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A B737-800 first officer found the crew oxygen system pressure dropped from a static 1800 psi to 500 psi during use; the tank isolation valve was only partially open.

Narrative:

First officer found during preflight that the crew O2 pressure dropped precipitously during the O2 flow check. The initial pressure was 1800 psi. During the flow check; the pressure was observed to drop as low as 500 psi. Mechanic found the O2 tank isolation valve only partially open instead of fully open; causing a restriction induced pressure drop and a significant lower flow of O2 available.

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Original NASA ASRS Text

Title: A B737-800 First Officer found the crew oxygen system pressure dropped from a static 1800 PSI to 500 PSI during use; the tank isolation valve was only partially open.

Narrative: First Officer found during preflight that the crew O2 pressure dropped precipitously during the O2 flow check. The initial pressure was 1800 PSI. During the flow check; the pressure was observed to drop as low as 500 PSI. Mechanic found the O2 tank isolation valve only partially open instead of fully open; causing a restriction induced pressure drop and a significant lower flow of O2 available.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.