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.

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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.