Nitrox Question

[Jax]

I dislike this "auto-change to 50%" in that some people will actually believe a computer can 'keep' you 'from' doing anything.

I want my systems telling me facts, not making them up as I go.

 

[Cave Diver]

I dislike this "auto-change to 50%" in that some people will actually believe a computer can 'keep' you 'from' doing anything.

It seems to have kept the OP from going deeper.

I want my systems telling me facts, not making them up as I go.

Theres an old acronym. GIGO. If the computer is set correctly pre-dive, you'll get the correct information during the dive. Fail to set it and it will default to the most conservative setting. That's hardly making up facts.

 

[Jax]

I believe the OP decided to not go deeper.

There are many who would look at that and think - no way, not on 31%, and would ignore it.


Who would be "right"?

 

[Cave Diver]

I believe the OP decided to not go deeper.

There are many who would look at that and think - no way, not on 31%, and would ignore it.


Who would be "right"?

Why does someone need to be "right?"

But since you want to know, IMO, both would be right.

The diver who looked at it and was unsure exactly what was wrong, would be "right" not to put themselves further into a situation they didn't understand.

The diver who is experienced and goes "awww crap, I forgot to set my mix" but knows the tables for 32% and decides to dive based on that, relying on the computer only for depth and time info would be right also.

Who would be "wrong?"

 

[ScubaSteve]

If one does not possess the mental capacity to reason out what your computer is telling you then you should stick to tables IMO. If you are uncertain then call the dive. If you stick with dive out of ignorance then the results are yours to deal with.

 

[Lobzilla]

I believe the OP decided to not go deeper.

There are many who would look at that and think - no way, not on 31%, and would ignore it.


Who would be "right"?

The one who plans the dive and dives the plan. There is no excuse for jumping in the water without at least a rough plan in case the marvel of technology floods, hangs, runs out of power, etc.

 

[Majickyl]

Why does someone need to be "right?"

The diver who is experienced and goes "awww crap, I forgot to set my mix" but knows the tables for 32% and decides to dive based on that, relying on the computer only for depth and time info would be right also.

This happened to me not that long ago...
I was on 34% but my computer was set higher. I didn't have an exact number in mind for my NDL because I hadn't cut tables for what was essentially a shallow skills dive, but I knew:
(1) The dive plan was significantly shallower than my MOD.
(2) I would run out of gas before I came close to my NDL.

So with that info I knew my computer was just giving me garbage, but I'd be fine diving my plan until the gas was low.

Tricky thing to keep in mind in that situation is repetitive dives. For my next dive I still set my computer to the mix I was breathing, but I planned and dove the dive based on air tables to stay conservative in terms of nitrogen accumulation since I had accumulated more nitrogen than my computer knew about. As far as O2 accumulation, the computer was logging more O2 than I had actually accumulated so I was acting conservatively on that end as well.

Point is, you have to have enough understanding of the tables that if you can't count on the computer you know what to do. If you don't know for a fact that you're okay, like when the OP saw that pO2 climbing, you've got to err on the side of caution and call the dive.

And as you continue to dive, pay attention to your numbers. Experience will only teach you if you pay attention.

 

[Cave Diver]

Experience will only teach you if you pay attention.

And I'd be willing to bet this experience has taught the OP the value of verifying settings before their next dive.

 

[eponym]

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.

That's one heavy-duty feature. Not necessarily a bad thing, mind you. As John says above, It's good to check everything before each entry, yes?

But it makes my computers' default settings look loosey-goosey by comparison. They switch to 21% (but still EAN clocked) after 24 hours, which exposes you to the opposite danger.

In both cases, it's good to RCMP (read and comprehend the manual, please).

-Bryan

PS. If I were charged with designing this "safety feature" there's no obvious set point between the fO2 of the previous dive and either 50% or 100% . . . so I guess I can understand the manufacturer's choice.

 

[Cave Diver]

That's one heavy-duty feature. Not necessarily a bad thing, mind you. As John says above, It's good to check everything before each entry, yes?

But it makes my computers' default settings look loosey-goosey by comparison. They switch to 21% (but still EAN clocked) after 24 hours, which exposes you to the opposite danger.

In both cases, it's good to RCMP (read and comprehend the manual, please).

-Bryan

PS. If I were charged with designing this "safety feature" there's no obvious set point between the fO2 of the previous dive and either 50% or 100% . . . so I guess I can understand the manufacturer's choice.

Several manufacturers do this. Here is the wording from the Nitek Duo manual:

Understanding FO2 default
In your entry-level Enriched Air Nitrox training, you learned that among the greatest risks Nitrox (EANx) use poses is CNS oxygen toxicity. CNS oxygen toxicity can occur if Nitrox is breathed at depths where the partial pressure of oxygen (PO2) exceeds safe limits.

The NiTek Duo is designed to help divers avoid such situations by presenting information and warnings regarding current PO2 levels and cumulative exposure to elevated PO2s. To do so, the NiTek Duo must be set to an FO2 that accurately matches the concentration of oxygen in the gas mixture being breathed.

Similarly, the NiTek Duo is designed to help divers avoid decompression illness by providing no-decompression limits (NDLs) or mandatory decompression stop information. Again, to do so, the NiTek Duo must be set in a manner that accurately matches the concentration of nitrogen being breathed.

When this information is not available, the NiTek Duo attempts to protect divers by basing its oxygen- and nitrogen-exposure calculations on a “worst case” assumption. This is, that calculations are based on a mixture containing up to 79 percent nitrogen and 99 percent oxygen. This “worst case” calculation is what is referred to as the FO2 default setting.