MtETHOIDS IN IETEHIMININC TE SIECIFIC
GRAVITY Or WOOED rOl AIIIPLANEI USE
/ December 1941
TillS IrIPOIT IS ONE OF A S1I$S ISSUElD
TO Air TIHE NATION'S DEfINSE EFFOIRT
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
In Cooperation with the University of Wisconsin
METHODS S FOR DETERMIn,11IJ THE SPECIFIC GRAVITY OF WOOD FOR AIRPLAIT V2P-
The specific gravity or density of wood, exclusive of the mois-
ture it contains, affords a good indication of the strength properties.
Hence, in the absence of actual strength tests to establish the quality of
individual pieces of wood for highly-stressed parts, such as spars, a m:ini-
mum specific gravity requirement has been established for each species.
There are several methods of expressing the specific gravity of
wood, but that used in aircraft inspection is the serecific gravity .sez "d.
the wei^jnv t and the volume when oven dry.
Either of the two methods described below is sug5-ested for de-
termining the specific gravity based on weight and volume when oven dry.
The second meth,-d should be used only when the apparatus essential to the
first method is not available.
First IK'th:d (Tm..?er;cn M&.h:-d)
Selection q.1d Prepraratin- crf Te,-t SpecLaP-.
At least one specimen should be cut from each piece of stock to
be examined. A convenient form of specimen is a section 1 to 2 inches in
length directionn of the grain) taken from a board, pla.n, or part. The vol-
ume of the specimen should preferably not be less than 5 cubic i'-hes-2 nor
more than 25 cubic inches. The object of limiting the length of the test
specimen in the direction of the grain is to reduce the time required for
drying. For the determination of specific gravity by this first method, the
specimen need not be regular in shape. All loose splinters should be removed.
determination of Sp ecific Gravity
After selecting a representative test speciuera, proceed as follows:
1) Put the specimen in an oven at 212 F. (100 0.) and dry until
the contained moisture is evaporated and the weight becomes constant. This
will require from 1 to 2 days. If a number of specimens are placed in the
oven for drying at the same time, the.y should be open-piled to allow free
access of air to all surfaces of each piece.
--This mimeograph is one of a series of progress reports issued by the Forest
Products Laboratory to aid the ThI-t.ion's defense effort.
-Satisfactoryr specific gravity determinatioILs can be made on specimens less
than 5 cubic inches in volume by using a suitably sensitive balance- such
as that shown in figure 2. Greater precision in technique is re-uired
with very sma.alXthan with large specimens to insure a Jiven standard of
Mimeo. lo. 1314
2) Feigh tho oven-dry soneci-pen whilp -rn. It is rost conv-nirnt
to record the weight in rmetric units (rirr.s nn!r 1. .ci-.1s th-r-of),
3) ,:'trrrin--. the volunm b,- the imrr.ersion me7hon' ter mrnist,:rr-
--cofin- the specir-,n, as follows:
(a) After the ov:-n-Iry wei.-ht ha3s been ob-trin- 1, qji- the
srrecimen, oreferabl.v '-hile still warrm, in hot rirarffin. 7ith- -oo. tech-
nioaue no surplus paraffin should adhere to the s'-r-nle, bu* in t'.-7 vent
that it does, nnTr surplus should be scrnped off,
(b) Find the volume of the sn'ucir.en by dct;rr:inin:! tn. wit.
o-" watpr it displaces when immersed.
A cort-.iner hol`in.-- sufficient a'atrr for the cor.pln-te I'b,.. sion
of the sr-.cimen is placed on one rpar. of a b.rlance. Thr con:-iin'.r -n:' -ater
are then balanced *"ith rights added to the other Tin.. :' .-, s of a shprp
pointed rod or other grip, the s.eci.r.. is '--li co-r+etI-.- s3b .:*..: nd not
touchin.- the container ihile th-' scales are -..--,in cnl:,ed. (fi--. 1). Tne
7'.iitior.nl -,eiht rcviired to restore aIl'nce is the -eiht of -'ater dis-
risced b.. the srecimnen, i-tn., if in f-r-rs, is nericll" equal to the oven-
dry vol'-1.. of the r-.'i2ir.--n in cubic centimeters.
It is irrcrtant that the determination of the v.olL-,- by -ei;-hing
be mn'1e as qr4ckly Ps possible after the imm!ersion of the sreci_-..n, since
any absorption of r-ater by the srecir.-n directly influences the accuracy of
the result. *y est1latirn the volume of the spr:ci-3nr. -n4 nlacinz on tne nnn
aprroximately the required *7ei:hts before the srecizmen is inmzerse.1, the time
necessary for balancing may be reduced to a minimum.
The rod or !-rip by menns of which h th; srpciren is e.ld in position
should be as small as practicable. Care should be taken not to lo--.r the
sr-cimen into the '-nter to a much L-r-ter depth thPn rp-rvir-" to sub:er.-e it
om.nletely; other'- -so, the "'ei7it of the v:ater Oisrl.cp the rod or Prip
may increase r.oreciably the -'eight r-aiire4 to bil-nce, n-1 h"r.-n Wy
seriously i.-r-ir the accuracy of the result.
4) Compute the sr'-cific gravity from the equation, 9"-c4.fic :rav-
ity = D wrr' D is the ov:n-dry -eipht of the specimen in r-., n' V is
the ov-r.-dry volume of the r.-'i-ten in cubic centirlters. 7hen. t*: .... -i-ht
as obtpinr*A under rar-..-rt.. 2, end the volume as obtain-.0 under 'rara':h
3 are in metric units ps su::.--'-sted, th values may :'- s-'' stituted ?ir','tly
in the f'ri.ili above. (.3.7. exar,-l ).
Figure l.--Apparatus for determining the volume of a wood specimen. This is accomplished by immers-
ing the specimen by means of a sharp pointed rod or other grip, and weighing the
buoyant force of water displaced. If the units are in grams, the weight required to
restore balance is numerically equal to the volume of the specimen expressed in cubic
LI.-.o 2.-- t to.' -ific Gravity determinations can be made on speci-
mens ess than 5 cubic inches in volume provided a suitably sensitive
blane, as shown here, is employed.
Second Method (Measurement !.' thod)
The second method differs from the first in that the volume is de-
termined by measuring the specimen, rather than by immersin- it in -'ater.
Selection and Preparation of
The general instructions for the selection of test specimens as
described for the first method should be followed with the additional require-
ment that the specimens be regular in shape. The cross section shioule' hence
be square or rectangular, and the ends should be parallel.
After selection: a representative test s-necimen, proceed as follows:
1) Put the specimen in an oven at 212 F. (100 0 C.) an'. dry un-
til the contained moisture is evaporated and the weight becomes constant.
2) Weigh each standard oven-dried specimen while warm. It is most
convenient to record the weight in metric units. If the specimen becomes out
of shape in drying to a degree that the volume cannot be determined accurately
by measurements, it should again be cut to regular shape before weighing.
3) Carefully measure the width, height, e-nd length of each oven-
dried specimen, taking average measurements vhen this aerears desirable.
From these cor.ciiute the volume of the specimen. If the volume is determined
in English units, it may be converted to metric units by the conversion fac-
4) Compute the specific gravity from the equation specific grav-
ity =, where D is the oven-dry weight of the specimen in grams, and V is
the volume in cubic centimeters.
In computing specific gravity by the above formiul, all measure-
ments must be expressed in metric units. When any of the measurements are
expressed in English units, the conversion factors given below may be usl::
1 inch = 2.54 centimeters
1 cubic inch = 16.4 cubic centimeters
1 ounce = 28.4 grams
1 pound = 453.6 grams
In or.It r to insur, r-ood results, the vei.-nts r n vo1-,Ts r either
:,7 t:iod sho*:ld b.? outnireI to an accurqc, of at lenst on'--hi'f of 1 rr nt,
The moisture content of the so' cimen is uslA1'y o;t1in. 1 K:o:v- with
the ,.-cific :-r-'vity. ThL- only a. :itional st is to o'tain the wei.--.t of
the sr,-ci.tfn i.cmediat,'ly after sa-vin-. It is i-nort-nt thit t"! ,-: "t be
t,"r.n i:r'di*tely rifter 9a'"innr, since wood is s-..'ject to roist',i- c-.nrnrs on
S:.:-rosure to the air. Th dceir'-e and rnoidity of ch-.an-e ar-' dr.-'. 'ent on the
moisture content of the piece and the condition of the iir to nhich it is
Ti'- r.'rc'nt-e moisture content of the --oo from 'high A test sneci-
men is cut is expre-ssed by the formula
'oisture content,in percent x 100
S= ori;-i:-,nl wei,:ht of the -'cimen .fo,'.-. *.-
D oven-drv weight of the speci-. r.
3a jnnle 1
Calculate the .r,-cific -ravity, '"as-ed on '7ei -'.-t -in volue pn
ov.r. dry, of samole of Sitka spruce on whic', t.,i follo-in' ,-ta have .een
obtained by the first method discussed.
cei.-'.t n.en ov.n ir;: (par. 2) = 57.4 .-razs
Volume (-eifht in ri.rs re-
quired to nl-.ncc aft'r im-
mcrir.r- snccir:en) (par. 3) = 140-.7 cu. cm.
Srcific gravity V 7 0.41
C'lculate the srecific or-v'ty, bt-- or. weilt ar.d vol-uma h.en
ovn .r-,, of a sa1irl- of Sitka sr.ruce or. ".chc + -. folloi'-..- 'n-ta hve
'" n .c'taint d by the s. cor.i meI'ho. descr-ibed.
Weight whvn oven dry (par. 2) = 57.4 grams
Measurements when removed from
oven (par. 3);
Ave. length = l.Cq inches
Ave. vidth = 2.47 "
Ave. thickness = 3.27 "
Volume in cu. cm. =
1.O0 x 2.47 x 3.27 x 16.4 = 143.0 cu. cm.
Specific gravity = = 5- = 0.40
Calculate the specific gravity, based on weight and volume when
oven dry, of a 3amrle of Sitka spruce of less than 5 cubic inches on which h
the following data have been obtained by the first method described.
Weight vwhen oven dry (par. 2) = 5.-9 Tr-I s
Volume (weiht in grams required
to balance after immersing
specimen ) (-par. 3) = 1i.29 cuii. cm.
Specific gravity = D -.9 C .44
Calculate the moisture content of the sample of Sitka spruce
used in Example 1. The weight of the sample, immediately after sai:in <,
was found to be 63.9 grams.
Weight when oven dry = 57.4 grams
Moisture content, in percent = W D x 100 39 57.4 x 100 11.3
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