Multiple dives, multiple days..Nitrox

[wetnorm]

Hi,

My buddy and I are thinking of using Nitrox for 7 straight days of atleast 4 dives a day. We both have Nitrox compatible computers, but have never dove over multiple days with Nitrox.

We are going to Bonaire, so we probably could get away with using 36% the whole trip, but would be pushing the 24 hour exposure limits..according to the tables anyway.

Any deep dives like the Hooker would probably be done on air or 32%.

Has anyone dove this much exclusively on Nitrox for multiple days and used a computer? If so, what % did you use and were you even close to reaching your exposure limits?

We have multiple options on how to use Nitrox, like using 36% for our last few shallower dives of the day..etc.

Any insight would be appreciated.

Thanks,

Jason

Yes, dove a week on Nekton Cruise in P.R. in March with 29-31% EAN with 4-6 dives a day/night on singles, with Aeris 2 'puter. Never came close to NDL or O2 limits. Ten divers with similar experience this trip; all different 'puters -- all safe.
Membrane oxygen enricher on board was somewhat erratic and a low volume producer so that only approx. 10 of 30 divers were able to dive EAN. Twice as many wanted EAN.
No science to report here except to mention the closed study by DAN. Perhaps time and a great number of divers will yield statistical power to prove greater safety, but until then standard practices seem safe.
Without EAN, longer SI's, shallower dives and/or fewer dives would have been more likely.

Norm

 

[Damselfish]

4-5 a day on liveabaoards with 32%, Vyper never near being unhappy with it. You do have pretty good SIs in that situation.

 

[jepuskar]

Thanks for sharing your experiences. We are pretty positive we are going to do 32% for this entire trip, seems the way to go.

Jason

 

[peterf]

It's hard to say without looking at a specific series of profiles. Depending on the dives I might be inclined to throw a lighter day or so in there someplace. That really doesn't have anything to do with O2 exposure though.

What they usually don't teach in a basic notrox class is that the rest of us take SI credit for CNS exposure.

Chances are your computer will give you SI credit also. Check the book for the computer. A half time of 90 minutes is common

Several posters have talked about surface interval credit for CNS O2 toxicity. This is erroneous. CNS toxicity refers to high(1.4 - 1.6 and higher) partial pressures of O2 during the dive. There is no surface interval credit issue here. What you are concerned about when diving nitrox for long periods (multiple dives on multiple days) is not CNS but Pulmonary toxicity.

Most nitrox computers will track both CNS (whilst diving) and Pulmonary (both underwater and on the surface) toxicity issues.

 

[padiscubapro]

What I generally recommend to divers doing multiple dives a day (especially on nitrox) is to insure a minimum surface interval of 90mins.
If you are doing shallower stuff with 32 and 36% your oxygen exposure really isn;t that bad unless the dives are very long..

The "rules" vary agency to agency, but I would go with an absolute minimum SI of 60 minutes, and longer as the dive day progresses.

I always try to maintain a 90min SI myself, but I realize there are exceptions.. If you are on a dive boat thats trying to get in an am 2 tank your SI is probably on 60 minutes.. The 90min min SI gives you good recovery on both your nitrogen loading and Oxygen exposure recovery.

I always suggest divers further their training. If questions like this are comming up, I would seek an instructor that can offer another "advanced" nitrox to fill in the blanks..

%cns recovery is not often taught in basic nitrox clases because for most of these divers its quite difficult to actually reach the limits.

 

[padiscubapro]

Several posters have talked about surface interval credit for CNS O2 toxicity. This is erroneous. CNS toxicity refers to high(1.4 - 1.6 and higher) partial pressures of O2 during the dive. There is no surface interval credit issue here. What you are concerned about when diving nitrox for long periods (multiple dives on multiple days) is not CNS but Pulmonary toxicity.

Most nitrox computers will track both CNS (whilst diving) and Pulmonary (both underwater and on the surface) toxicity issues.

BULLS**T
*Abridged explanation*
There are 2 main types of toxicity, whole body and "CNS" whole body is for exposures at a relatively low PO2 while "CNS" comes in at "elevated" PO2s.. The start where limits are tracked is a PO2 of 0.60 (by NOAA tables).

These lower exposures take a considerable exposure and a pretty much beyond the exposures sport divers could experience. On the Higher end say a 1.5 and 1.6 these limits can be reached pretty easily... everything else falls in the middle..
at a level of 1.3-1.4 with the proper surface intervals its also quite hard to reach 100% CNS (if you figure in surface credit) but the 24 hour limits are reachable.)
Decompression divers who use Oxygen (or high FO2 mixes a deep change depths) as part of their decompression protocol can easily make these limits. But none of this is an exact science.. just reaching a 1.6 (as many recreational divers are lead to believe) or hitting the 100% limit will invoke a otox event.. This simply isnt true.. Otox is a combination of several factors PO2 exposure being just one of the factors.. Time and CO2 loading are the other 2 very critical factors.

I've been diving Nitrox since the days when a po2 of 1.6 was an acceptable bottom mix and 1.8 was routinely used for decompression.
On some deep dives with enormous deco requirements I'll deco on oxygen starting at 30fsw (with several precautions, most notably a FFM).. Thats a 1.9... I've done some very deep air dives (very long ago) that had exposures well above 1.6. The whole key is managing the risk not pretending there is some magic to it.

 

[MechDiver]

The whole key is managing the risk not pretending there is some magic to it.

Very well said.

 

[peterf]

BULLS**T
*Abridged explanation*
There are 2 main types of toxicity, whole body and "CNS" whole body is for exposures at a relatively low PO2 while "CNS" comes in at "elevated" PO2s.. The start where limits are tracked is a PO2 of 0.60 (by NOAA tables).

These lower exposures take a considerable exposure and a pretty much beyond the exposures sport divers could experience. On the Higher end say a 1.5 and 1.6 these limits can be reached pretty easily... everything else falls in the middle..
at a level of 1.3-1.4 with the proper surface intervals its also quite hard to reach 100% CNS (if you figure in surface credit) but the 24 hour limits are reachable.)
Decompression divers who use Oxygen (or high FO2 mixes a deep change depths) as part of their decompression protocol can easily make these limits. But none of this is an exact science.. just reaching a 1.6 (as many recreational divers are lead to believe) or hitting the 100% limit will invoke a otox event.. This simply isnt true.. Otox is a combination of several factors PO2 exposure being just one of the factors.. Time and CO2 loading are the other 2 very critical factors.

I've been diving Nitrox since the days when a po2 of 1.6 was an acceptable bottom mix and 1.8 was routinely used for decompression.
On some deep dives with enormous deco requirements I'll deco on oxygen starting at 30fsw (with several precautions, most notably a FFM).. Thats a 1.9... I've done some very deep air dives (very long ago) that had exposures well above 1.6. The whole key is managing the risk not pretending there is some magic to it.

I agree with what you say but you miss my point.

I was merely pointing out that up to this point in the thread no one had distinquished between Pulmonary (whole body) toxicity which will result in a lung irritation after long exposures to relatively low ppO2 and CNS toxicity which may show as muscle spasms (which underwater are likely to result in drowning) at high ppO2 (generally above 1.6 ppO2 although it may occur at lower or indeed higher ppO2).

For most recreational divers diving Nitrox the issue of surface interval credit relates to diminishing the risk of Pulmonary toxicity rather than CNS (central nervous system) toxicity. Keeping yourself within the 100% O2 exposure limit during any 24 hour period is generally not a problem when diving 4 or 5 dives a day even without allowing for SI credit. Keeping surface intervals at 1 hour or longer is always a prudent step. Ensuring that ppO2 stays under 1.4 should ensure that CNS toxicity is not a significant risk (although there are never guarantees).

I was not suggesting that there was any magic to removing risk simply trying to clear up the different issues regarding otox.

 

[MikeFerrara]

For most recreational divers diving Nitrox the issue of surface interval credit relates to diminishing the risk of Pulmonary toxicity rather than CNS (central nervous system) toxicity.
.

Not correct.
Surface interval credit is applied directly to the calculation of CNS% which is tracked primarily to avoid central nervous system tox not pulmonary tox. While time has not been shown to be a reliable predictor both PPO2 and the length of exposure is used in the CNS% calculation.

While the application of a SI credit isn't usually taught in recreational nitrox classes most computers do apply one (usually a 90 minute half time but I've seen 60 minutes used also). Rather recreational classes usually end the discussion with a recommendation for a minimum surface interval. This is just taking advantage of SI credit without teaching the calculations.

CNS SI can be calculated as CNS% (0.5) exp (t(min)/90)
when using a 90minute half time.

Pulmonary exposure is generally tracked in units of OTU (oxygen toxicity units) per the Repex method and no SI credit is applied. Usually the tracking of OTU's isn't taught in recreational nitrox classes because it would be almost impossible for a diver staying within recreational limits to get near Repex limits.

Where OTU = t(min)(PO2 - 0.5)/0.5 exp (0.83)

 

[padiscubapro]

Not correct.
Surface interval credit is applied directly to the calculation of CNS% which is tracked primarily to avoid central nervous system tox not pulmonary tox. While time has not been shown to be a reliable predictor both PPO2 and the length of exposure is used in the CNS% calculation.

While the application of a SI credit isn't usually taught in recreational nitrox classes most computers do apply one (usually a 90 minute half time but I've seen 60 minutes used also). Rather recreational classes usually end the discussion with a recommendation for a minimum surface interval. This is just taking advantage of SI credit without teaching the calculations.

CNS SI can be calculated as CNS% (0.5) exp (t(min)/90)
when using a 90minute half time.

Pulmonary exposure is generally tracked in units of OTU (oxygen toxicity units) per the Repex method and no SI credit is applied. Usually the tracking of OTU's isn't taught in recreational nitrox classes because it would be almost impossible for a diver staying within recreational limits to get near Repex limits.

Where OTU = t(min)(PO2 - 0.5)/0.5 exp (0.83)

Mike,
I couldn't have said it better!
You can also add, even taking into the account of multiday diving reducing the allowable limits pe replex method (OTUS) it would still be quite difficult.

Peterf
In hyperbaric therapy and those doing dives with high po2 exposures, Low po2 breaks are done to reduce the lilihood of a CNS event (not pulmonary tox).. Even a short break does wonders for reducing the chances.