What % Nitrox settting on dive computer?

[chrispete]

The point is that the calculations can be off. If you are diving in cool or cold water, use the reading on your SPG to get the amount of gas in your cylinder and then top it off using the tank that's been sitting in your truck bed nice and warm, you will end up with more O2 in your mix than the calc's will show. What if one of the calculations was done wrong? The bottom line is: Test your mix personally, it's not worth it to not test. Are you just trying to save $100 here or what? Buy the analyzer, most responses in this thread have told you that, but you refuse to listen.

Edited for add: Maybe I'm just a bit too conservative I guess...

 

[Big-t-2538]

chrispete once bubbled...
Are you just trying to save $100 here or what? Buy the analyzer, most responses in this thread have told you that, but you refuse to listen.

Edited for add: Maybe I'm just a bit too conservative I guess...

You also have to look at cost vs. benefit of buying an analyzer. I personally do not own one, but I know how to use one. I dive Nitrox on occasion. On those occasions, I analyze the tank at the shop before I leave...sign the log book, and go to the dive site. I will get them air topped, and I will guesstimate what is in the mix via calculation.

Now I also know what the MOD is of the site I'm diving, I also know ahead of time what the plan is, and I know that the MOD for 32 is deeper than any of the planned dives. The deepest quarry I dive Quarry is not deep enough to exceed 1.6 if I go to the bottom on dive 1 or 2 of the weekend with 32 in the tank...and I'm only going to get close to that if I've really really really F'd up somewhere along the way, my buddy has bailed and left me for dead, and the $h!t has already been rung through the fan twice.

What I am saying here is, and it is the same thing I said in the beginning of the thread, use your goddam head and think. Don't get hung up on details and just be safe.

 

[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.

As has been said, getting accurate readings, and having the fill station at a similar temperature to your tank is important. You will get an *approximation* of the mix in the tank this way. That being said, the advice of dive the most conservative MOD and most conservative N2 loading is very sage.

chipster once bubbled...
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: [/B]

You got it exactly backwards.

a. You can do the MOD calculation in your head/calculator/dive computer before you get in the water, and monitor it easily using your brain. Your intended depth should not approach your MOD. If you are waiting until you hear the computer beep at you to stop descending then you have a problem.

b. You cannot easily do the N2 loading in your head (GUE folks excepted), and that's where the computer can help you. However if you want the N2 loading to be conservative then you should round *down*, not up.

c. How does setting your computer to generate an alarm at 1.5 ppO2 give you wiggle room for N2 loading? I assume this is just a thinko? (like a typo..)

The only dissenting opinions are from your original post and ChrisPete's. There isn't a plethora of different conflicting answers on this.

That being said, you can probably get away with rounding either way without difficulty. Deco theory is a blunt tool. If you round up then you are intentionally erroring away from caution.

If you are using an inexpensive pressure gauge and doing air tops on a boat (or whatever) without an analyzer then you have a very real chance of exceeding your MOD, especially if you are intentionally diving to 1.5 on a mix that you unsure of.

If you'd like an inexpensive analyzer, take a look here. I believe the kit analyzer runs about $100. http://oxycheq.com/

Of course Big-T's advice is perfectly fine as well (know that you will not exceed the MOD of the richest gas).

Best of luck, and safe diving.

 

[OneBrightGator]

chipster once bubbled...
Glad I started this thread


Still lots of conflicting asnwers here, except maybe for the computer setting at 1.4 instead of 1.5. Yes Iantd excepts the standard as 1.5 then scale down under either a working dive or cold conditons. Cold being your are going to be cold not just the water temp. Because if I am wearing a dry suit I will not be cold.

Why does everyone here think calculating your gas is rocket science.

Ex. Start with a 30% mix at 3000# end with 1000# we will have 300# of O2 left . No topp it up and lets just it is now 3000# for ease of calculation. At 21% O2 we will have 420#. Thats 21% of the top up of 2000#. now we add the two and we get a total of 720#. Divide this by our total PSI which is 3000# and you get 24%. This is fact and you cannot change that unless you are diving on the summit of a mountain somewhere .

Sorry Chip... I gotta disagree with you... very infrequently do you have exactly 1000psi left in your tank... your average spg has increments of 250 psi, so I would say you'd be lucky to get within 125 psi. The same applies after your top off, you can't get an exact number.

Bottom line: if you can't analyze, dive it as air with original MOD, it's not worth the risk because none of these (analyzers, spgs, oxtox, etc) are exact sciences.

Ben

 

[chrispete]

The only dissenting opinions are from your original post and ChrisPete's. There isn't a plethora of different conflicting answers on this.

Actually, after I stewed about my opinion for a bit, I realized that I was in fact backwards and said so in a post a couple of posts below my original one...

 

[sheck33]

as stated by others already this is not exact science, 27.5% O2 or 28% O2, no difference there is no reason to even enter O2 fractions into a nitrox computer, not that there is a reason to use a nitrox computer in the first place. If your analyzer screen displays a mix of 32.45% would one enter that number into a nitroxcomputer? of course not, the measurement wasnt that accurate to begin with. Many people using simple calculators make the same mistake, believing all the beautiful digits displayed on the screen.
turn on the brain

 

[Atticus]

chrispete once bubbled...


Actually, after I stewed about my opinion for a bit, I realized that I was in fact backwards and said so in a post a couple of posts below my original one...

Oh! You're right. My apologies.

I take back what I said about ChrisPete agreeing with you, Chipster

 

[dagodiver]

im new here and dont know anybody but my two
cents is always round your computer up because
ox-tox will kill you period, but a niggle from dcs
can be fixed with a 5 minute safety stop if your
that concerned.
thats not in the books thats in your brain....

 

[Atticus]

Welcome to the board.

1. I'm not horribly concerned about nitrogen loading with a 1% error, but it is the one thing that the computer is actually useful for calculating on a recreational profile.

2. Using a computer to remind you to stay away from your MOD is a problem - one should be aware of their MOD and stay above it consciously - not because a computer is beeping at them.

3. If the concern is CNS %, please consider that it's difficult to approach 100% oxclock on a recreational, single tank, non-staged deco profile.

At the end of the day it's a question of how you manage your resources (of which the dive computer is just one), and how much you trust yourself to remember your MOD vs. needing an electronic reminder.

I will suggest that for non staged-deco diving that exposing oneself to ppO2's above 1.4 is not particularly useful. If you really want to go a little deeper then use a different mix.

 

[chipster]

Thanks everybody....

Don't slam me if I make a point, thats why I posted here to get some good sound advice. Ok If I understand the general consesus. Round down the computer mix setting. Calculate MOD using the rounded up mix.
Yes I watch my computer to check everything, not waiting for the alarm to go off. The same for air left in tank (present, just using Air intergration on Uwatec) going to buy back up analog preasure guage.
I have about 60 dives under my belt, and I just want to absorb as much info as I can to make safe decision on what I should and should not do.

THANKS AGAIN

GO DOWN, GET IT WET... COME UP FOR AIR AND GO DOWN AGAIN