What was found
CREW STATED THAT DURING THE PREFLIGHT OXYGEN WAS OBSERVED TO BE AT THE BOTTOM OF THE GREEN ARC. GIVEN THE 40 DEGREE F TEMPERATURE, THIS WAS NOT UNEXPECTED. WHILE AT FL430 CREW NOTICED THE OXYGEN GAUGE INDICATING APPROXIMATELY 1300 PSI. A LITTLE OVER AN HOUR FROM KDAL. CREW HAD HAD BEEN WATCHING THE NEEDLE STEADILY DECREASE. AT THIS POINT THE GAUGE INDICATED APPROXIMATELY 1100 PSI. CAPTAIN REFERENCE THE USABLE OXYGEN CHART, MAKING THE APPROPRIATE CORRECTION FOR TEMPERATURE AND DETERMINED THERE WAS APPROXIMATELY TEN MINUTES WORTH OF USABLE OXYGEN. CREW DISCUSSED A SUITABLE AIRPORT TO DIVERT TO AFTER WHICH CAPTAIN CALLED THE OPS SUPERVISOR VIA ARINC PHONE PATCH. WHEN THE GAUGE INDICATED 700 PSI DURING THE DESCENT AROUND FL360, CREW DECLARED AN EMERGENCY. THEY WERE GIVEN DIRECT KMEM AND LANDED WITHOUT INCIDENT. WHEN THEY SHUT DOWN THE OXYGEN GAUGE INDICATED 500 PSI.