The physical mechanisms of spreading of smoke and gas during
a fire event must not be necessarily the same. Due to the heat smoke particles
are more likely transported by convection with a minor contribution of diffusion
effects whereas gas is transported by both convection and presumably with a
higher fraction of diffusion.
In order to investigate these differences we selected a
variety of gas sensors and placed them at different positions in a fire test
room. Gas sensitive field effect transistor (GasFET)-arrays, metal oxide
sensors (MOS) and electrochemical cells (ECs) were used for gas measurements in
test fire scenarios. Beside the investigation of the performance of the sensor
elements itself, we additionally focused our investigations on the propagation
behavior of different aerosol and gas components of standardized (EN54) test
fires in time and space.
We mounted different gas sensors on PCBs which were setup
into different vertical “multi-sensor” chains representing a measurement grid
in space. The metal oxide sensors showed the fastest response whereas the
GasFET-array was responding slower but shows advantages with respect to
low-power consumption and pattern recognition capabilities. The EC carbon monoxide
sensor has a good selectivity but a high price comparing to semiconductors. We
observed that smoke aerosols mainly stay beneath the ceiling whereas the fire
related gases also are transported in regions below the ceiling and near to the
floor.
As a very surprising and promising result we observed that
gases also diffuse through heat layers which may occur during fires right below
the ceiling and which smoke particles are not able to pass through and thus are
not able to enter into the detector to reach the optical measurement chamber.
In our case CO reached the gas sensors over 4 min earlier before any smoke
could be detected by optical smoke detectors.
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