Monitoring of nitrogen gas purity by the hot filament method

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 O-2 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 O-2 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 U-fil=6.3 V, is determined to S=21 ppm O-2/(mA/min). (C) 1997 American Institute of Physics.