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**Source: **STP48825S

ASTM G-124 seeks to evaluate combustion characteristics of metals in high-purity (>99 %) oxygen atmospheres. ASTM G-124 provides the following equation to determine the minimum number of purges required to reach this level of purity in a test chamber: *n* = −4∕log^{10}(*P*^{a}∕*P*^{h}), where *n* is the total number of purge cycles required, *P*^{h} is the absolute pressure used for the purge on each cycle, and *P*^{a} is the atmospheric pressure or the vent pressure. The origin of this equation is not known and has been the source of frequent questions as to its accuracy and reliability. This paper provides the derivation of the G-124 purge equation, and experimentally explores the equation to determine if it accurately predicts the number of cycles required.

**Keywords:**

purge equation, promoted combustion chamber, purge cycles, oxygen concentration, oxygen percentage, Dalton, partial pressures, high purity oxygen atmosphere

**Author Information:**

Robbins, Katherine E.*Student Researcher, Vanderbilt Univ., Nashville, TN*

Davis, Samuel Eddie*Materials Engineer, NASA George C. Marshall Space Flight Center, Huntsville, AL*

Herald, Stephen D.*Test Group Lead Engineer, NASA George C. Marshall Space Flight Center, Huntsville, AL*

**Committee/Subcommittee:** G04.02

**DOI:** 10.1520/STP48825S