Nitrox Question

[Laurie S.]

I took my nitrox class more than a year ago through SSI and until last month had not had the opportunity to dive nitrox. I just returned from a two-week dive trip in French Polynesia and was able to use it six times. The first day, in Tahiti, the divemaster went over everything again with me as we checked the percentage and I set my dive computer. I told him it had been a year since my class. My dive buddy and I had two great dives that day with no problems. The computer remained at 1.4 for the PO2 reading.

Two days later, at Rangiroa, we again dove nitrox. Again, we checked the percentages and I set my computer. The first dive was in Tiputa Pass (yay, we did see a hammerhead), where I think we were at about 80 feet or so. The computer had indicated a maximum depth of 116 for the nitrox concentration. However, on the next dive, which was in the 60-70 foot range, I noticed that the reading started climbing from 1.4 to 1.6 to 1.7, all the way to 1.81. When I saw that, I immediately went shallower and watched the gauge until it again reached 1.4. I also signaled to my buddy (who has no computer) to get up with me. The divemaster remained at the lower depth, but she had not been on the previous deeper dive. She finally looked at her gauge and brought everyone else up with us.

Please forgive a nitrox newbie question, but am I understanding this right? Did we exceed the nitrox safety levels with the second dive even though both dives did not individually exceed the safe depth levels? We again dove nitrox a few days later at Moorea but had no such problems there. We also weren't as deep.

 

[MauiScubaSteve]

Need more input!

Is the computer you used your computer? What computer are we talking about? What percentages of Oxygen are we talking about (for each dive)?

I'm assuming your buddy was not using EAN since they had no computer.

 

[Cave Diver]

The computer had indicated a maximum depth of 116 for the nitrox concentration. However, on the next dive, which was in the 60-70 foot range, I noticed that the reading started climbing from 1.4 to 1.6 to 1.7, all the way to 1.81. When I saw that, I immediately went shallower and watched the gauge until it again reached 1.4.

My *guess* is that the mix on your computer got reset or changed. Some automatically default to a certain mix if you don't set it for every dive.

Hopefully your class covered the formulas for how to calculate the mod (max operating depth) for a given mix and you manually confirm that after analyzing your mix and before diving it.

 

[Majickyl]

What mix of nitrox were you using? What kind of computer do you have? It doesn't make sense to me why your pO2 should have been climbing that much if you were staying within a 10 ft window at 60-70 ft.

On EAN36, 60 ft is a pO2 of 1.0 and at 70 ft it's 1.1.
On EAN32, 60 ft is a pO2 of 0.9 and at 70 ft it's 1.0.

So the pO2s you were seeing don't make sense unless your mix was set high or your depth was much deeper.

On EAN50, 60 ft is a pO2 of 1.4 and at 70 ft it's 1.6.
A depth of 86 feet on EAN50 would get your 1.81.
I have heard of computers that default to EAN50 for a repetitive dive (I think Aeris computers do this). Perhaps that's what happened?

Yes, you can approach oxygen saturation with repeated dives on nitrox but that is generally expressed as a % exposure. And it takes A LOT of diving close to MOD to get anywhere near that, not 2 dives.

And ditto what CD said... a GOOD nitrox class should have covered the formulas to calculate MOD, pO2, best mix, etc. You should be able to calculate these yourself and make a practice of doing it, not relying on the computer... otherwise you're a few wrong button pushes to ox-tox.

 

[Jax]

The computer had indicated a maximum depth of 116 for the nitrox concentration. However, on the next dive, which was in the 60-70 foot range, I noticed that the reading started climbing from 1.4 to 1.6 to 1.7, all the way to 1.81.

Something isn't adding up.

A Maximum Operating Depth (MOD) of 116, assuming a PO2 max of 1.4, says your gas was 31% . . . or that was what was put in for planning . . . .

116 / 33 = 3.5, +1 = 4.5ATA . . . PO2 / PT = FG so 1.4 / 4.5 = 31.1111%



However, if your computer ran 1.8 PPO2 at 70 feet:

70 / 33 = 2.12, +1 = 3.12 ATA

1.8 / 3.12 = .5769, or 58% or EAN58.

Good for you for doing the right thing and ascending! You might want to think long and hard about how the numbers got into your computer. This could have been very bad for you.

(Someone check my numbers, please . . . . )

 

[Laurie S.]

I don't have my dive log here at the office, but I believe the concentration was at 31 percent. I use an OC-1, and, yes, my dive buddy was diving nitrox without a computer (sigh.....). I didn't make any changes to my computer between the two dives as both tanks were at 31 percent. I was just curious about this so posted now. I'll check the readings on my computer and in my dive log tonight and post them. I really didn't think two dives would create a problem, but it sure scared me when I saw those figures start climbing.

 

[Jax]

BTW, Laurie; the best manual for really understanding Nitrox, IMO, is the TDI Nitrox manual. Not the advanced Nitrox, but the TDI basic Nitrox. If you can work your way through that, and do the tests without consulting the book, you will understand Nitrox.

(speaking from experience with 3 other agencies)

Please forgive a nitrox newbie question, but am I understanding this right? Did we exceed the nitrox safety levels with the second dive even though both dives did not individually exceed the safe depth levels?

To answer this question, you need to state what was the mix you analyzed / dived.

There are two parts here: What is the truth - what mix did you dive to what depth -- and what was put in the computer?

Cave Diver stated one possibility - your settings were somehow changed. The second possibility is your computer had a glitch. Me, personally, I would never dive that computer again, if this were true.

 

[Majickyl]

The Oc1 has a 50% Defaut feature where, unless you turn this feature off, 10 minutes after your first nitrox dive it will set to EAN50 automatically.

Page 87 of this manual...
http://www.oceanicworldwide.com/pdf/12-2761-r02 Eng.pdf

My guess is when you review the log the mix will be set to 50 and the max depth for that dive will be closer to 86.

You should be able to turn that feature off, just be sure to check your mix before every dive!

 

[Cave Diver]

I don't have my dive log here at the office, but I believe the concentration was at 31 percent. I use an OC-1, and, yes, my dive buddy was diving nitrox without a computer (sigh.....). I didn't make any changes to my computer between the two dives as both tanks were at 31 percent. I was just curious about this so posted now. I'll check the readings on my computer and in my dive log tonight and post them. I really didn't think two dives would create a problem, but it sure scared me when I saw those figures start climbing.

The manual for that computer says on pg 86 that it will default to 50%. That's my guess as to what happened to you.

http://www.oceanicworldwide.com/pdf/12-2761-r02 Eng.pdf

 

[Lobzilla]

The Oc1 has a 50% Defaut feature where, unless you turn this feature off, 10 minutes after your first nitrox dive it will set to EAN50 automatically.

Page 87 of this manual...
http://www.oceanicworldwide.com/pdf/12-2761-r02 Eng.pdf

My guess is when you review the log the mix will be set to 50 and the max depth for that dive will be closer to 86.

You should be able to turn that feature off, just be sure to check your mix before every dive!

Other Oceanic models like the Atom 2.0 and VT3 have this (IMO stupid) feature as well. You can turn this feature off.

If or when the computer still goes berserk during a dive you realize how vital it is to have the important numbers (MOD, max BT, and later deco schedule) in your head.