Abstract
A hot filament can be used as a simple, reliable, and inexpensive sensor to measure quantitatively ppm-level concentrations of oxidizing species present in high-purity nitrogen. A coiled tungsten filament of a 1 W signal lamp acts as a sensing element, which, by high-temperature reaction with O2 and H2O, forms volatile oxides. The resulting decrease in filament diameter and rise in filament resistance is monitored by measuring the current at constant voltage. The sensor’s response is linear with the O2 concentration over an extended range of “burn-down” and is relatively insensitive to the gas velocity. The reaction appears to be rate limited by the diffusion of oxygen along its concentration gradient toward the filament. The sensor’s sensitivity, operated in nitrogen at Ufil=6.3 V, is determined to S=21 ppm O2/(mA/min).
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