Nitrox for alitude compensation

[Wijbrandus]

My local diving is at 6000 ft., and toasty warm (55F), with Carribean vis (10').

I've very recently been certified on Nitrox, and one of the advantages that was mentioned was that you can use Nitrox to make your dive non-altitude adjustable. Meaning diving on a certain mix makes the dive equivalent to a sea-level dive on air. Where can I find out more about this? Is there a table, or am I doomed to doing the math?

Also, on a related note, I'm considering diving this weekend on 40%. My reasoning for this is that the pricing is only four bucks more for the tank, I'm not going deeper than 45 FFW, and I like the conservative element when I plan the dive on the altitude adjusted air tables. A friend of mine said there's no point to doing so. I figure even if there's no benefit, it absolutely can't hurt. Opinions?

 

[Charlie99]

See the article at http://divermag.com/archives/dec2000/divedoctor_dec00.html .

The Diving Doctor comes to the conclusion that diving EAN33 to 70' at 6,000' altitude is equivalent to diving air to 70' at sea level.

 

[DA Aquamaster]

On the one hand you have a theoretical depth at altitude that is greater than your actual depth. On the other hand you have an equivalent air depth on nitrox that is less than your actual depth.

Obviously there will be altidue and nitrox combinations where the two concel each other out.

But...you really need to fully understand both nitrox diving and altitude diving to do a high altitude nitrox dive safely and you also need to take each portion (nitrox and altitude) of the dive into account separately.

 

[Dive-aholic]

Diving to 45ffw at 6000' elevation gives you an equivalent depth of 62'. A 40% mix gives you a MOD of 80' (I think - I don't have my tables with me; I also think that might be deco MOD, not working MOD). So it sounds like it's a prudent choice.

Heed DA's advice. What's not prudent is feeling "doomed" to do the math. It's your life. If you trust someone else, your dive may be doomed. Learn the math. After a few times doing it, it will be 2nd nature. It's your life. You don't want to exceed your MOD and go into O2 tox and have a siezure underwater. If you're not willing to do the math, don't dive nitrox.

 

[TeddyDiver]

Do the math, but get a dive computer (with the altitude and Nitrox)!

 

[Wijbrandus]

80' is the working Mod of 40% (1.4 PPO2). I'm not going to be anywhere near it. I understand how to plan my nitrox diving, I understand how to make my altitude adjustments, and I understand how to combine the two.

Look, don't read more into my commentary than what's there. Yes, I was exaggerating when I said "doomed". It's not that big of a deal to run the numbers. I was primarily wondering if there was a table that said x% = x Altitude. There isn't. No big deal.

I ran the numbers yesterday while PMing with Charlie99. 33% is what cancels out my altitude. Is that a rational percentage to ask for when getting tanks? Not likely. I asked last night when I went to pick up some gear from my LDS, and the guy just looked at me like I was crazy. I was just asking, yet I walked away feeling like an idiot for even asking the question. Seems to me all they want to fill is 32 or 36, which works too. I just wonder how often they get asked for an alternate fill.

 

[DA Aquamaster]

Locally 32% is probably the most popular choice. It fits well with the 5000 ft altitude of the lake of choice and from an MOD standpoint, it allows relatively deep diving.

Asking for 33% instead of 32% anywhere would be a little odd. The accuracy of most analyzers is about 1% so you are asking for a difference that is virtually within the expected range of error anyway.

 

[Dive-aholic]

80' is the working Mod of 40% (1.4 PPO2). I'm not going to be anywhere near it. I understand how to plan my nitrox diving, I understand how to make my altitude adjustments, and I understand how to combine the two.

Look, don't read more into my commentary than what's there. Yes, I was exaggerating when I said "doomed". It's not that big of a deal to run the numbers. I was primarily wondering if there was a table that said x% = x Altitude. There isn't. No big deal.

I ran the numbers yesterday while PMing with Charlie99. 33% is what cancels out my altitude. Is that a rational percentage to ask for when getting tanks? Not likely. I asked last night when I went to pick up some gear from my LDS, and the guy just looked at me like I was crazy. I was just asking, yet I walked away feeling like an idiot for even asking the question. Seems to me all they want to fill is 32 or 36, which works too. I just wonder how often they get asked for an alternate fill.

No offense meant. It was just a little concerning. And not knowing exactly what you meant, I'd rather read more into things. Better to be safe.

Anyway, like DA said 32 and 36 are the norms. PADI produces tables for those specific mixes. They also come premixed. If your LDS is banking mix then trying to do a PPB means work, and if you don't do it on a regular basis, it can take a while.

But some LDSs won't give you the problems you got. Last time I went to one of my LDSs for Nitrox, I told them I wanted 32%. When I picked up the bottles, the owner told me it took him 5 hours to get the fills because he kept getting the mix off. I told him I would have been okay with 33 or 34 and he looked surprised.

The only time mix needs to be exact is if you are diving a specific site at a depth that may exceed an MOD (for deeper depths within NDL you would get 28%) or when planning deco dives and you need a specific mix greater than 40% for a specific deco stop.

Hope this helps.

 

[Wijbrandus]

None taken, I got your point. Thanks for your time to respond.