>>794885Your question is actually pretty good, and while I'm not really able to answer it, I'd just like to mention how it's not very much of an issue for scuba divers, in that we're taught to breathe slowly and steadily all the time, to make sure that we're breathing "fresh" air - That is to say, air that's basically on the same depth as you are (Sorry if I'm unclear, might be a language barrier going on).
You can read this:
https://www.bookyourdive.com/blog/2012/6/28/never-hold-your-breath to see how the air can be dangerous if you don't breathe proper.
Of course, you COULD, TECHNICALLY, save your breaths as long as you don't ascend or descend, but just like you technically can drive in the wrong lane on a road, I wouldn't take the chance. If you want to save air, work on your breathing and finning technique. If you're diving in cold waters (even with a dry suit), the tank might even outlast you. I've gone up because my mates were cold, and had 140 bar to spare out of a 300 bar/10 liter tank (and the plan was to turn around at 100). If you're diving for long enough, there's also the risk of decompression sickness, and you might have to cut your dive short to make a decompression stop on the way up, or risk some nasty health problems.
If you already know this and just wanted the answer to a curious science question, I'm sorry I can't help you - You might have better luck hearing with someone else, or asking the blokes at /sci/.
>>795135Do you feel like your instructor(s) didn't teach you properly, seemed incompetent about the subjects, or couldn't answer your questions? Do you feel like you're not ready to out and dive?
It's all about your instructor IMHO. The organisations follow the same ISO standards when they teach their courses (Some info on that here:
https://www.bookyourdive.com/blog/2015/10/9/alternative-padi ).
If you just dive and interact with other divers and acquire skills, it doesn't really matter much who cleared you to do so in the first place.
