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

[wgw04024]

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


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.

If the skipper put him in nitrox intro so he could use it, and gave him the 10 minute monologue about it, in my mind that is a prudent and nice thing to do. Let's face it, nitrox (for better or worse) is simply a card that says 1. i know how to check my tank with an analyzer, and 2. I know there's a maximum depth I can go on said nitrox. Not doing nitrox when you are going to do multiple deep dives, on multiple days, is simply bad planning. Alot of diving accidents seem to start with a small manageable issue, that grows in size when not dealt with appropriately in the first place. This was exactly that. Not being able to use nitrox was the first failure...

 

[DandyDon]

Let's face it, nitrox (for better or worse) is simply a card that says 1. i know how to check my tank with an analyzer, and 2. I know there's a maximum depth I can go on said nitrox.

And know how to change my computer settings. Yeah, the course is way overpriced around here, with the shop requiring unneeded dives to justify the cost.

 

[wgw04024]

Your shop requires an actual dives(s) to complete the nitrox course??

 

[Marie13]

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.

Note: you have to have your Advanced Nitrox card to get 50%. Standard Nitrox card only gets you up to 40%. So it’s mostly tech trained folks who are going to have this option, unless they have “under the counter” access to O2.

 

[Marie13]

Your shop requires an actual dives(s) to complete the nitrox course??

Yeah, I have the same reaction. I’ve even come across people elsewhere online saying they wanted to dive under an instructor for their first Nitrox dives as it made them more comfortable. SMH.

 

[BackAfter30]

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.

Thanks for the reply. I realized that my confusion came from not thinking it through in this context. I think the fact that on gassing/off gassing rates will be the same for the same gas is what tripped me up. I think that kept me from thinking about the fact that N2 LOAD is the pertinent factor, rather than on gassing RATE. I understood the nitrox mixes used for deco, I guess I was not making the connection because of the gas switch involved was fooling me and not completing the thought properly. Thanks.

 

[Graeme Fraser]

I completely agree - the point of not going into deco is stressed so much that many bad decisions are being made as a result. I've experienced boat briefings from more pragmatic ops where the point has been explicitly made 'don't deliberately go into deco, but if you do, follow your computer and meet the obligations' rather than the seemingly more popular 'if you go into deco the world will end'. It would benefit everybody if this small change in phrasing/intent were more universal.

It would also seem prudent that when relying solely on a piece of technology for ones safety one should know exactly what happens when a warning/process-change threshold is violated. The last thing you want to be is sitting there worrying about how you just screwed up and suddenly have your computer giving a bunch of signals and messages you've never seen before. I would advocate that everyone should take their computer just a bit into deco - in friendly conditions - see what happens and then do what it says to get familiar with the process of ascending safely. Perhaps this should be part of AOW level training. It's going to happen at some point - why not plan for it?

Although I agree that divers need to be able to interpret what their computers are telling them if they inadvertently slip into deco, I would still be very wary about encouraging untrained divers to experiment, even a little bit. The problem is, once that line has been crossed where does it end? The big issue isn't just understanding the computer and holding a stop but also gas management. 5 minutes once could easily become 10 minutes next time and so on. Eventually the standard NDL approach of leaving the bottom with 80 bar just ain't going to get you to the surface.

I was teaching a tech course last week, running through various dive plans with the student. Each plan had to include a lost deco gas contingent. Eventually the student settled on a proposed plan with what he felt was sufficient reserve. I then asked him to run a +5 minutes plan, again with lost gas, and he was genuinely shocked to discover that the additional gas required would leave him well short, with the options of either blown deco stops or OOG.

I know I may come across as one of those deco gloom mongers, but maybe sometimes you need to draw a line and perhaps NDL is a good a place as any.

 

[Jcp2]

If the skipper put him in nitrox intro so he could use it, and gave him the 10 minute monologue about it, in my mind that is a prudent and nice thing to do. Let's face it, nitrox (for better or worse) is simply a card that says 1. i know how to check my tank with an analyzer, and 2. I know there's a maximum depth I can go on said nitrox. Not doing nitrox when you are going to do multiple deep dives, on multiple days, is simply bad planning. Alot of diving accidents seem to start with a small manageable issue, that grows in size when not dealt with appropriately in the first place. This was exactly that. Not being able to use nitrox was the first failure...

That’s kind of like discover nitrox, where the student dives nitrox with instructor, but is not credentialed to use nitrox independently.

 

[RyanT]

Note: you have to have your Advanced Nitrox card to get 50%. Standard Nitrox card only gets you up to 40%. So it’s mostly tech trained folks who are going to have this option, unless they have “under the counter” access to O2.

Thanks for pointing that out Marie, I probably should have added that disclaimer.

 

[NAM001]

Your primary computer was on liberal and backup was on conservative. the backup says in deco and you had a problem???? What if you had not had the backup. You would have surfaced long before NDL hit and you would have still had the problem. The puter did not give you a problem so what did. I could understand if the computers were on nitrox and you were diving air but not the other way around. you should have been able to do the dive at p1 ( if that is the most liberal but apparently before you hit NDL on the most liberal computer you got DCS???? Something else is in play. P0 and P1 gives more NDL than P2.

Could it be possible that you could have been sunburned and in salt water generating the skin itch. DCS just doesn't seem to fit the symptoms. I just returned form a lake that is a bit alkaline and have been recovering form sunburn. Its now 2 days later and iI am still itching a bit. Lake bottom is <30 ft and I went through perhaps 50 ct ft of gas.

Not to mention that diving any nitrox with the computer set to air is a conservative action. Your computer was on air YES???