@freemo So technically some Trimixes could be available (the ones where the ratio of nitrogen to oxygen is at least 4:1, so they can be mixed out of air, nitrogen and helium)?

@robryk in theory they could mix helium into air and get a trimix. But by the depths I'd use trimix I'd need pure O2 to deco safely or timely. Even with pure oxygen deco takes about an hour after 10 - 15 minutes at the bottom. To deco on just air would take forever.

@freemo Is the "safely" part due to nitrogen narcosis, something else, or both?

@robryk I just did the math, a 400 ft dive with proper gases would be a 2 hour deco with 15 minutes bottom time. The same deco using just air would be 4 hours. So not as bad as I though but still not cool. I would also likely run out of air on standard tanks.

Less safe simply because your doing inefficient deco. More time int he water means more risk. It is within the algo, so "safe" enough, but its a riskier dive profile than it needs to be is all.

@freemo I'm somewhat confused by this aspect of safety. Would doing the longer (for air) profile using EAN be just as (un)safe as doing the profile for air with air? (I'm trying to figure out if this is safer just because there is a risk that's ~proportional to time spent underwater, or if there's something specific that e.g. makes slower deco less well understood.)

@robryk There is a lot of debate as to what is safest honestly and much of it is a matter of opinion. We have multiple algorithms and people seem to have varying opinions on what is best.

Consider the same dives on a buhman deco algorithm (the earlier numbers I gave was using the VPM model). With the ideal gases your deco has now become closer to 3 hours, but with just air your deco is 8 hours on a buhlman model.

So using air you are kinda in this area where one algo tells you you are going to die and the other says youll be fine. The margins between algos are much less when using ideal gases.

While this doesnt clearly explain WHY there is a greater risk it does sort of show that there is a big risk there.

Now is spending that 8 hours of deco more risky than doing it with proper gases and spending 1/4 the time in deco, probably. You can follow an algorithm perfectly and still get decompression sickness and die, the risk is always there. The more you push the limits the more risk there is. A 400ft dive is always going to be more risk than a 100ft dive, and similarly a five where you are using gases that slow your ability to off gas are inherently riskier too.

It isnt about the time int he water though. I actually double my last stop at 20 feet when i dive even thought he deco algo says im good to surface. But thats because I'm spending more time off gasing.

Now if i am to speculate as to why longer dives on air is more dangerous I think the best explanation would be saturation of deep tissue compartments. Deep tissue has to go through several layers of tissue to off gas. So what happens is that while at first your shallow tissues are more saturated that moderately deep tissues as you surface your moderately deep tissues diffuse to both deeper and more shallow tissue, and they must off cas before the deep tissue will start off gasing. The longer it takes to deco the more the shallow saturation will leak into deeper tissue before reversing and off gasing into the shallower tissue. This takes a much longer time, and more time means more possible for error in the algorithm if your metabolism isnt as expected. All this is adding risk.

@robryk Also keep in mind most of this is moot as I'd run out of gas LONG before 8 hours is up anyway.