Recreational NDL times ?

[Big Stig]

Trying to get an understanding of pros V cons of recreational rebreathers such as the Poseidon Mk6
ref comments on extended dive time.
Is this primarily due to not running low on air.
Or due to the better control of PP02 in the rebreather does it actually increase your NDL times??

Eg on Air at 110ft I have 16mins NDL
on 32% nitrox I can extend that to 20 mins NDL.
what would the NDL time be on a Rebreather such as the mark 6.??
also in regards to the hollis hybrid one that uses just one tank of Nitrox assuming we see it retail eventually would that be any different to either OC or the Mk6

will the rebreather reduce the amount of Nitrogen build up in my system ??

Obviously still a rookie by these questions.


Thanks for info.
Cheers SteveP

 

[Peter Guy]

You have actually hit on one of the interesting issues regarding recreational rebreathers -- what is the advantage? Without doing the actual math, I'm going to guess that from an NDL standpoint, there isn't that much advantage to having a 'breather for a "No Stop Dive" over just a big tank of 32%.

One of the reasons I keep not going there is I can't find many dives I do or want to do where having a 'breather makes more sense than open circuit.

 

[darushin]

This is from TDI Generic CCR manual:

the formula for calculating the EAD must for closed-circuit rebreather dives for which the PO2 is known, but the FIO2 is dependent on depth
P = the absolute ambient pressure at the target depth (in bar/ata).



so for 32% at110 ft (assuming no change in depth) you have a PP02 of 1.37. So unless you dive you rebreather at 1.4 or 1.5, you won't see any difference.

Daru

 

[ianr33]

If you are diving square profiles and use a "Best Mix" of nitrox then the rebreather offers no advantage from an NDL perspective.

Might be some use on multilevel dives.

With the possible exception of photography, I don't see the point of rebreathers on recreational dives. More cost.More complexity. Greater risk. Why bother??

 

[KD8NPB]

will the rebreather reduce the amount of Nitrogen build up in my system ??

In short, yes.

It's the equivalent of having multiple mixes available to you as you dive...from 99% O2, all the way to 1% hypoxic mixes (with a dilulent).

As you ascend, the % O2 in the gas concentration increases, which aids your body in flushing out N2.

The downside is technical complexity. Rebreathers are very appealing to the technical savvy geeks : Engineers mainly.

 

[Dive-aholic]

Steve, a rebreather such as the MKVI will allow you to control your ppO2 so that you can extend your NDLs. I'm guessing you are already nitrox trained/certified and understand the concept of best mix. Well, with a closed circuit rebreather (CCR) you control the nitrox percentage in your loop so you are always breathing the best mix regardless of your depth. In that regard you can extend your NDLs to the maximum allowable time. There are also passive semi-closed rebreathers (PSCRs) on the market that do not do this. They act more like open circuit and actually drop the percentage of the gas you are breathing by up to 5%. These rebreathers are better suited for divers who have higher respiratory minute volumes and are always the first one back on the boat while everyone else is still in the water. The KISS GEM is one such model. But they can also be used in different ways. I dive a KISS GEM (actually a couple of different models of the KISS GEM) and use it primarily as a gas extender during decompression dives.

There are a lot of advantages to diving rebreathers and a lot of different ways they can be used.

 

[Peter Guy]

Rob, while it is obviously true that keeping your O2 at a constant 1.3 (or whatever) during your dive is possible with a CCR and that will lesson your inert gas loading, the real question is, how much NDL time would a recreational diver actually gain in a dive to, say 100 feet wreck -- i.e., a fairly square profile as opposed to the time on say, 32%.

 

[Big Stig]

Thanks guys for some great information.
This is really helping me better understand.
So see if I am reading this correctly.
1) on a recreational square profile dive probably no bennifit unless I am an air hog and constantly run out of gas. (personally I am not )
2) On a multi level Dive I may see some extended NDL times because of the improved gas mix on a constant basis.
3) If I am a UW photographer (Which I am) I can see bennifit from reduced bubbles and noise
4) There is a reduced amount of Nitrogen taken into the system because of the constant optimal gas mix blend change. (This one is quite interesting to me. although I am fairly phsically fit I am fat so I seem to obsorb more Nitrogen and do get some aches if I dont watch my accent and do extra additioanl and longer safety stops. this is a primary reason I use Nitrox) I actually use Nitrox but stick to air tables for extra safety.
Not that I want to change those prcatice but it seems this can be an additional safety factor for me.

I am not scared by the technology. It seems to me that almost all the incidents with rebreathers over the last few years have been attributed to some kind of user error.

 

[Allan C Smith]

Rob, while it is obviously true that keeping your O2 at a constant 1.3 (or whatever) during your dive is possible with a CCR and that will lesson your inert gas loading, the real question is, how much NDL time would a recreational diver actually gain in a dive to, say 100 feet wreck -- i.e., a fairly square profile as opposed to the time on say, 32%.

On a True Square profile you would gain back some of the no stop time lost On the decent. more or less a small amount. However on a multi level dive you would gain no stop time all the way up as your pp02% would stay the same limiting the amount Nitrogen absorbed at every level and allowing you tissue compartments to off gas quicker.

 

[Peter Guy]

Allan, again, yes, of course having the constant PPO2 at the CCR set point will reduce inert gas loading. That isn't the real question. The real question (at least as I see it), is how much benefit would one actually get in reducing N2 loading/increasing no stop times, on a typical recreational dive?

While there may be many reasons to go to a CCR for recreational profiles, is no stop time/reducing signficant N2 loading, really one of those reasons?