Narrative:

During preflight of flight deck oxygen; noted a slight dip in the O2 pressure (less than the 100 psi allowed per the preflight procedure) while testing the emergency demand function. Had first officer and myself test the emergency function at the same time and the system pressure dropped 1000 lbs. Called maintenance to investigate. Mechanic discovered O2 valve for system was only open 3 turns vs. 8 turns required for full open. I strongly believe that if emergency oxygen had been required during flight at high altitudes it would probably have been insufficient. This is the third time in two years that I have discovered this same discrepancy. Each time there had been a recent O2 bottle change and the airplane had flown numerous times in this condition. Also; note that each time the first officer had completed his O2 check and not discovered the discrepancy due to lack of observing the O2 gauge during the test. I have also been informed of similar incidents from my fos.corrective action - I strongly recommend that:1) the maintenance department review its O2 bottle installation procedures with an immediate bulletin calling awareness to this reoccurring discrepancy and identify the individual mechanics involved for retraining.2) the flight department review current procedures for preflighting the O2 system with an immediate bulletin calling awareness to the discrepancy.3) security be notified of the incident(s) as possible sabotage of our aircraft. The perpetrator; knowing that the O2 bottle must be opened sufficiently to indicate bottle pressure on the flight deck; but would likely provide insufficient flow in the event of a real emergency requiring demand oxygen.

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Original NASA ASRS Text

Title: A B737-800 Captain discovered during preflight that individual oxygen mask tests allowed the oxygen pressure indication to remain normal; but simultaneous masks tests dropped the pressure 1;000 PSI. Maintenance discovered the bottle valve only open 3 turns versus the 8 required for full pressure.

Narrative: During preflight of flight deck oxygen; noted a slight dip in the O2 pressure (less than the 100 psi allowed per the preflight procedure) while testing the Emergency demand function. Had FO and myself test the Emergency function at the same time and the system pressure dropped 1000 lbs. Called maintenance to investigate. Mechanic discovered O2 valve for system was only open 3 turns vs. 8 turns required for full open. I strongly believe that if Emergency Oxygen had been required during flight at high altitudes it would probably have been insufficient. This is the third time in two years that I have discovered this same discrepancy. Each time there had been a recent O2 bottle change and the airplane had flown numerous times in this condition. Also; note that each time the FO had completed his O2 check and not discovered the discrepancy due to lack of observing the O2 gauge during the test. I have also been informed of similar incidents from my FOs.Corrective action - I strongly recommend that:1) The Maintenance department review its O2 bottle installation procedures with an immediate bulletin calling awareness to this reoccurring discrepancy and identify the individual mechanics involved for retraining.2) The Flight department review current procedures for preflighting the O2 system with an immediate bulletin calling awareness to the discrepancy.3) Security be notified of the incident(s) as possible sabotage of our aircraft. The perpetrator; knowing that the O2 bottle must be opened sufficiently to indicate bottle pressure on the flight deck; but would likely provide insufficient flow in the event of a real emergency requiring demand oxygen.

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.