Narrative:

During our first flight of the day cockpit preflight both crew masks tested good per the fom and the crew O2 gauge indicated completely full at 2000 pounds pressure. However; my first officer did show some concern as to the sound of the O2 during the test; so he pulled the mask out for a visual inspection and found the mask harness not inflating quite as large as usual; but he put it on and said it seemed ok. Upon our arrival in ZZZ after my first officer was ready for our next flight; his mind drifted back to the mask so he examined it again. He was showing more concern this time so I told him to go ahead and call maintenance. Maintenance sat in the first officer's seat; pulled the mask out; tested it and said it was 'ok;' but he would change it out if we wanted. At this point I said; 'let's compare the captain and first officer masks.' mine too tested; but did not seem normal. The mask harness was not inflating all the way and when set to emergency flow it too sounded weak; but the O2 gauge still indicated 2000 pounds full. The mechanic now said he would check downstairs. He came back a few minutes later to inform us that the crew O2 bottle was empty! It was also found that the pressure transducer was inoperative! When the transducer was uncoupled; the O2 gauge now went to the correct indication of zero pressure! I have never heard of this event before; empty bottle with a full gauge indication. If this is happening at some known frequency it certainly needs to be addressed. If the transducer fails; the gauge needs to show zero; not full. Maybe an O2 gauge test should be developed like we now have for the fuel; and oil gauges? And I certainly should have investigated further at the point when my first officer first had some concern about the functionality of his O2 system. Lesson learned.

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Original NASA ASRS Text

Title: ALTHOUGH THEIR CREW OXYGEN PRESSURE INDICATED 2000 PSI; A B737-300 FLT CREW HAD CONCERNS ABOUT THEIR OXYGEN MASKS NOT INFLATING ALL THE WAY. THE PRESSURE TRANSDUCER WAS FAULTY; AND CREW OXYGEN WAS ACTUALLY AT 'ZERO' PRESSURE.

Narrative: DURING OUR FIRST FLT OF THE DAY COCKPIT PREFLT BOTH CREW MASKS TESTED GOOD PER THE FOM AND THE CREW O2 GAUGE INDICATED COMPLETELY FULL AT 2000 LBS PRESSURE. HOWEVER; MY FO DID SHOW SOME CONCERN AS TO THE SOUND OF THE O2 DURING THE TEST; SO HE PULLED THE MASK OUT FOR A VISUAL INSPECTION AND FOUND THE MASK HARNESS NOT INFLATING QUITE AS LARGE AS USUAL; BUT HE PUT IT ON AND SAID IT SEEMED OK. UPON OUR ARR IN ZZZ AFTER MY FO WAS READY FOR OUR NEXT FLT; HIS MIND DRIFTED BACK TO THE MASK SO HE EXAMINED IT AGAIN. HE WAS SHOWING MORE CONCERN THIS TIME SO I TOLD HIM TO GO AHEAD AND CALL MAINT. MAINT SAT IN THE FO'S SEAT; PULLED THE MASK OUT; TESTED IT AND SAID IT WAS 'OK;' BUT HE WOULD CHANGE IT OUT IF WE WANTED. AT THIS POINT I SAID; 'LET'S COMPARE THE CAPTAIN AND FO MASKS.' MINE TOO TESTED; BUT DID NOT SEEM NORMAL. THE MASK HARNESS WAS NOT INFLATING ALL THE WAY AND WHEN SET TO EMER FLOW IT TOO SOUNDED WEAK; BUT THE O2 GAUGE STILL INDICATED 2000 LBS FULL. THE MECHANIC NOW SAID HE WOULD CHECK DOWNSTAIRS. HE CAME BACK A FEW MINUTES LATER TO INFORM US THAT THE CREW O2 BOTTLE WAS EMPTY! IT WAS ALSO FOUND THAT THE PRESSURE TRANSDUCER WAS INOP! WHEN THE TRANSDUCER WAS UNCOUPLED; THE O2 GAUGE NOW WENT TO THE CORRECT INDICATION OF ZERO PRESSURE! I HAVE NEVER HEARD OF THIS EVENT BEFORE; EMPTY BOTTLE WITH A FULL GAUGE INDICATION. IF THIS IS HAPPENING AT SOME KNOWN FREQUENCY IT CERTAINLY NEEDS TO BE ADDRESSED. IF THE TRANSDUCER FAILS; THE GAUGE NEEDS TO SHOW ZERO; NOT FULL. MAYBE AN O2 GAUGE TEST SHOULD BE DEVELOPED LIKE WE NOW HAVE FOR THE FUEL; AND OIL GAUGES? AND I CERTAINLY SHOULD HAVE INVESTIGATED FURTHER AT THE POINT WHEN MY FO FIRST HAD SOME CONCERN ABOUT THE FUNCTIONALITY OF HIS O2 SYSTEM. LESSON LEARNED.

Data retrieved from NASA's ASRS site as of May 2009 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.