Reduced MOD on EAN

[ccrprospect]

Hello,

I just bought a new Suunto D4i. Used it for the first time on my nitrox/deep speciality courses last week.

For some odd reason however, it would not allow me to plan dives deeper than 36.0 meters at 1.4 pO2/EAN 28. This is even though in theory, the 1.4 limit is reached at 40 meters on EAN 28. I had to re-adust the pO2 to 1.5 so that alarms don't start going off beyond 36.0 meters.

Is this just conservative accounting for a possible margin of error or is there some other reason?

Many thanks in advance.

 

[RainPilot]

Seems odd. 1.4 PPO2 at 36 m is EAN30, very strange that the computer gave you a hard time. Was it only in planning mode or did it give an accurate PPO2 during the dive?

If its a conservatism setting then maybe set your alarms for 1.6 and plan your gasses to dive around 1.4?

FWIW I dive 32% almost exclusively. Its a very common gas for a variety of reasons and it reaches 1.6 at 40m which means I don't really have to think too much about MOD on a rec dive. The END is also a hair over 30m at 40 m so using EAN32 means for anything less than 40m I don't worry too much about MOD or narcosis.

 

[Diving Dubai]

Suunto round down their MOD - it's in the manual effectively 31% has the same depth MOD as 32% on other computers

My Eon gives me 38.5m at PPO2 1.6 for a 32% mix whereas 31% is 40m at 1.6 PPO2

If memory serves me correctly, it's because they suggest if say you have a mix of 31.6% instead of rounding it up to 32% you round it down to 31% (I could be wrong as I don't have the manual to hand)

 

[ccrprospect]

Thanks all. Diving Dubai, I just checked your numbers on mine, and they correspond exactly as described. Rainpilot, this is in dive plan mode - I tinkered with the numbers of ppO2 before the dive as I didn't want alarms going off on my first excursion to 40 meters.

 

[JohnN]

Thanks all. Diving Dubai, I just checked your numbers on mine, and they correspond exactly as described. Rainpilot, this is in dive plan mode - I tinkered with the numbers of ppO2 before the dive as I didn't want alarms going off on my first excursion to 40 meters.

There is no magic (or science) out there that says 1.4 ppO2 is safe and 1.5 isn't (or 1.3 is measurably safer). Too much depends on the dive conditions and you. Use the computer as a guide, use your brain, and turn off that silly alarm

 

[guruboy]

There is no magic (or science) out there that says 1.4 ppO2 is safe and 1.5 isn't (or 1.3 is measurably safer). Too much depends on the dive conditions and you. Use the computer as a guide, use your brain, and turn off that silly alarm

I'm pretty sure the science shows that a lower ppO2 is safer (down to the minimum)

 

[Lorenzoid]

I'm pretty sure the science shows that a lower ppO2 is safer (down to the minimum)

All other things being equal, a ppO2 that's lower than another ppO2 by all of 0.1 is theoretically "safer." But all other things are never equal in the real world. I interpreted @JohnN's comment as saying that it's impossible to draw a line between ppO2 X and ppO2 Y to indicate that X is "safe" but Y, being just 0.1 greater than X, is "not safe," since there are other factors involved, such as amount of time spent at any given ppO2, as well as exertion and other personal physiological factors, some of which are unpredictable. These factors may be at least as significant as the 0.1 reduction.

 

[ccrprospect]

The diver's personal physiology is a constant (or very nearly so) except if during a particularly strenuos dive, or following significant improvement, efficiency, and relaxation during diving after accumulating experience.

The 1.4/1.6 limits were developed based on a conservative estimate of the likelihood of a diver with a less diving friendly physiological makeup suffering oxygen toxicity. The limits can and are pushed. It is my understanding that combat divers limit exposure at 2.0. Given the potentially sudden nature of the "oxtox" hit however, I will stick to the 1.4, allowing for some unintentional loss of bouyancy to 1.6. It doesn't excuse Suunto's further over conservative approximation however, so the alarm will be set at 1.5 from now on.

Also, it's not merely a .1 - it is also a 10% increase in oxygen partial pressure. This should not be underestimated.

 

[Lorenzoid]

Yes. And some recommend 1.2. And others choose to use 1.3. Even though it represents a 10% difference, there is no basis for drawing a "safe/not safe" bright line between any two of these options.

 

[bl6394]

Consider too that your risk of of a CNS hit is a combination of level of exposure (PO2) and time. Also factoring in (level of exertion, age , health, medication, etc.) One has a hard time picking out a single PO2 as "safe" without looking at the context of the dive - and other factors.