(1) Grade designation. That grade shall be assigned to a sample of wool which corresponds, in the measurement schedule for designating grades of wool, to the measured average and standard deviation of fiber diameter, provided this is determined in accordance with the procedure set forth in paragraph (a) of this section. Example: Measured average fiber diameter equals 27.25 microns; number of fiber diameter measurements equal 1200; standard deviation equals 6.72 microns; grade designation equals 56's. If the measured standard deviation exceeds the maximum specified for the grade to which the measured average fiber diameter corresponds, assign to the wool the next coarser grade. Example: Measured average fiber diameter equals 27.25 microns; number of fiber diameter measurements equal 1200; standard deviation equals 7.80 microns; grade designation equals 54's. Example: Measured average fiber diameter equals 27.25 microns; number of fiber diameter measurements equals 1200; standard deviation equals 8.50 microns; grade designation equals 54's. (2) Interpretation. Since all the wool in a lot may not be of the same grade, the grade determined by measurement represents only the average grade of the entire lot. It should not be construed to represent the grade of any component part of the lot selected on a nonrandom basis.

METHODS FOR DETERMINING GRADE OF WOOL TOP

SOURCE: Sections 31.300 to 31.302 appear at 33 FR 19074, Dec. 21, 1968, unless otherwise noted.

The official standards of the United States for grades of wool top as defined in §§ 31.100-31.116 shall be the basis for determining the grade of wool top. The provisions in §§ 31.30131.302 prescribe two methods for making such determinations-by measurement and by inspection. Both methods for determining grade shall be official; however, if the grade as determined by inspection differs from that determined by measurement, the grade determined by measurement shall prevail.

§ 31.301 Measurement method.

The determination of the grade of wool top by measurement shall be by comparison of the measured average fiber diameter and fiber diameter dispersion with the specifications of the U.S. standards. This determination shall be made in accordance with the procedure for determining average fiber diameter and fiber diameter dispersion provided in paragraph (a) of this section and the procedure for designating grade provided in paragraph (b) of this section.

(a) Procedure for determining average fiber diameter and fiber diameter dispersion—(1) Principle of procedure. The average fiber diameter and fiber diameter dispersion are determined by sectioning the fibers in a sample to a designated short length, mounting the sections of fibers on a slide, projecting the magnified image onto a scale, and measuring the diameter of a minimum number of fibers, as specified in this section.

(2) Apparatus and material. The following apparatus and material are needed and shall comply with the following provisions:

(i) Microprojector. The microscope shall be equipped with a fixed body tube, a focusable stage responsive to a coarse and fine adjustment, and a focusable substage with condenser and iris diaphragm. It shall be vertically installed with adequate light source, eyepiece, and objective to give a precise magnification of 500 × as determined by use of a stage micrometer. A magnification of 500 x can be obtained when the microscope is adjust

ed at a proper projection distance and equipped with a searchlight microprojector bulb, a 10-15 × eyepiece, and a 20-21 × objective of good quality with an aperture of approximately 0.50 centimeter.

(ii) Stage micrometer. Calibrated glass slide used for accurate setting and control of the magnification.

(iii) Cross-sectioning device, heavy duty. An instrument approximately 5 cm. (2 inches) in height, consisting essentially of a metal plate with slot for holding a quantity of fibers, a key for compressing the fibers, and a tonguepropelling arrangement by which the fiber bundle may be extruded for sectioning.

(iv) Microscope slides. 25 × 75 mm. 1" x 3").

(v) Cover glasses. No. 1 thickness, 22 x 50 mm. (8′′ × 2′′).

Colorless

(vi) Mounting medium. mineral oil with a refractive index between 1.53 and 1.43 and of suitable viscosity

(vii) Wedge scales. Strips of heavy paper or Bristol board imprinted with a wedge for use at a magnification of 500 × 0. The wedge is usually divided into 2.5 micron intervals.

(3) Calibration. The microscope shall be adjusted to give a magnification of 500 x in the plane of the projected image. This may be accomplished by placing a stage micrometer on the stage of the microprojector and bringing the microscope into such adjustment that an interval of 0.20 mm. on the stage micrometer will measure 100 mm. when sharply focused in the center of the image plane.

(4) Sampling. Sample the lot of top by drawing from each 20,000 pounds (9,072 kilograms), or fraction thereof, four sections of sliver (test specimen) each of which shall be at least 1 yard (0.91 meter) in length and taken from different balls of top selected at random. Take one ball only from any one bale or carton. For broken top take an equivalent aggregate length of sliver at random. The four test specimens shall constitute a sample.

(5) Test condition. Precondition all samples to approximate equilibrium in an atmosphere of 5-25 percent relative humidity at a temperature less than 50° C. (122° F.). Then condition them

for at least 4 hours in the standard atmosphere for testing-65±2 percent relative humidity at 21° ±1.1° C. (70° ±2° F.).

(6) Preparation of slides—(i) Fillingcross-section device. Each sliver (test specimen) of top making up the sample shall be placed individually in the slot of the cross-section device far enough from either end of the sliver to assure sectioning at an undisturbed area. The sliver shall be compacted firmly with the compression key and the latter secured with the set screw.

(ii) Preliminary section. The gripped fibers shall be cut off at the upper and lower surfaces of the plate. The fiber bundle shall be extruded to the extent of approximately 0.50 mm. in order to take up slack in the fibers and the propulsion mechanism. The projecting fibers shall be moistened with a few drops of mineral oil. This projecting fiber bundle shall be cut off with a razor blade flush with the upper surface of the fiber holder plate and the section discarded.

(iii) Final section. The fiber bundle shall again be extruded, approximately 0.25 mm., the equivalent of 250 microns. The fiber bundle shall be moistened with a few drops of mineral oil and the excess blotted off. The projecting fibers shall be cut off with a sharp razor blade flush with the holder plate. The fiber pieces should adhere to the razor blade.

(iv) Mounting the fibers. A few drops of mineral oil shall be placed on a clean glass slide. With a dissecting needle the fiber pieces shall be scraped from the razor blade onto the slide. The fibers shall be thoroughly dispersed in the oil with the dissecting needle and the slide completed with a cover glass. Sufficient oil should be used in the preparation of the slide to insure thorough distribution of the fibers, but an excess must be avoided, as practically no oil should be permitted to flow out or be squeezed out beyond the borders of the cover glass. If the number of fibers is too great to permit proper distribution on the slide, or if an excess of oil has been used, a portion of the mixture, after thorough dispersion of the fibers, may be wiped away with a piece of tissue or cloth.

(v) Finished slide. The slide shall be placed on the stage of the microprojector, cover glass toward the objective. The measurement courses shall be planned across the slide so that the far, near, and intermediate areas will be reached. Slides shall be measured the day they are prepared.

(7) Measurement of fibers. The midlength portion of the fiber to be measured shall be brought into sharp focus on the wedge scale. Fiber edges appear as fine lines without borders when they are uniformly in focus. It is unusual, however, for both edges of the fiber to be in focus at the same time. If both edges of the fiber are not uniformly in focus, adjustment shall be made so that one edge of the fiber is in focus and the other shows as a bright line. The measurements of 100 fibers are recorded on one wedge by marking on the wedge scale the point where the wedge corresponds with the fiber image as determined by (i) the fine lines of both edges when they are uniformly in focus or (ii) the fine line of one edge and the inner side of the bright line at the other edge when they are not uniformly in focus. The slide shall be traversed and successive fibers measured in the planned courses, with only those fibers being measured whose midpoints come within the field-a circle 4 inches in diameter, centrally located in the projected area. Fibers shorter than 200 microns or longer than 300 microns and those having distorted images shall be excluded from measurement. The marks on the wedge indicating the diameter of fibers measured are counted and combined into class intervals for calculation as indicated in paragraph (a)(10) of this section. Occasionally a fiber diameter will be less or greater than the extreme limits of the wedge scale. When this occurs, the image of the fiber is projected onto the border of the wedge scale and lines are drawn on the scale at the edges of the fiber image. The distance between the lines is later measured with a

metric ruler to obtain the correct average diameter of the fiber. In using the metric scale in this manner, 1 mm. is equal to 2 microns at a magnification of 500 X.

(8) Nature of test. One test shall consist of the measurement by two operators of the same four slivers (test specimens) of top. The measurement of both operators shall be combined for calculation of average fiber diameter and fiber diameter dispersion.

(9) Number of slides and fibers. Each operator shall make a slide from each test specimen for a total of four slides per operator. The number of fibers to be measured per slide shall be determined by dividing the total number of fibers to be measured per test by 8 (the total number of slides prepared per test). The minimum number of fiber measurements required for each test shall be the number for the respective grade as prescribed in the measurement schedule for designating grades of wool top set forth in paragraph (c) of this section. Each operator shall measure approximately onehalf the required number of fibers. In lots that are assigned a dual grade designation, the minimum number of fibers measured shall be that specified for the coarser of the two grades.

(10) Calculations. From the observations recorded on the wedge scales, compute the total number of measurements (n), the distribution of fiber diameter frequencies, and the average diameter of fiber (X).

(i) The average diameter of fiber (X) shall be determined by the following formula: X=A+mE1. In this formula

A Class interval midpoint
m-Class interval
E1=Σfx/n, where
Σ=Summation
f=Observed frequency

x=Deviation in class intervals from A
n=Total number of measurements

An example of the calculations is set forth below, based on an arbitrary selection of a class interval midpoint of 6.25 microns:

Number of measurements (n)=800

A (class interval midpoint)=6.25 microns m (class interval)=2.5 microns

Average

(E1=(Σfx/n)=(5977/800)=7.47

diameter, X=A+ME1=6.25+

2.5(7.47)=24.93 microns 1

'Round off the calculated values of average fiber diameter to two decimal places as follows: If the figure in the third decimal place is 4 or less, retain the figure in the second decimal place unchanged; otherwise, increase the figure in the second decimal place by 1.

(b) Procedure for designating grade. A grade shall be assigned to a lot of wool top which corresponds to the average fiber diameter and fiber diameter dispersion requirements specified in §§ 31.100-31.116 and paragraph (c) of this section.

(1) Single grade designation. If the measured average diameter and fiber diameter dispersion correspond to a single grade, that shall be the grade assigned to the sample.

Example: Average fiber diameter-28.10 microns.

Fiber diameter dispersion:

30 microns and under-64 percent. 30.1 microns and over-36 percent. 50.1 microns and over-1 percent. Grade designation-56s.

(2) Dual grade designation. If the fiber diameter dispersion does not meet the requirements for the grade to which the average fiber diameter corresponds, the wool top shall be assigned a dual grade designation, the second designation being one grade coarser than the grade to which the average fiber diameter corresponds.

Example: Average fiber diameter-28.10 microns.

Fiber diameter dispersion:

30 microns and under-61 percent. 30.1 microns and over-39 percent. 50.1 microns and over-2 percent. Grade designation-56s-54s.

(c) Measurement schedule for designating grades of wool top.

1 The second maximum percent shown for any grade is a part of, and not in addition to, the first maximum percent. In each grade, the minimum percent and the first maximum percent total 100 percent. 2 Research has shown that when wools of average uniformity in fiber diameter are measured, the prescribed number of fibers to measure per test will result in confidence limits of the mean ranging from approximately ±0.4 to ±0.5 micron at a probability of 95 percent.