Merits & limitations of cotton fibre length measuring instruments

Considerable divergence is found between the opinions about the level of agreement between the results of different length measuring instruments including Baer sorter, HVI fibrograph and AFIS, discloses N Balasubramanian.

Fibre length is one of the most important quality of cotton contributing to higher yarn strength spinnability and reduced end breakages in spinning. Short fibre content has pronounced effect on waste, yarn irregularity, appearance and end breakages Conventional and HVI instruments used for testing fibre length parameters of cottons are briefly reviewed here. Merits and limitations of these instruments, precautions to be taken and the agreement between the results of these instruments are discussed based on research conducted over the years.

• Oil plate method: Fibre straightened by liquid paraffin on a glass plate is measured for length by a scale. Merits – Accurate measurement Limitation – Laborious and time consuming.
• Balls sorter: Determines frequency length distribution of fibres from which mean, CVs are estimated. Fibres are laid as per their length on a plush table by a moving carriage and fibres falling in different length groups are weighed to give weight length distribution (f(l)). Time consuming and is not commonly used.
• Baer sorter: The fibres are fractionated into different length groups by a set of parallel combs and top comb and uniform array of fibres is prepared in descending order of length to get cumulative fibre length distribution . Effective length, mean length, upper quarter length and % short fibres are determined. Suter web sorter is a similar American instrument where tufts are weighed instead of being laid on a plush board to prepare a diagram. ASTM Standard D 1447-07, details the Standard Test Method for length and length distribution of cotton fibers (Array Method). Cumulative frequency by Baer sorter is q(l) and is related to frequency f(l) of fibres by (balls sorter)

Merits: 1.Effective length is close to Graders staple length 2. Provides accurate estimate of short fibre content

Limitations. 1. Time consuming (about 2 hrs per sample) 2. Requires considerable operator skill.

• Shirley Photo Electric Stapler: Light is made to fall on a moving tuft of fibres aligned at one end and the reflected light is made to fall on two photo cells. Distance between two maximum gradient points in the current generated in the two photo cells, as the tuft moved on a traversing tape gives staple length.
• Peyer Almeter: Fibroliner prepares a beard of aligned fibres held in the needle field of a transfer equipment. The clamped beard is passed between two capacitance plates and the change in capacitance is measured. Both cumulative and histogram of length frequency curve are shown in a printer. Time taken for measurement of length is about 15- 20 min. ASTM D5332- 92 stipulates test method of measuring length by Almeter. Limitation – Finite gauge of Fibroliner does not allow full gripping of short fibres. Fibroliner allows short fibres to move to the centre of beard instead of staying at the aligned end.
• Fibrograph: Beard of fibres is prepared by picking the fibres randomly from the sample by a comb and loose fibres brushed aside. The beard is optically scanned from the base to tip from which a fibrogram is drawn. The comb has 28 needles/inch. Since long fibres have a proportionally higher probability to be caught by comb this results in a length biased sample. The instrument based on this principle was developed by Hertel. Upper half mean length, mean length and Uniformity index are determined from the fibrogram by drawing tangents to the curve.

ASTM D1447 – 07(2012)e1 gives the Standard Test Method for Length and Length Uniformity of Cotton Fibers by instruments like Fibrograph. Mechanisation of measurement of length in the instrument resulted in servo fibrogr