HIGH VOLUME INSTRUMENT SYSTEM
The testing of fibres was always of importance to the spinner.
It has been known for a long time that the fibre characteristics have a decisive impact on the running behaviour of the production machines, as well as on the yarn quality and manufacturing costs. In spite of the fact that fibre characteristics are very important for yarn yarn proudction, the sample size for testing fibre characteristics is not big enough. This is due to the following
- The labour and time involvement for the testing of a representativesample was too expensive. The results were often available much too late to take corective action.
- The results often depended on the operator and / or the instrument, and could therefore not be considered objective
- one failed in trying to rationally administer the flood of the rawmaterial data, to evaluate such data and to introduce the necessary corrective measures.
Only recently technical achievements have made possible the development of automatic computer-controlled testing equipment. With their use, it is possible to quickly determine the more important fibre characteristics.
Recent developments in HVI technology are the result of requests made by textile manufacturers for additional and more precise fibre property information. Worldwide competitive pressure on product price and product quality dictates close control of all resources used in the manufacturing process.
Following are the advantages of HVI testing
- the results are practically independent of the operator
- the results are based on large volume samples, and are therefore more significant
- the respective fibre data are immediately available
- the data are clearly arranged in summerised reports
- they make possible the best utilisation of rawmaterial data
- problems as a result of fibre material can be predicted, and corrective measures instituted before such problems can occur
Cotton classification does not only mean how fine or clean, or how long a fibre is, but rather whether it meets the requirements of the finished product. To be more precise, the fibre characteristics must be classified according to a certain sequence of importance with respect to the end product and the spinning process.
The ability to obtain complete information with single operator HVI systems further underscores the economic and useful nature of HVI testing.
Two instrument companies located in the US manufacture these HVI systems. Both the systems include instruments to measure micronaire, length, length uniformity, strength, colour, trash, maturity, sugar content etc.
The length measure by HVI systems used by the USDA is called upper-half-mean length. This is the average or mean length of the longest one-half of the fibres in the sample. The spinlab system uses the fibrosampler device to load the fibres on needles, the motion control system uses the Specimen Loader to capture the fibres in a pinch clamp. However the preparation of the length specimen for both systems includes combin to straighten and parallel the fibres, and brushing to remove fibre crimp. The length measurement is then made by the instrument scanning along the length of the specimen to determine the length data.
The insturments are calibrated to to read in staple length. Length measurements obtained from the instrument are considerably more repeatable than the staple length determination by the classer. In one experiment the instrument repeated the same staple length determination 44% of the time while the classer repeated this determination only 29% of the time. Similarly, the instrument repeated to 1/32" on 76% of the samples, while the classer agreed on 71% of the samples to within 1/31".
The precision of the HVI length measurement has been improved over the last few years. If we take the same bale of cotton used in the earlier example and repeatedly measure length with an HVI system, over two-thirds of measurements will be in a range of only about 1/32 nd of an inch: 95% of the individual readings will be within 1/32nd of an inch of the bale average. In the 77000 bales tested, the length readings were repeated within 0.02" on 71% of the bales between laboratories.
The HVI system gives an indication of the fibre length distribution in the bale by use of a length uniformity index. This uniformity index is obtained by dividing the mean fibre length by the upper-half-mean length and expressing the ratio as a percent. A reading of 80% is considered average length uniformity. Higher numbers mean better length uniformity and lower numbers poorer length uniformity. A cotton with a length uniformity index of 83 and above is considered to have good length uniformity, a length uniformity index below 78 is considered to show poor length uniformity.
Repeated measurements on a single bale of cotton show the length uniformity index measurement to have relatively low precision. About two-thirds of the measurements will occur within one unit of length uniformity; thus a bale with an average length uniformity index of 80 would have 68% of the readings occuring between 79 and 81, and 95% of hte readings occuring between 78 and 82. This does not seem too bad until one considers that most US upland cottons will have a length uniformity reading between 75 and 85.
Most organizations operate their HVI systems to use an average of 2 or 4 readings per bale for the length uniformity index. Using that number tests per bale, the USDA test of 77000 bales showed that laboratoriesat different locations agreed 68% of the time to within one length uniformity index unit.
In some cases low length uniformity has correlated with high short fibre content. However, in general the correlations between length uniformity index and short fibre content have not been very good. One important reason why the length uniformity index is a not a very good indicator of the short fibre content has to do with the fact that the HVI systems do not measure the length of any fibres shorter than about 4mm.
Another reason for the poor correlations between length uniformity index and short fibre content is that the short fibre content is related to staple length while the length uniformity index is fairly independent of staple length. As an example, the shorter staple cottons tend to contain higher amounts of short fibre than the longer staple cottons. Howeer, many short staple cottons have length uniformity index readings above 80.
The micronaire reading given by the HVI systems is the same as has been used in the commercial marketing of cotton for almost 25 years. The repeatability of the data and the operator ease of performing the test have been improved slightly in the HVI micronaire measurement over the original instruments by elimination of the requirement of exactly weighing the test specimen. The micronaire instruments available today use microcomputers to adjust the reading for a range of test specimen sizes.
The micronaire reading is considered both precise and reperable. For example, if we have a bale of cotton that has an average micronaire of 4.2 and repeatedly test samples from that bale, over two-thirds of thet micronaire readings will be between 4.1 and 4.3 and 95 %of the readings between and 4.0 and 4.4. Thus, with only one or two tests per bale we can get a very precise measure of the average micronaire of the bale.
This reading is also very repeatable from laboratory to laboratory. In USDA approx 77000 bales were tested per day in each laboratory, micronaire measurements made in different laboratories agreed with each other within 0.1 micronaire units on 77% of the bales.
The reading is influenced by both fibre maturity and fibre fineness. For a given growing area, the cotton variety generally sets the fibre fineness, and the environmental factors control or influence the fibre maturity. Thus , within a growing area the micronaire value is usually highly related to the maturity value. However, on an international scale, it cannot be known from the micronaire readings alone if cottons with different micronaire are of different fineness or if they have different maturity levels.
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