Merits and Limitations of Cotton Fiber Length Measuring Instruments (Part-1)

Merits and Limitations of Cotton Fiber Length Measuring Instruments (Part-1)
N.Balasubramanian
Retd Jt. Director (BTRA) and Consultant
I, Rajeswari, 36, 17th Road, Chembur, Mumbai 400071, 9869716298
Email: balajamuna@gmail.com




Abstract
Baer sorter is the most accurate instrument for measuring short fibre content but is time consuming and requires operator training. HVI fibrograph though much faster suffers from 1overestimationdue to scanning some distance from clamping point 2. Underestimation as sampling in the clamped beard is not truly length biased and 3 highly unreliable in estimating short fibre content. AFIS is also rapid in measuring fibre length but suffers from fibre breakages during opening. Further fibres are not fully straightened during measurement. Almeter has the merit of giving both frequency distribution and cumulative frequency distribution but short fibres are not fully gripped and move to the centre. Image analyses is more accurate but is time consuming. Considerable divergence is found between the results of various authors about the level of agreement between the results of different length measuring instruments.

Introduction:
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, CV 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 Grader's 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 end1.

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 has28 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 Hertel2. Upper half mean length, mean length and Uniformity index are determined from the fibrogram by drawing tangents to the curve2. 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 fibrograph where fibrogram is drawn automatically. Manual models could also be converted to servo automatic model by this conversion3. Time required per test is reduced from 7.5 min to 4 min and operator fatigue reduced. Further improvements were suggested by Tallant to improve accuracy in trace and higher operator speed4. Rouse used 2 dial gauges to determine relative length of fibres and number of fibres in a fibrogram instead of drawing the fibrogram5 for improving accuracy. Ewald and Worley6 developed modifications to enable same fibrograph instrument to work as manual, an automatic curve drawing or an automatic dial or digital type. Hertel and Craven7 introduced the concept of span length measurements which led to Digital fibrograph. Digital counters with push button systems were used in place of dial gauges and the entire operation of length measurement is atomised. 2.5%, 50% span length, uniformity index and short fibre%, short fibre content(SFL) and Floating fibre index are determined in digital fibrograph. Fibro sampler is used in later models to clamp the fibres on the comb. Fibre sample is put inside the cylinder of sampler. Fibre comb, with 13 needles/inch, is rotated around the fibro sampler, with pressure applied on the cotton, during which it picks up fibres projecting from the holes of sampler. Time taken for measurement is reduced to 1 min per sample with reproducibility of .1 to .18 %.

Merits
  1. Simulates beard formed by fibres held by back or front roller nip of a drafting system
  2. Very rapid (about 1 - 4 min per sample)
  3. Does not depend much upon operator skill.
Limitation

1. Because of fibre breakages and length involved in clamping, estimates of length are lower.

2. Holding length calculated mathematically is found to be around 4.06 mm8. Scanning starts at 3.81 mm from comb. As a result actual scanning starts at 7.87 mm from the comb because of this. The holding length also varies from cotton to cotton. Krowwicki and Ramay9 showed that 50 % span length, 2.5 % span length and Uniformity ratio in Digital Fibrograph are overestimated as scanning starts at 3.81 mm from clamping point. 50 % and 2.5 % span lengths are overestimated to the extent given below:

50% span length = 0.5 L + 1.905
2.5 % span length = 0.975 L + 0.09525

Maximum possible Uniformity Index is increased as a result from 51.28 to 57.6 for 30 mm and 51.28 to 56 for 40 mm cotton. The extent to which fibrogram is affected by scanning at 4 mm from clamp position instead of clamp is shown in Fig 1 and Table 1 below. Both 2.5% and 50 % span length are increased but the effect is more on the latter. As a result uniformity Index is increased.
Fig 1 : Comparison of Fibrograms with scanning from clamp and scanning from 4mm length
Table-1: Span lengths and uniformity ratios with scanning from clamp and scanning from 4mm length

Span Length
Scanning from Clamp (Mm)
Scanning 4mm from clamp
2.5 %
31.67 mm
32.05 mm
50 %
12.03  mm
 14.32  mm
UI %
40
44.7

3. Crimp in the fibre leads to underestimation.

4. Highly unreliable for estimating short fibre content. Precautions required in fibre sampler for getting reliable beards are discussed by Carpenter10. 1 Even pressure should be applied on the sample inside cylinder 2. The cylinder should be rotated slowly and gently 3 at least one half of perforations should be covered by sample 4 beard should be uniform without thick and thin places 5 A fresh sample surface should be used for each comb 6. Excessive brushing of beard should be avoided. Yoakum11 reported a higher fibre length with fibro sampler beards than hand prepared beards. Correlation of yarn strength with 2.5 % span length is as good as the upper quarter length of comb sorter and upper half mean of servo fibrograph. 2.5 % span length by digital fibrograph and short fibre % by comb sorter were better related to ring frame end breakage rate than uniformity ratio by servo fibrograph.

Frequency of fibres of length l, r(l), by fibrograph is a cumulative distribution of Baer sorter diagram and is given by
Relationship between span lengths and mean fibre length are critically influenced by the variability and short fibre content. 
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