AN A32O CAPT RPTED THAT THERE IS A SIGNIFICANT PROB WITH THE CREW OXYGEN SYS WHICH DOES NOT DIRECTLY INDICATE WHEN THERE IS A SLOW LEAK. SUCH A PROB CAUSED HIM TO DECLARE AN EMER AND DIVERT TO SLC.

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AT ABOUT 2 HRS 20 MINS INTO THE FLT; THE CAPT AND FO NEEDED TO LEAVE THE FLT DECK FOR PHYSIOLOGICAL REASONS. THE CAPT WAS TO GO FIRST. THE LEAD FLT ATTENDANT WAS CALLED INTO THE FLT DECK AS PER SOP AND THE CAPT PREPARED TO LEAVE. THE FO PULLED HIS OXYGEN MASK AND NOTICED NO OXYGEN PRESSURE. THE CAPT CHKED HIS MASK THEN AND HAD THE SAME RESULTS. CHKING THE DOOR ECAM PAGE; THE OXYGEN SYS SHOWED 'OXY-XX; REGULATOR LOW PRESSURE.' THE CAPT IMMEDIATELY ASKED THE LEAD TO RETURN TO THE CABIN AND PROCURE 2 PORTABLE OXYGEN BOTTLES. THE CAPT CHKED THE COM OXYGEN SECTION. ITS INSTRUCTIONS LEFT LITTLE DOUBT AS TO THE NEXT COURSE OF ACTION. THE CAPT DECLARED AN EMER WITH ZLC AND REQUESTED DIVERSION AND DSCNT TO SLC. THE CAPT CALLED DISPATCH AND MAINT TO GET CONCURRENCE IN THE PLAN OF ACTION. THEY AGREED AND DISPATCH AUTHORIZED DIVERSION. AN UNEVENTFUL APCH AND LNDG WERE CONDUCTED INTO SLC. THE EMER WAS CANCELED ON THE GND. THE CAPT ENTERED THE DISCREPANCY INTO THE SHIP'S LOGBOOK. BOTH THE CAPT AND FO ARE HIGHLY EXPERIENCED IN THE AIRBUS. THEY ARE FAMILIAR WITH THE PREVIOUS A320 BULLETINS THAT HIGHLIGHTED PREFLT FAULTS THAT COULD LEAD TO THIS PROB. THEY CONDUCTED THEIR PREFLTS NORMALLY FOR THE FLT. THE FO NOTICED THAT DURING HIS PREFLT; HIS OXYGEN WHITE TAB WAS OUT AS HE STARTED HIS PREFLT. HE TESTED HIS MASK NORMALLY AND RESET THE MASK TO THE CORRECT STOWED POS WITH THE TAB OUT OF SIGHT. OXYGEN PRESSURE DURING PREFLT WAS 1050 PSI. THE LOGBOOK FOR SHIP HAD A PREVIOUS WRITE-UP THAT STATED THE CAPT'S OXYGEN MASK WAS LEAKING. IT WAS REPLACED. THE CAPT'S PREFLT WAS NORMAL. RECOMMENDATION: THE AIRBUS 319/320 HAS NO WARNING FOR THE GRADUAL LOSS OF CREW OXYGEN. THE ONLY READOUT IS ON THE DOOR ECAM PAGE. THE ONLY ALERT OCCURS WHEN IT IS TOO LATE. IT SHOULD BE DISPLAYED ON THE CRUISE ECAM; SO THAT IT CAN BE MONITORED.