Fiber Fineness is Measured by the Airflow Method

Principle:
In this method, fiber fineness is measured by air flow. If large amount of air is blown, the fiber will be coarse and if small amount of air is blown, the fiber will be fine. The method based on this principle.

This is an indirect method of measuring fibre fineness which is based on the fact that the airflow at a given pressure difference through a uniformly distributed mass of fibres is determined by the total surface area of the fibres . 

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Fig: Fiber Fineness is Measured by the Airflow Method

The surface area of a fibre (length X circumference) is proportional to its diameter but for a given weight of sample the number of fibres increases with the fibre fineness so that the specific surface area (area per unit weight) is inversely proportional to fibre diameter; Fig. shows this diagrammatically. Because the airflow varies with pressure difference it is the ratio of airflow to differential pressure that is determined by the fibre diameter. Therefore the method can be used to measure either the airflow at constant pressure or the pressure drop at constant airflow.

The measurement of airflow at constant pressure is the more usual form of apparatus with wool. For fibres of approximately circular cross-section and constant overall density such as unmedullated wool, the estimate of fineness corresponds to the average fibre diameter as determined by the projection microscope with a good degree of accuracy.