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This note reports the results of work being carries out, for the Factory Department of the Ministry of Labour and National Service on the development of a method of testing explosimeters. One type of instruments was used in the investigation. This instrument measures the increase of temperature when the flammable atmosphere is burned on a heated platinum filament forming one arm of a Wheatstone's bridge circuit. A comparison of six detector units supplied with two explosimeters and using pentane/air mixtures showed that the instrument readings obtained at first were close to or greater than the actual concentrations, the readings being linear throughout the working range of concentrations. Chemical ageing tests and other usages have shown that changes in the sensitivity of the detector unit occur, and readings obtained with the explosimeters appeared to be dependent to a certain extent upon the history of the detector. Electrical heating of the detector appeared to have the effect of increasing the detector sensitivity. In tests with various flammable mixtures the differences between the indicated and actual concentrations varied with the flammable constituent. Satisfactory indications of flammability were obtained with mixtures containing pentane, benzene, hexane, ethyl alcohol, diethyl ether and coal gas. Methyl ethyl ketone gave readings which were much lesser than the actual concentrations tested, and no consistent results could be obtained with mixtures containing acetone or amyl acetate. It is considered that the instrument provides a satisfactory warning of the existence of possibly flammable concentrations of a number of gases and vapours, but, because of the variation of the readings with different solvents it could not be used for precise measurement of the concentration of a vapour for which is had not been calibrated. The instrument could not be used for acetone or amyl acetate. It is suggested that the red warning band on the indicator diel be extended to cover a lower concentration than the existing 60 per cent of the lower flammability limit. This would provide a safeguard against inadequate sampling in compartments where pockets of higher concentrations might be missed, and also would cover the use of the instrument for vapours which give low readings. It is also suggested that a label on the instrument should state the types of vapour for which it is suitable.