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

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.

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.

--

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

Would you believe they're both right? If you plan your PPO2 based on 32% instead
of 31 (or 31.8) you have increased your margin of safety. If you do your tables
(or computer) at 31%, then you've reduced your allowable bottom time and again
increased your margin of safety. IMHO Unless you're diving right to the limits,
the 1% difference shouldn't matter in real life.