First Nitrox and computer alarmed

Basics:

I have over 100 dives and I have always dived Air until this weekend. I switched to a steel 120 with Nitrox and had my first dive with it offshore Savannah, Ga saturday.

Conditions were not ideal, 4 ft chop and visibility was 8-10ft (normal is about 25-40ft)

Planned depth was 70ft (wreck starts at 55ft and bottom is at 70ft) for a bottom time of 40 minutes. The mix was EAN 32 and I was spearfishing. That would have kept me well within limits.

At frst I struggled a bit with bouyancy because of the new tank but I got that sorted out pretty quickly, I was just more negative than I planned for. The current was pretty bad and I was paying much more attention to the fish then my computer (which is an Uwatec Aladdin by the way and it is NOT air integrated)

So at 25 minutes into the dive my computer beeped and the O2 percentage flashed at 75%. I had never had that alarm and wasn't sure what it meant. I got my buddy's attention and signaled an ascent. I ascended perfectly with a 3 minute safety stop at 15ft

I actually did feel very tired and had a headache. I attribute a lot of that to the conditions and having to work so hard, and the new tank.

I downloaded the dive and went through through the details. at 25 minutes into the number in the bottom right said 75% and then on ascent 2 minutes later it said 100%. During the dive it was all over the place from 7% to typically around 30%.

Does anyone have an idea why the toxicity alarm would go off when I should have been well within limits?

I will b glad to email the screen shot of the dive profile.

... did you correctly input the mix?

Congratulations on your first dive with a steel tank. As you've realized, it does take a little getting used to. The weight benefits are noticeable in that you can shed some weight from a weight belt/integrated weight pockets.

Initially I would suggest that you check to make sure you entered your mix properly. Some computers will defalut to a mix that maximizes your oxygen and nitrogen content if the mix is not entered correctly (default to 50% oxygen/79% nitrogen). This was done as a built in safety factor to prevent divers doing repetitive dives from absorbing too much oxygen or nitrogen by placing them in the "worse case" scenario. My Nitek computer used to do that. It was really only an issue during the repetitive dive. Did you dive with it in the pool first? Was there another mix programmed into the computer prior to your first OW dive with it? Assuming the electronics and internal components are working correclty, I would start there. From the dive you have described, the alarms should not have gone off giving you those oxygen readings.

I have used this computer for quite a while and it was programmed for 21%. I changed the mix to 32% percent prior to the dive and double checked the setting on the boat after the dive. I am going to run by my local dive shop and show the profile to the owner (where I bought the computer) and see what he has to say. I just want to be sure that I'm not missing anything before I dive that plan again.

ScubaDocER:

(default to 50% oxygen/79% nitrogen)

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So you have 29% extra gas in your tank! And I thought my dive computer had a stupid algorithm. One more reason to dive gauge mode.

ScubaDocER:

Some computers will defalut to 50%

Click to expand...

This is how my Oceanic Geo is if you dont set it correctly. I also see you said you checked it again once back on the boat, so I assume this is not the case. I would like to know the outcome, please keep us posted.

Hi there,

I just went through my recent dives on a Liveaboard, on a day that I did 4 relatively deep dives, the data was as follows.

Dive # - 1
O₂ - 32%
Surface Int - n/a
Max Depth - 30.3m
Ave Depth - 17.7m
Dive Time - 42 min
CNS After - 12%


Dive # - 2
O₂ - 32%
Surface Int - 03h26
Max Depth -25.9m
Ave Depth - 11.9m
Dive Time - 46 min
CNS After - 9 %


Dive # - 3
O₂ - 32%
Surface Int - 03h45
Max Depth - 24.7m
Ave Depth -15.2m
Dive Time - 48 min
CNS After - 12%


Dive # - 4
O₂ - 32%
Surface Int - 03h01
Max Depth - 17.1m
Ave Depth - 9.9m
Dive Time - 47 min
CNS After - 7%

That might give you an indication of my CNS from those profiles, I expect (hope) that is pretty normal.

If you would like any other data from the dives let me know and I'll pull it up.

Best Regards
Richard

PS : As far as my understanding and experience with CNS, it will rise as you dive and reduce between dives, it should (under normal circumstances) be a relatively smooth climb and drop, there should be no "bouncing around" during the course of a single dive. If I am mistaken, I trust the whizz kids will see me right.

Sounds like you are confusing the ascent alarm with the O2 %?

Mr Carcharodon:

So you have 29% extra gas in your tank! And I thought my dive computer had a stupid algorithm. One more reason to dive gauge mode.

Click to expand...

No, the mix in the tank is whatever it is: 32%, 36%, 40%, AIR, who knows (precisely the point)! Since the computer has not been set by the diver, the algorithm is based on the worst possible scenario for both gasses. The Oxygen calculations are based on a mix containing 50% oxygen content (presumably the worst case scenario), and the nitrogen calculations are based on 79% nitrogen content (the worst case scenario). Most computers manufactured in the last 5 years or so have this function as a user selection. The user can either choose to have the computer default to this "impossible" mix, or they can choose to have the computer stay set on whatever previous mix was set.

SparticleBrane:

Sounds like you are confusing the ascent alarm with the O2 %?

Click to expand...

Oh yah, great point.

If his computer uses the portion of the LCD designated for CNS% to also show ascent rate warnings (often displayed as percent of maximum allowable), he could just be confusing them.

Best thing is to download the logs (if possible) and see what it was saying. If it really thinks you were at 75%, check the mix it assumed.