18 October 2012
Fiber Fineness Measurement by Projection Microscope
The projection microscope is the
standard method for measuring wool fibre diameter, and all other methods
have to be checked for accuracy against it. The method is also
applicable to any other fibres with a circular cross-section. The method
involves preparing a microscope slide of short lengths of fibre which
is then viewed using a microscope that projects an image of the fibres
onto a horizontal screen for ease of measurement. The apparatus is shown
diagrammatically in Fig. Techniques are followed that avoid bias and
ensure a truly random sample.
Method of Test
A suitable random and representative sample is conditioned for 24 h in a standard testing atmosphere. Using a modified Hardy microtome the fibres are cut to a suitable length (0.4mm for fibres below 27 (im) and a slide is prepared by carefully mixing the fibres into the mountant. The use of short fibres gives a length-biased sample so that proportionally more of the longer fibres will have their diameter measured. The mounting agent should be non-swelling and have a suitable refractive index (for example liquid paraffin). The mixture of fibres and mountant is spread thinly on the slide and covered with a cover glass, carefully avoiding air bubbles and finger prints.
The slide is placed on
the stage, coverglass down (microscope inverted) and fibres are selected
for measurement in the following way. The slide is traversed in a
zigzag fashion, measuring every fibre that complies with the following
requirements: 1 has more than half its length visible in the 7.5cm
circle which is drawn in the centre of the field of view; 2 is not in
contact with any other fibre at the point of measurement. The traverse
of the slide is continued until the required number of fibres
has been measured. The magnification of the microscope is adjusted to
be 50Ox so that on the scale used to measure the fibres each millimetre
represents 2 um.
For accurate tests three slides should be measured from randomly selected areas of the material and not less than 150 fibres per slide should be measured. The coefficient of variation of diameter for unblended wool lies between 20% and 28%. From this value the number of tests to give certain confidence limits has been calculated .
http://textilelearner.blogspot.com/
A suitable random and representative sample is conditioned for 24 h in a standard testing atmosphere. Using a modified Hardy microtome the fibres are cut to a suitable length (0.4mm for fibres below 27 (im) and a slide is prepared by carefully mixing the fibres into the mountant. The use of short fibres gives a length-biased sample so that proportionally more of the longer fibres will have their diameter measured. The mounting agent should be non-swelling and have a suitable refractive index (for example liquid paraffin). The mixture of fibres and mountant is spread thinly on the slide and covered with a cover glass, carefully avoiding air bubbles and finger prints.
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| The projection microscope |
For accurate tests three slides should be measured from randomly selected areas of the material and not less than 150 fibres per slide should be measured. The coefficient of variation of diameter for unblended wool lies between 20% and 28%. From this value the number of tests to give certain confidence limits has been calculated .
http://textilelearner.blogspot.com/
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