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r~rCTIRICIl MDISTLUIE M[T[I $ FOl WOOD
IRevised, July 1944
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
la Cooperatim with the Un'rwity of WifConMi
ELECTRICAL MOIST'JRE 0--MRS FOR ?WOE
By M. E. LUMLAP, Senior Engineer
:h. measurement of moisture in wood by rapid electrical methods
is now, fairly well established in thL United States; about seven or eight
years havc elapsed since electrical moisture meters for wood first made
their appearance in a practical form. The Forest Products Laboratory has
liev. lor.-d several typec of electrical moisture meters, including the
"blinker,." which has becn on the market for A number of years; a caracity-
tyi-c mashinv, which has never been commercialized; and a recently patented
rLsistancc type employing a novel Wheatstone bridge balancing circuit
(sec 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. ~TeLd, the time required to determine the amount of moisture in any
piece of wornd being only a few seconds, They are therefore adaptable for
sorting lumber and veneers on the basis of moisture content. Furthermore,
th,.y offer the only practical means thus far developed of determining the
moisture content of finished woodwork in place without serious injury to
Two general types of instrument are available, one evaluating
the moisture content by measuring the electrical resistance of th. wood
and the oth.r \y measuring the electrical capacity of the wood (see list
of mak.-rs at end of article).
B.I.w the fiber-saturation point (about 25 percent moisture con-
tent) the el-ctrical resistance of wc,:dd varies widely with changes in
moisture cJr.tent, the reslstanre increasing as the moisture content de-
creases, Thi-n enqables on,.- to evaluate the moisture content ly measuring
the electrical resista.ee of wood. The resistance increases with a fall-
Ing trm -.raturc and decreases as the tLmreraturc rises. Furthermore,
there are variation., among different species that must also be taken
into account. Fortunately, reasonably accurate corrections fgr tempera-
tur, p-rd fuirr.nish correction iata with their instruirments.
Eli.ctriral contact is generally made by driving four necd.le
pointss into the wo-,d t- be tested so that the fl:w of current is :-araillCi
to th: rlr.i. The two i points of likc *-lkl dtI arc U=RV .raced nL-half
inch ( irr t, -r.d the ro'ints of u.liko' ,elarity abut !-i/4 inches apart in
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 wood 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 that# 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. When 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 j.5 or 4 percent
up to 16 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 resistance-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 moisture 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
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 actslR measurement is made by balancing the 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 another, flashing
at a sta,.daid 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.
Crqjcity ue ters
'ihe electrical capacity of wood varies directly with tne amount
of moisture in it, throughout the entire range of moisture conte..t irom
grtti to oven dry. Therefore, the moisture content may be evaluated by
measairing the electrical capacity. Temperature effects are so small as to
be negligible for ordinary use, and theie are no errors introduced into
t-.e readers by variations in properties inherent amone different species.
The capacity method is, in principle, an excellent means for
evaluatinE tne quar.tity, by weight, of water in wood. However, it is not
possible to convert this weight into a percentage without 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
assxme the specific gravity of the individual piece when dry to be the
same as the average for the species and to calibrate tne moisture meter
*ccurdin1/j. Each rendinE on a meter so calibrated carries an error
proportioial to the actual error in the assu-.ed specific aiavity of the
severall specific forms of condenser plates for obtaiini measure-
r.ents on wood have been developed. One fowm consists of two suitably in-
salated plates that are placed on opposite faces of the piece under test.
In another foi:I, four 1uadrant-shaped plates aie assembled in the shape of
a flat circular disk several inches in diameter, In use this assembly of
plates is pressed aeainrst one su-face of the piece ander test.
meters usin2 condensers with plates on opposite sides of the
piece mnder tests evaluate the total quantity of water in the wood, irre-
sptctive of moisture gradient, and the readings are therefore inherently
true av--r',res. Moisture bxadients may well affect the readings of meters
si;. d cL.deisez, plates on one side only of the piece under test. However,
it is not feasible to actually determine moisture &radients or to measure
moisture contents at various depths with any capacity-type meter.
The capacity-type meter now on the Amezricnn market uses the
q4adrant-shappd condenser plates and is calibrated to read from 0 to about
(; pc'r'-nt ;..oisture content,
hIlo -j -
Electrical Moisture Meter Makers and Dealers
Makers and dealers Trade name
Colloid, Equipment Co., Inc. Delmhorst Moisture
50 Church St., Detector
New York City
Hart Moisture Gauges, Inc, Hart Moisture Gauge
126 Liberty St.,
New York City
Raymond S. Hart Moisture Meter
1950 Grand Central Terminal
New York City
Industrial Instruments, Inc. Megohm Bridge
156 Culver Ave.
Jersey City, N. J.
Measurements, Inc. Delmhorst Moisture
Boonton, V. J. Meter
Moisture Register Co., Moisture Register
133 5. Garfield Ave.
Moore Dry Kiln Co., Tag-Heppenstall
Jacksonville, Fla. Moisture Meter
also at !orth
C. M. Lovsted & Co. Moisture Register
Standard Dry Kiln Co., Moisture Register
C. J. Tagliabue Mfg. Co.,
Park & Nostrand Aves.,
Brooklyn, N. Y.
Vilbur Instrument Co.,
1123 T. W. Gilson St.,
National Engineering Co.
P. 0. Box 1475
References to Published Articles on Electrical
Moisture Meters by the Forest Products Laboratory
Thc Electrical Resistance of Wood as a Measure of Its Moisture Content,
by A. J. StamT, p. 1021, Indus. & Eng. Chem., V.19, No. 9, Sept. 1927.
An Electrical Conductivity Method for Determining the Moisture Content
of 'food, By A. JT. Stnm. p. 240, Analytical Ed. Indus. & Eng. Chem.,
V.2, July 15, 1930.
Lessons in Kiln Drying (Nos. 18 to 24 and 30). by H. D. Tiemann. Pub-
lished in the Southern Lumberman in 1936, 1937. and 1938. (how
available in book form from the Southern Lumberman, Nashville, Tenn.)
A Wide Range Vacuum Tube Resistance Bridge, by I. I. Davies. p. 261,
Instruments, Oct. 1937.
A wide range vacuum tube resistance bridge U. S. Patent 2055843 -
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,
Wood Construction, July 1, 1930,
Amcr. Ltrn.n., July 5, 1930,
Lbr. Trade Jour., July 15, 1930,
Timberman, July 1930,
Barrel & Box & Packages, July 1930,
Furn. YMfr., Aug. 1930,
Wood 'forking Indus., Aug. 1930,
Instruments, Aug. 15, 1930,
Natl, Assn. of Commission Lbr.
Salesmen Annual, 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 1879359 assigned to the U. S.
Governr.ment for the free use of the public.
LUPJI. E RSIT V OF FL ORIDA
I 13 1262 08929 0737111
3 1262 O8929 0737
Electronics Applied to Moisture Determinations in Lumber, Anonymous,
cDescribes the Moisture Register, American Lumberman, p. 34, June 10,19WA.