ISO 5659 Smoke Density Test: Principle, Procedure, and Applications

The ISO 5659 smoke density test is a widely recognized small-scale laboratory method that quantifies the smoke generated by materials exposed to controlled radiant heat in a closed chamber. As specified in ISO 5659-2, the test measures the obscuration of a vertical light beam to determine parameters such as specific optical density (Ds), VOF4, and Dmax, which are critical for fire safety compliance in railway, marine, and mass-transit applications. This article explains the technical principle, sample requirements, measured parameters, operating procedure, and typical applications of the ISO 5659 smoke density test.

Technical Overview

ISO 5659 is an international standard series titled "Plastics — Smoke generation," developed to assess the smoke production characteristics of polymeric and composite materials under fire-like thermal exposure. The series consists of two parts: ISO 5659-1, which provides general guidance on optical-density testing for smoke measurement, and ISO 5659-2, which specifies the single-chamber test method that is most commonly referenced in commercial and regulatory testing.

In the ISO 5659-2 test, an essentially flat specimen is placed horizontally beneath a conical electric radiant heater inside a closed chamber with a volume of approximately 0.51 cubic meters. The specimen is typically exposed to an irradiance level of 25 kW/m² or 50 kW/m², either with a small pilot flame (flaming mode) or without a pilot flame (non-flaming mode). As the specimen decomposes and produces smoke, the smoke accumulates within the sealed chamber and attenuates a vertical light beam passing through it. The continuous measurement of light transmittance is converted into specific optical density, providing a quantitative indication of the smoke hazard potential of the material.

The ISO 5659-2 method is technically comparable to ASTM E662 (the NBS smoke chamber test) and is directly cited as the reference test method for smoke density requirements in major fire safety specifications, including EN 45545-2 for railway applications and the IMO FTP Code for marine applications.

Testing Samples

Standard Specimens

The standard test specimen is a square plaque measuring 75 mm × 75 mm, with a thickness not exceeding 25 mm. The material should be essentially flat and representative of its end-use configuration, including any surface layers, coatings, laminates, or printed decorations. Each specimen is wrapped in aluminum foil on all faces except the surface directly exposed to the radiant heater, and then mounted in a dedicated specimen holder with a backing board.

Sample Conditioning

Prior to testing, specimens shall be conditioned at a temperature of 23 ± 2 °C and a relative humidity of 50 ± 5% for a minimum of 24 hours, in accordance with ISO 291 or the specific requirements of the applicable product standard. Proper conditioning ensures reproducible results by stabilizing the moisture content of hygroscopic materials.

Special Material Forms

In addition to rigid plastic sheets, the method is applicable to flexible materials such as fabrics, elastomers, foams, and multilayer composites used in rail vehicle interiors. For materials thicker than 25 mm, the specimen is cut to the maximum allowable thickness while preserving the exposed surface structure. Materials that melt, drip, or intumesce are tested in the same holder configuration, with observations of such behavior recorded in the test report.

Testing Items

The ISO 5659-2 test delivers a complete smoke generation profile of the specimen. The principal reported results include:

• Light transmittance as a function of time during the full test duration
• Specific optical density Ds as a function of time, derived from transmittance data
• Maximum specific optical density (Dmax or Dm)
• Ds(4): the specific optical density measured at 4 minutes
• VOF4: the cumulative smoke density value calculated as the sum of Ds values at 1, 2, 3, and 4 minutes, a key acceptance parameter in EN 45545-2
• Time to reach a specific optical density of 16, where applicable, corresponding to the obscuration level at which exit signage becomes difficult to perceive
• Specimen mass loss during the test, when the chamber is equipped with a load cell
• Observations of ignition behavior, flaming time, and specimen physical response

Testing Methods

Test Apparatus

The test is performed in a sealed smoke density chamber with internal dimensions of approximately 914 mm × 610 mm × 914 mm, giving a volume of about 0.51 cubic meters. A conical electric radiant heater is positioned above the horizontally mounted specimen and can deliver irradiance levels from 10 kW/m² up to 50 kW/m². A small propane pilot flame is used for flaming-mode tests. The photometric system consists of a stable light source at the base of the chamber and a photomultiplier detector at the top, with a vertical optical path length of 914 mm. Calibration of the photometric system is verified using neutral density filters.

Test Procedure

The radiant heater output is first calibrated using a water-cooled heat flux meter positioned at the specimen plane. The conditioned and foil-wrapped specimen is then placed on the holder, which may rest on a load cell for continuous mass measurement. After the chamber door is closed and the photometric system is confirmed to read 100% transmittance, the exposure begins. In flaming mode, the pilot burner impinges on the specimen surface; in non-flaming mode, the pilot flame is used only to ignite combustible gases in the chamber when necessary, or is excluded entirely depending on the specification. Light transmittance is recorded continuously, typically for a total test duration of 10 minutes. A minimum of three valid specimens is tested for each exposure condition.

Data Reduction

The specific optical density is calculated from the measured transmittance using the relationship 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 at the given time. For the standard chamber geometry, this formula yields the characteristic Ds curve. VOF4 is then obtained by summing the Ds values recorded at minutes one through four. Average values and individual results are reported for each test condition.

Operating Modes and Exposure Conditions

The most commonly specified exposure conditions are 25 kW/m² with pilot flame (flaming mode) and 25 kW/m² without pilot flame (non-flaming mode), which are both required by EN 45545-2 and the IMO FTP Code. A 50 kW/m² non-flaming exposure is also permitted by the standard for higher-severity evaluation or research purposes.

Applications

The ISO 5659-2 smoke density test is a cornerstone of fire safety verification for materials used in confined spaces where smoke obscuration directly threatens occupant evacuation. Its principal applications include:

• Railway rolling stock: compliance demonstration for the smoke density requirement R10 of EN 45545-2, covering interior and exterior materials for mainline trains, metro vehicles, and trams
• Marine and shipbuilding: assessment of surface materials under Part 2 of the IMO 2010 FTP Code for fire test procedures on ships
• Mass transit and buses: smoke performance qualification of seating, panels, flooring, and decorative components
• Aerospace and other transport sectors: comparative smoke evaluation of cabin and interior materials
• Material research and development: ranking the smoke generation tendency of polymers, flame-retardant formulations, and composite systems
• Quality control: batch verification of production materials against qualified smoke performance baselines

FAQ

What is the difference between ISO 5659-2 and ASTM E662?

Both methods are based on the NBS smoke chamber concept and measure specific optical density in a closed 0.51 cubic meter chamber. ASTM E662 specifies a fixed irradiance of 25 kW/m², while ISO 5659-2 permits a range of irradiance levels, typically 25 kW/m² and 50 kW/m², and defines additional reporting parameters such as VOF4. Results are broadly comparable, but the two methods are not fully interchangeable, and the applicable product standard determines which one must be used.

What do the flaming and non-flaming modes represent?

The flaming mode uses a pilot flame to force ignition of the specimen, simulating exposure to a developed fire. The non-flaming mode exposes the specimen to radiant heat without sustained pilot ignition, simulating thermal decomposition in a pre-flashover or smoldering fire scenario. Many specifications, including EN 45545-2, require passing both modes.

What are typical acceptance criteria?

Acceptance limits depend on the applicable specification. For example, EN 45545-2 sets flaming-mode VOF4 limits of 240, 150, and 120 for hazard levels HL1, HL2, and HL3 respectively, with additional limits on Ds(4) and Dmax, and a non-flaming mode VOF4 limit of 600. Always verify the exact limits against the governing product standard and hazard level.

How many specimens are required?

A minimum of three specimens per exposure condition is generally required. Additional specimens may be tested if the scatter between results exceeds the reproducibility limits defined in the standard or the invoking specification.

How does ISO 5659-1 relate to ISO 5659-2?

ISO 5659-1 is a guidance document that describes the general principles of optical density smoke measurement and the factors influencing test results. ISO 5659-2 is the operative test method that specifies the apparatus, procedure, and calculations, and it is the part referenced for compliance testing.