Nitrox Common Mixes

[conway400]

Very helpful. Most of my desire to use is for the added safety (if we use air tables) or to go to a few sites that are a little deeper than we would normally go. I wish extending bottom time was my primary need, but I am usually limited there by my higher than desired air consumption rate.

 

[scubadiver888]

conway400, two reasons to end a dive are (1) you run out of air or (2) you reach your NDL. If you use nitrox it will extend the time until NDL. So if you are ending a dive because you are reaching NDL nitrox is beneficial.

If you are ending a dive now because you are running out of air then you are not getting close to the NDL for air, that in itself is a safety margin. I would believe using nitrox would be overkill. If you are close to your NDL or ending a dive because of NDL then using nitrox with air tables would make more sense to me.

Nitrox is not all benefit. Oxygen can cause convulsions. Underwater this can result in losing your regulator and drowning. The deeper you go the higher the concentration of oxygen you will get. Remember that at 33 feet you are breathing twice as much air as at the surface. At 66 feet you are breathing triple the air. Air becomes toxic well below 140 feet. So not a worry when breathing air. However, if you were breathing say nitrox 36 it can be toxic below 95 feet and nitrox 40 can be toxic below 80 feet. So air can actually be safer than nitrox for deep dives.

 

[Tortuga68]

Air becomes toxic well below 140 feet. So not a worry when breathing air. However, if you were breathing say nitrox 36 it can be toxic below 95 feet and nitrox 40 can be toxic below 80 feet

O2 can be toxic at much shallower depths. It's not quite as simple as <1.4 PPO2 is safe and above >1.4 may not be... a lot of other factors come into it like exposure and exertion, and the bottom line is that, like DCS, we don't really understand what the 'rules' are

Anyway... otherwise you're spot on: EAN is for extending bottom time, is of limited benefit if your gas consumption is high, & is not for diving deeper

 

[Mossman]

Actually, if "deeper" is considered the deeper end of recreational diving at 90-130', nitrox is excellent for "diving deeper". I often won't bother with nitrox on a shallower dive because the NDC limits are usually long enough with air, but welcome nitrox if I'm going below 90' because of the extended bottom time it allows, plus the superior off-gassing on ascent.

 

[Aqua-Andy]

Well most of the dives I do are beach dives that max out at around 60' and more often than not the avg. depth for the dive is in the 25'-35' range. So that would make my standard mix 21%, plus our dive shop has an air club $125 covers you for air fills for one year(ya I'm cheep). If I'm doing a deeper dive and I want the max bottom time I will use best mix. Around here if the dive is blown out, either the boat doesn't leave the dock or it will be a much shallower dive site.

 

[conway400]

All in line with what I am interested in. And yes, you each are correct. The 'deeper' that I mean is getting some useful bottom time out of the few places I would like to go that are at the end of the recreational limit - like the Oriskany. For shallower dives, I understand that me and air are fine - especially on the more taxing dives where air consumption is my limiting factor. Although taking up less nitrogen even then seems to be a plus.

 

[Colliam7]

What percentage mixtures are you commonly using and why? pros and cons?

When I use nitrox in NC, I use 30%, primarily because that is what our shop banks. I can blend whatever I want (my tanks are all O2 clean and I can use the shop fill station to blend when I want to), and there are times when I am going a little deeper, and will blend a lighter mixture (27%, for example). But, 30% is fine for the recreational diving I do.

Just wanting to learn more about certain limits placed on certifications (32 and 40 being common) and if it matters much to me as a rec only diver.

I am not sure I understand this statement ('limits').

 

[DaleC]

I prefer best mix (or whatever the guy is selling cheapest on the streetcorner) when I use nitrox, which is pretty rare these days. Air, it turns out, is often adequate for my needs.
For me best mix includes consideration of:

The need for extended NDL's or limiting nitrogen ongassing (same thing but looked at two different ways).
Nitrogen offgassing
The expected Max depth
The potential for exceeding Max depth

For a hard bottom dive to 100' where I need more NDL or I'm doing repetitive dives 32% might be best mix. On a wreck dive where the deck I plan to dive on is 100' but the bottom is 130' perhaps 28% might be best. If I'm altitude diving and want to limit my ongassing and don't need depth 40% might be best. I may stock air in my stage/pony, 32% or even 40% depending on how I see it's function.

I understand the rationale for standard mixes and don't argue against anyone using that method but I think it can lead to mental laziness in regards to dive planning if one is not careful. I also think it's a hard sell to nix computers in favor of BT's and ratio deco then insist best mix is too edgy for rec diving.

Your gas mixture is just another tool in your toolbox. Some people like to use a lot of different combinations of tools and some people just like to stock a hammer screwdriver and crescent wrench. Whatever you choose, just make sure you know how and when to use it.

 

[vladimir]

I almost always dive the resort or liveaboard's best effort at 32%, which is usually between 30 and 33%. That is all they offer, for the most part, without special arrangements, so the decision for me is usually between that and air.

 

[conway400]

By 'limits' I meant that the local SSI place here offers a nitrox cert that is limited to 32%. It has a through lab and test after the study work. But they also offer a 40% max cert if you include dives. Little more expensive and it is getting colder here. Not sure if this is standard everywhere (I haven't gotten the same answer twice when I call around), but I love this shop and their customer service.