BS ISO 29903-1:2020 pdf download – Comparison of toxic gas data from different tests Part 1: Guidance and requirements
4 Combustion conditions
The yields and nature of the fire effluent component from a fire test of any scale are determined by the involved fuels and the prevalent thermal and oxidative conditions in the current stage of the fire. These conditions also determine the burning rate of the products/materials and thus the rate of effluent generation. See ISO 16312‑1. During a fire test of a finished product, the combustion conditions are likely to change. These changes include the chemistry of the combustible item and the sufficiency of the ventilation. Whether decomposition is flaming or non-flaming is a dominant factor in the production of toxic gases. The combustion conditions under which toxic gas data are developed shall be as close to equivalent as possible between the physical fire models or test scales compared (see Clause 6).
NOTE 1 A large change in the rate of combustion can affect the degree of oxidation of the emitted effluent. Smaller changes in combustion rate can have no significant effect.
NOTE 2 Fire stages and the corresponding combustion conditions are described in ISO 19706.
4.2 Thermal environment
The thermal boundary conditions in a test include the external applied heat flux and the heat flux from any flaming combustion. Also of importance is the heat flux distribution among radiation, convection, and conduction. The thermal environment sensed by the test specimen during combustion includes both gas temperature and the temperature of the sample material, as defined by the thermal boundary conditions.
The oxygen availability (ventilation) in the physical fire models compared determines the combustion conditions. Comparison among different methods requires characterization of the ventilation conditions in order to assess the degree of similarity.
For a given experiment, it is necessary to identify how the ventilation is characterized and whether the characterization is local or global. For a physical fire model in which the fuel gasification rate and the entering oxygen flow and concentration are each controlled independently, the relative oxygen availability can be characterized by a fuel/oxygen equivalence ratio. For other models and real-scale fire tests, one or both of the terms in the equivalence ratio may not be well‑known. In those cases, a broader characterization is used. This could be a global equivalence ratio or a term such as “underventilated burning” or “well ventilated burning”.
NOTE 1 Methods for calculating equivalence ratios for physical fire models are given in ISO 19703.
NOTE 2 The local air speed rate can be a significant factor in some fire tests. This applies especially for a tube furnace, where the air speed can affect the results of the combustion.
4.4 Characteristics of test specimens
The test specimens used for comparison of gas yields among physical fire models or between a physical fire model and a larger scale test shall be prepared from a single batch of the finished product or a single batch of each of the component materials. Alternatively, it shall be demonstrated that any differences in composition among the test specimens, tested in the different apparatus, do not affect the test outcome significantly.
For finished products that consist of a single, homogeneous material, the test specimen used in a physical fire model shall be prepared to accommodate the constraints of the test apparatus. For specimens from non‑homogeneous products, the test specimen shall also contain the same portions of the different materials present in the finished product in both tests compared. For layered commercial products, an ideal physical fire model accommodates specimens that preserve the relationship of the layers. When this is not possible within the constraints of the model, the rationale for the configuration of the layers shall be documented.
NOTE The yields of toxic gases can depend on the surface exposed, and the timing and extent of penetration of the layers.BS ISO 29903-1 pdf download.