On Thursday, 7 April 2016 17:15:05 UTC+2, Dan Marotta wrote:
Be sure to use a mask above 18,000'.

And have a plan B if you fly really high.
O2 systems are not infallible so a small handheld emergency backup O2 supply like the Mountain High Co-Pilot Portable is probably a good idea unless your glider is capable of descending at 20,000 feet per minute (200 knots vertical dive).
Time of useful consciousness at 30,000 feet could be as little as 1 minute.