Nitrox discussion (Split from "Reasons not to use Enriched air" thread in Basic)

[nereas]

...Question: Is there any reason why I should not use Enriched Air for every dive? Is there a reason why I would want to sometimes use AIR opposed to Enriched Air?

From my studys in the Nitrox class it seems ok for me to just stick to Enriched Air whenever I can...

If you are diving really shallow, such as 20 to 50 ft along the coast or in the tropics, then EANx would be a waste of oxygen and money. The only time that diving shallower than 50 ft would make sense with nitrox would be for diving in the mountains (high altitude).

The general rule is that EANx is ideal, and better than air, for any depth between 50 ft and 150 ft.

For dives in the range of 50 ft to 95 ft, EAN 36 is the best mix.

For dives in the range of 100 ft to 130 ft, EAN 32 is the best mix.

For dives in the range of 130 ft to 150 ft, EAN 25 is the best mix. However this is normally considered a technical deco dive, since your NDL would only be 5 mins.

For anything deeper than 150 ft, you whould be diving with trimix, and now you would be outside the range of nitrox.

A basic nitrox certification would allow you to use EAN 36 or EAN 32, for diving to depths of 130 ft (or shallower, per the above limits).

 

[nereas]

If you aren't at 100% of your O₂ Clock and simply a CNS seizure from a high PO₂ then ascending will likely reverse the seizure. If you've been diving on EANx all day and you're pushing 100%+ O₂ toxicity, you probably even run the risk of seizing at the surface.

Sorry, I meant to Rec dive limits.

With 1 hour surface intervals, like you were taught in your basic open water class, you will not be able to add up sufficient CNS to affect your CNS OxTox risk, one way or the other.

CNS Ox Tox is only a risk if you dive too deep with nitrox. That is why 90 ft and 130 ft are the depth limits taught for EAN 36 and EAN 32, respectively.

An IF you do have a seizure, then you are dead. There is no chance to ascend to "reverse the seizure."

With nitrox, you need to make sure you obey your depth limits, and you need to make sure you allow a 1 hour surface interval between repetitive dives.

 

[HowardE]

For dives in the range of 50 ft to 90 ft, EAN 36 is the best mix.

Why not 40% ?? 40% 1.4 is 83' 1.6 is 99' Wouldn't this be best mix for a dive to 80 FSW?

 

[tschmitt]

I'm not sure about that...

In fact, I'm not sure if anyone is sure about that

Can anyone else confirm this? I'm not suggesting that you yourself just perform a controlled ascent while seizing, but if someone can keep you reg in and get you well above the MOD, you can reverse the toxic affects high PO2 O2 is having on your CNS. I'm pretty sure this is in the recent PADI Enriched Air Diver manual.

 

[Anti-Hero]

Can anyone else confirm this? I'm not suggesting that you yourself just perform a controlled ascent while seizing, but if someone can keep you reg in and get you well above the MOD, you can reverse the toxic affects high PO2 O2 is having on your CNS. I'm pretty sure this is in the recent PADI Enriched Air Diver manual.

If I remember correctly the procedure was not to reverse the seizure, but the correct way to ascend with the diver experiencing the seizure was to:

Hold regulator in their mouth.
Tilt head backwards to open the glottis and prevent expansion injuries.
Make a controlled ascent to the surface.

I don't recall anything about stopping / reversing a seizure. A diver who suffered a seizure would be in no condition for quite some time afterward to be able to dive self sufficiently.

 

[Gombessa]

Also, in order to make that controlled ascent, the rescuing diver needs to manage 2-4 air sources; he should make himself slightly negative, make the toxing diver slightly positive, and control the ascent primarily using the unconscious diver's BC. I don't know how long a typical oxtox seizure lasts, or assuming the best case whether/when they will/can wake up during the ascent. I think the general assumption is that they're going to be out cold, at best, all the way to the surface and beyond.

 

[String]

Do remember to not to try to lift someone while still in the convulsion phase of the attack (up to 2 mins)- the airways is closed and they'll embolise.

 

[Catt99]

The general rule is that EANx is ideal, and better than air, for any depth between 50 ft and 150 ft.

For dives in the range of 50 ft to 90 ft, EAN 36 is the best mix.

For dives in the range of 100 ft to 130 ft, EAN 32 is the best mix.

For dives in the range of 130 ft to 150 ft, EAN 25 is the best mix. However this is normally considered a technical deco dive, since your NDL would only be 5 mins.

nereas - I don't think that there's any "general rule" as you described. And I think you should probably either (1) brush up on MODs or (2) clearly spell out for the poster and other readers that you choose not to observe the more standard MOD calculations on your mixes. I'm guessing that diving EANx32 to 130 feet isn't part of most divers regular planning let alone what most would consider "the best mix" at those depths.

 

[John_B]

I said this in another post yesterday, I'll try to summarize it here:

32% is the perfect gas for recreational dives. The difference between air and 32% is enough that its noticeable and measurable in terms of longer bottom times either for a deeper 90' dive, or for a series of dives day after day after day.

Also, the MOD for a 1.3 pO2 is 101', the MOD for a 1.4 pO2 is 111', and even for a 1.6 pO2 it is 132' (contingent depth, for an emergency only). And 132' just happens to be the absolute limit of recreational diving according to most agencies (BSAC and IANTD not withstanding).

It may not be the perfect "best mix" for a specific dive but its the most flexible by far for all rec dives, IMO.

So for any dives I consider a recreational dive (as deep as 100' or 110') I reach for the 32%. When I get fills that means I get 32% because its the right mix for pretty much any dive I'll do. No surprise, it is what I have in my tanks right now.

For dives deeper than than 100' or 110', I don't personally consider those to be normal recreational dives. And not just from the perspective of how lean the nitrox should be, but also my buddy and their training, being narc'ed, what is rock bottom for that depth, etc. So while I wouldn't use 32% for a 120' dive, there are a lot of other adjustments I'd be making at the same time.

YMMV

 

[TSandM]

Most seizures last two to three minutes and resolve, but there is a period of reduced awareness and confusion afterward which varies tremendously in time. If the stimulus causing the seizure is not removed, you can have repetitive or continuous seizures, known as status epilepticus. A person who is actively seizing probably has a closed glottis, and should not be moved upward in the water column until the active convulsion has ended. Subsequent to that, it would be desirable to move them upward as quickly as is safe, to reduce the ppO2.

Few people have survived oxygen toxicity seizures underwater. It requires the immediate presence of a buddy who recognizes the problem and takes precisely the right actions immediately.

That said, oxygen toxicity seizures are extremely unlikely if O2 concentrations are kept below 1.4 ATA. It's easy to do the vast majority of dives that recreational divers will do on 32% and respect that limit. One might lose out on a few, deeper dives. Use of a standard mix in this way also permits keeping one's computer (or one's mental computations) set the same way all the time. Cumulative OTUs are not much of a concern unless one is doing multiple deep or long dives on multiple days.

I do all my diving on 32% if I can (which I can at home). But I pay a blanket fee for a year's Nitrox which is an extraordinary deal, so it makes it very reasonable financially for me. I do dive 32% on trips if I can, although I have shaken my head at myself for paying the extra for Nitrox to do 15 foot deep cave dives in Mexico

So, to answer the OP, there are disadvantages to diving Nitrox all the time if a) you want to do particularly deep dives; b) you are diving multiple times over multiple days, or c) if it substantially increases your cost for shallow dives where the increased bottom time is irrelevant.