I've heard a few comments of danger whith rebreathers, what are they and why.
That's the big question that nobody wants to address when they start talking about recreational rebreathers. OC rec diving has inherent risks, but it is more or less a safe activity. Technical diving (deep, mixed gas, hard and soft overhead) is an inherently dangerous activity that can be done with relative safety. Doing these dives on OC has more static risks that are hard to mitigate fully (e.g. loss of gas, having to switch gases, etc.). Doing them on CCR makes them more dynamic, but you have many more options to mitigate them. That, and the fact that CCR's are much, much more effiicient and flexible than OC, is why they are gaining popularity. But I wouldn't call them the wave of the future for recreational diving.
Here's a quick break-down on the dangers that you would never have to think of in recreational diving:
1) They can kill you in 30 feet of water in several ways; hypoxia- not enough O2, hyperoxia- too much O2, hypercapnia- too much CO2
2) Right now, there is no reliable way to measure CO2 in the loop. You can be breathing happily along, well within the limits of the unit and BANG- CO2 hit that you never saw coming. CO2 is a very dangerous gas and this is extremely difficult to recover from. So if you pack the scrubber incorrectly, you can take a hit. If the sorb settles wierd on the way to the dive site, you can get breakthrough and take a hit. If you forget an o-ring, you can take a hit. The inspo/ evo vision has a temp stick that uses a secondary reaction to guage scrubber life and there are a few fledgeling technologies that hold some promise, but this largely remains a guessing game.
3) The O2 sensors that we all use were not designed to be used in rebreathers. The leading supplier just pulled out of the rebreather market due to law suits. Current-limited cells can be deadly. Doing a long dive that builds up condensation will have your cells reading low and slow, sometimes really low. So you have to know enough about your unit to fly it properly in its current condition and pad your deco and CNS tracking accordingly. This is why most units have 3-4 cells. You need to implement voting logic and cross-checking in order to have a good guess at what you are breathing.
Lot's of inexperienced and/or ill-trained people have perished on CCR's. But lots of very experienced, very well-trained, and well-respected divers have also perished on CCR's. We usually never find out the cause and it's often blamed on medical events.
Compare this to filling an AL80, throwing it on your back and going diving.
Now let's look at the operational aspects:
1- Diving prep. Pack the scrubber(s), get gas, analyze gas, assemble the unit, stereo check on the DSV, positive/ negative check, test every function on the unit, then calibrate. Any one of these things can go wrong and then you are off trouble-shooting to get it right. My rEvo has been very reliable in this aspect, but I have several friends who are not so lucky.
2- Pre-dive. Start up the unit, flood the loop with o2 and make sure the calibration held. Pre-breathe the unit for 5 mins to ensure everything is working propertly. When you jump in, do a 20ft linearity check at 20ft (if you're smart) to ensure your cells are up to snuff. Then you can go dive. The only things you have to worry about during the dive are monitoring and maintaining your PO2 levels, O2 pressure, dil pressure, scrubber duration, and bail-out supply.
3- Post-dive. You have to break the unit down and clean it after a day of diving. This doesn't really take a lot of time but you have to do it religeously. Disinfect the unit every few days of diving (if diving multiple days, otherwise I do it everytime).
4- Skills maintenance. There are a few dozen skills you need to stay on top of to be proficient at CCR diving. You can't dive once or twice a year and expect to have fun on one of these things.
Compare this to filling an AL80, throwing it on your back and going diving.
Now let's look at the practical aspects:
1- Cost. CCR's cost WAY more in every way, shape, and form than recreational OC diving. And unless you are diving helium on a regular basis, they are more expensive than CCR's as well. On top of the unit and training cost, you have to pay for all of the consumables; dil and O2, cells, sorb, batteries, disinfectant, it all adds up.
2- Time. Time to set up, prepare to dive, and break-down after diving are ridiculous compared to OC recreational diving. Plus if you travel with the thing, you will spend a lot of time organizing logistics. CCR divers are treated like lepers just about everywhere.
Compare this to filling an AL80, throwing it on your back and going diving.
So this is just something to think about before jumping on the bandwagon. Don't get me wrong, CCR diving is very rewarding and it is a game-changer for technical diving (deep, wreck, caves, etc.). If I had to do it all over again I would, but there is sooo much more to consider than the cool factor. Put it this way, if I can do the dive on a single 80 and I'm out to dive for fun, I'm not doing it on a rebreather. Just my $.02.