A319 FLT CREW REPORTS DISCOVERING ENROUTE THAT CREW OXYGEN IS INSUFFICIENT WITH VARYING PSI INDICATIONS. AFTER CONFERRING WITH MAINT AND DISPATCH; THE DECISION IS MADE TO DIVERT.

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ENRTE PART 91 REPOSITION FLT; IN CRUISE AT FL380; FO DONNED OXYGEN MASK IN PREPARATION FOR CAPT PHYSIOLOGICAL BREAK. FO RPTED NO OXYGEN FLOW TO MASK AND COCKPIT OXYGEN INDICATION DROPPED FROM 1700 PSI TO 40 PSI. WHEN FO STOWED MASK; COCKPIT OXYGEN SLOWLY INCREASED TO 1600-1700 PSI. I THEN TESTED THE CAPT AND OBSERVER OXYGEN MASKS WITH THE SAME RESULT. FO BROUGHT 2 PAX PORTABLE OXYGEN BOTTLES TO COCKPIT AS TEMPORARY EMER BACK-UP WHILE WE CONTACTED DISPATCH/MAINT CTL VIA ACARS. DIVERTED AT REQUEST OF DISPATCH/CHIEF PLT/MAINT CTL. UPON INSPECTION; CONTRACT MAINT ADVISED THAT CREW OXYGEN BOTTLE SHUT OFF AT BOTTLE. I BRIEFED THE FO PRIOR TO THE PREFLT WALKAROUND THAT SINCE THE ACFT WAS LEAVING A MAINT FACILITY THAT WE NEEDED TO BE EXTRA DILIGENT DURING THE PREFLT CHKS AND INSPECTION. I COMPLETED A NORMAL OXYGEN MASK CHK DURING THE PREFLT AND HEARD THE FO COMPLETING HIS OXYGEN MASK CHK. IN ADDITION; I REMOVED MY MASK FROM STOWAGE DURING THE PREFLT CHK AND DONNED IT TO CHK NORMAL OP. THE MIKE OPERATED NORMALLY AND I TOOK 2 DEEP BREATHS WITH NORMAL INDICATIONS. COCKPIT OXYGEN WAS APPROX 1700 PSI WITH NO ABNORMAL INDICATIONS. THIS NEED FOR THIS DIVERSION WAS SOMEWHAT SURPRISING AND FRUSTRATING FOR ME AS I BELIEVED THAT I HAD USED EXTRA DILIGENCE IN TESTING MY OXYGEN MASK. CONTRACT MAINT ADVISED THAT THE MASKS MAY HAVE TESTED OK DUE TO RESIDUAL PRESSURE IN THE LINES. AS THERE IS NO OTHER INDICATION IN THE COCKPIT OF THE OXYGEN BOTTLE CLOSED AT THE BOTTLE; IN THE FUTURE I WILL HOLD THE EMER PRESSURE SELECTOR FOR A LONGER PERIOD DURING THE PREFLT CHK TO VERIFY NORMAL OP.