nitrox MOD/TOD

[Atticus]

One thing I'd like to add that really bothers me: I've been seeing people marking an MOD at 1.4ata, then during a dive going deeper than that "because they have a deeper contingency depth". Then when I ask them how deep their "contingency depth" is they say "you know, a little deeper than the MOD". Which tells me that they don't know what their ppO2 is at the deepest point of the dive.

Please know how deep you can safely breath your gas. I have no problem with people planning their dive to a shallower dive than the maximum (in fact I encourage it), but know the limits of the gas before the dive!

For this reason I mark the Maximum depth I will breath a gas on the tank. That way it's firmly planted in my mind. If your maximum is at 1.4ata then that's fine, but don't end up at 1.5, 1.6, or 1.9 taking pictures.

 

[Kim]

you could have 21% 02 and die ... if you exceed a safe PPO2 (let's say 1.6)

and you could have 100% O2 and be fine ... if you don't exceed a safe PPO2 (let's say 1.6)

That's true - except that the limit (MOD) is expressed as a depth. That's easier for most people to understand than PPO2 because it's what's actually on their gauge or computer. Now...if you had a gauge that told you your current PPO2 (assuming you could dial your mix into it) - then I'd agree with you. Of course - such a PPO2 gauge would only keep you informed of your O2 risk......not your Nitrogen NDL. Depth handles both.

 

[H2Andy]

no question... but it helps to understand how you get MOD. it's all about PP02.

you're basically pulling a PADI here ("don't confuse them! just give them a number!")

 

[Kim]

you're basically pulling a PADI here ("don't confuse them! just give them a number!")

Not at all, but as Atticus just posted:

Please know how deep you can safely breath your gas. I have no problem with people planning their dive to a shallower dive than the maximum (in fact I encourage it), but know the limits of the gas before the dive!

For this reason I mark the Maximum depth I will breath a gas on the tank. That way it's firmly planted in my mind.

I also find it easier to work out my TOD, and MOD before the dive as I then just have to carry two numbers in my head. It's just easier.

 

[H2Andy]

lol

kim, how do you get to the TOD and the MOD?



it's PPO2, baby

 

[Atticus]

lol

kim, how do you get to the TOD and the MOD?



it's PPO2, baby

Andy - I completely agree that ppO2 is the factor we're discussing. However from a divers perspective after they have entered the water with the tank(s) they will breath they generally convert the fO2 and maximum ppO2 they are willing to subject themselves to into a maximum depth for that gas.

I think we're really saying the same thing.

 

[H2Andy]

this is the statement i was responding to:

An MOD goes together with a mix. The Maximum Operating Depth is the theoretical
maximum depth a given mix is safe to dive to - it has nothing to do with the PPO2 EXCEPT for the argument/divergence of views as to which PPO2 is safe or not.

and i responded by saying:

one further thing that struck me:

percentage of oxygen means nothing. PPO2 means everything.

you could have 21% 02 and die ... if you exceed a safe PPO2 (let's say 1.6)

and you could have 100% O2 and be fine ... if you don't exceed a safe PPO2 (let's say 1.6)

it's the partial pressure that matters, not the percentage in the mix. and partial
pressure has everything to do with depth.

just to clarify what i am arguing about. MOD is all about PPO2 (what PPO2
are you willing to live with?) That will determine the MOD, not the particular
mix you have.

the same mix on two different dives can have different MOD's because MOD
depends on what PPO2 you are willing to live with.

it's a subtle difference, but an important one.

 

[Atticus]

Andy - I understand your point. It is about ppO2 which is a factor of fO2 and pressure(depth).

 

[Charlie99]

I think what's coming out in this discussion is what comes out from time to time in discussion of the NDLs, and that is that none of these numbers is cut in stone. ..... There is a ppO2 where nobody toxes, and a ppO2 where most everybody would, and in between there is a large grey area of increasing -- but not exactly quantifiable -- risk. Where you draw the line depends on how much risk you are willing to take. The agencies have picked their numbers -- IANTD says it's okay to go to 1.6 and stay there, NAUI says 1.4.

Another similarity is that oxtox, just like DCS/NDLs is a function of TIME. When talking about NDLs there is a TIME associated with a particular depth. The NOAA CNS clock runs much faster at 1.6ata than at 1.4, the same way that the NDL countdown runs faster at 100' than 60'.

We all learned this in our nitrox classes, but many divers seem to immediately forget it.

 

[H2Andy]

you're talking about pulmonary oxtox

we were discussing Central Nervous System (CNS) oxtox, which can be triggered immediately if a certain PPO2 treshhold is exceeded

(i.e. there are two types of oxtox)