swimmer_spe
Contributor
I know there are many options for breathing gas. There are also various systems to breath from. So, what is the smallest gear for the longest bottom time? Air, Nitrox, Trimix, Rebreather, etc?
Getting the most bottom time out of the available gear
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high/baric
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That would depend on at what depth the bottom is. As long as you can substitute oxygen for an inert gas (meaning nitrox for all intents and purposes) without risking oxygen toxicity, you gain more bottom time for the same amount of decompression, or with no decompression obligation, if that's what you're asking. The deeper you go, the less oxygen your mix can have and the shorter the time you can spend there without having to decompress afterwards. Technical divers start to substitute helium for nitrogen after 40 meters (some even before that), but the main reason for this is avoiding nitrogen narcosis, not extending bottom time, although using both helium and nitrogen mixes at certain points of the dive should shorten your decompression time according to current decompression theory (although some clinical trials don't approve).
So simple answer: Nitrox with a higher than 21% oxygen content is the only way to extend NDL time. I'm sure someone with more insight and better command of the diving vocabulary (in English) can give you a more thorough answer.
PS: A rebreather is the most versatile tool to manage your breathing gas composition, but I would not exactly call it small (and certainly not cheap).
So simple answer: Nitrox with a higher than 21% oxygen content is the only way to extend NDL time. I'm sure someone with more insight and better command of the diving vocabulary (in English) can give you a more thorough answer.
PS: A rebreather is the most versatile tool to manage your breathing gas composition, but I would not exactly call it small (and certainly not cheap).
Every tool has it's specific application(s). Every smart mechanic, uses the right tool for the job. If all you own is a hammer, then the entire world becomes a nail.
Rebreathers give you the longest bottom time because they are adjust the mix to always give you the desired PO2. Set it for 1.3 or 1.2 and the PO2 changes to achieve that mix. They are typically lighter than heavy twinsets but they require a lot more attention to detail. If you fall into the "If at first you don't succeed" category, then this is not for you!
For most dives, to Air is human, but to NitrOx: divine! I dive %32 almost exclusively unless I'm going deep. Then I dive what's going to give the best MOD. I don't much care about the quantity of time down below though. I concentrate on the quality. What am I going to see? Who am I going to see it with? And so on...
Rebreathers give you the longest bottom time because they are adjust the mix to always give you the desired PO2. Set it for 1.3 or 1.2 and the PO2 changes to achieve that mix. They are typically lighter than heavy twinsets but they require a lot more attention to detail. If you fall into the "If at first you don't succeed" category, then this is not for you!
For most dives, to Air is human, but to NitrOx: divine! I dive %32 almost exclusively unless I'm going deep. Then I dive what's going to give the best MOD. I don't much care about the quantity of time down below though. I concentrate on the quality. What am I going to see? Who am I going to see it with? And so on...
high/baric
Contributor
Just in case the OP is not familiar with the ideology behind rebreathers, let's say that yes, theoretically they could offer you a longer bottom time than any other solution I know of simply because they can dynamically adjust pO2 to a higher value than compressed air like NetDoc said (and I hinted). However, I suspect a vast majority of people who end up getting a rebreather do it for its ability to give a lower pO2 than compressed air and a rebreather would be quite an overkill for recreational diving. I'm still not sure what you (the OP) are after (a practical or theoretical solution), but if you want to extend your bottom time in practice I would suggest you look into nitrox and do the course; it is cheap, easily understood, readily available almost everywhere, and specifically developed for achieving longer bottom times with the added benefit of a lower risk of DCS 
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smallest gear for the longest bottom time at recreational depths are the SCR's like the Hollis Explorer. No way to argue that one, if you are going deeper, then CCR, but there is only so much of a size advantage with them because you still have to carry all of your bailout so it's still not a small rig. The Explorer is pretty much self contained and mostly idiot proof.
high/baric
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^- I stand corrected. The Explorer actually seems to be smaller than your average scuba tank. Still quite an investment compared to regular nitrox, though. But that should answer the original question.
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Explorer Rebreather - Dive Right in Scuba
Dans Dive Shop should be able to get them as well. Not uncommon to have free training with rebreather purchases, but if you want the most bottom time in the smallest package, that's the cheapest way to get there. It's somewhat limited compared to full CCR's since it only has a 2 hour duration on the scrubber, where 4 hours isn't uncommon on a full CCR, but they're also 2x the cost, and require a lot of other stuff to get right. For comparison, at 100ft, an AL80 will last you about 40 minutes if you surface with 500psi and have a really good SAC rate...
Dans Dive Shop should be able to get them as well. Not uncommon to have free training with rebreather purchases, but if you want the most bottom time in the smallest package, that's the cheapest way to get there. It's somewhat limited compared to full CCR's since it only has a 2 hour duration on the scrubber, where 4 hours isn't uncommon on a full CCR, but they're also 2x the cost, and require a lot of other stuff to get right. For comparison, at 100ft, an AL80 will last you about 40 minutes if you surface with 500psi and have a really good SAC rate...
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high/baric
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All valuable info about the rebreathers and thanks for it. Don't want to nitpick, but where did you get the 6 mins from? When I calculate my bottom time for 30 meters (100 feet) using my average SAC I get about 25 mins, with 50 bars left in the tank. Still way less than 2 hours for sure.
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sorry, let google run calculations for me and accidentally hit *.4 instead of /.4 for sac rate
high/baric
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As an interesting side note, with a SAC like that and a typical rig that people use around here (double 12 liter / 230 bar) you would get that same 2 hours at 30 meters at a fraction of the cost, but at twice the weight of the Explorer. 2 hours at 30 meters would put you so far into deco though that it kind of defeats the point.
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