A B737-800 FLT CREW MADE LOG WRITE-UP STATING THE OXYGEN GAUGE 'DIPS' WHEN TESTING OXYGEN MASK.

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FLT CREW WRITE UP 'CREW OXYGEN GAUGE 'DIPS' WHEN TESTING OXYGEN MASK.' FOUND OVERHEAD INDICATOR 1500 PSI BOTTLE GAUGE AT 1000 PSI. INDICATOR APPEARED TO BE INCORRECT 500 PSI. APPLIED MEL FOR CREW OXYGEN INDICATOR INOP. FLT CREW HAD CONCERN ABOUT THE O2 BOTTLE VALVE POSITION ON HIS PREVIOUS EXPERIENCE. SO I INSPECTED THE O2 BOTTLE AND IT APPEARS TO BE IN NORMAL CONDITION AND THE VALVE WAS SAFETIED. TO SATISFY THE FLT CREW CONCERN I PROCEEDED TO CUT THE SAFETY WIRE ON THE O2 BOTTLE VALVE AND EXERCISE VALVE CLOSED APPROX 3 TURNS TO FULL OPEN AND FULL CLOSED TWICE AND THEN POSITIONED VALVE PER MAINT MANUAL AND ALL CONDITIONS RETURNED TO NORMAL; REMOVED MEL. AND TO THIS WHOLE PROCESS THERE WAS A WRITE-UP ISSUED AND CLRED ON THIS LINE MAINT FORM.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE COCKPIT OVERHEAD CREW OXYGEN INDICATOR APPEARED TO BE INCORRECT AND APPROX 500 PSI HIGHER THAN THE BOTTLE GAUGE PRESSURE READING OF 1000 PSI. HOWEVER; AFTER OPERATING THE 'OPEN/CLOSE' HANDLE THREE FULL TURNS TO WHAT FELT LIKE FULL OPEN; THE COCKPIT INDICATOR AND CREW BOTTLE WERE IN AGREEMENT AND WITHIN OPS LIMITS. IF A CREW OXYGEN BOTTLE REQUIRED SERVICING; THEN HE WOULD REMOVE THE CREW OXYGEN BOTTLE AND A NEARBY OUTSIDE VENDOR HIS COMPANY HAS CONTRACTED WITH; WILL PROVIDE BOTTLE SERVICING.