Nitrox as CCR diluent

[wedivebc]

Hi all,

Please help this rb wannabe in his continuing education...

If I’m understanding all this right, the hyperoxia concerns stem from the fact that adding diluent with a higher FO2 (relative to air) will cause a larger positive swing in the loop PO2. Please correct me if I’m worng.



Your above statement implies (by it’s converse) that there is some benefit to using NITROX 32 for manually-controlled RBs. The only benefit I can see would be not having to manually inject O2 as often. Are there other benefits?

I guess, in the middle of a dive, if you hit your low PO2 limit just before an anticipated decent (where diluent will be added) you could refrain from adding O2. In contrast, an eCCR would add O2 solely based on the current state of the loop, not being able to anticipate.

Also, by ‘constant manual’ CCR, you are talking about a constant mass flow, manual add (KISS-style) O2 feed, correct?

With a constant flow of O2 it seems to me that the positive PO2 swing (relative to setpoint) would have a longer duration due to the fact that it’s being constantly ‘fed’ O2. Wouldn’t this increased time above the setpoint additionally increase your risk of hyperoxia? Is there some way to easily increase the duration of your loop’s low PO2 state to counterbalance the high PO2 state? Or is this slight deviation trivial when talking about the OxTox timeframe?

Thanks, again.

Don't want to insult your intelligence but I need to lay a little ground work for those following this thread.
-if you inspire .21 po2 at the surface and exhale .17 po2 you have used up .04 po2
-if you go to 100 fsw or 4ata you will inhale 4*.21 or .84 po2 but you will only use up[ the same amount of O2 as you did on the surface so you will exhale .83 po2 and use up .01 (or 1%) of the available O2.
-this example assumes air diluent and no O2 injection so far.
-let's say we want our setpoint to be 1.3 we need to add O2 to get it from .83 to 1.3
-if we have a constant mass flow system, the orifice will replace the 1l/min or so we use up at rest and we will manually replace O2 used due to higher workloads and when the diluent is added.
-if we have a higher o2 in the diluent we will have to add less O2 during a descent but after a few minutes it will make little difference because at constant depth only the O2 needs to be replaced and the diluent will stay constant except for any amount absorbed by our tissue which is minimal (but not zero).
-we will be limited by the MOD of the diluent due to PO2 spikes when flushing or changing depths
-so what all this is saying is for my money air diluent is better because it allows me more depth (up to 150fsw) with little benefit from an O2 point of view.

I hope I have explained that clearly (at least it makes sense to me )

 

[Pawel]

With a constant flow of O2 it seems to me that the positive PO2 swing (relative to setpoint) would have a longer duration due to the fact that it’s being constantly ‘fed’ O2.

No, this is completely wrong. wedivebc explained well.
However, I still disagree with wedivebc on the conclusion.

The only benefit I can see would be not having to manually inject O2 as often. Are there other benefits?

I suggested running Nitrox as diluent in a CCR several months ago on DiveOz (while I could), which generated a lot of undesirable comments from some "experts" :monkeydan.

Anyway, another benefit in more efficient SCR function in case of O2 feed failure (most likely ran out of gas). In such case you vent every n-th breath depending on mix, depth and exertion rate. Another (and most obvious) benefit is when using diluent as OC bailout.

But, there can be drawbacks too. If your diluent is too rich for the depth (but still within MOD), you may actually need to purge the loop to lower PPO2 on descent, which is not good because you are wasting gas. I'll give you example:

You are on the surface with 100% O2 in the loop. You have 60% diluent mix, which has MOD, say 18m. You descent to 5m. You add O2 to compensate for depth. Your PPO2 is 1.5, fine. So far so good! Then you go down to 15m. You add diluent to compensate for pressure change. Your mix, assuming constant volume of the loop, at 15m would be 84%, which at 15m gives you PPO2= 2.1, Crikey! :death2:

Also, in case of O2 injector stuck open (or other cause of O2 spike), you will have hell of the time managing PPO2 with rich diluent (before you shut off O2 supply).

Summing up, on shallow dives and not "too rich" Nitrox, I think it is a good idea.

Cheers,

Pawel

 

[Curt Bowen]

I have a different train of thought.

Normally I use only air or a helium mix such as Tx10/50 as a diluent.

Air for standard dives from 150 and up and Tx below

Why, you ask?

1. Lower PO2 diluents allow me to control oxygen spiking if depths change rapidly. i.e. multi level caves of wrecks.

2. Helium in the diluent allows me to explore safely whatever depths we might discover during cave exploration or deep wall dives. (Someplace where you do not know what your max depth might be)

3. Air and Heliair is easy to mix. (So is Nitrox)

4. Air is always available.

CCR Rebreathers run on a set point that you choose. No matter the diluent mix, your PO2 should always remain the same. Your decompression schedual will change slightly according if you use helium or nitrogen.

I do not think a nitrox mix as a diluent will give you any benifit. except bail out.

 

[Pawel]

I have a different train of thought....I do not think a nitrox mix as a diluent will give you any benifit. except bail out.

Greetings Curt,

I think we are debating whether the glass is half full or half empty

(I said that nitrox as diluent will give you benefit as bailout and would not give any benefit otherwise.)

Anyway, in practice I would use 32% Nitrox as diluent for extended, flat profile dives up to 20-25m. Otherwise, I fully agree with you, specially with 1st point you made.

Cheers,

Pawel

 

[wedivebc]

Greetings Curt,

I think we are debating whether the glass is half full or half empty

(I said that nitrox as diluent will give you benefit as bailout and would not give any benefit otherwise.)

Anyway, in practice I would use 32% Nitrox as diluent for extended, flat profile dives up to 20-25m. Otherwise, I fully agree with you, specially with 1st point you made.

Cheers,

Pawel

Intresting, I had an experience today that made me glad I had air diluent. As you are probably aware with a KISS system your O2 regs IP is how you set the O2 flow rate and use of a non-compensating reg is essential. I think my O2 reg went balanced on me.
For some reason today on a 100ft dive just after descent I noticed my PO2 was 1.5. Hmmm, that's weird I thought I better keep an eye on that. A few seconds later it was 1.6. OK this is not great but no worries, a quick flush and my PO2 is down to .9 and so I watched it as it slowly climbed after about 3 min.
The point is it was nice having a good range to work within to get the po2 down quickly. I see your point Pawel but SCR bailout is just 1 tool available for a rebreather diver and high PO2 diluent limits the use of other tools such as a quick way to get the loop down. I don't have the option like shoulder mounted CLs to squeeze all the bad stuff out so I keep my cl volume at a minimum and it still might take a few breaths to clear out the bad gas.

 

[Pawel]

I see your point Pawel but SCR bailout is just 1 tool available for a rebreather diver and high PO2 diluent limits the use of other tools such as a quick way to get the loop down. I don't have the option like shoulder mounted CLs to squeeze all the bad stuff out so I keep my cl volume at a minimum and it still might take a few breaths to clear out the bad gas.

Hi Dave,

True, but, this is what I said exactly: "...Also, in case of O2 injector stuck open (or other cause of O2 spike), you will have hell of the time managing PPO2 with rich diluent (before you shut off O2 supply).... " And, if it was 100ft dive I would not have rich oxygen content for exactly those reasons.

Also, an interesting point you made about being able "to squeeze all the bad stuff out". You are right, in some rebreathers you can't do it without overinflating CL, in others you can.

Cheers,

Pawel

 

[teksimple]

Thanks for all the feedback, guys. On any other occasion, I would not be considering nitrox as a diluent. However, in the Galapagos--I am told--the currents, downdrafts, updrafts, and surge can be very strong, and thus vertical movement would be quite common. Decoweenie: definitely different than diving in the Arabian Gulf, as you you would agree. With vertical changes requiring regular diluent additions, and the fact that I have only one K bottle for two weeks of hard core diving, I want every advantage I can get.

BTW, has anyone done a two week hard core liveaboard trip with a CCR? If so, how much O2 did you use? I realize that everyone is different, but a ball park figure would be nice to contemplate.

 

[Curt Bowen]

BTW, has anyone done a two week hard core liveaboard trip with a CCR? If so, how much O2 did you use? I realize that everyone is different, but a ball park figure would be nice to contemplate.

I have done a couple CCR liveaboards. I would guess that I used maybe 60-80 cuft total O2? Just guessing at 4-6 cuft per dive X 10-15 dives.