TECHNICAL NOTE NUMBER B-14
UNIT STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE
FOREST PRODUCTS LABORATORY
DSON 5, WISCONSIN REMIBED October 1956
ME THODS OF DE TER ENING THE, "
SPECIFIC GRAVIITY)!F 'WOOD ,,
S U S. DEPOS To~h
Next to actual strength tests, the specific gravity of w oo -thest indication of its strength properties. The specific gravity of any piece of wood may easily be determined by the methods described in this note. With the aid of table 1, the strength of the piece as a beam or as a column, its shock-resisting ability, its hardness or wear-resistance, its toughness, its shearing strength, and its value in several other respects may be estimated.
The specific gravity of a substance is its weight divided by the weight of an equal volume of water. As both the weight and volume of wood vary with the amount of moisture in it, specific gravity as applied to wood is an indefinite quantity unless the circumstances under which it is determined are specified. The specific gravity of wood is almost always based on the weight when ovendry, but the volume may be that in the ovendry, air-dry, or green condition. The true specific gravity of wood is, of course, that based on volume when ovendry. For greater convenience inmaking determinations, however, the U. S' Forest Products Laboratory bases specific gravity on the volume of the specimen when tested and has determined the relation of strength to specific gravity separately for green and air-dry wood.
Inusing table 1 for estimating the properties of particular timber, therefore, it is necessary to determine specific gravity on the volume of the sample in a green condition or at a moisture content of about 12 percent (air-dry condition). The volume when ovendry may be obtained and converted into the volume when green by means of shrinkage figures.
Specific gravity determinations may be mad u should contain no more than 25 cubic inches. arj4eW E rus siderably more time for drying.
MAY 19 1973
I.FA.S. Univ. of Florida
After selecting a representative specimen, proceed as follows:
To find specific gravity of wood To find specific gravity of wood basedupon volume in a green or baseduponvolume when ovendry
1. Find volume of specimen by 1. Put specimenin oven at 103*
measurements or by immer- C. (plus or minus 2* C. ) and
sion method. dry until constant weight is
2. Put specimenin oven at 103 2. Weigh specimen.
C. (plus or minus 2 C.) and dry until constant weight is
3. Weigh specimen. 3. Find volume of specimen by
measurements or by an immersion method.
4. Compute specific gravity, using formula
Specific gravity = D
where D = weight in grams and V = volume
in cubic centimeters.
When weights or measures are not taken in metric units, use the following reduction factors:
Inches x 2.54 = centimeters Ounces x 28.4 = grams
Cubic inches x 16.4 = cubic Pounds x 454 = grams
Both the ovendry weight and the volume should be correct to within at least 1/2 of 1 percent.
Determining Volume of Specimens
Lineal measurement method. --The specimen must be regular in shape with right-angle corners for determination of its volume by lineal measurement. It should be measured carefully to determine its length and
.. . ......
cross sectionalarea. Its volume is equal to the product of its length and
cross sectional area.
Immersion methods. --The volume of a regular or irregular specimen may be found by determining the weight of water it displaces when immersed. This weight in grams is numerically equal to the volume of the
specimen in cubic centimeters.
Method A. --A container holding enough water to completely submerge
the specimen is placed on one pan of a balance scale. The combined weight of the container and water is then balanced with weights added to the other scale pan. By means of a sharp pointed rod, the specimen is held completely submerged without touching the container while the scales are again balanced. The weight that is added to restore balance is the
weight of water displaced by the specimen.
POIN TED ROD
CON TA INER
WA TER WEIGH
L - - -
Method B. -A container holding enough water to completely submerge
the specimen is placed below one pan of a balance scale from which a wire basket is suspended of sufficient weight to hold 'the specimen submerged. (See sketch.) The weight of the basket is balanced with weights added to the other scale pan. The specimen is first weighed in air and then placed in the basket and held completely submerged without touching the water container while the scales are again balanced. If the specimen has a tendency to float, the weight that is added to restore balance is added to the weight of the specimen in air to give the weight of an equal
volume of water., Conversely, if the specimen has a tendency to sink, the weight that is added to restore balance is subtracted from the weight of the specimen in air to determine the volume. In this case, the weights added to restore balance are added to the same pan as the weight applied
to balance the weight of the wire basket.
WEIGHTrS APPLIED TO WEIGHTS APPLIED TO
REBALANCE SPECIMEN BALANCE WEIGHT
WIRE A SKE r
SPECIMEN LMETHOD B
Green specimens may usually be immersed for volume determinations in the condition in which they are selected. The determination of volume should be made as quickly as possible after immersion of the specimen, because any absorptionof water by the specimens directly affects the accuracy of the result. To prevent ovendry specimens from taking up water when they are immersed, they may be dipped in hot paraffin wax. After the paraffin dip, the specimen should be weighed again before immersion and this second weight used in connection with the immersed weight for determining the volume of the specimen.
A quicker method of treating specimens to prevent water absorption, which is suitable only for softwoods and for hardwoods with small pores, is to dip them in a solution of paraffin wax in carbontetrachloride (1 gram of paraffin in 150 cubic centimeters of carbon tetrachloride). Allowa few minutes for the carbon tetrachloride to evaporate and then determine the volume by one of the immersion methods. The gain in weight due to the thin film of wax is negligible and may be ignored. An advantage of this method of treating the specimen is that it may be used satis-4-
factorily to determine the volume of air-dried specimens, since the thin film of wax does not appear to affect the subsequent shrinkage of specimens when ovendried.
Rapid Method for Determining Specific Gravity
The flotation method is a rapid method of determining specific gravity directly, but it is less accurate than the other methods described. A long piece of regular cross section is floated on end in a narrow vessel of water. The submerged portion of the piece, expressed as a decimal fraction of the total length, is numerically equal to the specific gravity of the specimen, based on weight and volume at the moisture condition when tested.
Determination of Moisture Content
If the specimen is weighed immediately when obtained as well as after ovendrying, the moisture content may be computed, thus affording both moisture and specific gravity determinations on the same piece.
Moisture content (percent) = green weight ovendr y weight x 100 ovendry weight
Weight per Cubic Foot
The weight per cubic foot of a specimen at the current moisture condition may be determined quite easily from the specific gravity, based on ovendiry weight and volume at test, by the following relationship:
Weight per cubic foot = Specific gravity x 62.4 x (100 + moisture content at test)
APR2 1 VJ PR 1 8 1973
Table 1. --Specific gravity-strength relations among different se
j:Air-dry (1aProperty : Unit Gre percent
Static bending: ZS: I.ZS
Fiber stress at-proportional limit..: Pounds per square inch ........: 10,200 : 16,700G
Modulus of rupture ...........................do 17,600G : 25,70G
Work to maximum load ........... Inch-pounds per cubic inch. ... 35.60 32.4G
: z 2
Total work ....................... ............ do .............. .103G : 1 .7G
Modulus of elasticity............... 1,000 pounds per square inch.. : 2,360G : 2,800G
Impact bending: :1.5 1.25
Fiber stress at proportional limit..: Pounds per square inch ....... 23,700G : 31,200G Modulus of elasticity .............. 1,000 pounds per square inch.. : 2,940G ; 3,380G
Height of drop ................... : Inches ...................... 114G : 94.6G
Compression parallel to grain:
Fiber stress at proportional limit..: Pounds per square inch ....... ..5,250G : 8,750G
Maximum crushing strength ....... ............ do .............. 6,730G : 12, 200G
Modulus of elasticity ............. :.1,000 pounds per square inch.: 2,910G : 3,380G
Compression perpendicular to grain:': Z25 .25
Fiber stress at proportional limit..: Pounds per square inch ........: 3,0 G 4,6300
Hardness: : a
End............................... Pounds........................ 3,7 G 4,
.Z 5 3 ?O 2.25
Side ............................. ............ do .............. 3,40G 3.7700
The values listed in this table are to be read as equations. For example, modulus f rupture for
green material is 17,600G1.25, where G represents-the specific gravity, ovendry, based on
at moisture condition indicated.
UNIVERSITY OF FORIA
Z M 28990 F