Dynamics of Trace Methane Diffusion/Flow Into Hollow Core Fiber Using Laser Absorption Spectroscopy

A technique is described for measurement of the flow and diffusion rates of trace gases in atmosphere into and out of hollow core fiber (HCF) using tunable diode laser absorption spectroscopy (TDLAS). Both diffusion and pressure-induced flow for methane through the fiber end faces obey simple equations. Measurements of the TDLAS signal of the trace gas optical absorption through the fiber as a function of time for small pressure differentials around atmospheric pressure across the fiber end faces can easily determine the times to fill and purge the fiber. The slope of these times as a function of the inverse pressure differential is a function of fiber core diameter, fiber length, and the dynamic viscosity of air. The data are in excellent agreement with the theory. Furthermore, it is found that highly sensitive trace gas concentration measurements are possible using relatively short lengths of HCF (<; 1 m) which can be filled with external gas samples using modest pressure differentials (~10 kPa or 100 mbar) within minutes or even seconds. This result is important for remote trace gas sensing using HCF.