I'm still not asking this right.
On a rebreather, how shallow is it a waste of helium to use 15% helium diluent:
always, 180', 160', 140', 120', 100', 80', 60', 40', 20'.
For narcosis and disregarding the negligible cost on a rebreather.
Said differently, when would you switch to air or nitrox, as the helium would be silly. Assuming you have bottles with 45, 30, 15, and 0% helium sitting ready and free. (and ignoring NDL for helium shallow)
Personally, beyond 40m (132') I would prefer to use Helium. Shallower, there is less point. Shallower than 30m, I wouldn't bother using Helium. That said, if you are topping off a partially used Trimix DIL, cylinder I would probably just add air and have a very light Trimix DIL, rather than dumping the remaining gas and replacing it with air.
One of the big factors is conditions. Low visibility, high workload dives, make using Trimix shallower a much better option. Narcosis is caused by a combination of factors, stress and depth being two of them.
I much prefer the benefits of Helium, on deeper dives. The reduction in Narcosis, and improved clarity, give you, not only a better and safer dive, but you remember more of the dive.
I haven't read through the whole thread. To answer the original question.
If you want longer no-stop bottom times, you have the option to dive shallower, and/or (dependent on PO2), reduce the Nitrogen by increasing the Oxygen (i.e. use Nitrox).
If you are using optimum Nitrox, and have insufficient bottom time. Then the only remaining option is a decompression stop dive.
The fundamental problem, is in North America, dives involving compulsory decompression are treated as if they have some special mystic. The transition from No-stop dives, to dives involving staged decompression, is less onerous, than the initial task of learning to dive. It is a modified application of existing skills and knowledge, rather than the requirement to learn new skills and knowledge.
A large number of the European agencies teach staged decompression as a progression within their core diver training programs.
Your next logical course is something like ADP and / or a twinset course. Initially, you don't need to do accelerated decompression. A straightforward dive, with limited decompression stops can easily be done on the back gas. (Assuming you have properly planned it, and have sufficient gas/redundancy).
The highest proportion of the diving in my club involves, compulsory decompression stops, without accelerated decompression!
Progressing to accelerated decompression can be done, (if required,) after practicing and becoming confident and comfortable doing 'light' deco dives.
One of the big advantages of decompression diving is the reduced compulsion to rush the ascent that comes with a focus on no-Stop diving. As long as you have the gas (and redundancy), you can focus on safe ascents, rather than a rush to the surface to ensure you don't break the no-stop restriction. If needed, you can pad out the stop with no concern.
You may, or may not progress to CCR. That is a decision for down the road. CCR becomes more attractive as you progress into Trimix. As others have said, Helium is expensive, but then a CCR is not exactly a cheap investment.
I have a CCR, and I 'like' diving it. Most of my twinsets are on loan to friends, I so seldom dive OC. It should be remembered, that you still need your OC skills on CCR in the event you have to bailout.
