O2 percentage - round up, round down?

[geoff w]

When setting the percentage of O2 in the breathing gas, is the recommendation to round the value up or down to the nearest whole percentage, truncate to the lowest whole percent, or something else, since O2% is entered as a whole number? I couldn't find anything in the manual. I'm used to a Suunto Vyper, and Suunto's guidance in their manual is to always round down, e.g., 31.8% O2 gets entered in the computer as 31

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A ScubaBoard Staff Message...

This thread was originally in the Shearwater forum. It was moved here because it is a more appropriate forum.

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[TONY CHANEY]

Always round down because the Nitrogen controls the decompression which all tables and computers are based upon. Keep in mind to know your MOD for the O2.

 

[NAM001]

I round up for mod calculation and round down for computer entry. Non of my diving pushes the time limits for O2 so rounding up to get more accuracy is a moot issue. Next rounding down has less negativne effect on the dive from the aspect of exceeding ppO2. If you push the mod when you round down you get perhaps a mod for 1.42 instead of 1.4. That litttle extra is no crime. rounding up provides a too shallow mod and rounding down gives a too deep of mod. That concern goes down the drain when you ask for a gas good for 10+ ft deeper than your intended dive. look at your standard ean32 and its mod. if you are only doing a 90 ft dive you will never get to the mod. YOu will hit the ndl long before you hit the O2 limit so rounding down gets a higher N2 factor and a slightly shorter ndl than reality. (NDL error in your safety advantage) I think the most inportant thing is to understand the errors you are injecting into the process so you know how to manage it for your given dive.

 

[tbone1004]

like everything in this sport, you round to the more conservative number for safety. In the case of O2 percentage it is always up for MOD calculations, and down for decompression calculations. Rounding up for MOD will give you a shallower depth, though it is splitting hairs at that point, but still good to round up, and rounding down for decompression calculations will give you a slightly longer run time, but you'll come with a bit less nitrogen loading.

This btw should have been covered in your nitrox class....

 

[Freewillow]

Are you sure that the apparatus that measures % of 02 is so precise? I have seen some funny results on some measuring tools. Otherwise, as said above, round up for MOD and round down for deco obligations. Now if you set up your PPO2 max @ 1.4, you can easily round down for both

 

[geoff w]

... This btw should have been covered in your nitrox class....

I took the PADI nitrox class about 10 years ago (only had the table version back then, no computer version of the class, and went back to the student book to confirm what I remembered from my training. The book says to round to the closest whole percentage, and gives flexibility to round up or down if the analyzer reads something point five. So the PADI guidance and Suunto guidance are different, which is why I asked the question to see what Shearwater recommends

Are you sure that the apparatus that measures % of 02 is so precise? I have seen some funny results on some measuring tools. Otherwise, as said above, round up for MOD and round down for deco obligations. Now if you set up your PPO2 max @ 1.4, you can easily round down for both

I have my Petrel set to max PP02 of 1.4. The datasheet for my Analox O2EII says "± 1% of reading, ± 0.2% of O2"

 

[boulderjohn]

I took the PADI nitrox class about 10 years ago (only had the table version back then, no computer version of the class, and went back to the student book to confirm what I remembered from my training. The book says to round to the closest whole percentage, and gives flexibility to round up or down if the analyzer reads something point five. So the PADI guidance and Suunto guidance are different, which is why I asked the question to see what Shearwater recommends

I round to the nearest whole percent myself, mostly because of what tbone1004 explained and partially because I don't think the analyzers are so accurate that we should be splitting hairs on the percentages.

I don't see that this is really a question for either Shearwater or Suunto. The question is up to your understanding of dealing safely with nitrox diving. The brand of the computer you are using while diving does not change that.

 

[tbone1004]

John, why don't you pull this out of the Shearwater forum then since they won't be able to answer for liability reasons?

 

[boulderjohn]

John, why don't you pull this out of the Shearwater forum then since they won't be able to answer for liability reasons?

Good idea. It's heading for Basic Scuba.

 

[Patoux01]

Disclaimer: This is MY view, and it may, or may not, be shared by the agencies I've been trained by. It may also be considered inappropriate by the mods. However I don't see any issue with my thinking and therefore feel free to share it. Do not go on "trust dives", doing something "because some guy on internet said so".

I round to whatever I feel like (ie lower if I happen to stop clicking the button too soon, or higher if I'm a bit slow to stop. Heck I generally even leave it on air when diving shallower than 25-30m and less than 3 a day)
- Deco is not rocket science.
- Your analyser is most likely off by a value that can be anywhere from 0 to 1 or 2% (because when calibrating we ASSUME the "voltage to %" curve is a straight line, which is not the case).
- Ox tox is not rocket science.

So in other words: Do as you want, you won't die because of any of those choices.


An example to explain why I think like this:
For deco: let's say 30 minutes at 33meters (yes this is a completely random profile), using N32 and an algorithm that's unlikely to be the best (gf 30/70 on buhlmann), I get on my simulator a total run time (ie those 30 minutes at the bottom + deco + ascent time, yes I mention deco in the basic forum, and those values seem long to me, but heck it's just an example) of 48 minutes. N31 gives 50 minutes, N30 gives 51 minutes. It is up to you to then decide if you think those 2 minutes will get you bent or not. I personally believe they wont kill me, simply because I use the plans as an enveloppe, in the same way you tend to not dive your computer exactly to the NDL, I do not dive my plans exactly to the limit.

For MOD: Assuming a maximum pO2 of 1.4. N32 gives us a mod of 10*(1.4/0.32 -1) = 33.8m, N31 gives a mod of 10*(1.4/0.31 - 1) = 35.2m, N30 gives us a mod of 10*(1.4/0.3 -1) = 36.7m.
And the pO2s at the maximum depth, ie what happens if I took a N32 to 36.6m. 0.32*(36.6/10 +1) = 1.49b, N31 gives us 0.31*(36/10 + 1) = 1.44b.

To me, these differences are small enough to not worry about it.


Again, this is only my view.
I just feel like there's too much worrying about minor things in diving, when major issues are overlooked (are you able to give air ANYTIME to your buddy without him having to come and grab you? are you able to deal with a stuck inflator in whatever position at any time? etc.)