A FIELD METHOD OF DETERMINING 

SPECIFIC GRAVITY BY USE OF 

INCREMENT CORES 

OR AUGER CHIPS 

Information Reviewed and Reaffirmed 

March 1956 

No. 1587 
- - - - - - 

A FIELD METHOD OF DETERMINING SPECIFIC GRAVITY 

BY USE OF INCREMENT CORES OR AUGER CHIPS1 

By 

BENSON H. PAUL, Senior Silviculturist 

and 

MARTIN E. BAUDENDISTEL, Associate Forest Products Technologist 

Forest Products Laboratory, 2 Forest Service 

U. S. Department of Agriculture 

-

Aside from results of actual mechanical tests, specific gravity is one 

of the best indicators of the strength of wood that is free from 

weakening defects. Wartime requirements of wood place exacting 

demands upon its quality for many uses, An easy and rapid method of 

determining the specific gravity of wood before it is processed is 

highly desirable in order to eliminate unnecessary cost and delay and 

also to put timber supplies to those uses to which their quality best 

adapts them. One such method, developed at the Forest Products 

Laboratory, is described herein for use with standing trees, round 

logs, and timbers of various dimensions. This method has been found 

to compare closely in accuracy with standard methods. 

Method 

The Forest Products Laboratory method consists of extracting a core of 

wood from a log or tree trunk with a standard increment borer of 

known caliber, carefully measuring its length when green, drying it to 

a moisture-free condition in a portable oven, and then, by means of a 

spring of the Jolly balance type and a specially prepared specific 

1This is one of a series of progress reports prepared by the Forest 

Products Laboratory relating to the use of wood in aircraft issued 

in cooperation with the Army-Navy-Civil Committee on Aircraft 

Design Criteria. Original report published in 1943. 

2 

Maintained at Madison, Wis., in cooperation with the University of 

Wisconsin. 

Report No. 1587 -1- Agriculture-Madison 
gravity chart, determining the specific gravity in terms of the distance 

the core stretches the spring. The specific gravity of the wood thus 

obtained is in terms of its weight when oven dry and its volume when 

green.. 

The chips from an auger hole of known diameter and depth can be used 

instead of an increment core. 

The volume of the core is computed from its length when green and the 

diameter of the cutting edge of the increment borer used. The weight 

may be determined directly by use of the spring, which is calibrated 

to a weight scale. In practice, actual weighing and computations of 

volume may be omitted because the chart is designed for direct reading 

of specific gravity values for cores having a definite diameter and 

within the range of lengths for which the chart is prepared. 

When auger chips are used, the volume of the auger hole is computed 

from the depth of the hole and its diameter. The latter may be 

obtained for a given auger by measurement of its cutting diameter. 

With the greater weights obtained by chips, a spring of greater capacity 

is required than for increment cores. It is possible, however, to 

make holes in sizes proportional to increment cores, so that both 

springs may be calibrated to one chart but at different scales of weight. 

The specific gravity of the chips is read from the chart in the same 

manner as the specific gravity of a core. 

Equipment and Use 

The equipment required consists of the following: 

1. A standard increment borer or an auger approximately 1/2 

inch in diameter. For testing the clear portions of standing trees or 

of logs, the length of the boring device used should equal at least onefourth the diameter of the largest trees or logs to be sampled. 

2. A narrow scale graduated to 0.01 inch for determining length 

of cores or depth of bored holes. 

3. A micrometer caliper for determining the exact diameter of 

bore of the increment borer, or the cutting diameter of augers. 

4. Suitable containers for carrying cores to prevent drying and 

breaking or for collecting chips. 

5. A sensitive spring of the type used in a Jolly balance. The 

spring should have an elongation of about 5 centimeters per gram within 

Report No. 1587 -2-
a range of about 9 grams for use with cores or an elongation of about 

1 centimeter per gram within a range of 45 grams for use with chips. 

6. A strong piece of thread to be suspended from the spring with 

a small clip at the lower end for holding cores or with an envelope or 

other device for holding chips. The thread hangs across the face of 

the chart and carries an indicator which, after being checked at zero 

chart reading, shows on the chart the specific gravity value of the 

suspended sample. 

7. A suitable support or vertical surface, such as a wall, from 

which the spring can be suspended (by means of screw-eyes) and upon 

which the specific gravity chart may be fastened. 

8. A 5-gram weight for calibrating the elongation of the light 

spring, and a weight of 20 grams for calibrating the elongation of the 

heavier spring. A weight scale is prepared for each chart to 

facilitate accurate calibration of the spring being used. 

9. A specific gravity chart constructed in accordance with details 

given in following paragraphs. 

10. A simple portable drier for cores or chips. It may consist 

of a sheet-iron box with a hinged cover lined with asbestos 1/4 inch 

thick. Two electric lamps are fitted in the bottom of the box and a 

connection provided for plugging into any 110- to 120-volt electric 

circuit. Temperature may be regulated by cutting suitable ventilating 

holes in the box, by varying the wattage of the electric bulbs used, or 

by employing automatic flickering devices to make and break the 

contact at required intervals. 

Preparation of a Weight Scale 

The use of the helical spring depends upon its uniform elongation per 

unit weight within its elastic limit, There are two springs listed in 

laboratory equipment catalogues, one with an elongation of about 5 

centimeters per gram and another with an elongation of about 1 centimeter per gram. 

The weight scale to which the lighter spring is calibrated is based on 

an elongation of 4. 5 centimeters per gram. The capacity of the spring 

being only 9 grams (within its elastic limit) the weight scale should 

have a working limit of not more than 7 grams in order to allow for 

the weight of the thread and holding attachment for cores. The length 

of the weight scale, therefore, is 31.5 centimeters. This distance 

may be divided into seven equal parts, which can be subdivided further 

Report No. 1587 -3-
for smaller units, such as 0.1 gram, etc. By making the scale somewhat below the rated unit elongation of the spring, springs with slightly 

varying elongation units may be calibrated to it, as described later. 

For the same reason, the scale chosen for elongation of the heavier 

spring may be 0.9 centimeter or less per gram, with the weight scale 

prepared in the same way. The weight scale forms a basis for 

calibration of a spring to the specific gravity chart. 

Calibration of the Spring 

The spring may be calibrated by adjusting its elongation to the weight 

scale, as follows: place the small end of the spring through a screweye or other similar support and insert a thin strip of metal (an old 

razor blade will serve) over the screw-eye and into a coil of the spring 

near its extremity. Fasten the thread to the spring with the attachment 

for holding a 5-gram weight in place, and set the indicator at the zero 

line of the weight scale. Add the 5-gram weight and note whether the 

indicator reaches the 5-gram mark on the weight scale. If the indicator is carried past the 5-gram mark, shorten the effective length 

of the spring by intercepting it several coils farther down with the metal 

strip. Reset the indicator on zero and again note the displacement made 

by the 5-gram weight. Continue adjustment of the spring's length until 

the 5-gram weight extends the spring just the right amount to bring 

the indicator from the zero point to the 5-gram line. In like manner 

the heavier spring is calibrated with a 20-gram weight and a corresponding weight scale. 

In use, it is a good plan to insure against overloading the spring by 

passing the thread through a small screw-eye that will arrest the spring 

at the point beyond which it should not be stretched. 

Preparation of Specific Gravity Charts 

For increment cores from a borer of a predetermined diameter, a 

chart may be prepared from which the specific gravity may be read 

directly for cores having lengths within the designated range of the 

chart. 

The chart is prepared by locating a certain specific gravity value based 

on a given volume and weight. Intermediate lines are drawn at 

proportional distances to show intermediate values. For example, on 

a sheet of paper 18 inches wide plot the weight scale vertically with 

the zero near the upper left corner. On a horizontal line originating 

at the zero and extending to the right, indicate various lengths of 

Report No. 1587 -4-
cores or depths of holes at equal intervals of 1 inch, 2 inches, 3 

inches, and so on. 

The volume of a core 0.185 inch in diameter and 11.35 inches in length 

is 5 cubic centimeters. If the weight of this core equals 5 grams, 

its specific gravity is 1.00 and it extends the calibrated spring 22. 5 

centimeters. This point should be plotted just 22. 5 centimeters below 

the 11. 35-inch value on the horizontal zero line. The diagonal line 

determined by this point and the zero of the weight scale represents 

a specific gravity of 1.00 for cores of varying length. The intervening 

vertical space is divided at equal intervals for plotting diagonal lines 

showing specific gravity values less than 1.00. The chart based on the 

above values can be used only with cores cut 0.185 inch in diameter. 

Borers vary in diameter and length. The following formula, used to 

compute the volume of the above core, can be used for other sizes as 

well by substituting the proper dimensions: 

In the same way, a chart for chips may be constructed on the basis 

of holes of a given depth and cross section. Thus, a hole 12.9 inches 

deep and 0.505 inch in diameter has a volume of 40 cubic centimeters. 

If the oven-dry weight of the chips were 30 grams, the specific 

gravity of the wood removed would equal 0.75 on a basis of oven-dry 

weight and green volume. A diagonal line is drawn and the intervening 

vertical space is subdivided as in making the core chart. The chart 

based on the values given can be used only with chips from holes cut 

by an auger 0.505 inch in diameter. 

The chart illustrated in figure 1 was constructed for use with either 

cores or chips by means of springs that may be calibrated to weight 

scale A for cores or to weight scale B for chips. In figure 1, the 

elongation on scale A is 7.45 times that of scale B, since for any 

given length on this chart the hole from which the chips are taken has 

7.45 times the volume of a core. A diagrammatic illustration of the 

equipment in use is shown in figure 2. 

Procedure 

Steps in determining specific gravity by the increment core or chip 

method are: 

Report No. 1587 -5-
1. Take an increment core from a log or tree trunk and at 

once measure and record its total green length; or, if values are 

desired for different parts of the radius, divide the core into convenient lengths and measure and record the green length of each piece. 

If prompt measurement cannot be made, cores can be kept moist and 

measured at a more convenient time. For chips, measure the depth 

of the bored hole accurately. Care must be taken to collect all of the 

chips of a given hole. Cores or chips should be given identifying 

marks. 

2. Dry the cores or the chips at a temperature of approximately 

212° F. for 2 to 4 hours, or until moisture-free. 

3. Using the calibrated spring and chart, adjust the indicator 

on the thread to the zero line of the chart. This should be done after 

the core or chip holder is fastened to the thread. 

4. Place the core or chips in the holder and adjust the chart BO 

that the vertical line corresponding to the length of the core or to the 

depth of the auger hole is directly behind the thread. Read the specific 

gravity from the chart at the point shown by the indicator. 

Report No. 1587 - 6- .3-9 

Figure 2.--Equipment for determining specific gravity by 

means of increment cores or auger chips. 

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