Nitrox Computer Setting

[Eastbayfe]

The PADI guide for Enriched Air Diving states to round to the nearest oxygen percentage when setting your dive computer to the analyzed tank reading (31.8 would be set to 32). The Sunnto dive computer guide states to never round up (31.8 percent should be set to 31). I see an argument for both views but tend to lean toward the computers guide direction. What is the best practice?

 

[SteveAD]

The computer being off the actual mix either high or low could cause problems. It is up to you to determine which problem is more likely.

Actual mix higher than computer set could cause O2 tox shallower than computers MOD.

Actual mix lower than computer set could cause NDL shorter than computer reads.

If you are pushing max depth for your mix round up.

If you are pushing max time for your mix round down.

The usual 1.4 PPO2 is so conservative that oxtox is very unlikely.

Most computers NDLs are also quite conservative.

Don't push any limits.

 

[don Francisco]

I'll probably get flamed for this, but I think you should round to the nearest whole number.
Humans aren't digital, and an error of half a percent isn't likely to be critical, unless you're seriously pushing the envelope. Even then, remember the limits don't represent an absolute line between safe and dangerous, but rather a grey zone of statisticly greater risk.

You might also make your rounding decision according to the gas for which you want the greater safety margin. Rounding up, increases the margin for Oxygen, rounding down for Nitrogen. If I were planning a dive at a depth near the MOD, I might round up, If I was planning a mid depth dive near the NDL I might round down.

Either way, as I said it isn't super critical, you're not digital, and the readings from your analyzer aren't as accurate as you'd believe, so there's a certain tolerance for error built into it all.

 

[LowVizWiz]

That's odd, the Vyper manual shows what you're talking about while the Cobra doesn't say a thing about it. Most people I know would round up 31.8 to 32 including myself. However, I agree with DF assessment. it does depend on your dive plan.

Wys

 

[Charlie99]

Do like the Suunto manual says.

If you enter 31% they will use 31% for N2 calculations and 32% for O2 calculations, which is exactly what you want for any mix between 31.0 and 31.9%.

You can easily verify this by checking the calculated MOD for various combinations of max ppO2 and FO2. It can be a bit confusing at first, because the Suunto computers might seem to be incorrectly calculating MOD if you aren't aware that they add 1% to the entered number before doing MOD calc.

Other computers handle it differently. For example, my Oceanic Data Plus 2 takes whatever I enter and uses that for both N2 and O2 calculations. I ordinarily round down, since my limiting factor is always deco, not CNS/O2.

 

[1_T_Submariner]

The handheld analyzer you are using to analyze probably isn't that accurate to worry about it. If my computer is already set to 32 and I get a 31.2 to 32.8 I just leave it. I usually don't come close to limits anyway.

 

[Eastbayfe]

All good points to work with. Thanks.

 

[John_B]

You can easily verify this by checking the calculated MOD for various combinations of max ppO2 and FO2. It can be a bit confusing at first, because the Suunto computers might seem to be incorrectly calculating MOD if you aren't aware that they add 1% to the entered number before doing MOD calc.

This was one of the two things I found really annoying about my old Vytec. Like I wouldn't notice they played lawyer with the MOD of a standard gas.

 

[Lone Frogman]

No problem, I just set my Cochran to 31.8.

 

[Damselfish]

I agree with rounding down because NDL is so much more likely to be an issue then O2. I actually tend to set my computer to 31 and leave it there for a week of the usual scattering of tanks around 32 unless something is way off. As said, the accuracy of the analyzer is not usually that great anyway. I don't care about the depth alarm itself being wrong, I'm perfectly capable of knowing and not going below my MOD without that.