A simple method to determine the purity of an inert gas

A simple and economical method to determine the overall purity of inert gases such as Ar and He is described which uses the filament of commercial light bulbs. The lifetime of the incandescent filament is sensitive to gas impurities such as O-2, H2O, and some hydrocarbons, but insensitive to N-2 and, to first order, to the applied voltage. The relationship between the filament lifetime, tau, and the O-2 and H2O concentrations was carefully calibrated. The relationship follows a power law: tau=AC(-gamma), where A and gamma are positive constants depending on the impurity type. For mixtures of O-2 and H2O, a simple superposition rule for the overall lifetime, tau(overall), as 1/tau(overall) 1/tau O-2 + 1/tau H2O, applies. A mechanism for the concentration dependence of the filament lifetime is proposed in which the creep of the filament is enhanced by the impurity solutes in the filament. With a lifetime spanning from a few seconds to a few days, the filament acts as a simple gauge for the determination of impurity concentrations in an inert atmosphere. As an example, for O-2, the measurable concentrations vary from 10(5) ppm to 10(-2) ppm. (C) 1997 American Institute of Physics.