Just a general O2 rebreather question from someone who knows very little about them, (we are in the basic scuba section after all). Why exactly is it that you can't substitute 50% nitrox for O2 and go to deeper depths?
I'm sure it has to do with the rebreather just adding O2 based on volume and nitrogen not being consumed by the lungs, however it would seem to me to be a simple matter to just have a constant "leak" in your bladder to accommodate for this. Either that or you could start with say 32% nitrox and just add O2 to your original mix to replace the oxygen as it gets used.
I know there have been some pretty bright folks studying rebreathers for several decades now, so I'm going to assume it's already been tried dozens of times, I'm just kind of wondering why it wouldn't work. It seems to me that rebreathers won't really take off with the general public until someone comes up with something simple, affordable, reliable and able to reach normal recreational scuba depths.
I'm not qualified to answer questions about mixed-gas CCR. That's the realm of a very technical class of divers, however, you are correct MCCR and ECCR rebreathers are capable of far exceeding the depths of my little pure O2 CCR rigs or any compressed-air SCUBA divers or EAN (Nitrox) SCUBA divers. Nitrogen not only dissolves into the blood-stream at depths below 120-160 FSW, but becomes extremely toxic to the extent a diver will become intoxicated (like the effects of Nitrous Gas at the dentist's office). Called "rapture of the deep", you get goofy and spit your regulator, thinking you're a fish (joke: but similar). That's where inert gases come into play like Helium, when you really want to get down deep. I'm just now becoming interested in working with Helium, so I can't answer any technical questions about mixes, depths, bottom times or decompression algorithm specifics as yet, but in a few months I'll have a better understanding of the issues involved. I have a dive buddy deployed in the Gulf right now, who has promised to educate and bring me up to speed on all this mixed-gas technology for deep diving, when he gets home.
But I'll to try and answer your first questions, yes, CCR (closed-circuit rebreathers) are capable of depths far greater than 20 FSW. I just like pure O2 at the shallow depths because that's where I usually hang out anyway. I'm a photographer, and also do studies of local fish populations in the rivers, streams, and lakes of Georgia. So these rigs allow me to gather my data without spooking the fish with OC SCUBA bubbles.
Now, if you want to go deeper, you first need oxygen partial-pressure reading electronics, (3-cells for 3 separate readings of PPO2 for comparison and redundancy) in order to keep from killing yourself, and plan your decompression stops VERY CAREFULLY! Very expensive, training intensive, and out of the realm of most sport divers abilities or needs. Diluent (air or EAN) is added when needed to keep your O2 partial pressure at an acceptable level, somewhere between 0.21-1.40 (somewhere in the low-middle range is the "set-point" or target PPO2) but NEVER OVER 1.60. That's why my pure O2 rig is limited to 19.6 FSW-----that's when I hit the magic 1.60 PPO2 upper threshold. And with that PPO2 you're limited to less than 2 hrs. duration before things could get dangerous. The US Navy fudges these numbers a bit, but I'm no SEAL so I dive civilian (conservative) tables, to be a little safer. I want to live to dive another day.
Even at sport diving depths if you add oxygen into your mix EAN / Nitrox, you've got a whole new set of depth / time parameters to keep close watch on----If you want to live to dive another day. If you're Nitrox certified, you're already aware of this, and being vigilant / cautious.
Ditch the Nitrogen and switch to Helium, and that's when things get real interesting, REAL TECHNICAL, and very dangerous. That's why I'm waiting for my dive buddy to return to the states before I go playing around with gas mixing technologies. So I certainly can't answer any technical questions about this matter---yet. I'm quite happy playing in the shallows with my pure O2 CCR for now, and switching to standard air OC SCUBA or EAN when I need to go a little deeper.
If you're not the sort of diver who enjoys studying advanced gas physics, and learning and adhering to a much stricter technical dive regimen, then this realm of diving is better left alone. After all, we need to be having fun diving---not attending funerals. And lastly, rebreathers HAVE come a long way in just the past 5 years or so. There are some fantastic rigs on the market---they ARE the future of diving, so stay tuned. But, and here's where I drop out---they start at around $8500 and go as high as $30,000 if you want to play hard. Too rich for me, and too rich for most recreational diving enthusiasts. As man moves deeper onto the oceans, you're going to see some fantastic new equipment. OC SCUBA like we old-school boys use just can't do it. Our kids will be exploring realms we never dreamed of.
Cheers,
Jack
