The original question was what one would do ,if at 410,and the press' gauge read zero???I agree SRB that the political(sensible)correct answer would be Divert and Fix!!
The zero gauge press would force on to check the mask(inop gauge)at 100% flow one would get the 'breeze' flow??
As Smokey says the O2 rules are different at Fl250,and the TUC,ergo I would still consider 'salvaging' the flight,with the availabilty of supplemental O2..All one would have to do,in the worst case'structural'(explosive),would select Alt and Cruise change(Idle open for the Airbus)and suck on the bottle(Have one each)
As you put it ,one has to explain to the CP why one diverted because of a faulty gauge,or no Crew O2, which had been checked prior to the flight????(leaks happen)
It vaguely reminds me of the Asian Carrier that dumped fuel and returned to base because the captains windshield wiper flew off at top o'climb,and rainshowers were in the Destination Forecast.
