Standard Specification
IWTO-8 governs the determination of wool fibre diameter through airflow measurement, establishing an empirical relationship between resistance to fluid flow and mean fibre thickness within raw and sliver consignments. Commercial topmakers apply this protocol during commercial transfer between early-stage processing facilities and spinning mills to fix billing masses without individual filament measurement. Air passes through a compressed plug of scoured wool of fixed mass and volume, generating a pressure drop that registers directly on a calibrated micron scale.
Slippage and packing irregularities alter the permeability coefficient, so test chambers demand strict adherence to mass and volume parameters to prevent commercial disputes over fineness grades.
Sample Conditioning
Atmospheric moisture alters wool fibre dimensions significantly, forcing laboratories to condition testing lots under standard conditions of twenty degrees Celsius and sixty-five percent relative humidity prior to airflow testing. Hydroscopic swelling modifies the cross-sectional area of individual fibres, which changes the specific surface area and skews permeability readings if ambient humidity shifts outside specified tolerances. Technicians place scoured wool samples inside conditioning racks until mass stabilization occurs over consecutive weighing intervals.
Conditioning eliminates moisture bias from the raw material before operators transfer the mass into the permeability barrel.
Measurement Mechanics
Compressed air flows at a regulated velocity through the cylindrical chamber containing the prepared wool plug, and a differential manometer records the resulting back pressure. Resistance rises as fibre diameter decreases, because finer lots present a greater specific surface area that impedes fluid passage through the inter-fibre capillary network. Calibration relies on reference tops of known fineness established by inter-laboratory trials, converting pressure units directly into micron values without optical microscopy.
Instrument calibration drift invalidates billing data across entire production runs, requiring daily checks against primary standards before commercial lots enter the testing queue.
Commercial Limits
Airflow testing fails to detect diameter distribution width or fibre crimp frequency, measuring only the mean fibre thickness of the bulk material. Topmakers accept this limitation because downstream spinning performance correlates strongly with mean diameter rather than single-fibre variance, keeping processing costs predictable for mill buyers. Mixed lots containing divergent micron profiles produce misleading permeability averages that mask the presence of coarse contamination within fine parcels.
Contractual agreements therefore specify maximum allowable variance limits alongside the baseline IWTO-8 value to protect spinning mills from processing failures caused by heterogeneous consignments.
