Observations on one-dimensional counterflow diffusion problem

A time-dependent, one-dimensional counterflow diffusion model was developed to gain insights into the issue of helium exposure of Hemispherical Resonating Gyros located in the Inertial Reference Unit used in the Earth Observing-1 spacecraft. Although the IRU enclosure was purged with high-purity liquid boil-off nitrogen, the HRG's were still quite susceptible to contamination generated by exposure of the enclosure to levels of helium above atmospheric background levels. This helium would be preferentially absorbed by the HRG's, changing their mass, and hence the required operational driving voltage. The paper presents a comparison of theoretical results with test data for a purged IRU enclosure, and how contaminant gases can enter vent holes despite the presence of a purge. These observations are then used to describe a possible improvement for purge effectiveness.