[sigmoid10]

90 seconds of breath hold is still way too little to see oxygen drops in a finger pulse oxi. Also explains why you didn't die free-diving. Even with no training you can go much longer than that before risking a blackout (at least under normal circumstances). In fact training free-diving is all about CO2 tolerance and relaxation, so you won't even be able to store more O2. When I was reaching the 80s, I was doing 3+ minute breath holds. The first ~2 minutes my oxi stayed at full O2 sat with basically no change. In principle you should be able to induce a blackout yourself using this method without feeling an urgent need to breathe - if you do the prep stage hard enough. Just make sure you only do this when lying down in a safe position.