6.1 This test method for the determination of evenness of textile strands is used extensively for acceptance testing of commercial shipments of filament or spun staple yarn, comber laps, roving, sliver, or tops.

6.2 Values of strand evenness are also used in quality control, process optimization, and together with yarn strength measurements, as the first appraisal of a strand's quality. A low evenness value is, in general, preferred. Higher evenness values generally indicate poor yarn manufacturing practices, lower yarn strength, and poorer fabric appearance. Experience has shown that the relationship of evenness to the prediction of yarn performance and to fabric appearance is not a simple one. An evenness value must, therefore, be used cautiously and be supplemented by additional evenness information, such as mid-term and long-term mass variations, thin, thick, and nep imperfection counts, diagram chart spectrogram chart, length variation curve, and histogram analyses.

6.3 Continuous filament yarns should be tested for mass variation on evenness testers that are specifically designed to test this yarn type; failure to do so will result in inaccurate test results. Further, low-twist, continuous filament yarns tend to flatten to a ribbon configuration while passing through the sensor of a capacitance instrument. This flattening effect will cause false mass variation measurements by the capacitive sensor (commonly referred to as shape effect). Evenness testers that are specifically designed to test continuous filament yarns insert a false twist to the yarn strand during testing to overcome the flattening effect and thus ensure accurate mass variation measurements.

6.4 Strands made from fiber blends should be tested only if the different fibers are uniformly distributed throughout the strand. Non-uniform blending may cause a higher reading of mass variation than the true value if the component fibers differ in dielectric constant.

6.5 Evenness values obtained on different instruments will be comparable for strands from the same sample provided the following parameters are the same in all cases: (1) the measure of evenness used (CV% or U%); (2) the capacitive length zone L_c; (3) the sample length, Ls; (4) instrument test speed, (5) laboratory temperature and humidity conditions (see 10.1); and (6) test specimen preparation, and (7) test specimen variation. When different models of an instrument are used, and one or more of the seven parameters are not identical, test results may differ.

6.6 If there are any differences of practical significance between reported test results for two (or more) laboratories, comparative tests should be performed using an agreed upon number of samples that are homogeneous and randomly assigned. Competent statistical assistance should be used to determine if there is a statistically significant difference between the laboratories. If a bias is found, either its cause must be found and corrected, or future testing for that material must be adjusted in consideration of the statistically significant differences found.