Rebreather Nirtogen?

[frank]

So if a rebreather can basically recycle your air and make it last hours longer, is there a significant difference in absorbed nitrogen as well? ie. longer bottom time to correspond with longer air supply?

 

[rainreg]

Hello Frank as far as your question it depends on what type of set-up you are using. there are 3 major types of closed circuit repreathers, and they are:

1. 100% o2 ( a shallow water rig with a max depth of around 35') there is obviously no additional N2 problem because no n2 is used at all in the gas loop.

2. N2O2 (a medium depth rig usually a depth of 150')the rig is set up to maintain a PPO2 (Partial pressure of 02 at .7 - 1.2) this is done electrically by sweetening or souring of you gas loop)to sweeten the gas it will add o2 to maintain proper PPO2 limits, to sour the gas loop it will add Air (your diluent gas) to help lower and maintain you PPO2. Either way you look at it the rigs are designed to maintain your setpoint PPo2 limits. by maintaining .7 - 1.2 PPO2 you reduce the amount of nitrogen in the loop and hence reduce the amount of absorbed nitrogen. this alows you greater bottom time with reduced decompression obligations.

3. HEO2 ( a deep depth rig usually 300') this rig is setup to maintain a PO2 level like the above listed rig but using HE as your dileunt gas, there is no N2 used so longer bottom times are possible at deeper depths with a way reduced decompression obligation.

I hope this answers your question feel free to ask for clarification on any points.

 

[exnewwavedivers]

How does helium affect a diver physiologically?

 

[Felix]

Originally posted by rainreg
Hello Frank as far as your question it depends on what type of set-up you are using. there are 3 major types of closed circuit repreathers, and they are:

1. 100% o2 ( a shallow water rig with a max depth of around 35') there is obviously no additional N2 problem because no n2 is used at all in the gas loop.

2. N2O2 (a medium depth rig usually a depth of 150')the rig is set up to maintain a PPO2 (Partial pressure of 02 at .7 - 1.2) this is done electrically by sweetening or souring of you gas loop)to sweeten the gas it will add o2 to maintain proper PPO2 limits, to sour the gas loop it will add Air (your diluent gas) to help lower and maintain you PPO2. Either way you look at it the rigs are designed to maintain your setpoint PPo2 limits. by maintaining .7 - 1.2 PPO2 you reduce the amount of nitrogen in the loop and hence reduce the amount of absorbed nitrogen. this alows you greater bottom time with reduced decompression obligations.

3. HEO2 ( a deep depth rig usually 300') this rig is setup to maintain a PO2 level like the above listed rig but using HE as your dileunt gas, there is no N2 used so longer bottom times are possible at deeper depths with a way reduced decompression obligation.

I hope this answers your question feel free to ask for clarification on any points.

Hello Rainreg,

Isn't Helium lighter than N2?
Doesn't our body absorbs more Helium than it would absorb N2 during the same dive, but breathing air?
Doesn't this result in longer decompression obligations using HEO2 (Heliox) or HEO2N2 (Normoxic - Trimix) than using air on the same depth?

 

[Kane(adian)]

Felix,
Helium is indeed lighter than nitrogen, that is why blimps float in a predominatly nitrogen atmosphere. The difference between absorbing nitrogen and absorbing helium is not a matter of lighter or heavier. It is a matter of biology, chemistry and physics. I would recommend that you spend some time reading on rebreather sites. I have read a few, and although I don't have any handy, you will be able to find diagrams and more detailed descriptions of the gases and whatnot than I can describe. As far as I know, the only bad part about helium is that you get a funny voice, which is perhaps a concern if you had underwater communication. Squeaky voices are not just for teenagers, they are for rebreathers too.

 

[mikeblitch]

Of course one molecule of inert helium is lighter (less dense) than two molecules of nitrogen. Look at the table of the elements.
The body does absorb helium more quickly, but it also off gasses more quickly as well.

The decompression, however, is not going to be significantly longer if at all. In fact, helium is a much better gas in nearly all aspects. Even on shallow (70-110) dives, I (and other I know) have been using trimix for no decompression dives. We'll generally use a 21/25 mix and just used a regular air computer.

We've found that we remember the dives much more vividly, have less fatigue, and able to catch more lobsters than otherwise. All one nees to do is increase the depth of the stops and slow the ascent rate which should be done no matter what one is breathing. This conception that helium is more difficult to decompression from is old school and needs to be tossed. Same misconceptions/redirections were made for nitrox.

 

[tekdiver1]

One other thing that you need to look at is the types of rebreathers. Mechanical vs. Electronic or something like Drager vs. Cis-Lunar. Here is the deal with bottom times.

I got TRIMIX certified this summer. I was wearing double 80's on my back and 65's under each arm. My instructor was using the Buddy Insperation rebreather.

I had set deco stops for set times because I worked out the dive plan for me before getting in the water. He switched his rebreather to auto pilot at set a PO2 of 1.3 and the rebreathers computers did the rest. Because he maintained a "constent" PO2 his O2 content was optimal the whole time he was down so is deco stops were alot shorter.

Now using a drager semi-closed "Mechanical" rebreather, you wouldn't have the option of auto pilot. It is a variable PO2 unit, which means you PO2 changes at every depth just like it would on open circut gear.

By the way. I had a 20 minute bottom time @ 220fsw and my runtime from start to finish was something like 58 to 61 minutes. My instructor was back on the boat in about 40 minutes.

If you have to hang on a deco line, bring a book. It gets pretty dull down there.

P.s. I just ordered a buddy insperation and my open circut gear is going on e-bay tonight.

 

[King_Neptune]

As mikeblitch pointed out, "The body does absorb helium more quickly, but it also off gasses more quickly as well" ...

Is the difference in speed slight or is it a pretty big difference in the gassing off between the two?

 

[jmsdiver]

If trimix is a wonderful invention, and judging by the literature it is, do rebreathers do trimix? And if so, do they do the different types fo trimix for the different applications (aren't there like two or three different mixes?)? Furthermore, why do some divers dis the recreational rebreathers (ie Drager vs Cis-luner and others)?

I only ask these questions because I've been eyeing rebreathers and technical diving and weighing the pros and cons of each. I like the discussions thus far, but more data is needed. Are there some recommended readings and websites that could be tossed out that I can do further readings in regards to rebreathers aside from the manufacturers' sites due to what I feel is biased (eg Buddy Inspiration versus the stats)?

Tanks all