Narrative:

During preflight activities; both the first officer and I noticed when checking the oxygen that the psi indication on the overhead panel would drop considerably when oxygen was flowing. When the first officer pointed this out to me; I ran my mask on emergency for 10 seconds and verified continuous flow. At cruise; we both donned and wore oxygen during lav breaks and had oxygen flowing from the mask. We both remarked that each time we inhaled; we would take notice of the overhead indicator as it dropped towards zero but would immediately return to around 1200 psi. While we both thought it odd; we took no further action. Upon arrival we learned that the crew operating the next flight had written it up and maintenance had found the valve to be not in the full open position. When receiving this information; the flight manual's 'normal' section was reviewed and the indications we observed should have pointed us towards a call to maintenance as the drop in psi was not normal. Contributing to our failure to address this with maintenance was the normal flow of oxygen which led me to believe it might not be a system malfunction.

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Original NASA ASRS Text

Title: B737 flight crew reported the crew oxygen system pressure dropped excessively when testing. Later; Maintenance found the oxygen valve not fully open.

Narrative: During preflight activities; both the first officer and I noticed when checking the oxygen that the PSI indication on the overhead panel would drop considerably when oxygen was flowing. When the first officer pointed this out to me; I ran my mask on Emergency for 10 seconds and verified continuous flow. At cruise; we both donned and wore oxygen during lav breaks and had oxygen flowing from the mask. We both remarked that each time we inhaled; we would take notice of the overhead indicator as it dropped towards zero but would immediately return to around 1200 PSI. While we both thought it odd; we took no further action. Upon arrival we learned that the crew operating the next flight had written it up and maintenance had found the valve to be not in the full open position. When receiving this information; the Flight Manual's 'normal' section was reviewed and the indications we observed should have pointed us towards a call to maintenance as the drop in PSI was not normal. Contributing to our failure to address this with maintenance was the normal flow of oxygen which led me to believe it might not be a system malfunction.

Data retrieved from NASA's ASRS site 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.