Nitrox Question

Teller · Mar 17, 2010

I use the Suunto Cobra and the Sunnto Gekko wrist, when my computer is set to 32% o2 it says I have MOD of 127 with the po2 of 1.6. My SSI dive tables say the MOD for 32% is 135 with a po2 of 1.6. I know its just a few feet but I want to know what I can really do safely. Can I go to 130 or not. If I do what will my computers do at that depth.

H2O 70 · Mar 17, 2010

MOD=Partial Pressure Gas/Percentage of Gas

MOD 32% O₂ with a partial pressure of 1.6=1.6/.32=5ata=132fsw

I wouldn't suggest diving to such a high PPO₂. 1.6 is fine for deco. 1.4 is the current accepted mass PPO₂ for normal diving.

Your computer is likely going to start beeping at you.

GrumpyOldGuy · Mar 17, 2010

Most tables have rounding errors. The computer may also be allowing for 32%+/- 1/2% or some reasonable deviation from nominal. Thus the difference.

Also, if you are at altitude the computer may be adjusting for this.

Lastly, 1.6 is a very aggressive limit, normally used for deco. I would use 1.4 max.

tstormdiver · Mar 17, 2010

The mathematical formula for MOD is (PPO2/ FO2-1)X 33 (or 34 for fresh water) . Using that formula the maximum operating depth for 32% O2 at a PPO2 of 1.6 is 132fsw (salt water) or 136 ffw (fresh water).

That being said, it is not recommended to excede a PPO of 1.4; which there the MOD would be 111.4 fsw or 114.8 ffw. Going beyond a PPO2 of 1.4 should only be done as an emergency contingency or for decompression.

Guy Alcala · Mar 17, 2010

DUI_Diver:
I use the Suunto Cobra and the Sunnto Gekko wrist, when my computer is set to 32% o2 it says I have MOD of 127 with the po2 of 1.6. My SSI dive tables say the MOD for 32% is 135 with a po2 of 1.6. I know its just a few feet but I want to know what I can really do safely. Can I go to 130 or not. If I do what will my computers do at that depth.

Suunto computers have what I call a nanny function; they always calculate your oxygen loading and MOD at the next higher integer percentage to the one you've set. This is explicitly stated in the manual. This isn't a problem if you're actual FO2% is XX.5% or higher, but is a pain if it's XX.4% or below. In the latter case, in order to get the same MOD result as a table you have to set the computer for the next lower FO2%. Example, if your FO2 analyzes 32.2% and you set the computer for 32%, it will do all calculations and give you MOD for 33%. Which is why it gives you a result of 127' and not 132' for EAN32 @ 1.6 PO2.

Your SSI dive tables should show MOD @ 32%/1.6 as 132', not 135'. If you look in the SSI Nitrox manual, there should be a table (in Chapter 2 IIRR) which lists MOD for PO2s of 1.3 to 1.6. And as others have mentioned, it's considered a good idea to use a working PO2 max. of <=1.4 rather than 1.6.

Guy

Rhone Man · Mar 17, 2010

Guy Alcala:
Suunto computers have what I call a nanny function

I tend to refer to it is a "101%" mix. So if you are diving Nitrox that measures at 32.5%, it rounds both fractions up, so it treats the O2 as being 33% for oxygen toxicity, but the N2 at 68% for decompression limits.

Guy Alcala · Mar 17, 2010

Rhone Man:
I tend to refer to it is a "101%" mix. So if you are diving Nitrox that measures at 32.5%, it rounds both fractions up, so it treats the O2 as being 33% for oxygen toxicity, but the N2 at 68% for decompression limits.

Pisses me off regardless. I'm perfectly capable of rounding by myself to whichever I think is the best setting. I see no need to round 32.1% up to 33%, so I'm forced to remember to set the computer 'wrong' (i.e. 31%) in that case. I guess it's too much to ask that Suunto treat me like an adult.

Guy

Rhone Man · Mar 17, 2010

Guy Alcala:
I guess it's too much to ask that Suunto treat me like an adult.

I am begging you - don't get me started.

Mike Boswell · Mar 17, 2010

Guy Alcala:
Suunto computers have what I call a nanny function; they always calculate your oxygen loading and MOD at the next higher integer percentage to the one you've set. This is explicitly stated in the manual. This isn't a problem if you're actual FO2% is XX.5% or higher, but is a pain if it's XX.4% or below. In the latter case, in order to get the same MOD result as a table you have to set the computer for the next lower FO2%. Example, if your FO2 analyzes 32.2% and you set the computer for 32%, it will do all calculations and give you MOD for 33%. Which is why it gives you a result of 127' and not 132' for EAN32 @ 1.6 PO2.

Your SSI dive tables should show MOD @ 32%/1.6 as 132', not 135'. If you look in the SSI Nitrox manual, there should be a table (in Chapter 2 IIRR) which lists MOD for PO2s of 1.3 to 1.6. And as others have mentioned, it's considered a good idea to use a working PO2 max. of <=1.4 rather than 1.6.

Guy

Hi Guy,

I'm perhaps not the only one who does this, but when the O2 analyzer readout starts to level off, I don't hold it there for another minute or two to get the last few tenths of a percent accuracy - I get a little impatient and end the test. Maybe Suunto is trying to take into account the fact that actual O2 levels tend to be a bit higher than the readings some of us take.

ianr33 · Mar 17, 2010

Rhone Man:
I am begging you - don't get me started.

Started on what ??

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