Design Of A Smoke Concentration Measurement System Based On Light Extinction Method

Smoke concentration, as one of the basic light attenuation characteristics of fire-produced smoke, is a key parameter in fire science. Accurate measurement results of light extinction by smoke are commonly used for purposes of smoke control, building and fire code regulatory, life safety calculations, advanced material development, plume study, combustion diagnostics, design and testing of fire detectors, etc.However, the methods used for measuring light extinction in various fire tests and experiments are not consistent. Motivated by this, after studying previous literature, existing methods, published systems accepted by standards organizations and measurement device features that are necessary for the accurate and precise measurement of extinction, this paper establishes design characteristics for extinction instrument performance requirements. Design and testing of a new smoke concentration meter is presented. Key design features include: the use of a reflecting light path to avoid large mechanical error usually caused by the one-way light path structure, balanced optical and circuit design being adopted to monitor the incident and transmitted light intensity simultaneously and to amplify the differential signal of light intensity while rejecting common-mode noise signals caused by light source and change of temperature, maximum control of the driving and amplifying circuits'linearity, stability, bandwidth, noise level and temperature drift through thorough insight into the circuits'response models and careful testings. The system achieves satisfying performance, including: under room temperature, with the sensitivity of 0.12%/m (confidence level: 99.7%) and absolute accuracy of 0.12%/m (confidence level: 99.7%), also with errors no more than 6%/m (under 85 C) and 1%/m (under 45 C).This device contains following characteristics that are consistent with the goals previously stated in many standards: use of a changeable single wavelength light source which makes the results easy to be calibrated; adjustable measuring light path length, enabling the system adaptive in all scales of fire experiments; an acceptance angle less than 2 degrees, satisfying the Lambert-Beer's law; and detailed testing results which minimizes the uncertainty of using; low cost and easy to be duplicated.This measuring system has basically solved the problem in many fire tests and experiments, and can be widely applied in many places.