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Mean Fibre Diameter
Airflow (IWTO 6)
The principle of the Airflow method is to measure the flow of air through a fixed mass of wool. The surface area of the wool fibres creates a resistance to the flow of air. The finer the wool, the greater the surface area and the greater the resistance to the flow of air. The volume of flow reduction can be accurately measured on an Airflow meter to equate to mean fibre diameter.
Internationally accepted calibration wools of known fibre diameter are used to calibrate the Airflow meter to produce very accurate and consistent results. The measurement theory of the airflow method assumes that fibres have a constant density. Therefore, a fixed weight of wool of the same average diameter will always give the same amount of fibre surface and average micron result.
An exception to this rule is created by the presence of medullated fibres that have a hollow inner core. In these fibres, the overall density is often lower than that of non-medullated fibres and a slightly finer result is possible than if the fibres were measured using the LASERSCAN instrument. Similarly, lambs wool may differ in density to adult wool which can have, to a lesser extent, a similar result.
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