DCI and the perils of diving in a mixed EAN/Air Group

[RyanT]

Nitrox is not an ultimate panacea

Agreed, nothing is in diving. Here's something else to consider though, and I don't hear this discussed much for recreational diving. When diving nitrox, especially a richer mix like 36%, you're going to get more efficient off-gassing on ascent and during your safety stops. Even if you're diving nitrox to the NDL, the added off-gassing efficiency is valuable relative to diving to the NDL on air. I'm always glad to have nitrox on recreational dives for this reason.

 

[doctormike]

IMO, The biggest takeaway to me (that I did not see mentioned) is never under any circumstances, ever skip a decompression obligation from any computer.

I agree, but of course the bigger point is that we are arbitrarily drawing a bright line through a grey area.

His backup computer showed a deco obligation, his primary didn't. He followed his primary and got bent.

What if he hadn't had the backup and done exactly the same thing? He would have followed his primary, ascended "within NDLs" and gotten bent. And some would have called it an "undeserved hit".

Nitrox doesn't make you less likely to get bent unless you are gas limited. If you are doing dives where you are ascending when your NDL gets close to zero, nitrox will give you a longer dive, but your decompression stress and tissue loading will be the same. If you want to build in a DCS buffer, then you need to load less inert gas or extend your decompression profile.

 

[BackAfter30]

Agreed, nothing is in diving. Here's something else to consider though, and I don't hear this discussed much for recreational diving. When diving nitrox, especially a richer mix like 36%, you're going to get more efficient off-gassing on ascent and during your safety stops. Even if you're diving nitrox to the NDL, the added off-gassing efficiency is valuable relative to diving to the NDL on air. I'm always glad to have nitrox on recreational dives for this reason.

I initially balked at this. I thought about a bit, though, and think I see how this would be true. I don’t remember reading it in any of the lit I’ve looked at. To you have any references you could point me to for this?

 

[Dr Herring]

Great replies by both RyanT and doctor Mike.

I’m glad you got away with skin bends only.

I very much like to add conservatism to my deco/dive plan(there’s no such thing as no decompression dives in my book).

So more times than not, I’ll increase conservatism if I do repeated dives or have any risk factor for DCS.
I’ll take these extra minutes of deco any day over DCS.

Diving nitrox could’ve increased your conservatism. Ignoring the more conservative computer certainly did not.

Just the other day I had to wait 15 min for my OC buddy to finish his deco obligation, while mine was cleared already (diving CCR). He apologized after for it, but I was happy about it, since I knew I added conservatism for myself.

I wish more divers saw adding conservatism as something good, rather than a nuisance. You diving air might theoretically have saved one of your buddies from DCS.

 

[Luis H]

I agree, but of course the bigger point is that we are arbitrarily drawing a bright line through a grey area.

His backup computer showed a deco obligation, his primary didn't. He followed his primary and got bent.

What if he hadn't had the backup and done exactly the same thing? He would have followed his primary, ascended "within NDLs" and gotten bent. And some would have called it an "undeserved hit".

Nitrox doesn't make you less likely to get bent unless you are gas limited. If you are doing dives where you are ascending when your NDL gets close to zero, nitrox will give you a longer dive, but your decompression stress and tissue loading will be the same. If you want to build in a DCS buffer, then you need to load less inert gas or extend your decompression profile.

I think we are in the same page. But, I do not "arbitrarily drawing a bright line through a grey area" and that is why I said that "All dives are decompression dives". This is the case whether you have a decompression stop obligation or not.

You may also notice one of my last sentence:
"All dives should include deco stops, but some divers are only comfortable calling them safety stops. In the real world they are the same... it is just a continuum."


I totally agree with your last paragraph on Nitrox.

I use Nitrox to extend dive times.
I use "extended decompression/ safety stops" to mitigate the risk of DCS.

 

[doctormike]

I think we are in the same page. But, I do not "arbitrarily drawing a bright line through a grey area" and that is why I said that "All dives are decompression dives". This is the case whether you have a decompression stop obligation or not.

You may also notice one of my last sentence:
"All dives should include deco stops, but some divers are only comfortable calling them safety stops. In the real world they are the same...

They are both stops, they both reduce decompression stress, but they aren’t the same.

My point is that whether you have an obligatory deco stop or not often depends on the choice of computer and/or algorithm, which of course has no effect on your personal physiology...

 

[RyanT]

I initially balked at this. I thought about a bit, though, and think I see how this would be true. I don’t remember reading it in any of the lit I’ve looked at. To you have any references you could point me to for this?

Check out "Deco for Divers" by Mark Powell. From SCUBA 101, we know that when we dive, we are under added pressure, so the nitrogen in our breathing gas begins to be absorbed into the body. As we ascend and release the pressure, the nitrogen in our tissues begins to come out of solution because it is at a greater concentration than the nitrogen in the gas we are breathing.

Here's the interesting part. Gas diffusion is influenced by pressure gradients; the higher the gas pressure on one side compared to the other, (e.g. air in your lungs vs blood) the faster diffusion will happen. AND, the different gases (O2 and N) will effectively diffuse independently of each other, only affected by their own concentrations. So what adding oxygen to your mix does, is actually reduce the amount of nitrogen in your breathing gas. This reduced nitrogen makes the concentration gradient between the nitrogen in your tissues and lungs steeper. So breathing EANx 36%, for example, means that your fraction of N is 64%. That 64% nitrogen fraction is lower than the 79% you get with air. So the end result is that the lower fraction of nitrogen in your lungs generates a steeper gradient and your body off-gasses the N faster.

This is exactly why tech divers use high O2 mixes for decompression. For a typical 200' decompression dive, for example, divers commonly switch to 50% and then 100% O2. This increases the steepness of the concentration gradient and dramatically reduces decompression times.

 

[Luis H]

They are both stops, they both reduce decompression stress, but they aren’t the same.

My point is that whether you have an obligatory deco stop or not often depends on the choice of computer and/or algorithm, which of course has no effect on your personal physiology...

Well this is interesting. I thought we agree that sometimes we draw an arbitrary line on a grey area.

I said, it is a continuum. You just change the GF in your computer and the safety stop becomes a deco stop. The diver has not changed. It is the same diver. You just changed the math a bit and made it more conservative.

I also agree with your point that a computer can tell you that you are within your NDL. Well IMHO, since real life is not digital (on/off), to me getting close to an NDL limit means that I have to extend my deco stop. The computer calculates a threshold number for NDL, real life analog creatures do not.

I consider all dives to have obligatory deco stops. I don't have a problem calling them safety stop, but the only difference is a "reduced" risk of DSC if a safety stop is violated.


Note: I hope that I am expressing my thoughts clearly. Maybe I am not.

 

[RyanT]

Like @Searcaigh, my buddy and I also commonly sling a 40 cf tank of 50% on our recreational dives. Even without going into decompression, we'll switch to that 50% mix at 70' and finish our ascent/safety stops.

 

[DandyDon]

I have to say that spending the money to go a liveaboard half way around the world and then not taking a Nitrox class there if you don't already have it is just silly. Depending on the trip, it's easy to do 4-5 dives daily for multiple days in a row.

I wouldn't want to risk insulting the diver, but yes - it would seem like a prudent plan.

....and often one of the guides is also an instructor and happy to do a nitrox course onboard. Seen it a few times.

On one of the few liveaboards I've done, we had only one who was not Nitrox certified. The highly respected Captain put him down for Intro Nitrox and showed him how to reset his computer. I don't know what other instruction he got, but he wasn't carded.

I knew the guy from previous group trips and his medical problems unknown by the skipper and had vowed to never dive with him again, but he showed up as a surprise diver. The group organizer hated to lose on an empty seat and had allowed him on group trips he was unqualifed from the first. My choices were to not board after booking & traveling, or be the tattletale, or grit my teeth and silently hope he didn't die. I still wonder if opting for the latter was the best choice, but we were lucky.