40% O2 mix at the safety stop

[OldNSalty]

So I have a question, years ago I use to dive with a friend/instructor and when he took groups of friends out on his boat we would always hang a bottle at 20 feet with a 40% O2 mix (might have been even higher-don’t recall). He said it was there for us to use at our safety stop if we wanted to. Some of us would be diving NITROX some air but no one would necessarily be breathing the same % as the safety bottle.

I know zero about planning a dive when you use different mixes (if such things are even allowed). I asked him about O2 toxicity and how to calculate it seeing how I would be going from a 32% mix to a 40% mix at the end. He told me it wasn’t an issue as long as you weren’t pushing the limit anyway (which you shouldn’t). He said the extra O2 at 20 feet for 3 to 5 minutes was mathematically insignificant. He claimed the benefit was that you off gassed more nitrogen at the safety stop. (I can see this benefit as you would be breathing in less nitrogen so I suppose that is correct)

Of course, he wasn’t suggesting that you change you dive plan; he said it just added an extra margin of safety and that personally, he always felt better when diving like this.

So the question is, was this a good idea or an accident waiting to happen? I like having the staged bottle but I am not sure I like having a different mix in it.

[hypertech]

In my nitrox class, the instructor commented about how the top notch dive ops sometimes hang a 100% O2 reg or bottle for you at the safety stop.

I didn't/haven't done the calculations for your O2 loading after a dive and how much that would affect it (sounds like a good idea to do though), but your old instructors comment sounds reasonable. I wouldn't think a couple of minutes would be that big of a deal for a conservatively planned dive. If you are already on the edge, diving near MOD for your mix, you may need to be more careful.

If you choose to take advantage of this though, check your depth guage and make darn sure that bottle is at 20 feet or more prefereably at 15 feet and isn't any deeper. MOD for EAN100 using a ppo2 of 1.6 is 20 feet.

[String]

Not convinced 40% makes enough of a difference compared to back gas to make it worthwhile as a safety bottle.

50% possibly, 80% or 100% at that depth would make more sense for emergency deco.

Its quite common here for people to carry a richer mix and use it on the stop to allegedly increase the safety margin (and agencies teach it). I tend to do it myself if ive got some rich mix left over as well as its not going to do any harm.

[Blackwood]

I agree with Sting. Going from 32% to 40% is well into the realm of diminished returns (from a decompression standpoint).

Is it safe (from an OxTox standpoint)? Most likely. What you learned in your basic nitrox course is completely adequate to verify. That you are changing mixes makes no difference.

[TSandM]

If you are diving 32% and switching to 40, the advantage is so minor as to be worthless. The risk is also minimal, as the MOD of 40% at 1.6 (for deco) is 99 feet.

Pure O2 is a different story. There, you do have a significantly increased gradient for offgassing, but a lot more potential for toxicity. As I have said before, at that point you are trading a very small possibility of DCS for a small but lethal risk of seizures, and it doesn't pencil out for recreational diving.

[String]

As I have said before, at that point you are trading a very small possibility of DCS for a small but lethal risk of seizures, and it doesn't pencil out for recreational diving.

People dont seem to be dropping like flies here where its done (and here they generally carry the bottle below its MOD not as a hang bottle).

80% at 6m would seem fine to me.

[Deefstes]

If you are diving 32% and switching to 40, the advantage is so minor as to be worthless. The risk is also minimal, as the MOD of 40% at 1.6 (for deco) is 99 feet.

Pure O2 is a different story. There, you do have a significantly increased gradient for offgassing, but a lot more potential for toxicity. As I have said before, at that point you are trading a very small possibility of DCS for a small but lethal risk of seizures, and it doesn't pencil out for recreational diving.

OK, I'm curious. I don't want to hijack the thread but then I don't want to create another thread either.

Could someone explain some of this to me please. I am only OW qualified and have done no courses on gas mixing and MOD etc. However, I'm always curious and have done a fair bit of reading on it.

From what I understand oxygen becomes toxic at certain pressures and the partial pressure of O2 in your mix should always be lower than 1.4 or 1.6 depending on some factors. That's how the MOD is calculated for a mix.

But how does that affect a 100% O2 deco stop bottle? I mean if pure O2 is breathed at 5m we're talking PPO2 of 1.5. Is that unsafe? Why would that cause oxtox? Does it have something to do with residual oxygen buildup from your dive? Does oxygen build up in tissue like Nitrogen? I thought your body metabolises oxygen.

Just curious.

[String]

1.5, low workload its more than likely fine. But what if buoyancy isnt great and the diver drops?

On planned stages deco dives CNS % exposure maybe an issue but not for no stop dives being talked about here.

[cave_diver]

As a general rule, it is always good to make safety-stops even if they are not required by standard dive tables. If you breathe a higher percentage of Oxygen, it helps to get rid of excessive Nitrogen and potentially CO2 that has accumulated in your body.

The higher the percentage of Oxygen in the gas-mix is, the faster the “cleaning”. However, you have to watch your depth since higher percentages of Oxygen become “toxic” at different depths. You will definitely need additional training to understand the process, and know what a PO2 of 1.6 or MOD, and OUT’s mean.

I would say breathing 100 percent O2 at 20 feet for 3 – 5 minutes is beneficial after every open water dive. Cost and equipment considerations and the lack of proper understanding of decompression procedures prevent the implementation.

Just think about it – 10 years ago or so most training agencies condemned using anything but air (21 percent O2) in diving.

[OldNSalty]

OK, thanks for all the input. I haven't done NITROX in years so I will probably take a course soon before diving it again. I think cave_diver summed it up good and thanks String and everyone else.

What exactly is the $$$ of this should I ever get a boat. A dedicated bottle and reg with a couple of seconds on it, equipment to suspend the bottle and the O2 fill? Is the 80-100% O2 fill $$$ (of course, if you are only using it for 3-5 minutes that tank should last for awhile-maybe not, depends on how many people used it...)