Standard MOD

[ScubaFeenD]

Only shallow deco gases are planned at 1.6.

Backgas is planned at 1.4 or lower.

Explain how 1.6 is better when shallow then when deeper? I am not a deco diver or anything, but I always imagined it was more acceptable because activity on deco stops is (i think) much much less. As a result higher pO2s can be tolerated because of less muscle activity.

Im not trying to be antagonistic, but I am honestly curious.

 

[rjack321]

But they would be LABELED with the MOD for 1.6 ,even though you would not choose to actually dive them that deep?

Bottom gases labeled generally 1.4
That stays the same even though you might not plan on using them that deep.

For instance, for a short ocean dive to 170ft 21/35 is fine and labeled 190. A long dive to the same depth might chose 18/45 labeled 220. The depth of the planned dive doesn't change the depth the gas is increasingly trying to kill you at.

21/35 has a practical use depth of 100/130 (depending on agency) down to ~165ft due to narcosis while 18/45's useful depth range 150-200ft or so.

By and large, only deco gases get used to their labeled MOD. Bottom gases have a squishy practical use depth which is above the "increasingly actively trying to kill you" MOD. This practical use depth varies based on the dive duration and conditions (cold, current, vis, penetration, etc).

 

[rjack321]

Explain how 1.6 is better when shallow then when deeper? I am not a deco diver or anything, but I always imagined it was more acceptable because activity on deco stops is (i think) much much less. As a result higher pO2s can be tolerated because of less muscle activity.

Im not trying to be antagonistic, but I am honestly curious.

Deep deco gases are mostly carried from a rock bottom perspective. There's only a very modest amount of offgassing you can do switching to 21/35 and 190ft coming up from a 300+ft dive. Using a ppO2 of 1.6 that deep is more risk than its worth from a deco perspective (you might still need to swim, elevates your total CNS% by the end of the dive, you wouldn't have a habitat yet, less support, etc)

 

[ScubaFeenD]

Deep deco gases are mostly carried from a rock bottom perspective. There's only a very modest amount of offgassing you can do switching to 21/35 and 190ft coming up from a 300+ft dive. Using a ppO2 of 1.6 that deep is more risk than its worth from a deco perspective (you might still need to swim, elevates your total CNS% by the end of the dive, you wouldn't have a habitat yet, less support, etc)

OK, so judging from this response I am guessing that nitrox cert isnt enough to fully grasp the different aspects of it. Thanks for the answer though

 

[Rainer]

Plus you're already running pretty low PPO2s on the bottom for such extremely deep dives.

Deep deco gases are mostly carried from a rock bottom perspective. There's only a very modest amount of offgassing you can do switching to 21/35 and 190ft coming up from a 300+ft dive. Using a ppO2 of 1.6 that deep is more risk than its worth from a deco perspective (you might still need to swim, elevates your total CNS% by the end of the dive, you wouldn't have a habitat yet, less support, etc)

 

[rjack321]

Bottom gases don't need to be labeled as the mix should be appropriate to the dive to begin with.

How many dives have you done with bottom stage(s)?

For me in a cave, they are gonna get MOD labels. For me on my boat or any charter I might use they are gonna be marked with a MOD even if its just duct tape. (although the 3" mailbox letters look way better)

Compared to the price of a roll of tape and a fat sharpie, not marking them is really just nickel rocket laziness.

 

[rjack321]

OK, so judging from this response I am guessing that nitrox cert isnt enough to fully grasp the different aspects of it. Thanks for the answer though

No worries.

Imagine running repetitive 90ft dives on 36% with 30min SIs. By the end of a 6 dive day you'd be pretty fried from both pulmonary and CNS% perspectives.

A long deco dive is similar to this scenario except that instead of a short SI you get a slight lowering of ppO2 as you ascend. And a backgas break before switching to the next deco gas.

Given the choice would you rather dive air (low ppO2 gas) for dive1 of the series or dive6? You should chose to dive the low ppO2 gas for the first dive and then have the shallower EAD for all the subsequent dives. Vs. diving shallow EADs for dives 1-5 and having a relatively deep EAD on the last dive.

Not sure this makes sense but I hope so.

 

[ucfdiver]

OK, so judging from this response I am guessing that nitrox cert isnt enough to fully grasp the different aspects of it. Thanks for the answer though

It should be enough to understand it, but perhaps Richard's response was a bit lacking on the detail because he assumed you had some deco training or understanding. Unfortunately nitrox courses cut out a lot of information that I believe really needs to be introduced early on.

Rock bottom is what we refer to as the gas to get a diver and buddy to the surface. For instance, if you plan a 2min safety stop at 20ft, and a 1min stop at 50ft, you'll want to have enough gas at the lowest portion of your dive to share air with your buddy long enough to ascend to 50ft, stick there for 1min, ascend to 20ft, and stay there for 2 min.

For a deco dive, if we had 30 minutes of deco from 120-20ft, you need enough gas to carry your buddy at least until his first bottle switch. The more bottle switches you have, the less distance you need to concern yourself with supporting a buddy for.

Let's say we're doing a 160ft dive, and we only have o2. If deco starts at 80ft, that means that from 160ft to 20ft, we're sharing gas in a worst case scenario. Now, if we add a 50% / 70ft bottle, our "rock bottom" has reduced, as we only need to carry a buddy from 160ft to 70ft.

Now, as for deco benefit....at 120ft, we're not seeing a huge pressure difference, so even with a .2 higher ppo2, off gassing isn't all *that* fast. The extra CNS loading really adds up by the end of the dive (we often run over 100% CNS out of necessity), so doing it in areas where there's no huge benefit isn't worth it.

Finally, until you get shallow, there's a very real possibility of having to swim. Particularly in caves, a 120 bottle might be 1000ft back. Usually by the time you're on pure o2 or even a 70 bottle, things have calmed down.

 

[ScubaFeenD]

OK, so I think I just misunderstood, and actually my original thought is correct. Higher PO2 is acceptable at shallower depths due to probable less energy expenditure and the increased benefit gained during deco. It makes sense; the reason I asked initially is because it seemed someone was suggesting that pO2 of 1.6 was somehow not the same at shallower depth as it is as deeper depths (that wasnt jiving with what i understand about partial pressures). It might be different, but not because of physical properties.

Also, your right, I should have learned (officially) about rock bottom by now. I will have to wait for fundies for that I think.

thanks rjack321 and ucfdiver

 

[lamont]

OK, so I think I just misunderstood, and actually my original thought is correct. Higher PO2 is acceptable at shallower depths due to probable less energy expenditure and the increased benefit gained during deco. It makes sense; the reason I asked initially is because it seemed someone was suggesting that pO2 of 1.6 was somehow not the same at shallower depth as it is as deeper depths (that wasnt jiving with what i understand about partial pressures). It might be different, but not because of physical properties.

whatever happened to the idea that higher ppN2s offset the tox risk from higher ppO2s? that was a popular meme from back in the days of > 2.0 ppO2s on 21%, but i don't know if that was just rationalization or had any actual science...