Measuring Moisture Content.

TM 38-400/NAVSUP PUB 572/AFMAN 23-210 MCO 4450.14/DLAM 4145.12

meters are now on the market. The features common to

distance along the length of the timber. Surface checks

the meters are two pairs of sharp metallic terminals that

are generally of minor importance except where they

can be embedded quickly in the wood, batteries for

develop into the relatively large cracks that are likely to

supplying an electric current through the wood

occur in boxed-heart timbers. Timbers may decay in

intervening between the two terminals, and a means for

storage because of fungi that were in the living tree, or

reading the resistance in the electric circuit directly in

infection may start during the storage period. Decay

terms of the moisture content of the wood holding the

may start on the surfaces of timber in solid piles

terminals. Different species of wood vary in their

because surface drying is retarded or in surface checks

electrical resistance for a given moisture content, and

or splits into which rain or snow water penetrates.

this fact must be taken into consideration in making

(1) Outdoor storage of timber. Timbers stored in

moisture determinations. Usually, manufacturers supply

piles outdoors are usually exposed to sunshine and

proper correction factors. The range of the present

wetting because it is impractical to provide roofs.

meters is about 7 to 24 percent moisture content.

Timbers stored outdoors should be end-coated for

Moisture meters are now available with needles that are

protection from end-checking. If the coating is applied

insulated with a nonconductive coating except for the

before end-checking has started, it will greatly retard the

extreme tip. The insulated needles are preferred since

drying of the end-grain that causes checking and

they can measure the moisture content at various

splitting. The various types of antisplitting irons that are

depths within a piece of lumber.

used commonly on railroad ties may be used on timbers

The electrical-resistance method has an advantage

to prevent the opening up of splits that may develop

over the oven-drying and distillation methods,

from end checks. Timbers stored outdoors should be

principally, because of its speed and convenience as

piled so that the air has access to all faces. The timbers

only a few seconds are required to determine the

in each layer, of course, should be placed several

amount of moisture in any piece of wood. Therefore, it

inches apart and the courses should be separated by 2-

is adaptable for sorting lumber on the basis of its

by-4s. The stickers should be spaced 6 to 10 feet apart.

moisture content. The moisture content is determined

Pile foundations should slope and the pile of timbers

by inserting the prongs of the meter in the back of the

should be built with a forward pitch much like hand-

piece somewhere near the center. This avoids marring

stacked piles of boards or dimension lumber.

the face of the board when the metallic terminal points

(2) Open shed storage of timbers. Timber stored in

are inserted. The electrical methods are the only

an open shed are subjected to practically the same

practical means thus far developed by which the

atmospheric conditions as timbers stored outdoors, but

moisture content of finished woodwork in place can be

they are protected from sunshine and wetting by the roof

determined without serious injury to the wood.

of the shed. Piles are level, rather than sloped and

c. Methods of determining moisture content in

pitched.

timbers. Since wood acts as a resistance element in the

electrical circuit of a moisture meter, the current flowing

5-10. Measuring Moisture Content.

between electrode needles will follow the path of least

resistance which is the path with the greatest amount of

a. General. Ordinarily, the amount of moisture in

moisture. Thus, a meter with uninsulated electrodes will

wood is expressed as a percentage of the weight of the

show the moisture content of the wettest wood

wood when oven dry. The three distinct methods of

contacted by the electrode needles.

Therefore,

determining moisture content are the oven-drying

electrical meters should not be used on lumber that has

method, the distillation method, and the electrical

been recently wet by rainfall since the meter may

moisture meter method. The latter of the three methods

indicate too high a moisture content. Meters equipped

is the most rapid and does not require cutting the

with insulated needles are preferred. The insulated

material. The only accurate and rapid test is by using a

needles respond only to the moisture content of the

lumber moisture meter in accordance with MILSTD-

wood in contact with the uninsulated tips, hence, it is

1363, Measurement of Wood Moisture Content.

possible to take readings at different depths to obtain a

b. Use of the electrical-resistance method. When

more complete indication of the moisture distribution. If

the rapid indication of the moisture content of wood is

moisture meters with uninsulated needles are not

desired for inspection purposes, the electrical-resistance

available, the moisture content at a depth of one-fifth of

method is the most suitable. This method is based upon

the thickness

the well-known fact that the electrical resistance of wood

changes with moisture content.

Several types of

portable electric moisture

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