The water vapor sensor based on tunable laser diode absorption spectroscopy, capable of measuring concentration of water vapor in real time at multiple locations in fire environment under extreme obscuration was designed. A three-tier detection sensitivity scheme was implemented using various levels of laser power to overcome continuously changing smoke obscuration. Simulations based on HITRAN database were used to quantify water vapor concentration in real-time by comparing the absorbance values after accounting for temperature correction based on local temperatures monitored using thermocouples. Water vapor concentration in training fire scenarios carried out in three different structures, i.e metal container, concrete and drywall were studied. The effect of fuel load on water vapor concentration was compared by considering three distinct fuel loads namely, pallet and straw, pallet, straw and oriented standard board (OSB) and lightweight furnishings. Suppression by water application caused an increase in water vapor concentration in the metal structure but the maximum water vapor concentration was observed during the evolution of fire for most of the scenarios. Among the fuel loads investigated, the highest temperature and water vapor concentration was observed when pallet, straw and OSB were used as fuel load, irrespective of the structure. However, for the same fuel load, fires in concrete structure recorded the highest temperature while fires in dry wall structure generated higher water vapor concentration. Concrete structure was found to absorb the least amount of energy from the fire. Dry wall structure, made of gypsum (CaSO4∙2H2O) when exposed to high temperature fire environment, dehydrated resulting in higher observed water vapor concentration.
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Development and testing of real-time tunable diode laser based water vapor measurement system
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