Overview
ISO 16549:2021 - Textiles - Unevenness of textile strands - Capacitance method - specifies a standardized method to determine the unevenness (variation of linear density) along textile strands using capacitance measuring equipment. The method applies to tops, slivers, rovings, spun yarns and continuous-filament yarns (natural or man‑made) within the ranges:
- Staple-fibre strands: 4 tex to 80 ktex
- Continuous-filament yarns: 1 tex to 600 tex
The standard is not applicable to fancy yarns, conductive-metal-containing strands (these require optical sensors), or raw silk filaments (covered by other standards).
Key topics
- Measurement principle: specimen passes between capacitor plates; capacitance changes are proportional to local mass variation → instrument reports unevenness as CVu (coefficient-of-variation) or Uu (mean deviation) - CVu is preferred.
- Length parameters: both the capacitor (length between, Lb) and length within (Lw) must be known and reported (examples: Lb commonly 8 mm for yarns, 12 mm for rovings, 20 mm for slivers; Lw = total measured length).
- Variance-length curve: plotting CVu vs. Lb (often on log–log paper) to analyze short-term vs long-term unevenness.
- Periodicities / spectrogram: Fourier-based analysis to detect periodic density variations and process faults.
- Imperfection counting: detection and counting of neps, thick places and thin places with adjustable thresholds (typical default thresholds: ±50% for thick/thin; higher fixed options for neps).
- Apparatus & calibration: condenser assembly, yarn guides/pretensioners, adjustable-speed motor, signal processing unit (with spectrogram and counters), optional printer and twisting device for filament yarns; calibration via instrument procedures or manufacturer standards.
- Influencing factors: dielectric constant, fiber blends, sizing or moisture distribution can affect capacitance readings - interpretation requires caution.
- Conditioning: testing atmosphere per ISO 139 (standard atmospheres for conditioning and testing).
Applications
ISO 16549:2021 is used for:
- Quality control in spinning mills and yarn production
- Incoming/outgoing inspection in textile supply chains
- R&D and process troubleshooting (identifying periodic faults)
- Third‑party textile testing laboratories and certification bodies
- Calibration and performance checks of unevenness testers (instrument manufacturers)
Practical benefits include consistent reporting of yarn evenness, detection of neps and thin/thick places, and actionable diagnostics to improve spinning consistency and fabric appearance.
Related standards
- ISO 139 - Standard atmospheres for conditioning and testing (referenced for test environment)
- Historical/related methods: ISO 2649 and IWTO methods (background and alternative approaches)
- Optical sensor methods (recommended for conductive strands or specialty yarns) - see Annex A of ISO 16549 for other methods
Keywords: ISO 16549:2021, capacitance method, unevenness, yarn testing, linear density, variance‑length curve, spectrogram, neps, thick and thin places, textile standards.
