938. (Now available in book form from the Southern Lumberman, Nashville, Tenn.) A Wide Range Vacuum Tube Resistance Bridge, by I. I. Davies. p. 26l, Instruments, Oct. 1937. A wide range vacuum tube resistance bridge - U. S, Patent 20888^3 _ assigned to U. S. Government for the free use of the public. Articles describing the Blinker Moisture meter by Suits and Dunlap were published in the following periodicals. The most complete descrip- tion will be found in the General Electric Review mentioned below: South. Lbrman. , July 1, 1930, Furn. Mfr. , Aug. 1930, Wood Construction, July 1, 1930, Wood Working Indus., Aug. 1930, Amcr. Lbrman., July 5, 1930, Instruments, Aug. 15. 1930, Lbr. Trade Jour., July 15, 1930, Natl, Assn. of Commission Lbr. Timberman, July 1930, Salesmen Annual, 1930, Barrel & Box & Packages, July 1930, General Elec. Review, Dec. 1931 (p. 706-13 - Determination of the moisture content of wood by electrical means) Blinker moisture meter - U. S. Patent 187^359 - assigned to the U. S. Government for the free use of the -public. Other Publications UNIVERSITY OF FlORlDA Hill 1 1 III II III II 3 1262 08929 0737 Electronics Applied to Moisture Determinations in Lumber, Anonymous, Describes the Moisture Register, American Lumberman, p. 3^» June 10,19^. RllU6 -5-