CCR vs SCR

[diveandclimb]

What is the difference between a closed circuit rebreather and a semi-closed circuit rebreather other then the fact that one lets out a few bubbles now and then. I know CCRs are better but what are some specific advantages?

 

[Jimmer]

CCR's maintain a constant partial pressure of oxygen with the fraction of oxygen fluctuating as depth changes, whereas an SCR maintains a constant fraction of oxygen, with the partial pressure fluctuating with depth changes..

 

[Rhone Man]

I know nothing about rebreathers, but in non-technical terms: I read in Gary Gentile's Technical Diving Handbook that a CCR can keep you underwater for about 48 hours, whereas a SCR gives you slightly longer than a big set of doubles.

Gary's rhetorical question was: why would anyone bother with a SCR?

 

[almity1]

I know nothing about rebreathers, but in non-technical terms: I read in Gary Gentile's Technical Diving Handbook that a CCR can keep you underwater for about 48 hours, whereas a SCR gives you slightly longer than a big set of doubles.

Gary's rhetorical question was: why would anyone bother with a SCR?

48 hours!!!!!!!!

You must mean 4 - 8 hours...

Is he talking about 1 CCR or more???

Note to the other RB divers....

A scrubber that last 48 hours???? Is that a Holy Grail or What!!!!!

 

[wedivebc]

CCR's maintain a constant partial pressure of oxygen with the fraction of oxygen fluctuating as depth changes, whereas an SCR maintains a constant fraction of oxygen, with the partial pressure fluctuating with depth changes..

This is true but the real significance of it is that with fixed PPO2 you get a larger range of available depths with optimal O2 exposure for that depth. This gives you a deco advantage in most cases.
The SCR not only restricts you to the MOD of the oxygen content of the carry gas but you lose even the deco advantage of the nitrox mix since the inspired oxygen will be somewhat less than the oxygen present in the carry gas.
For most applications SCR have run their course and less expensive CCRs have made them obsolete.

 

[Cave Diver]

In addition to what was already posted above, SCR is about 5-7 times more efficient than OC and CCR can be up to 20-30 times more efficient.

SCR is typically a single gas system, vs. the constant gas mixing provided by CCR.

 

[Dr. Doug Ebersole]

I have always considered SCRs as "gas efficient nitrox". You get the warm moist air, minimal bubbles, and significant gas efficiency compared to open circuit. It varies on your mix and depth but in general I could get about 4 hours of dive time on the 50 cu ft cylinder I had on my old Drager SCR. However, as you have a cylinder of nitrox you have to worry about your MOD just like with open circuit nitrox diving. Also, one of the properties of SCR is that there is a constant flow of nitrox into the system. As you are metabolizing more or less of this based on your workload, the nitrox blend you are breathing is variable. This makes calculating your decompression obligation a little tricky. The Drager came with a single oxygen sensor and is recommended for no-decompression diving. There is a two sensor Oxygauge from Uwatec which when paired with a Uwatec AirZO2 can give you live deco, but again these units are not designed for decompression diving.

CCR on the other hand I consider as "blending on the fly". You are basically always diving the "best" mix for the given depth which minimizes your decompression obligation. Decompression can be calculated with an in-line computer, an off-board CCR computer, or tables. CCRs are even more gas effiicient as I now get about 4 hours of dive time from my 14 cu ft aluminum cylinder of oxygen. And they are not limited by the MOD of nitrox like SCRs are. Additionally, if you want to go deep, just use trimix as your diluent instead of air (with the proper training, obviously).

SCRs used to be very useful for "recreational" divers who just wanted to get closer to marine life or simply have a very gas efficient system with no bubbles. However, as the cost of several CCRs now approach SCRs, the role of SCRs is now very limited.

Just my 2 cents but if you're interested in rebreather diving, I'd skip over SCR and go to CCR. This is coming from someone who teaches both SCR (Drager & Azimuth) and CCR (Kiss).

Doug

 

[ScubaDadMiami]

I completely agree with Doug.

 

[Dr. Doug Ebersole]

Thanks, Howard!
Doug

 

[diveandclimb]

Thanks for the answers. I get it now. Anyway, open circuit it where I'm going to be at for a while but CCR might be in my distant future.