The flaming combustion smoke density test measures the optical density of smoke generated by materials burning under direct flame exposure in a closed chamber, providing critical data for fire hazard assessment and product classification. Performed according to standards such as GB/T 8627, ASTM D2843, and ISO 5659-2, this test is widely required for building materials, plastics, and wire and cable products. This article explains the test principle, applicable sample types, measured parameters, procedures, and typical applications.
The flaming combustion smoke density test is a fundamental fire performance evaluation that quantifies how much smoke a material produces when it burns under an open flame. Because smoke generation strongly affects visibility, evacuation time, and survivability in real fires, this test serves as a key acceptance criterion in building codes, transportation specifications, and cable standards worldwide.
Technical Overview
The flaming combustion smoke density test determines the amount of smoke released when a test specimen is ignited and allowed to burn in a sealed chamber of known volume. A horizontal light beam passes through the chamber from a stabilized light source to a photometric detector positioned on the opposite wall. As smoke accumulates, the light transmittance decreases, and the continuous transmittance-versus-time record is converted into specific optical density values that characterize the smoke-producing tendency of the material.
The most widely used realization of the flaming-mode method is the single-chamber technique described in GB/T 8627 and ASTM D2843. The test chamber is a cube with interior dimensions of 914 mm, giving a volume of approximately 0.76 cubic meters. The specimen is supported on a holder with its exposed face facing a propane-fuelled burner, and the flame is applied continuously for the standard exposure period of four minutes. The specific optical density is calculated as Ds = (V / (A × L)) × log10(100/T), where V is the chamber volume, A is the exposed specimen area, L is the optical path length, and T is the percentage light transmittance; for the standard chamber geometry this expression reduces to Ds = 132 × log10(100/T).
The flaming combustion mode is distinguished from the non-flaming, or smoldering, mode employed in radiant-chamber methods such as ASTM E662 and ISO 5659-2. In those methods, specimens are heated by a radiant flux, optionally with a small pilot flame to induce flaming ignition. Flaming-mode data represent the smoke produced once a material is fully involved in fire, which for many thermoplastics corresponds to the most severe smoke generation condition and is therefore the more frequently specified test mode.
Testing Samples
Building and Construction Materials
This category includes thermal insulation boards, decorative wall and ceiling panels, sandwich panel facings, floor coverings, wall coatings, sealants, and adhesives. These materials are commonly evaluated against the smoke density rating requirements referenced in building material classification systems such as GB 8624.
Plastic and Polymeric Materials
Thermoplastics such as PVC, polyethylene, polypropylene, and ABS, together with engineering plastics, foams, and fiber-reinforced composites, are routinely tested. Specimens are typically small square plaques on the order of 25 mm × 25 mm × 6 mm for the single-chamber flaming method, with the actual dimensions and thickness defined by the governing product standard.
Wire and Cable Materials
Insulation compounds, sheathing compounds, and finished low-smoke cable constructions are assessed using larger-scale flaming smoke tests such as IEC 61034-2, in which horizontal cable samples are burned in a 27 cubic meter chamber and the minimum light transmittance is recorded.
Transportation Interior Materials
Interior finish materials for railway vehicles, ships, and mass transit systems are frequently subjected to flaming-mode smoke density requirements because reduced visibility in enclosed spaces is a primary escape hazard.
Specimen Preparation and Conditioning
Specimens are cut to the specified dimensions, conditioned to constant mass at 23 ± 2 °C and 50 ± 5% relative humidity, and weighed before testing. Tests are normally performed on multiple specimens, and the reported results represent the average behavior of the material.
Testing Items
- Maximum specific optical density (Dm or Ds max)
- Smoke density rating (SDR) per GB/T 8627, calculated from the light transmittance values recorded at the end of the first, second, third, and fourth minutes of exposure
- Complete light transmittance versus time curve and specific optical density curve
- Smoke density at four minutes, Ds(4), and the cumulative value VOF4 per ISO 5659-2
- Minimum percentage light transmittance per IEC 61034-2 for finished cables
- Comparative flaming versus non-flaming smoke behavior
- Supporting data for fire performance classification of building materials
Testing Methods
The standard single-chamber flaming procedure is carried out as follows:
- The chamber, light source, and detector are stabilized, the optical system is calibrated, and the baseline transmittance is verified at 100 percent.
- The conditioned specimen is wrapped or supported according to the standard and mounted in the holder inside the chamber.
- The propane burner is ignited, verified for stable flame geometry, and positioned so that the flame impinges on the specimen as specified.
- The exposure continues for four minutes while the photometric system continuously records light transmittance.
- The test is terminated, the chamber is purged and cleaned, and the specimen residue is documented.
- Specific optical density, maximum density, and, where applicable, the smoke density rating are calculated and reported together with the transmittance curve.
Related Flaming-Mode Smoke Test Methods
In addition to the GB/T 8627 and ASTM D2843 single-chamber method, flaming smoke generation can be evaluated using ISO 5659-2 (also published as EN ISO 5659-2), which combines a 25 kW/m² radiant furnace with a pilot flame; ASTM E662, the NBS chamber method with flaming and smoldering modes; IEC 61034-2 for cables; and the smoke developed index obtained from tunnel furnace methods such as ASTM E84 and UL 723. Because each method uses a different apparatus, exposure condition, and calculation basis, results are not directly interchangeable between standards.
Applications
- Regulatory compliance and fire performance classification of building materials under systems such as GB 8624
- Verification of low-smoke claims for cables installed in tunnels, stations, and other confined public spaces
- Material research and development, including comparison of flame retardant formulations and smoke suppressant additives
- Factory quality control and batch acceptance testing for polymeric products
- Input data for fire safety engineering analyses of visibility and tenability during evacuation
- Qualification of interior materials for railway, marine, and mass transit applications
FAQ
What is the difference between flaming and non-flaming smoke density tests?
In the flaming mode, the specimen is ignited and burns with an open flame, typically from a propane burner or a pilot flame combined with radiant heating. In the non-flaming mode, the specimen is heated without ignition and decomposes or smolders. Flaming conditions generally produce the highest smoke yields for many materials and are the more commonly specified mode.
What is the smoke density rating and what values are typically required?
The smoke density rating (SDR) is a derived index calculated from the transmittance readings taken during the first four minutes of the GB/T 8627 test. Classification systems such as GB 8624 set upper limits on SDR, for example a value not exceeding 75 for certain higher fire performance grades; the applicable limit depends on the product and the declared class.
How long does a single test take?
The standard flaming exposure lasts four minutes. Including chamber stabilization, specimen mounting, and purge cleaning between runs, a complete test sequence on one material, typically involving several specimens, is generally completed within a few hours in accredited third-party laboratories.
Do results from different smoke density standards correlate?
Only qualitatively. Chamber volume, exposure condition, specimen orientation, and calculation constants differ between GB/T 8627, ASTM D2843, ISO 5659-2, and IEC 61034-2, so numeric results must always be referenced to the specific method required by the specification.
Why do some materials pass flame spread tests but fail smoke density tests?
Flame retardancy and smoke generation are distinct properties. Halogenated polymers, for example, can be highly flame resistant yet release dense smoke when they do burn, which is why smoke density is assessed as an independent criterion in most fire safety frameworks.