Suunto D4i User Manual page 14

It is not possible to change between AIR, NITROX and FREE
NOTE
modes before the instrument has counted down the no-fly time.
There is one exception to this: You can change from AIR to NI-
TROX mode even during the no-fly time.
When planning both air and nitrox dives during the same dive
series, you should set the instrument in NITROX mode and
modify the gas mix accordingly.
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