THE FLOTATION METHOD Of DETERMINING THE SPECIFIC GRAVITY OF WOOD Revised July 1946 1 0f mm umn * nmm rroipbsrroR^ This Report is One of a Scries Issued In Cooperation tuith the \RMT-NA\T- CIVIl COMMUTE! on AIRCRAFT DESIGN CRITERIA Under the Supervision off the AfPCNAlJTICAl BOARD Nc. 1398 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the University of Wisconsin THE FLOTATION IJSTKOD 0? DETERMINING THE SPECIFIC GRAVITY OF WOOD^ By BENSON H. PAUL, Senior Silviculturist A method of estimating the specific gravity of wood quickly and to a reason- ably accurate degree with a minimum of time and equipment has been developed at the Forest Products Laboratory to assist suppliers of lumber used in war production in meeting specification requirements for weight. While the method may not be so accurate as the standard process described in Army-Navy Specification AIT- , "/-4a, its time and labor saving features should make it useful for producers seeking a quick specific gravity test to guard against the use of underweight material and, at the sane time, avoid the rejection of wood of acceptable specific gravity. The method is believed accurate enough to detect up to 90 percent of stock which should be resectable for light weight. Use of the more accurate method outlined in Specification AN-W-4a is, of course, recognized as necessary in making final assessment of the quality of wood in such border-line cases as may arise from time to time. Essentially, the method consists of determining the proportion of a piece of wood with parallel sides that is submerged when it is floated in water; To facilitate this determination, the method calls for a test specimen 1 inch square in cross section and 10 inches long, marked into 10 equal divisions of 1 inch. When a piece so marked is floated upright in a cylinder of water (fig. l) its specific gravity at current moisture content can be ascertained by observing the proportion of the piece that is submerged. For example, if six divisions of the piece are under v:ater, the specific gravity is 0.60, while if it sinks to the seventh mark, the specific gravity is 0.70. When the water level reaches a point somewhere between two divid- ing marks on the piece, specific gravity may be visually estimated with reasonable accuracy; a water level half way between 6 and 7, for example, may be read as 0.65. Other xn i mJdiate levels may be rather closely estimated as 0.32, 0.67, and so on. Any container, of suitable size to hold the specimen vertically or nearly so and yet allow the specimen to float freely may be used (fig. l). If the container is filled with water so that it will overflow, the specimen can be quickly marked at the water level before absorption of water takes place. —This is one of a series of progress reports prepared by the Forest Products Laboratory relating to the use of wood in aircraft. Results here reported arc preliminary and may be revised as additional data become available. Report No. 1398 -1_ Tests of many samples of equal length can be speeded by use of a scale of the same length marked In 10 equal units. The proportion submerged is read by placin he specimen "beside the scale. For rapid sorting, the required i terslon depth may be ..:urked on specimens before test. For comparison with standard specific gravity values, specimens should be as nearly moisture free as possible unless the moisture content is kn nd the appropriate correction made. The method is not applicable to green vood. Specimens for test may be cross cut from the ends of boards. If it is not convenient tc cut specimens 1 by 1 by 10 inches in size-, another size may be chosen — a length of 5 inches, perhaps, with a uniform cross section, '.'/here possible, specimens should be dried before test to an oven-dry condition. n it is desirable to determine the specific gravity of specimens of vari- ous lengths (but, of course, with parallel sides) the specimen is floated and the water level marked. The immersed 1 asured, and this value, .1 divided by the total length of the specimen, gives its specific gravity. '.'/hen testing samples that are not oven dry, but for which the current mois- ture content is known, a formula developed at the Forest Products Laboratory may be used to make a simple correction calculation from current volume and moisture content tc oven-dry weight and volume. In this formula a correction factor, K, is employed as a constant for each change of 1 percent in -.eisture content for a given species. This factor compensates for shrinkage also. For a sample tested at X percent moisture content, the formula is: Specific gravity equals current specific gravity at X moisture cont minus KX. Assuming a moisture content of 15 percent for a spruce specimen, and a current specific gravity reading by the flotation method of 0.42, the formula may be used as follows: First from the accompanying table-, obtain the correction factor, (0.00193) for spruce. Substituting the value in the formula, the following equation is obtained: Specific gravity equals 0.42 minus (C. 00193 times 15) By simple arithmetic, the approximate specific gravity cf the specimen at oven- dry weight and volume is found to be 0.39. values in the last two columns of the table have been computed to adjust the current specific gravity (grams per cubic centimeter, col. II) and pounds per cubic foot (col. 12) for each 1 percent change in moisture content Of aircraft woods. -Table 2-4 in 1943 editioi. of Wood Aircraft Inspection and Fabrication Lanual. Report No. 1393 -2- a '^ t3 -a « -a « »■ 3 3 3 3 3 3 -O ■ CD ooooo"Sm "* C C C C C J3 • aa 333a 3 * Tlgure 1. — Flotation method of determining •peclflc grarlty. The oyllnder •hould be filled to overflowing with water, and the water Una narked on the specimen immedi- Z II .1631 r ately after immersion. 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