AbstractMeasured Critical Oxygen Index values at 40°C for Opal polymethylmethacrylate (Perspex) sheet have been found to be remarkably dependent on the gas velocity within the column of the Hot Flammability Test Apparatus. Investigation of the atmosphere present in the column has indicated that the effect is almost solely attributable to varying quantities of ambient air entering the column as the flow rate is altered. A modification to the apparatus is proposed that successfully prevents this entrainment of air at normal testing flow rates