ISO 4589-4:2021

Overview

ISO 4589-4:2021 - Plastics - Determination of burning behaviour by oxygen index - Part 4: High gas velocity test specifies a laboratory method to measure the minimum oxygen volume fraction that supports flaming combustion of small vertical sheet specimens under high gas velocity. The result is reported as the high gas velocity oxygen index (HOI) and is intended for plastic sheets up to 2 mm thick, including flexible sheet materials supported vertically. This part of the ISO 4589 series extends oxygen-index testing to conditions where forced ventilation (higher upward gas velocity) governs oxygen supply.

Key topics and technical requirements

• HOI definition and scope: HOI is the minimum oxygen fraction (oxygen + nitrogen mixture) at a specified gas velocity that supports sustained flaming; typical test velocities are 600–1 000 mm/s (specified gas velocity > 40 mm/s).
• Test principle: A vertical specimen is mounted inside a transparent test chimney; an oxygen/nitrogen mixture flows upward at a controlled velocity while the specimen’s top edge is ignited. Burning behaviour and burnt length are observed to determine HOI.
• Apparatus and dimensions: Recommended chimney height 450–500 mm and inside diameter 75–100 mm; open upper outlet and even gas distribution at the base. Specimen holders (suspended or supported) have reference marks at key heights.
• Gas control and measurement:
  • Gas velocity control 600–1 000 mm/s, controllable within ±50 mm/s.
  • Oxygen fraction resolution 0.1%, accuracy requirements (e.g., ±0.2% at 800 mm/s).
  • Oxygen analyser accuracy typically ±0.1% (paramagnetic analysers recommended).
• Ignition: Propane fuel (≥98% purity) used with a specified flame outlet and flame height (document gives target flame height 16 mm ± 4 mm).
• Test procedure elements: specimen preparation and conditioning, calibration of equipment, stepwise oxygen fraction adjustments, calculation of HOI, precision and reporting requirements.
• Supporting material: normative annex on calibration and informative annexes with example data, interlaboratory results, and guidance on blow-off behaviour at high gas velocity.

Applications and users

• Who uses it: materials test laboratories, fire-safety engineers, plastics manufacturers, product design teams, regulatory compliance bodies, and researchers assessing flammability under forced-ventilation scenarios.
• Why use it: to evaluate flammability of plastic sheets where forced ventilation or elevated airflow affects combustion risk, to compare materials, support product selection, perform risk assessments, or demonstrate compliance for applications (e.g., enclosures, ducts, transportation interiors) where high gas velocities may occur.

Related standards

• ISO 4589 series (other parts on oxygen index tests, e.g., ISO 4589-2 for lower gas velocity)
• Referenced standards used for conditioning and terminology: ISO 291, ISO 13943, and sampling standards (ISO 2859 series).

Keywords: ISO 4589-4:2021, oxygen index, HOI, high gas velocity, flammability testing, plastics, oxygen/nitrogen mixture, test chimney, gas velocity, burning behaviour.