O2: Round up or Round Down

"George Cathcart" <gcathcar@nospam.umd.edu> wrote in message
news:cog57b$a7a$1@grapevine.wam.umd.edu...
> In my NAUI Advanced Nitrox class, I was taught that I should always round
> the percentage of oxygen up to the next whole number when calculating MOD,
> PO2, EAD, OTU, CNS, etc. In other words, if the analyzer says I've got
> 31.8 percent O2, I should calculate on 32.
>
> My Suunto Vyper Manual, however, says I should always round down to
> minimize the risk of DCS.
>
> My instructor's advice frankly makes more sense, because the gas we're
> concerned about is oxygen, and underestimating oxygen in Nitrox is more
> likely to cause a problem than underestimating nitrogen.
>
> Or am I wrong? If not, why would Suunto recommend rounding down?

There are some assumptions involved either way. How you, personally, assess
the risks, determines how you will decide. Your instructor is correct. So
is Suunto. It's not a bad idea to consider both in your dive plan. If you
don't plan your dives, it's not a bad idea to adjust your level of
conservatism (yours, not your computer's) to account for all of the risks.

For the average nitrox diver, it's probably six of one, half a dozen of the
other. If you set your MOD for a PPO2 of 1.4, you're not going to get in
much trouble no matter which way your round. Much the same thing applies to
DCS risk. If you're in the green according to your computer, you're pretty
safe, any way you look at it.

Since you mentioned an advanced Nitrox course, you may want to be a bit more
careful. If pushing your decompression rate up by using higher PPO2, you're
pushing your CNS risk up with it. There's more than one answer to the
question of where the PPO2 becomes critical, but most feel comfortable that
1.6 is far enough. There have been CNS hits reported at 1.6 and less as
well as people that pushed things well beyond that for brief periods. Of
course, if you're doing this kind of diving, you don't care what Suunto has
to say. You're not using their computer to plan your dive anyway.

There's one more thing worth thinking about. Suunto's nitrox computers are
known for being quite conservative. It may be that they've adjusted enough
for PPO2 issues to make DCS a more critical measure. That is not true of
all computers.

Lee

On Mon, 29 Nov 2004 16:44:13 -0500, George Cathcart
<gcathcar@nospam.umd.edu> wrote:

>In my NAUI Advanced Nitrox class, I was taught that I should always
>round the percentage of oxygen up to the next whole number when
>calculating MOD, PO2, EAD, OTU, CNS, etc. In other words, if the
>analyzer says I've got 31.8 percent O2, I should calculate on 32.
>
>My Suunto Vyper Manual, however, says I should always round down to
>minimize the risk of DCS.
>
>My instructor's advice frankly makes more sense, because the gas we're
>concerned about is oxygen, and underestimating oxygen in Nitrox is more
>likely to cause a problem than underestimating nitrogen.
>
>Or am I wrong? If not, why would Suunto recommend rounding down?
>
>Thanks,
>
>gc

Could it be because the Suunto doesn't calculate O2 exposure and only
models N2 for DCS purposes?

In this case you round UP to calculate your EAD, MOD and PO2 and O2 time in
order to plan your dive and reduce the risk of CNS O2 toxicity. Now it's up
to you to dive your plan.

You would round DOWN to enter the EAN into the computer since it will tell
you on that basis when you run the risk of entering into a decompression
scenario. This would minimize your risk of DCS.

Geoff wrote:
>
>
> Could it be because the Suunto doesn't calculate O2 exposure and only
> models N2 for DCS purposes?
>
>

Thanks, Geoff,

Good guess, but the Suunto Vyper does calculate O2 exposure, both OTU
and CNS, and it graphs the one that is closest to maxed. It also tracks
ND time constantly. And it does both quite conservatively. MOD for 32%
at PO2 of 1.4, for example, is 107 feet, not the widely accepted 111.

If I round up, plan my dive and dive my plan, I'll be well within the
safety limits of the Nitrox tables and the computer, I'm pretty sure.

Thanks,

gc

"George Cathcart" wrote

> Good guess, but the Suunto Vyper does calculate O2 exposure, both OTU and
> CNS, and it graphs the one that is closest to maxed. It also tracks ND
> time constantly. And it does both quite conservatively. MOD for 32% at PO2
> of 1.4, for example, is 107 feet, not the widely accepted 111.

111 is widely accepted because it's the accurately calculated MOD for 32% O2
at a PPO2 of 1.4. ((1.4/.32)-1)x33=111.4

> If I round up, plan my dive and dive my plan, I'll be well within the
> safety limits of the Nitrox tables and the computer, I'm pretty sure.

If you round up, plan your dive and dive your plan, you have no need of a
computer.

Lee

In article <cog57b$a7a$1@grapevine.wam.umd.edu>,
George Cathcart <gcathcar@nospam.umd.edu> wrote:
>In my NAUI Advanced Nitrox class, I was taught that I should always
>round the percentage of oxygen up to the next whole number when
>calculating MOD, PO2, EAD, OTU, CNS, etc. In other words, if the
>analyzer says I've got 31.8 percent O2, I should calculate on 32.
>
>My Suunto Vyper Manual, however, says I should always round down to
>minimize the risk of DCS.
>
>My instructor's advice frankly makes more sense, because the gas we're
>concerned about is oxygen, and underestimating oxygen in Nitrox is more
>likely to cause a problem than underestimating nitrogen.
>
>Or am I wrong? If not, why would Suunto recommend rounding down?
>
>Thanks,
>
>gc

Suunto overstates O2% by 1% already internal to their software. If you
set for a 31% FO2, the unit actually computes for a 32% FO2.

Try it - you will find that a 32% FO2 toxes you at 107' (PO2 = 1.4) In
reality the MOD is 111 - which just happens to be what you get if you set
the FO2 to 31%.

HOWEVER, and this is a big "if", Suunto does NOT overstate the FN2 by the
same 1%! So if you round up, you both get toxed early by the Vyper AND
you understate your FN2 exposure, which is not good from a DCS standpoint.

Second, be aware that the Vyper, irrespective of the PO2 limit you set,
refuses to respect that limit and radically (like 10x NOAA tables)
accelerates your CNS loading if you exceed what IT thinks of a PO2 of 1.4
- with its rounding up.

The result of this is that the unit will tox you inside of 20 minutes at
107' on a 32% mix, which is pure bullshit. Oh, and due to rounding errors
at 107' you won't get a PO2 alarm for 1.4 either......

The ugly part is that if you set an allowed PO2 of 1.5, you STILL get
toxed by the computer at 107' in the same less-than- 20 minutes.

The Vytec does NOT have this same stupidity included.

I have emailed Suunto about the Vyper's behavior. They think its just fine
even though it is in no way documented that they have included this nice
"acceleration feature" - and there is no way to turn it off.

--
--
Karl Denninger (karl@denninger.net) Internet Consultant & Kids Rights Activist
http://www.denninger.net My home on the net - links to everything I do!
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On Mon, 29 Nov 2004 16:44:13 -0500, George Cathcart <gcathcar@nospam.umd.edu> wrote:
>My instructor's advice frankly makes more sense, because the gas we're
>concerned about is oxygen, and underestimating oxygen in Nitrox is more
>likely to cause a problem than underestimating nitrogen.

It doesn't make any difference at all unless your dive plans include square profiles
at MOD for NDL.

You don't dive Nitrox the way they teach theory in class.
Forget all the optimum mix crap. it was a waste of your time.

safe diving,

bullshark
safe diving,

bullshark

bullshark wrote:
> On Mon, 29 Nov 2004 16:44:13 -0500, George Cathcart <gcathcar@nospam.umd.edu> wrote:
>
>>My instructor's advice frankly makes more sense, because the gas we're
>>concerned about is oxygen, and underestimating oxygen in Nitrox is more
>>likely to cause a problem than underestimating nitrogen.
>
>
> It doesn't make any difference at all unless your dive plans include square profiles
> at MOD for NDL.
>
> You don't dive Nitrox the way they teach theory in class.
> Forget all the optimum mix crap. it was a waste of your time.
>
> safe diving,
>
> bullshark
> safe diving,
>
> bullshark

Thanks to everyone for their thoughts on my question. I figured out what
was going on shortly before Karl posted his answer, when I realized that
the Vyper's MOD for EANx 32 is 107', which is actually the MOD for 33%
on the tables.

Interestingly, I think everyone who responded was right, except the one
person who wondered if the Vyper only tracks nitrogen and not oxygen
loading. I know Bullshark is right, too. I would just hate to get locked
out of a dive because the computer is too conservative, and the Vyper is
pretty conservative. I rarely do dive square profiles, although on some
wrecks it does happen.

I think the most important thing I got out of the Nitrox class was the
reinforcement of the importance of dive planning and planned diving.
Yeah, I knew it from OW, AOW and rescue, but Nitrox definitely
emphasizes the planning piece.

Again, thanks for the good thoughtful replies.

But how come nobody said anything about guns, sex or Michael Moore???

gc

"Karl Denninger" wrote

> Suunto overstates O2% by 1% already internal to their software. If you
> set for a 31% FO2, the unit actually computes for a 32% FO2.
>
> Try it - you will find that a 32% FO2 toxes you at 107' (PO2 = 1.4) In
> reality the MOD is 111 - which just happens to be what you get if you set
> the FO2 to 31%.
>
> HOWEVER, and this is a big "if", Suunto does NOT overstate the FN2 by the
> same 1%! So if you round up, you both get toxed early by the Vyper AND
> you understate your FN2 exposure, which is not good from a DCS standpoint.
>
> Second, be aware that the Vyper, irrespective of the PO2 limit you set,
> refuses to respect that limit and radically (like 10x NOAA tables)
> accelerates your CNS loading if you exceed what IT thinks of a PO2 of 1.4
> - with its rounding up.
>
> The result of this is that the unit will tox you inside of 20 minutes at
> 107' on a 32% mix, which is pure bullshit. Oh, and due to rounding errors
> at 107' you won't get a PO2 alarm for 1.4 either......
>
> The ugly part is that if you set an allowed PO2 of 1.5, you STILL get
> toxed by the computer at 107' in the same less-than- 20 minutes.
>
> The Vytec does NOT have this same stupidity included.
>
> I have emailed Suunto about the Vyper's behavior. They think its just
> fine
> even though it is in no way documented that they have included this nice
> "acceleration feature" - and there is no way to turn it off.

I hope they think it's fine enough to lose customers because of it. I know
two instructors who tried Vypers and won't use them again. That means that
every student they advice won't either. I presume you won't and I know I
won't. Somewhere along the line, the nice little gauge mode feature isn't
going to be enough to offset the quirks this computer has.

Lee

"bullshark" wrote

> Forget all the optimum mix crap. it was a waste of your time.

For pretty much all of the diving we normally do here.

"George Cathcart" <gcathcar@nospam.umd.edu> wrote in message
news:coidn9$amk$1@grapevine.wam.umd.edu...

> But how come nobody said anything about guns, sex or Michael Moore???

We're not done yet.