A B737-200 FLC AT FL310 CONTINUES THEIR FLT TO DEST AFTER DISCOVERING EITHER A GAUGE PROB OR A POSSIBLE LOSS OF OXYGEN IN THE CREW'S SYS WHEN NEAR DCA; DC.

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AT CRUISING ALT OF FL310; I LOOKED UP AT THE OVERHEAD PANEL AND NOTICED THE CREW OXYGEN WAS READING 100 PSI. THE CAPT AND I RAN THE LOSS OF OXYGEN CHKLIST. THE CHKLIST SAYS IF THE GAUGE READS ZERO; THE BOTTLE IS DEPLETED. THE CHKLIST DOES NOT SAY TO DIVERT. WE IMMEDIATELY REQUESTED A LOWER ALT TO 10000 FT AND DSNDED AND CHKED FOR OXYGEN LEAKS IN THE CAPT'S MASK; FO'S MASK AND OBSERVER. THERE WERE NO LEAKS. WE ALSO TESTED AT LENGTH ALL MASKS; AND FOUND THE PRESSURE TO BE FREE FLOWING AND CONSTANT. WE ASSUMED THE PROB TO BE AN INACCURATE GAUGE AND CONTINUED TO DEST OF EWR WITHOUT FURTHER PROB. WE DID ADVISE MAINT AND OUR DISPATCHER OF THE PROB IN A TIMELY MANNER; AND OUR DISPATCHER SENT US ONLY A MESSAGE THAT HE WILL ADVISE MAINT. ON APCH TO NEWARK; ABOUT 15 MINS PRIOR TO LNDG; I LOOKED UP AGAIN AT THE CREW OXYGEN GAUGE; AND IT SHOWED 1400 PSI; WHICH IS THE NORMAL PRESSURE; WHICH VERIFIED THAT WE HAD A GAUGE THAT WAS NOT ACCURATE. IN HINDSIGHT; PERHAPS WE SHOULD HAVE LANDED AT THE NEAREST SUITABLE ARPT AND HAD THE PROB LOOKED AT. WE WERE AT 10000 FT IN CASE WE LOST PRESSURIZATION; BUT IF WE HAD SMOKE; THEN THAT WOULD HAVE BEEN A PROB IF WE HAD NO OXYGEN PRESSURE.