ISO 2648:2020 pdf download – Wool – Determination of fibre length distribution parameters – Capacitance method

03-05-2022 comment

ISO 2648:2020 pdf download – Wool – Determination of fibre length distribution parameters – Capacitance method
4  Principle
The capacitive sensor-based machine tests the length of textile fibres, using a test specimen of fibres made up with the aid of the mechanical grip. The grip, fed with slivers or rovings, prepares a numerical specimen of fibres, where the number of fibres in each length class is represented in the same numerical proportion as in the original sliver. This test specimen is arranged in the form of a draw of fibres, with all the fibres having one of their ends (their base) situated approximately on the same line, perpendicular to the direction of the fibres. The test specimen formed in this way is then transferred from the grip to the capacitive sensor-based apparatus, where it is inserted between two thin plastic sheets. The carriage containing the test specimen is either moved at a constant speed through a measuring condenser, or the measuring condenser moves over a stable test specimen at a constant speed. The variation in capacitance so produced is due to the partial replacement of the dielectric “air” and fibres between the dielectric plates of the condenser. Knowing the formation of the sample, it can be shown that the measured signal (proportional to this increase in capacitance) is equivalent to a cumulative Hauteur (H) diagram, which is automatically traced. The following length distribution parameters are calculated, Hauteur (H), coefficient of variation of hauteur (CV H ), Barbe (B), coefficient of variation of barbe (CV B ), L values and K values.
5.1.3  Recorder.
In the analogue system, the record is a galvanometric recorder, which automatically traces the cumulative Hauteur diagram on squared paper during measurement. In the digital system, the data are captured by a computer which is capable of outputting the results to either the screen or to a printer. The diagram ordinate gives the percentage of fibres (biased by cross- section) of length greater than the length indicated on the corresponding abscissa. (The percentage by cross-section is very close to the percentage by number).
6.1.1 General
The sample, kept in the form of a twisted hank, is exposed to the conditioning atmosphere for a minimum period as indicated below. This period can vary according to the type of the material and the sampling conditions. Generally, regardless of the origin of the sample of sliver, the preliminary conditioning period is 24 h in the standard atmosphere for testing as defined in ISO 139. In order to standardize the procedure, this period of 24 h may be adopted for all cases where no urgency exists.
6.1.2 For slivers coming from a process involving soaking, drying or oiling, a conditioning period of 24 h in standard atmosphere is to be observed.
6.1.3 For tops sampled in the normal way at a passage following combing, and drawing slivers and rovings sampled from a machine where fibre lubricant is not applied, the period of conditioning in standard atmosphere can be reduced to a minimum of 4 h.
6.1.4 In some cases, this period can be reduced still further; for instance, if a rapid conditioning enclosure is available in which the sample hank can be placed for 30 min followed by a further 30 min in the standard atmosphere.

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