Alpaca Diameter Measurement

Abstract

Dear Sir :While measuring the fineness of Alpaca fiber with the Coulter Counter [4] we noted discrepancies which required explanation. Several methods of measurement have now been compared to resolve these differences. Because Alpaca fiber is generally medullated (that is, it is often hollow) and ellipsoidal in cross section, the apparent or eflective diameter can vary according to the method of measurement.The standard micrdprojection method [la], ASTM D 2130, is liable to error due to fiber ellipticity (as discussed in Appendix A2 [la] ) but is not concerned with medullation as it is a method for measuring diameter per se. The Coulter Counter indirectly gives an average diameter calculated from the cross sectional area, assumed to be circular. Deviation from circularity, accordingly, may produce differences from the microscopic method. Differences would also result when diameters are calculated from measurements based on linear density, as found with the vibroscope or by cutting and weighing [lb]. With respect to medullation, the Coulter tnethod might conceivably &dquo;see through&dquo; a hollow fiber section, sensing only solid material. According to Berg [2]. this is unlikely. Methods based on the linear density measure only solid material.Besides the methods already mentioned, major and . minor axes of cross sections were measured on photographs of known enlargement. To convert these measurements to area, a shape factor was calculated from planimeter readings on 200 sections; the factor 0.80 was obtained. If the sections were truly elliptical, the calm culated factor would be ir/4 T 0.78.The fiber sample tested was of Haucayo Alpaca obtained from W. von Bergen and<H. J. Ortstein of J. P. Stevens Company. Measurements were made at 65% RH and 70°F after conditioning from the dry side. In each case, an extent biased sample r51 was taken. Results of the various tests are given in Table I ; the average diameter, D in microns, is that calculated for the equivalent (right circular) cylinder. ' The first three methods all measure the solid material . present and give similar results. In the cut-and-weigh . method, average area or average (diameter) 2 its obtained; a standard deviation of 8 microns was assumed to convert to average diameter. In the second method, the area of the medulla was subtracted from the overall cross-sectional area for each fiber, In the vibroscopic method, the second-order correction for fiber stiffness was made r31 for each fiber to improve accuracy.The fourth and fifth methods should agree in that both measure area. The slightly higher standard deviation in the Coulter method reflects the variability in. cut length of the fiber sections; this has been noted [4]. The possibility that Coulter method may &dquo;see through&dquo; tubular sections cannot now be resolved because of random error. TABLE I. Relationship of Measurement Method md Resnlting Statistics No. of sections weighed, as sections were not weighed individually, S could not be calculated. t Average No. of sections per pass thr...