Basic questions regarding CCR technology...

[ianw2]

Can anyone recommend a source of general information about rebreathers that will get into the technical aspects of rebreather use, function and maintenance?

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).

I got re-certified last summer (2006) after a 32 year surface interval. My wife and I dive together. We’ve averaged just over 7 dives each month since getting our OW cards (not extreme, but respectable). We’ve both gone through AOW, Nitrox and Rescue. I completed DM classes and have my DM cert. Laura will probably do her DM next Spring. We both have PADI Deep and Wreck specialties. Laura did her equipment specialty a few months back; I do mine in December. In short, we dive. We dive a lot. We’re serious addicts. Unfortunately, we’re both nearly 50 and, although Laura fantasizes about diving on the Andrea Doria, realistically, I’m not so sure.

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. I’d love to do Advanced Nitrox training this next year, too.


Now, the questions about CCR:

If the system is closed, it must form a complex system consisting of biological (lungs, trachea, mouth, lips, etc.) and mechanical (reg, hoses, scrubber vessel) portions. 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?

How do you maintain buoyancy with a CCR? It would seem to me that this would be a major difficulty.

The technology for air "scrubbing" is very old. I understand that rebreathers use a scrubber system relying on calcium hydroxide as the primary reactant to remove carbon dioxide. I’ve 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?

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?

I have seen a number of divers on Southern California dive boats using CCRs. I can’t recall ever seeing a CCR being used from shore. Is this a correct observation or have I just missed the CCR shore divers? If this is correct, why aren’t CCRs used from shore?

One of the significant advantages to CCRs as I see it is the fact that the breathing gas can be "adjusted" so that the gas is higher in oxygen/lower in nitrogen. Since the "bottom times" with CCRs are significantly longer than those with Open Circuit Scuba, this would be a major advantage. Even "newbie" divers would benefit from longer NDLs availble due to decreased nitrogen in the breathing gas. 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?

What are the differences between the available "recreational" (read non-military/commercial) CCRs on the market? Do the manufacturers or CCRs have "Demo Days" similar to dry suit manufacturers? If so, what requirements do they have in order to try out CCRs?

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?

Thanks in advance…

And, please be gentle... and thanks for reading!


Ian

 

[SCUBA_Phil]

Ian,
Go to Rebreather World - Rebreathers for Scuba Diving - the next step and most of your questions will be answered. I have dove my Meg at both Vet Park (Redondo Beach) and Laguna. Usually I just use up some leftover scrubber material from other dives. One thing about a RB - you need to dive it regularly to keep proficient. I'm still new at it with only 35 hours on it to date. To do it right, plan on spending $10K. Last but not least, you should have a good amount of dive experience before transitioning. I had around 1000 dives and was making over 100 dives per year when I transitioned. You should have also mastered a dry suit and doubles before the switch over. my $0.02

Phil

 

[Rogue_Diver]

The best source I have found for information in this area is rebreatherworld.com You will find all the answers to your questions there.

 

[PerroneFord]

I am going to take a stab at this for fun. I am not a CCR diver, though I do spend a fair bit of time around the units and CCR divers. I am sure someone will point you to comprehensive sources, and likely to the rebreatherworld forum. I am answering becase 2 years ago, I had many of the same questions and took it upon myself to educate myself a bit.

Now, the questions about CCR:

If the system is closed, it must form a complex system consisting of biological (lungs, trachea, mouth, lips, etc.) and mechanical (reg, hoses, scrubber vessel) portions. 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?

The closed loop is connected to normal open circuit tanks. Though they are generally much smaller that what open circuit divers use. When ascending, excess gas is vented. The idea of it being a completely closed loop is a misnomer of sorts. The system relies on the mouth and the nose to close the loop, and that can be regulated. The counterlungs also have exhaust valves to relieve excess pressure if desired.

How do you maintain buoyancy with a CCR? It would seem to me that this would be a major difficulty.

It requires a bit of practice, but its not as hard as you might think. Took me an hour in the pool to get the hang of it.

The technology for air "scrubbing" is very old. I understand that rebreathers use a scrubber system relying on calcium hydroxide as the primary reactant to remove carbon dioxide. I’ve 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?

The pellets will become pasty on their own. The air from your lungs has a wonderful amount of humidity.

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?

The water to form a caustic cocktail can come from anywhere if the loop is compromised. You might let the breathing loop come out of your mouth, the counterlungs may develop a leak, an sealing o-ring might fail, etc. Some CCRs are more tolerant to water intrusion than others. But a full on flood of the unit will compromise any CCR. Many CCRs have clever designs to mitigate water intrusion. Some have very little that stops it. The most avid CCR diver I know (200-300 dives per year) has only had a caustic cocktail on one unit. So the incidence can be quite low. However, failure to properly maintain the unit, or poor diving habits can increase the likelihood of it happening.

I have seen a number of divers on Southern California dive boats using CCRs. I can’t recall ever seeing a CCR being used from shore. Is this a correct observation or have I just missed the CCR shore divers? If this is correct, why aren’t CCRs used from shore?

Can't answer this one. I live in Florida, and I see CCRs all over the place.

One of the significant advantages to CCRs as I see it is the fact that the breathing gas can be "adjusted" so that the gas is higher in oxygen/lower in nitrogen. Since the "bottom times" with CCRs are significantly longer than those with Open Circuit Scuba, this would be a major advantage. Even "newbie" divers would benefit from longer NDLs availble due to decreased nitrogen in the breathing gas. 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?

Bottom times are not always significantly longer on CCRs. They have the potential to be, but sometimes logistics simply prevent it. A diver who is used to carrying stages and decompression gases may in fact spend more time in the water than a similar diver on CCR. To decrease nitrogen in the breathing mixture, one has to replace it with something. If that something is oxygen, then depth becomes a real factor. If the goal is to replace it with helium, then significantly more training is required, and this is beyond the scope of a "newbie". All CCR units I have ever seen allow the mixture to be adjusted during the dive. SCR units either cannot accommodate this, or you must jack in different tanks to accommodate it.

What are the differences between the available "recreational" (read non-military/commercial) CCRs on the market? Do the manufacturers or CCRs have "Demo Days" similar to dry suit manufacturers? If so, what requirements do they have in order to try out CCRs?

There are as many differences as there are units. They all do the same basic thing, but there are many, many ways to skin the cat. Each unit has it's pluses and minuses, and you must research which unit you feel best meets your needs. Demo days happen fairly regularly in some areas of the country. My shop hosted one last month that was a marvelous success. We had 8 units available (Megalodon, Inspiration, KISS sport and classic, and the Optima.) We did not bring the Ourborous. For our tryout you needed only to be a certified diver, and it helped if you were somewhat experienced, or well practiced. There is a lot going on with the rebreather, and comfort in the water should be a prerequisite.

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?

I started with a book by Jeff Bozanic, and then looked at a few CCR training manuals, and took a basic course. I am sure others will be able to give you some specific reading materials.

Thanks in advance…

Hope this made some sense. I am sure some real CCR folks will be along shortly to help even more and correct any mistakes I might have made.

 

[PerroneFord]

Ian,
Last but not least, you should have a good amount of dive experience before transitioning. I had around 1000 dives and was making over 100 dives per year when I transitioned.
Phil

Do you really feel this is the case? I've had the opportunity to watch a few dozen divers go through CCR training. Some with less than 200 dives, some with several thousand. At the end of 8 intense days, they all looked remarkable similar in the water. In fairness, I would say that all of them were fairly good divers at the start of class. Good buoyancy, most with good trim, most diving dry, all familiar with doubles.

 

[ianw2]

Woof! Jackpot! I must have been using crappy search keywords to not have found rebreather world, but I missed it. The printer is going to go trhough a few ink pots tonight, guys!

Phil, Rogue and Perone! THANKS! (all caps intended)

Perrone, what is the title of Jeff's book?

Ian

Ahmm...nevermind...

Amazon.com: Mastering Rebreathers: Books: Jeffrey E. Bozanic

 

[Jeff Toorish]

Very good questions --and looking at RebreatherWorld.com would be worth your while. There is also a good bit of rebreather information at thedecostop.com.

Much of what is written here has been accurate.

I'll add a few bits of information as well:

I shore dive with my Classic KISS all the time. Shallower shore dives do present some challenges as dives to less than about 60 feet require greater buoyancy control. I dive my KISS from shore because I really prefer it to open circuit and I also prefer to add experience. As shallower dives are more challenging in some ways, the experience is actually worthwhile.

For a caustic cocktail to form in a modern rebreather, it pretty much requires a system failure --for example, an O-ring failure of the scrubber unit. If the unit is pressurized, it would be very unlikely for enough water to enter the loop through the mouthpiece, even if the mouthpiece was in the water for a while.

There is no doubt, buoyancy is dramatically different. When I dive with friends on OC, I ask them to swim around objects whenever possible, rather than over. That's not always possible of course, and it's not a problem --but I like to stay at one depth for a while because on closed circuit, you can't "cheat" with your lungs.

I don't agree with the notion that you must have hundreds or thousands of open circuit dives before going to closed circuit. I would say it depends on the diver. CCR training is relatively intensive and certainly thorough. I know divers who went to CCR almost immediately after basic open water.

Frankly, I believe we will soon see a basic open water CCR course from a major instruction agency.

Jeff

 

[SCUBA_Phil]

PerroneFord,
Some divers will do good after a few hundred dives. Some are bad after 1000+. In general, it comes down to the type of dives you do, frequency of dives and the dive partners you have. In the last three years, I have learned 70% of my total dive knowledge (~300 dives) but I have been diving for 30 years. There are alot of RB deaths due to diver errors. When taking a CCR class, it is assumed that your not learing to dive, but learing how to deal with the complexity of the RB. So all in all, it depends - does that make sence?

Phil

 

[wedivebc]

I started with a book by Jeff Bozanic, and then looked at a few CCR training manuals, and took a basic course. I am sure others will be able to give you some specific reading materials.



Hope this made some sense. I am sure some real CCR folks will be along shortly to help even more and correct any mistakes I might have made.

Looks like you covered it pretty well, without picking any nits
What basic course did you take? I was not aware of any CCR training short of MOD1

Mastering Rebreathers by Jeff Bozanic is a great resource of RB information.

 

[PerroneFord]

Looks like you covered it pretty well, without picking any nits
What basic course did you take? I was not aware of any CCR training short of MOD1

More correct to say I started the 8 day course, and due to work constraints could not finish it. Over the past year and a half, I've sat through the entire lecture portion (2-3 days at a time!), but have not completed the in-water portion.

So,

Lecture - Yes
Strip and rebuild unit - Yes
Make Meg batteries - Yes
Pool portion - Yes
OW dives - No. (maybe in 2008)