Rebreathers, Partially closed or fully closed?

[silent running]

Hello all, back to the original question, whether tech or rec, I think it's no contest. CCR's are the way to go. I had 150 hrs on the Dolphin and I can tell you it was just as much work to get ready for a dive day as it is with the Prism. So other than the cost difference, which isn't that dramatic, I see no advantage in going with an SCR. Not to mention that the only way to really get any decent gas efficiency with the Dolphin is to mismatch the orifice to the gas mix, which is also a little risky and not condoned by Drager. If you use the unit the way they intend, you get a burp of bubbles every other breath and it doesn't exactly help if you're trying to better observe the wildlife. Which by the way, seems to me to be the only good reason to use a RB for recreational diving. I noticed a significant increase in the frequency and quality of my animal interactions when I switched from SCR to CCR. For tech diving it's a huge benefit to be able to maintain a given PO2. A CCR will extend your range and get you out of the water faster. And you can make a CCR as simple or as complex as you want. And nitrox is not always available, O2 is always near at hand. I don't see the point of SCR's in the year 2005.

 

[mxracer19]

Why would anyone want to go through all of this trouble to switch gasses, learn rebreather techniques, 19 and a half different bottles with 24 different gas mixtures...why doesnt the human race just build an under water city? that seems to me the more logical approach than a space station. why should we be in space when we dont know whats on the bottom of our planet? we should figure out whats happening here before we figure out whats happening there. Ill stick to quarrys...at 60 feet ...with one bottle for now

 

[silent running]

Hello Padiscubapro, I was reading over this thread and I just wanted to clarify the Prism cal procedure. Unless Mr. Taylor had a Prism which was very different from everybody else's, the calibration procedure is very simple and requires pressing exactly 1 button after filling the loop with O2. I've done it on many a pitching boat deck without any trouble. Changing the set point involves turning the switch pot inside the same cavity, which can be done with a dime. I have no idea how these procedures could be considered difficult. The cal pots for the secondary are a little smaller and require a little more care, but they almost never need to be adjusted and only pertain to the needle display itself. I have never had to do it more than once in a week. So I do it when there is less rocking and rolling, although I have been able to adjust them on the high seas as well. It may seem slightly troublesome and archaic, but this is the only way to design the system so that the secondary can be driven directly off the sensors, thus ensuring the most simple and thus reliable secondary preformance. I think it's well worth the small trouble. And the secondary calibration has nothing to do with the primary display/ECCR function. It is completely separate from the electronics and is only a physical calibration of the needle movement. Also, there is no way possible to knock the rig out of electronic calibration. So who knows what Mr. Taylor is talking about when he mentioned this supposed phenomena.

 

[padiscubapro]

Hello Padiscubapro, I was reading over this thread and I just wanted to clarify the Prism cal procedure. Unless Mr. Taylor had a Prism which was very different from everybody else's, the calibration procedure is very simple and requires pressing exactly 1 button after filling the loop with O2. I've done it on many a pitching boat deck without any trouble. Changing the set point involves turning the switch pot inside the same cavity, which can be done with a dime. I have no idea how these procedures could be considered difficult. The cal pots for the secondary are a little smaller and require a little more care, but they almost never need to be adjusted and only pertain to the needle display itself. I have never had to do it more than once in a week. So I do it when there is less rocking and rolling, although I have been able to adjust them on the high seas as well. It may seem slightly troublesome and archaic, but this is the only way to design the system so that the secondary can be driven directly off the sensors, thus ensuring the most simple and thus reliable secondary preformance. I think it's well worth the small trouble. And the secondary calibration has nothing to do with the primary display/ECCR function. It is completely separate from the electronics and is only a physical calibration of the needle movement. Also, there is no way possible to knock the rig out of electronic calibration. So who knows what Mr. Taylor is talking about when he mentioned this supposed phenomena.

I was aware of the procedure... He was usually fiddling with the secondary.. John wasn't exaggerating his problems because of all his issues (not just cal issues) it was getting to a point were some of the captains really didn't want rbs on their boats... but thats in the past and lets leave it there...

The two boats I most often ran into Johm was the Eagle's nest and a few times he joined a bunch of us on wreck valley.. He never had any issues (dive related) with the last rebreather he had, but didn't like a few design points...

 

[silent running]

Thanks for your reply padiscubapro. After reading all the chatter on this subject and talking to various people, it seems like Mr Taylor's situation with the Prism was a rather dramatic case of mismatch of product and consumer which was made worse by an early manufacturing problem. I think the point which may have been obscured in all this is the difference in the way different divers see themselves and their needs. For a mostly recreational diver like myself, the ease of use and size/weight are some of the most important factors in determining which CCRs to choose from. For others who want a 'bullet proof' unit, who are used to hauling around 120lb doubles and are willing to put up with a heavy, bulky unit, those factors are much less important. That's probably a big reason why some people would pick the 69lb all metal scubber/head Megalodon over the 47lb Prism. But differences in perception are always trumped by facts, which is how we get to the heart of the matter. A lot of entry level people who read this forum probably won't develop a proper understanding of the different design choices of the many rb manufacturers for sometime to come. That learning takes longer when some of the people on this forum resort to hyperbole and emphatic, emotional statements which then obscure the real, interesting discussions of the various design philosophies. This is the one of the only places to learn about rbs and which you have contributed much good information to. I don't believe that how much abuse a product(except something like a tent)can take is any real indication of it's worth, especially a sophisticated device like a CCR. I treat all my big ticket items(cars, motorcycles, etc)with much respect and they treat me well in return. Negligence is just another form of operator error. As I've said elsewhere, I've had very few problems with my unit and subjected it to 250 hrs of rigorous use all over the world. My Prism has been dropped and banged around above and below water and I never missed a dive. To me, that qualifies as plenty durable. There are several more heavily built, more complicated ECCRs out there, but that doesn't necessarily mean their design and the way they actually function, is superior. If I've learned one thing in my 400 plus hrs of rebreather diving and diving with other rb divers, it's the more simple the design, the less time you spend above water and the more time you get below.

 

[jmcgauhey]

Having just taken the course for Dolphin, I'm getting used to rebreather. I chose this because of cost. At $3K I could afford it. Including training and an extra tank. Bought used, of course. The Inspiration is probably best way for support etc, but it's $8 to $10K with training, and that can take a couple months.

Great post from Mole, I thought.

I'm getting into RB as I take video every dive. Trying to get closer to critters. Time underwater is a minor factor.

It's completely different. An hour before each dive for prep, plus more after the dive. They don't call it a "slime line" for nothing. Rebreathing your own spit for a couple hours makes it all ..... Breathing is different, often harder, always just different. Flood your open circuit, breathe out, go again. I forgot my valve was open, bent over to put on fins, flooded loop, there went the dive. I had to add weight to counter the counterlungs and water trap.

Great advice - think twice about why. Then consider $$, time, potential for a lot more aborted dives.

Still interested?

Jim McG
Washington State

I would like to hear everyones opinon on diving with a partially closed circuit or a fully closed circuit unit?

Thanks

Julie

 

[silent running]

Hello jmcgauhey, hold it, stop right there! Your best bet now that you've taken the Dolphin plunge(as I did once upoun a time)is to purchase a used unit and convert it to a KISS style CCR. If you don't know what this is, do a search of this thread or the internet. I say this because of the time, effort and money I assume you've spent to learn the Dolphin system. If you dive the unit the way you're taught, it's almost worthless-you will still discharge bubbles about every other breath, especially at shallow, critter friendly depths. To get the most out of the SCR Dolphin system you need to mismatch the oraffice for a slower flow rate than the actual corresponding nitrox mix, which is only optimal at 100 ft and beyond. BUT, in this configuration it's still much less efficient than a CCR, it's very imprecise without inline O2 sensor/computer interface, and you're asking for real trouble in normal recreational depths because of shallow water blackout. The reason for this will become apparent when you understand more about the physics/O2 fractions of an SCR. Don't trust me, do your homework. If you have bought a Dolphin, the good news is that they breathe better than other back mounted counter lung rebreathers and the conversion to CCR is simple and pretty cheap. If you have not bought a Dolphin, the Ray would be a better choice as it has over shoulder counter lungs and can also be changed to a CCR, but will breathe easier because of the optimal hydrostatic load of over the shoulder counter lungs. It's also lighter. There are 2 types of constant flow oraffices which are the main component of a conversion of the unit to CCR, one fixed and the other a variable needle oraffice which requires more diver attention, but which has more benefits: if you're photographing something and it takes a long time, a variable oraffice can be dialled down to flow so slowly as not to overtake your breathing rate and cause you to have to vent gas at a crucial photo moment to avoid floating upward. There can be problems with doing a CCR conversion to a Drager: most dive operators will not know the difference between an SCR and a CCR, but some will and they may be hesitant to let you dive on their boat with a modified unit. I can tell you that there are plenty of places, mostly in the South Pacific/Australia where others have converted their units to CCR's and they will be glad to dive with you. That said, I think it's still better to bite the bullet and go electronic CCR all the way from here on out. Your newly learned skills with still be relavant and you will have less hassel overall for many reasons, not the least of which is that nitrox is not always available as it requires a lot of hardware. O2 is never far away and requires almost no equipment to decant it to your CCR cylinders. Having started out on the Dolphin SCR, I can tell you I would have never chosen it over an electronic CCR had there been one worth buying at the time I decided to move over to rebreathers, and had known what I know now. Good luck.

 

[tlessard]

Here's the guy to talk to about the CCR Dolphin conversion on the west coast...gosh! Lucky...

www.tmishop.com

I bought one of his (Ron's) SS backplate conversions...it's totally rufuss (neat-o). I should have my CCR set-up done by August. I'm on the quest for no bubble diving! OC/Trimix is so the 90's way to go deep man...LOL!

-Trey