What % Nitrox settting on dive computer?

[Genesis]

which you have not personally analyzed, or witnessed being analyzed.

Yes, the situation posited, where you had a tank of a known mix and topped it with air, can be "kinda safely" dove, but to do it you really have to treat it as air for the Nitrogen content, and the original mix for the O2 content (MOD), in which case you may as well dump the tank and refill with air anyway.

For the original question, round DOWN on O2 percentage, and do not invade the MOD of the original mix. You should not need a PO2 alarm to know when/'where that is. If you do, you need to be rethinking your dive planning....

 

[Atticus]

chipster once bubbled...
Wow talk about conflicting responses...

with regards to having in analyzed, not always possible if you have a nitrox fill from the LDS than go somewhere where they do not have nitrox. Must to a calculation derived from the amount of top up of air in the tank for dive #2. I would tend to agree with mine and others here that rounding up would be more cautious then rounding down. Would think that if I rounded down might accidently exceed MOD. Also I have some wiggle room for % of obsorbing N2 as I have my computer set at 1.5ATA not 1.6. Needless to say it sounds here, that one should be cautious with there MOD If rounding up, and on the flip side dont push the max BT if rounding down.
Sound about right???????:doctor:

1. When you say you have some wiggle room for N2 because your computer is set to 1.5ata I get worried. I assume the 1.5ata is the maximum ppO2 setting. This isn't going to affect your N2 loading!

2. 1.5ata is actually a little high for recreational diving. 1.4ata is the standard number.

3. Rounding down when programming the computer is more conservative (NOT up). The primary job of the computer is to track nitrogen loading.

4. You should compute and note your MOD. Your dive profile should be well above your MOD. You should not need your computer to tell you when you approach the MOD.

5. For recreational single tank 40% (or less) 1.4 ppO2 max nitrox diving it's challenging to come up with profiles which create CNS % and/or OTU problems. At 1.4 ppO2 I believe the NOAA tables give you 150 minutes max per dive and 40%/hour. Even doing three boat dives a day it's hard to manage. If you believe in half time credit for oxygen during surface intervals then you're extra peachy.

6. For the top off scenario I would do as suggested - set the computer to AIR, but use the MOD from the original/richest mix. I often do this.

 

[diverbrian]

Genesis once bubbled...
which you have not personally analyzed, or witnessed being analyzed.

Yes, the situation posited, where you had a tank of a known mix and topped it with air, can be "kinda safely" dove, but to do it you really have to treat it as air for the Nitrogen content, and the original mix for the O2 content (MOD), in which case you may as well dump the tank and refill with air anyway.

For the original question, round DOWN on O2 percentage, and do not invade the MOD of the original mix. You should not need a PO2 alarm to know when/'where that is. If you do, you need to be rethinking your dive planning....

For instance, a couple of weekends ago, I was diving doubles that were EAN28 and I had them topped off with air from a dive a few weeks back (for 10 bucks, I couldn't say no ). I was diving Gilboa Quarry for my next dive and knew it. The bottom there is 132 ft. (maybe more depending on rain). Obviously MOD there is not an issue. I set my Cobra and VyTec to EAN21 and dove them them knowing that I had EAN24 or EAN23. This added a safety margin to my dive. The place that topped them off doesn't do nitrox (not uncommon up North) so I could not analyze the tanks. I don't own an analyzer. As it turned out, all of the divers at the quarry were diving air anyways (not Nitrox certified yet), so I was sort of restricted by their limits. It worked out well and I had an extra safety margin (which was nice as we did have a couple of minor incidents with the divers that I was with on that trip) and I didn't have to entirely waste an earlier nitrox fill by venting the tanks.

 

[OneBrightGator]

-Set your computer O2 to 26%

-Set your computer ppO2 to 1.4

-Know your MOD for 27% and don't push it

-Use your tables for CNS if you're worried about it

-If you top-off and don't analyze, dive MOD for original mix, O2 of 21%

-Review your nitrox procedures... throughly...

Ben

 

[DeepTechScuba]

You should have bought a Suunto, then you could dial in your EXACT nitrox fraction.

Now you are betwixt and between.

Either your computer computes your N2 loading right, or else it is set to the proper ppO2 warning limit.

Bad purchase. Take it back and see if they will take it in trade for a better nitrox computer.

 

[Atticus]

While most dive computers do allow for 1% increments in oxygen content, I wouldn't toss the computer you have just based on this. The analyzer probably has a 1% error in it as well, and unless you're good about letting the gas completely homogenize you probably have some error in that respect as well.

I certainly wouldn't give up on the computer just because you have to set it 1% lower half the time (the other half the time being when you have an even % of o2 in the analysis).

With respect to the ppO2 alarm and CNS %, I would manually calculate my MOD and stay *well above* it regardless of what the computer says, and I personally think new recreational nitrox divers spend more time worrying about CNS % than is justified given the profiles they run. If you're really worried about it assume a square profile and use the tables.

I've had Oceanic, Suunto, and Dive Rite nitrox computers, each of which offer 1% resolution on O2. I wouldn't change my mind about any of the purchases if the resolution was 2% (unless there were other issues that I didn't like).

 

[MikeFerrara]

I think I saw the right answer in here someplace. What are they teaching in nitrox classes these days. LOL

Know (in your head) what your MOD is. If you comming anywhere near CNS limits on recreational dives I would be very surprised. The only time I come close is when decompressing on 100% at 20 ft and how we handle that's a whole nother story.

Round down for N2 tracking. However, it's not that exact of a science in the first place so if you're close enough to the edge (whatever that is) that a percent or two makes a difference you're either doing something wrong or just kidding yourself. I the computer has settings every two percent the most you can be off is one percent right? Analyzers aren't that accurate anyway.

Like Genesis said with top offs the safest is to treat it as air for N2 planning and the original mix for tracking O2 exposure and MOD. Although, if you think about it...If the mix was 28% like in this example and you topped with air you know the MOD has to be deeper than 130 ft right? Is that even a factor for the diving you're doing. And again, I can't see you comming anywhere near the CNS limits.

I think computers fool people into thinking there is any real precision in all this.

 

[Atticus]

MikeFerrara once bubbled...
...I think computers fool people into thinking there is any real precision in all this.

I agree completely.. Uh oh ... Does this mean it's time to drink some cool aide?

Happily diving my computers as BT's and backup

Cheers,
Merlin

 

[Big-t-2538]

Atticus once bubbled...


I agree completely.. Uh oh ... Does this mean it's time to drink some cool aide?

Not yet...you have to spell it right first.

 

[Atticus]

Big-t-2538 once bubbled...
Not yet...you have to spell it right first.

:wacko:

Yer right.. Then again, there is this product http://rexfordrand.com/CoolAide.htm which claims to be a lubricant, perhaps that's what the dir guys are using to inject (ahem) their ideology.

That's tongue in cheek, no one go and get offended now ...