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Unevenness and Coefficient of Variation of Yarn

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Md Sohanur Rahman Sobuj
Student of Bangladesh University of Textiles (BUTex)/ Department: Apparel Engineering
Md Sohanur Rahman Sobuj
Md Sohanur Rahman Sobuj
Md Sohanur Rahman Sobuj

Unevenness and Coefficient of Variation of Yarn

Yarn Evenness-1   Yarn Evenness-2   Yarn Evenness-3  Yarn Evenness-4

Mathematical Expression of Yarn Irregularity or Mass Variation

Generally two parameters are used to express the irregularity of yarns. They are…

  • 1. Percentage of mean deviation (PMD) or Unevenness (U%)
  • 2. Coefficient of variation CV%

 Percentage of Mean Deviation (PMD) or Unevenness (U%)

The average value for all the deviations from the mean which is expressed as a percentage of the overall mean is called percentage of Mean Deviation (PMD). This is termed Um% by the Uster Company.

 Coefficient of mass Variation (CV%):

The coefficient of mass variation CV % is the ratio of standard deviation of mass variation divided by average mass variation.The higher the CV value is the more irregular the yarn.

eqn CV

A modern instrument, such as the Uster Evenness Tester, can measure the U and CV values of a fiber assembly at a high speed. The larger deviations from the mean value are much more intensively taken into consideration in the calculation of CV% rather than in U% (due to the squaring of the term). For this reason, the Coefficient of Variation CV% has received more recognition in
modern statistics than the irregularity value U%.

 

Graphical Representation of Unevenness and Coefficient of variation

unevenness and CV table

 Relation between Percentage of Mean Deviation (PMD) and Coefficient of Variation CV%

If the fiber assembly required to be tested in normally distributed with respect to the mass variation, then the two parameters of irregularity are related by the following equation:

unevenness eq1

Type of Fiber Assembly Conversion Factor
►Normal Distribution (contains purely random variations, symmetrical)

►Asymmetrical Distribution, periodic and random   variations present (faulty test material e.g. with long wavelength count variations, thick places, etc.)

►Symmetric Distribution with strong periodic variations

1.25

 

>1.25

 

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Md Sohanur Rahman Sobuj Student of Bangladesh University of Textiles (BUTex)/ Department: Apparel Engineering