A B737-800 OUTBOUND CREW OXYGEN CHECK WAS FOUND ABNORMAL. CONTRACT TECHNICIANS RPT BOTTLE CLOSED. INBOUND CREW RPTED NORMAL PRESSURE AND FOUR COMPLETE TESTS AND NORMAL OXYGEN FLOW. CREW OXYGEN BOTTLE SERVICED PRIOR TO FIRST FLT OF DAY AND TESTS ALL NORMAL.

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DURING A ROUTINE WALKAROUND FOR A SCHEDULED SVC CHK ON ACFT X; I NOTICED THE FLT DECK CREW OXYGEN GAUGE WAS READING LOW. PER THE ACFT MAINT MANUAL SUPPLEMENT I REMOVED THE BOTTLE FROM THE FORWARD CARGO PIT TO BE SERVICED AT A NEARBY OXYGEN SERVICING STATION. I TOOK NOTICE THAT THE BOTTLE I REMOVED WAS NOT THE NORMAL METALLIC BOTTLE I WAS USED TO BUT MADE OF A COMPOSITE MATERIAL OF SOME KIND. FUNCTIONALLY; HOWEVER; IT WAS THE SAME AND APPEARED IN GOOD SERVICEABLE CONDITION; I MERELY THOUGHT IT WAS AN INTERESTING DESIGN. AFTER SERVICING THE BOTTLE TO FULL I REINSTALLED THE BOTTLE IN THE ACFT. I TREAT THE OXYGEN BOTTLE VALVE WITH A BIT OF PARANOIA. I WILL CHECK AND RE-CHECK AND TRIPLE CHECK IT SEVERAL TIMES TO ENSURE IT IS FULLY OPEN AND THEN CLOSED 1/4 TURN PRIOR TO SAFETYING IT OPEN. I DISTINCTLY REMEMBER DOING THIS EXERCISE ON ACFT X. AFTER I CHECKED THIS BOTTLE AND SAFETIED IT; AND PRIOR TO CLOSING THE CARGO PIT ACCESS PANEL; I DOUBLE CHECKED THE TORQUE ON ALL THE CONNECTIONS I REMOVED AND IMMEDIATELY WENT TO THE COCKPIT TO VERIFY TWO THINGS; THE PRESSURE GAUGE ON THE OVERHEAD PANEL AND POSITIVE OXYGEN FLOW AT ALL FLIGHT DECK CREW OXYGEN MASKS. I DID THIS ON ACFT X AND FOUND ALL ITEMS TO BE SATISFACTORY. I THEN RETURNED TO THE FORWARD CARGO PIT TO CLOSE THE ACCESS PANEL. THE AFTERNOON FOLLOWING THIS PARTICULAR SHIFT 9/MON/06; I RECEIVED A CALL FROM THE HANGAR LINE SUPERVISOR ON DUTY AND GIVEN A HEADS UP THAT ACFT TOOK FLT TO ZZZ AND THAT IN ZZZ THERE HAD BEEN A PROBLEM WITH THE CREW OXYGEN. I WAS TOLD THAT WHEN CONTRACT MAINT IN ZZZ ACCESSED THE BOTTLE THEY SAID THAT THEY HAD FOUND THE BOTTLE SAFETIED SHUT. THIS DID NOT CORRELATE AT ALL TO MY RECOLLECTION OF EVENTS FROM THAT NIGHT. AT THAT TIME; IT WAS NOT EXPLAINED TO ME HOW THE ACFT LEFT ZZZ1 AND MADE IT ALL THE WAY TO ZZZ WITHOUT THE PLT CREW NOTICING THEIR OVERHEAD PANEL PRESSURE GAUGE OR A POSITIVE FLOW AT THEIR INDIVIDUAL MASKS. ISN'T THE FLT DECK CREW OXYGEN USED AT LEAST ONCE ON A RELATIVELY LONG FLT FROM ZZZ TO ZZZ1? I REMAIN VERY CONCERNED AND PERPLEXED AT THIS SITUATION. AFTER SPEAKING WITH THE SUPVR FROM MY HOME; I IMMEDIATELY SAT DOWN TO SUBMIT THIS RPT.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE CREW OXYGEN BOTTLE WAS SERVICED PRIOR TO THE FIRST FLT OF THE DAY. THE BOTTLE WAS REMOVED AND SERVICED PER THE MAINT MANUAL PROC. IT WAS NOTED ON REMOVAL FROM THE AIRPLANE THAT IT WAS A NEW TYPE COMPOSITE BOTTLE AND REQUIRED NO SPECIAL HANDLING OR SERVICING PROCS. THE SERVICED BOTTLE WAS INSTALLED; THE BOTTLE VALVE WAS TURNED TOWARD OPEN UNTIL THE VALVE COULD NOT BE TURNED ANY MORE; THEN BACKED OFF 1/4 TURN AND THEN SAFETIED IN THE OPEN POSITION. THE BOTTLE WAS TESTED IN THE COCKPIT FOR PRESSURE AND FLOW AND ALL WAS FOUND NORMAL. THE SAME BOTTLE WAS TESTED OK BY THE OUTBOUND CREW FOR PRESSURE AND FLOW TO THE MASKS. WHEN ASKED BY THE ANALYST IF DURING THE FILLING OF THE BOTTLE ANY HEAT WAS NOTICED IN THE VALVE AND WAS THE VALVE DIFFICULT TO CLOSE AND OPEN; THE ANSWER WAS 'NO.'SUPPLEMENTAL INFO FROM ACN 709583: DURING OUR CREW REST AT ZZZ; WAS NOTIFIED BY PHONE BY THE CAPT OF THE OUTBOUND FLT THAT THEY HAD AN ABNORMAL CREW OXYGEN CHK ON ACFT X AND THAT MAINT HAD BEEN CALLED OUT TO RESOLVE THE DISCREPANCY. ON OUR CREW OXYGEN PREFLT IN ZZZ1 FOR FLT; WE NOTICED A DROP IN OXYGEN GAUGE PRESSURE DURING BOTH THE CAPT AND FO 'INPLACE' MASK TEST BUT AN IMMEDIATE RETURN TO 1600 PSI AT COMPLETION OF THE TESTS. NEITHER ONE OF US HAD EVER SEEN THIS BEFORE SO WE BOTH CLEANED AND DONNED OUR RESPECTIVE OXYGEN MASKS AND TESTED THE MASKS IN THE CONVENTIONAL MANNER FOR BOTH FLOW AND COMMUNICATION. THERE WAS A PRESSURE DROP BUT NOT AS GREAT AS THE IN THE HOLDER TEST. ONCE AGAIN THE PRESSURE IMMEDIATELY RETURNED TO 1600 PSI. WITH THE PRESSURE WELL ABOVE DISPATCH LIMITS AND AN IMMEDIATE RETURN TO NORMAL PRE-TEST PRESSURE AFTER 4 COMPLETE TESTS AND NORMAL OXYGEN FLOW I ASSUMED THIS WAS SAFE FOR FLT AND CONTINUED.