Can anyone recommend a source of general information about rebreathers that will get into the technical aspects of rebreather use, function and maintenance?
Hi Ian, welcome to the rebreather forum.
Several good info sources have been mentioned in previous posts, however if you look through this forum you'll also find plenty of information on rebreathers. Generally, Scubaboard's RB forum actually suits people new to rebreathers quite well. Also, you'll find quite a few people here that also frequent the forums at The Deco Stop (technical diving board), Dive-Oz (Australian diving board) as well as RBW.
I also highly recommend Richard Pyle's articles on rebreathers.
On boards under the name "deepreef" he 's a scientist who's been successfully using CCRs to explore the twilight zone between 200 and 400 fsw, discovering unknown species. All along he's been very generous sharing experiences and writing several articles. Among them
"What is a rebreather" and the famous
A Learner's Guide To Closed-Circuit Rebreather Operations. Published more than a decade ago, I still consider it mandatory reading on rebreather diving.
I posted a simple description of rebreathers a while ago in the
What's a rebreather? thread which may answer some of your questions regarding function and gas addition.
My background: I am a former chemist, now a licensed land surveyor in California (meaning I have a strong technical background and significant forma education in chemistry and physics).
While that may help, there have been a lot of sad fatalilties while rebreather diving.
With anesthesiologist and physics nobel prize winner among the victims.
Skills, discipline and good health go a long way preventing those.
The technical aspects of diving fascinate me. As a classically trained scientist, the gas laws and their direct application to diving are a wonderful thing. Id love to do Advanced Nitrox training this next year, too.
Don't you wish physiology was as simple as physics when it comes to diving.
And using RBs sure doesn't make it any easier.
Now, the questions about CCR:
Since this is a closed system, where does the supply of additional gas volume come from, when descending, to maintain equal pressure between soft lungs and ambient water pressure? Where does it go when ascending?
Gas addition of various RBs you can find in that post of mine I linked to above.
Most rebreathers have adjustable overpressure relieve valves much like drysuits, as well as manual dump valves as you used on BCDs. Thinkig about it, most divers I know just vent around the mouthpiece or through their nose. I usually do the former.
How do you maintain buoyancy with a CCR? It would seem to me that this would be a major difficulty.
As pointed out, it takes some practice to handle an additional bouyancy source, and not being able to "fine tune" by filling or emptying your lungs with gas. Nice though, you can breath without that annoying up and down.
Ive watched rebreather divers dumping pellets into their scrubber vessels. Is there an additional solution added to turn this into a paste? Is the "breathed" gas simply passed over the Ca(OH)2 pellets?
When the pellets turn into paste, breathing resitance goes way up and becomes counterproductive, you generate more CO2 and have a harder time venting your lungs properly. The pellets used for diving are of a diffeent composition than those used in medical equipment for example, having a harder surface as well as a higher moisture content.
As I understand it, the "caustic cocktail" is created by sudden influx of water (seawater?) into the system. Where does this water come from? How does it enter the system? What steps are taken to prevent this from happening? Is this a frequent occurrence?
Leaks in the loop. Can be from improper assembly, bad maintainance or bad diving protocol. The rarest reason is likely to be actual damage to the unit while diving, such as cutting a hose in a wreck or scootering into a cave ceiling dislodging the scrubber/head assembly.
Proper maintainance and attention to pre-dive checks and dive protocol can usually prevent this.
It's a rare occurance, still it happens more often than it should. Complacency can happen easily, especially these days when units are pretty reliable ... Richard Pyle's Rebreather Paradox.
Depending on the unit, some have quite effective water traps that will prevent contaminated water ending up in the diver. Some units also allow water to be purged during the dive to quite some extend. One even had a flood proof cannister since it was designed for use with LiOH.
So, is it possible to adjust the oxygen levels while "in flight"? Must the levels be set prior to diving? Is this one of the things that separate higher end units from the lower end units?
Again, it depends on the unit. Usually they all have dual setpoint these days, one below 1.0 ata (usually 0.7 ata) as shallow water/surface setting, a second one for the actual dive. On some units the setpoint is chosen before the dive, others allow changing during the dive. As all production units have manual add valves the setpoint can always be raised, for example during deco.
What are the differences between the available "recreational" (read non-military/commercial) CCRs on the market?
Am not sure how many commercial RBs there actually are, probably very few. Seems most if not all commercial diving is done with umbilical gas supply, intercom, heated suits and remote monitoring.
Military units tend to be very simple and reliable, frequently using expensive, non-magnetic materials. They tend to be exorbitantly expensive (MK16 is in the $60,000 neighborhood) and the mixed gas CCR dive protocol requires several people in addition to a diver.
Recreational rebreathers tend to have more bells and whistles yet cost considerably less.
MK15.5 CCRs, based on the MK16's architecture, sold for around $15,000, including the rather expensive Inconel flasks. Recreational divers have a lower budget as they also supply the military's.
Do the manufacturers or CCRs have "Demo Days" similar to dry suit manufacturers?
Advanced Diver Magazine (which I highly recommend - they featured many rebreathers and related subjects, back issues available) organizes tryouts several times a year, as do some of the better dive stores. IANTD has one coming up, too, for example, and many rebreather instructors offer them. I've done it three times, with three different instructors, trying 5 different units. Good experience and fun, I wrote an article that's on RBW about it.
If so, what requirements do they have in order to try out CCRs?
Usually sign waivers up the whazoo, pay a fee, be OW certified.
You also want to have deep pockets. It's highly addictive ... .
Not just the $$$ rebreather and the $$$ training, but you'll want a $$$ dive computer, $$$ booster pump and on it goes.
As you can see, I have a heck of a lot more questions than I seem to be able to find solid answers for. Can anyone point me in a direction for some serious technical reading on CCR basics?
Well, just ask.
More reading available:
Manuals from manufacturers, Jetsam, Ambient Pressure, Closed Circuit Research, Dive Rite all have them online. C2R also has some intresting papers on their design perspective and goals worth reading.
Rebreathers Worldwide also has some very good info as well as an enormous amount of RB history.
Several very good articles in the RBW Library.
Training manuals can be quite good, I like the ANDI maunals myself. Have them even for units I don't dive.
IANTD has a some books, "Technical Diving Enzyclopedia" and "Tek Closed Circuit Rebreather".
Mel Clark has a new self-published book, "CCR TRIMIX SIMPLIFIED" which has gotten good reviews and is sold through the RBW Store. She also writes CCR manuals for TDI.
All I can think of right now, oughta keep you busy for a while.
