Variability of Smoke Particulate Concentrations in the NBS Smoke Chamber

We studied small-scale smoke yield measurements of 9.5-mm-thick three-ply Douglas-fir plywood in the Aminco-NBS smoke density chamber to determine smoke distribution patterns and observed the variability of these measurements. These data are needed to calculate the paramteres used in predicting large-scale smoke yield. Based on 66 test runs, the smoke particulate concentration exhibited little systematic change with position except near the chamber walls. The measurement of optical density, smoke particulate concentration, and specimen weight loss showed coefficients of variation of 14, 16.6, and 27.3 percent, respectively. The large amount of variation introduces large uncertainties into estimates of the parameters, thus making good predictions difficult to achieve. The inherent variability of wood samples, due to differences of grain direction, surface density, and thermal degradation, accounts for some of the observed variation, while variations of equipment and procedure account for the remainder. Since the specimens are inherently variable, any reduction of experimental variation in the future must depend on improving the measurement of weight loss and optical density.

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