Maximum Operating Depth (M.O.D.) S.C.U.B.A. Diving On Air.

[tbone1004]

I'm a little surprised that someone who, according to their profile, is EAN certified and an instructor says this.

I'd expect someone with credentials like that to be able to calculate MOD for any standard mix like EAN36, EAN32 and EAN28. Whether it's for 2 bar, 1.6 bar, 1.4 bar or any other pPO2. By extension, I'd expect that person to be able to calculate the MOD for any oxygen percentage, like 34% or 21%.

In other words, I'd expect someone who is EAN certified to be able to calculate the MOD for EAN21 at 1.4 bar O2. Perhaps even be able to reflect on the differences between air and EAN21. Me, I dive both nitrox and air. My PDC is permanently in nitrox mode, and if my tank contains air, I'll just tell my computer that I'm diving EAN21. Not unexpectedly, it gives me the same NDL times as if I told it I was diving air. It doesn't give me the MOD, though, because it doesn't have that functionality in air mode.

slightly in @Boston Breakwater 's defense, and at risk of being pedantic which he apparently had blocked be for, see my original post, #14 in this thread, where the MOD of air is a bit more complicated than nitrox. With the nitrox mixes that we use as standard, ppO2 is going to be the leading factor for setting the MOD and as such is the only variable that is really talked about in the nitrox classes. Some will talk about narcosis, but those are usually dated courses since our current understanding is that oxygen and nitrogen are comparably narcotic so there is no narcosis benefits from diving nitrox. What they don't do is talk about gas density, and CO2 retention. CO2 retention being a far larger factor in most peoples experience of narcosis than inert gases are, and with deep air, that should be your leading variable as you set your MOD.

 

[Storker]

slightly in @Boston Breakwater 's defense, and at risk of being pedantic which he apparently had blocked be for, see my original post, #14 in this thread, where the MOD of air is a bit more complicated than nitrox. With the nitrox mixes that we use as standard, ppO2 is going to be the leading factor for setting the MOD and as such is the only variable that is really talked about in the nitrox classes. Some will talk about narcosis, but those are usually dated courses since our current understanding is that oxygen and nitrogen are comparably narcotic so there is no narcosis benefits from diving nitrox. What they don't do is talk about gas density, and CO2 retention. CO2 retention being a far larger factor in most peoples experience of narcosis than inert gases are, and with deep air, that should be your leading variable as you set your MOD.

If gas density and/or narcosis is a concern, your MOD should be somewhere between 30m and 40m, no matter your mix. As long as you don't start faffing with He, of course. If pPO2 is your only concern, even EAN28 would allow you to go to 40m and EAN25 would allow you to go to 46m.

Which is why I love EAN32. My pPO2 MOD is about 30m, and I'm well within sensible limits for gas density and narcosis as well. And I get 50% longer NDL times, so I basically only have to worry about staying within min gas.

 

[tbone1004]

If gas density and/or narcosis is a concern, your MOD should be somewhere between 30m and 40m, no matter your mix. As long as you don't start faffing with He, of course. If pPO2 is your only concern, even EAN28 would allow you to go to 40m and EAN25 would allow you to go to 46m.

which is why I wrote my post the way that I did back in the beginning of the thread, but END/Gas Density is not something that is taught in nitrox courses.

 

[Storker]

END/Gas Density is not something that is taught in nitrox courses.

I know. Neither is narcosis, IIRC.

EDIT: If I go to e.g. France and flash my 3* card, they'll probably be just fine with me giving a plan with a max depth of 56m. Which I, personally, never would do. But those limits are set on pPO2, not on narcosis or gas density.

But since MOD usually refers to max pPO2, that's the parameter I'll use no matter the mix. And then I'll adjust my max depth according to the other factors.

 

[helodriver87]

I wish density was taught as a consideration in nitrox classes. The worst narc I've ever had was a function of gas density and workload. I was shallower than my MOD and felt more impaired than I ever have before. I treat Mitchell's recommendation of 5.2g/L as ideal density as a law now. Particularly on the rebreather, but open circuit as well. I don't feel comfortable with the density of any gas without a helium component below 110-120.

 

[Doc Harry]

Hello. The post has absolutely nothing to do with Nitrox/Enriched air. If, you want to engage me in conversation....you can P.M. me.
Cheers.

Effectively the math is the same.

 

[Storker]

Effectively the math is the same.

FTFY

 

[tursiops]

But since MOD usually refers to max pPO2, that's the parameter I'll use no matter the mix. And then I'll adjust my max depth according to the other factors.

Exactly. In fact, the way the OP worded his question rather directly implies an oxygen-only consideration.

I was curious what the MOD is for air (20.95%) using a PPO2 of 1.4

 

[NAM001]

If you I think the oxygen toxisity values (get the formula and do it on a spreadsheet.) and plot the results in .5 increments or less. you will see how the cns level changes form linear to logrythmic at about 1.65 or so.using a clock the line rind from center to 10 minutes until you get to PPO2 of 1.65+/- and the line turns to run upward to less than the clock center to the 5 minute.

User this link and go down to Pulmonary Oxygen Toxicity Calculations the formula is the one that has the exponent or -5 over 6 make the calculations and then make a graph of ht adnyou willl see what happens at about 1.65 PPO2

https://www.shearwater.com/wp-content/uploads/2012/08/Oxygen_Toxicity_Calculations.pdf

 

[NAM001]

for what it is worth. in the rec world we only look at O2 as teh bad guy because the evil N2 lives on the edge if not below rec diving depths. There is sound logic to say that if youare below 100 and being narced is symtomatic (I believe on everyone to different degrees) that at some point you have to start including N2 in re c diving as the O2' bad guy' cousin. In the rec world we are screwed in that to calal both gasses bad it leaves nothing that is inert. we can only control which one is previlant for the depth we are diving. to avoid that you have A./.. dive shallower and remove the N2 from the problem or go to trimix. For rebreathers trimix i assume is no big deal because you dont need a lot of it as you would in open circuit.