Reduced MOD on EAN

[KaptaniDerya]

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.

Yes, there is another reason. In theory, MOD of EAN28 is 40 m, but that is assuming ideal gas laws. However in real world, especially over 200 bars / 3000 psi, ideal gas laws deviate from "real" gas laws. Suunto's are using "real" gas laws, hence the numbers are slightly different than you expect them to be.

If you have the Dive Manager (as of version 3.1), you could change the gas laws in the settings and see the effects on the dive plans.

 

[Jack Hammer]

My Gekko did the same thing and I recall it was mentioned in the manual. In reality having your MOD a few feet shallower really shouldn't be a big deal. Considering the accuracy of most O2 analyzers having a little conservatism is wise, IMO.

 

[DuboisP]

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 like

my computers are set to 1.6 PpO²
always

 

[bl6394]

I like

my computers are set to 1.6 PpO²
always

As long as you understand the risks and consequences - sure.

A "safe" exposure at 1.6 is roughly a duration of 36 minutes - or roughly 80% of the NOAA limit of 45 min at this PO2 - generally during a non-working portion of the dive.

http://www.anaspides.net/documents/scuba_diving_documents/NOAA CNS Percentage Table.pdf

You almost certainly will be able to exceed that recommendation many times.

But as you get older, heavier, medicated, overexerted- or just have an off day - you'll run an increased risk of an oxygen toxicity hit. You probably won't survive it.

Safe diving.

 

[DuboisP]

yes, i must breathe oxygen 100% during 45 min
to have a deco stop at 6 m of 45 min, i must stay some hours at ... x meters.
as i'm not diving with a CCR, but with tanks, I will die out of gas before my deco stops.

a "standard" diver cannot reach these limits.

 

[DiverDownD3]

I have the same issue with my Mares Puck and EAN30. It tells me the MOD for 1.4 is 118'. I just take it for what it is, some conservatism built in. Most of the places I dive, are 115' or less to the sand anyway.

 

[bl6394]

yes, i must breathe oxygen 100% during 45 min
to have a deco stop at 6 m of 45 min, i must stay some hours at ... x meters.
as i'm not diving with a CCR, but with tanks, I will die out of gas before my deco stops.

a "standard" diver cannot reach these limits.

A recreational diver who exceeded his MOD could exhaust his oxygen clock very quickly...

Recreational diving practices generally factor in a safety factor that assumes a limited knowledge / skill on the part of the diver. When you exceed those recommendations - e.g. choosing a higher than normal MOD - your safety margin is decreased.

The important thing is that you understand the risks and consequences of the decisions you make.

 

[DuboisP]

A recreational diver who exceeded his MOD could exhaust his oxygen clock very quickly...

a recreational diver exceeding his MOD too longer/too high will have more serious problems to manage.