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Computational Model of Dissipation of Oxygen From an Outward Leak of a Closed-Circuit Breathing Device (NIST TN 1484)



Kathryn M. Butler


Closed-circuit breathing devices recycle exhaled air after scrubbing carbon dioxide and adding make-up oxygen from a tank of pure oxygen. Use of this equipment allows first responders to work for up to four hours without swapping out cylinders and scrubbing canisters. Firefighting situations in which these devices would be useful include tunnels, mines, ships, high-rise buildings, and environments contaminated with biological or chemical toxins. A risk perceived by firefighters entering environments containing open flame and high radiant heat is the possibility of fire ignition in the vicinity of the respirator caused by the outward leakage of oxygen around the facepiece. This paper presents the results of a computational fluid dynamics (CFD) study of oxygen dissipation into the environment surrounding a respirator facepiece. Actual heads and masks are scanned into a 3D data set for entry into the CFD software, providing a physical boundary for the problem to be solved. Leak geometries representing an imperfect seal are defined. Oxygen concentration fields and flow streamlines are presented for multiple combinations of fuel and air in the surrounding environment, for pure oxygen and air expelled from the leak, and for both normal and high stress breathing patterns. The flammability diagram for propane is used to estimate the flammable regions as a function of time during two breathing cycles for each case.
Technical Note (NIST TN) - 1484
Report Number


breathing apparatus, self contained breathing apparatus, dissipation, oxygen, leakage, first responders, carbon dioxide, fire fighting, risks, fire fighters, respirators, dissipation, computational fluid dynamics, ignition, face masks, seals, face protection, geometry, oxygen concentration, fuels, air, flammability, propane, turbulence, respiratory systems


Butler, K. (2007), Computational Model of Dissipation of Oxygen From an Outward Leak of a Closed-Circuit Breathing Device (NIST TN 1484), Technical Note (NIST TN), National Institute of Standards and Technology, Gaithersburg, MD, [online], (Accessed June 18, 2024)


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Created June 1, 2007, Updated November 10, 2018