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dannybot
11-30-2007, 19:43
Does anybody dive nitrox on an air computer? Can you actually get more bottom time an an 80 cf al tank? Just curious.

Defman
11-30-2007, 20:15
Ummmm.... No to both questions.....

Assuming that you're minding the reduced depth limits that come from Nitrox.... the computer in air mode (21% Nitrox) is going to assume you are taking on more nitrogen than you really are. Actually, the reverse will happen in regards to question 2.

WV Diver
11-30-2007, 20:20
If you dive nitrox (above 21%)with your computer set on 21% what you will gain is a conservative buffer against DCS concerns, not more bottom time.

Navy OnStar
11-30-2007, 20:30
Diving Nitrox on an air computer can be dangerous and lead to Oxygen poisoning. Our bodies can't handle high partial pressures of Oxygen. If you aren't diving the Nitrox tables or a nitrox computer you can have partial pressures of Oxygen get too high and it could lead to seizures and death. Make sure you take a class.

bperrybap
11-30-2007, 21:54
I'm assuming perhaps the main idea behind your question was that since
the air in nitrox has more oxygen in it than regular air would you be able to
breath slower and therefore be able to stay down longer? The quick answer is no.
Just as your body needs oxygen, it also needs to purge the CO2 it generates.
It is that need to purge the C02 that is responsible for most of our desire
to breath.
The brain mainly uses the C02 build up to detect that its time to
take a breath because the only way we can expel the C02 is to take a
breath and breath it back out.
So while nitrox does have more oxygen in it, your body still needs to
purge the CO2. Each breath will be the same size and just as often
when breathing nitrox as breathing air so the nitrox will be breathed
just as fast and won't last any longer.


Yes there are folks that dive nitrox without diving a "nitrox" computer.
Some of these people do have nitrox computers but set them to "air" mode
which is the same as diving on a air only computer.
When diving in this mode you don't extend your NDL time because your
computer still thinks you are diving on air. Some people do this to add
an extra margin of safety against DCS. The idea being that they will be
less loaded with nitrogen and thus less likely to take a hit.
When doing this, you have pay close attention to your depth, because the
higher the percentage of oxygen in the air you breath, the shallower the depth
that the air becomes toxic and the computer won't warn you because it
thinks you are diving air.
For normal air, that depth is deeper than most people go
(around 200ft or so). For Nitrox that depth can be cut in half
and starts to land where recreational divers do go.
Jump in with nitrox/EAN 32 and try to do Devils throat in Cozumel and you
could die as the 140-150ft depth is deeper than you can dive on nitrox/EAN 32.

Nitrox reduces your rate of nitrogen buildup and extends your NDL time.
But bottom time is not just limited by nitrogen buildup (NDL time),
It can also be limited by oxygen exposure, and more importantly air supply.

If a person is running out of air before they get near their NDL limit,
nitrox will not extend their bottom time at all.
Nitrox helps extend your bottom times when you are coming up because
your were reaching your NDL time and yet still have lots of air left.

There is lots of interesting information covered in a Nitrox class.
Ok perhaps more interesting to scientific types, but still quite useful.
If you have the extra cash, and time, you might consider taking
a nitrox class sometime.

--- bill

dannybot
12-01-2007, 07:57
This was actually intended as two different questions:
1) who dives nitrox on an air computer?
2) Using an aluminum 80 cf tank, it there any real advantage to diving nitrox?

Sorry for the confusion, appreciate responses.

Navy OnStar
12-01-2007, 10:36
The advantage is you can stay down longer not because you have any more air to breathe but because the nitrogen loading in your tissues is less. The limit of your bottom time may still be the size of your tank but on a day where you do several dives you will find that you can have more bottm time with less surface interval due to a lower residual nitrogen in your tissues.

Puffer Fish
12-01-2007, 11:04
This was actually intended as two different questions:
1) who dives nitrox on an air computer?
2) Using an aluminum 80 cf tank, it there any real advantage to diving nitrox?

Sorry for the confusion, appreciate responses.


To the first question... I would hope not... you are trading lower Nitrogen for potentially other issues with O2 (depending on the depth). Part of the "computer" part of a computer is indicating limits you need to stay less than or above.

Regarding the second...

1. First it depends on your air consumtion rate..for most new divers, it is usually a no for one dive.

2. Second it depends on the depth of the dives you are making.

3. It depends on how many dives you are making, and how long between dives.

On a single tank, with a surface air rate of say .6 cubic ft/min, an 80 cubic ft tank (with a safety margin) at 99 ft, would only last around 20 minutes (an AL80, by the way, does not hold 80 cf at 3,000psi). Mine is .4... I know one person with a .25 (and yes... I do hate her for that). At .25... that is 53 minutes... so yes she could easily.

Do several dives, and then everyone but the biggest air vacuum will benefit.

Please note, that the size of the most common tank is no accident... it is a very practical size for most people to avoid issues with.

cummings66
12-01-2007, 11:25
I wish I could add something, but pufferfish has said it all and as far as I'm concerned he's hit the topic all over the place, nothing more to say other than he's right and I agree with it.

Damselfish
12-01-2007, 11:42
1) who dives nitrox on an air computer?
2) Using an aluminum 80 cf tank, it there any real advantage to diving nitrox?

1) I did for a bit until I got a Nitrox computer. And some people will set a Nitrox computer to 21% or something lower than the mix to be extra conservative - I was even on a liveaboard years back that requested you do this.

It's true you need to be careful of your oxygen limits since the computer won't be telling you these in this case, but this is pretty easy to do without a computer. It's easy to know your depth limit and not exceed it - if you need an alarm to tell you this IMO you shouldn't be in the water. And exceeding your cummulative O2 exposure limit is actually pretty difficult to do in rec diving.

2) For a single dive if your air consumption isn't that great yet, not probably not. (But many people can easily exceed air NDLs on a single 80, I know I can.) For almost anyone doing a bunch of repetitive dives in a day, probably. In between it all depends.

Puffer Fish
12-01-2007, 11:59
I wish I could add something, but pufferfish has said it all and as far as I'm concerned he's hit the topic all over the place, nothing more to say other than he's right and I agree with it.
I would be happy to delete the post and let you say it...you have a much more powerful avatar..

Puffer Fish
12-01-2007, 14:51
1) who dives nitrox on an air computer?
2) Using an aluminum 80 cf tank, it there any real advantage to diving nitrox?

1) I did for a bit until I got a Nitrox computer. And some people will set a Nitrox computer to 21% or something lower than the mix to be extra conservative - I was even on a liveaboard years back that requested you do this.

It's true you need to be careful of your oxygen limits since the computer won't be telling you these in this case, but this is pretty easy to do without a computer. It's easy to know your depth limit and not exceed it - if you need an alarm to tell you this IMO you shouldn't be in the water. And exceeding your cummulative O2 exposure limit is actually pretty difficult to do in rec diving.

2) For a single dive if your air consumption isn't that great yet, not probably not. (But many people can easily exceed air NDLs on a single 80, I know I can.) For almost anyone doing a bunch of repetitive dives in a day, probably. In between it all depends.
I would strongly recommend not doing that...It seems like a very safe thing to do, and on a single tank would work reasonably well, but if you are making multiple dives...your computer will now be off, and if diving with others... leads to some terrible compromises...do you ignore a deco obligation to stay with your buddy? Guess at what the correct time should be?

Matt P
12-01-2007, 18:04
I have (once or twice) dived Nitrox with my Nitrox computer set to air. I've only done this when diving with a buddy who is on air and there is physically no possible way of exceeding the mix's MOD such as when I'm at the quarry.

dannybot
12-01-2007, 18:56
Thanks a lot for all of you input, I was thinking of taking a nitrox calss next month, becaude the next trip I'm taking offers free nitrox. I have a nitrox computer, but my wife doesn't. Just wieghing pros and cons of the calss, the diving, a new computer.....

ianr33
12-01-2007, 19:55
If you are going to be doing multiple dives each day on your trip take the class.Always good to learn stuff.

I would have no problem diving nitrox on an air computer if that is all I had,BUT you do need to understand PO2 issues.

Assuming you are diving 32% with single Al 80's and never go below 110 feet then oxygen toxicity is not an issue. Bounce down to,say,150 feet and it could be a very bad day.

If you dive nitrox up to the air NDL's you will have a large safety cushion.

Damselfish
12-01-2007, 21:13
1) who dives nitrox on an air computer?
2) Using an aluminum 80 cf tank, it there any real advantage to diving nitrox?

1) I did for a bit until I got a Nitrox computer. And some people will set a Nitrox computer to 21% or something lower than the mix to be extra conservative - I was even on a liveaboard years back that requested you do this.

It's true you need to be careful of your oxygen limits since the computer won't be telling you these in this case, but this is pretty easy to do without a computer. It's easy to know your depth limit and not exceed it - if you need an alarm to tell you this IMO you shouldn't be in the water. And exceeding your cummulative O2 exposure limit is actually pretty difficult to do in rec diving.

2) For a single dive if your air consumption isn't that great yet, not probably not. (But many people can easily exceed air NDLs on a single 80, I know I can.) For almost anyone doing a bunch of repetitive dives in a day, probably. In between it all depends.
I would strongly recommend not doing that...It seems like a very safe thing to do, and on a single tank would work reasonably well, but if you are making multiple dives...your computer will now be off, and if diving with others... leads to some terrible compromises...do you ignore a deco obligation to stay with your buddy? Guess at what the correct time should be?
Yes the computer will be off, but on the conservative side. It is a very safe thing to do. Just as safe as diving Nitrox on air tables. Some people choose to do that. What terrible compromises? And who said anything about deco obligations? If one person in a team has a Nitrox computer and the other has an air computer, the Nitrox computer should be set to 21%, or if not the air computer wins.

in_cavediver
12-01-2007, 21:36
I think the issue has been muddied a bit especially with regard to air on nitrox computer.

First, what is needed by a computer/person to plan a nitrox dive:
a) NDL info (N2 info if you will)
b) O2 info

Either could be a controlling factor on the dive.

If you dive a computer with the mix set to something other than what your breathing, you will need to manually make some calculations.

For instance, an Air computer when breathing Nitrox:

a) NDL infomation will be more conservitive since the computer is assuming more N2 uptake than actually occurs. This may seem to be a safety advantage but statistically, its not. If you fall in the 'outliers' of the statistics, it may be significant advantage to you.

b) O2 Information. The Air computer (or nitrox computer set to air) will have UNDERESTIMATED the o2 exposure. It is NOT safe to use this as the only means of calculating exposure limits. You must do this by hand for each dive and it may be possible for your computer to give you more NDL time when your out of O2 time.

The good news is that it is very difficult to run out of O2 time on most rec diving profiles. Its not impossible though in repetitive dives with high PO2's.

If you track your O2 correctly, diving an air computer is not dangerous. If you do not do this, then you are playing with fire and O2 hits tend to be fatal underwater.

texdiveguy
12-01-2007, 21:59
Thanks a lot for all of you input, I was thinking of taking a nitrox calss next month, becaude the next trip I'm taking offers free nitrox. I have a nitrox computer, but my wife doesn't. Just wieghing pros and cons of the calss, the diving, a new computer.....

Danny.... anything worth doing in scuba diving is worth doing it 'right'....by this I mean by all means you and your wife invest the small amount of time/$ in a basic nitrox class....and possibly look into trading her dive computer in for a nitrox model.

Puffer Fish
12-02-2007, 10:32
I think the issue has been muddied a bit especially with regard to air on nitrox computer.

First, what is needed by a computer/person to plan a nitrox dive:
a) NDL info (N2 info if you will)
b) O2 info

Either could be a controlling factor on the dive.

If you dive a computer with the mix set to something other than what your breathing, you will need to manually make some calculations.

For instance, an Air computer when breathing Nitrox:

a) NDL infomation will be more conservitive since the computer is assuming more N2 uptake than actually occurs. This may seem to be a safety advantage but statistically, its not. If you fall in the 'outliers' of the statistics, it may be significant advantage to you.

b) O2 Information. The Air computer (or nitrox computer set to air) will have UNDERESTIMATED the o2 exposure. It is NOT safe to use this as the only means of calculating exposure limits. You must do this by hand for each dive and it may be possible for your computer to give you more NDL time when your out of O2 time.

The good news is that it is very difficult to run out of O2 time on most rec diving profiles. Its not impossible though in repetitive dives with high PO2's.

If you track your O2 correctly, diving an air computer is not dangerous. If you do not do this, then you are playing with fire and O2 hits tend to be fatal underwater.


Well stated...thank you.

dannybot
12-02-2007, 12:46
Danny.... anything worth doing in scuba diving is worth doing it 'right'....by this I mean by all means you and your wife invest the small amount of time/$ in a basic nitrox class....and possibly look into trading her dive computer in for a nitrox model.[/QUOTE]

Thanks, that is all I was really after, now I just need to work a little more overtime to do what I knew I was going to wind up donig eventually anyway, but getting all of your opinions and input is valuable.