Nitrox when others are on air

[L13]

32%. 36m is just the deepest point and I'm happy to go to 1.5 PPO2 for a minute or two. Not sure how controversial this is, but from my research, 1.4 is just the "long term safe" limit, and tec divers are on 1.6 during deco regularly.

There are many factors effecting the risk from high PO2. Yes, 1.6 is accepted when there is relatively low work load and other risk factors are minimized during deco. Most of the evidence actually says that 1.2 or lower during the working part of the dive is actually safer. However, for Open Circuit Scuba where peak PO2 is fixed by fixed FO2 and depth, 1.4 is considered a reasonably safe compromise between High FO2 to reduce inert gas deco risks and low PO2 to reduce O2 risks.

An occasional short excursion to 1.5 is not a huge risk by itself, unless you are deep, cold, exerting heavily, and/or under stress(all of which raise your CO2 and O2 risks significantly). In the case you are discussing, you are already deep, and likely cold: make sure you are mitigating all the other factors and don't stay very long.

 

[OctopusLover]

There are many factors effecting the risk from high PO2. Yes, 1.6 is accepted when there is relatively low work load and other risk factors are minimized during deco. Most of the evidence actually says that 1.2 or lower during the working part of the dive is actually safer. However, for Open Circuit Scuba where peak PO2 is fixed by fixed FO2 and depth, 1.4 is considered a reasonably safe compromise between High FO2 to reduce inert gas deco risks and low PO2 to reduce O2 risks.

An occasional short excursion to 1.5 is not a huge risk by itself, unless you are deep, cold, exerting heavily, and/or under stress(all of which raise your CO2 and O2 risks significantly). In the case you are discussing, you are already deep, and likely cold: make sure you are mitigating all the other factors and don't stay very long.

Good to know all these risk factors. Water was actually 24° according to my peregrine, definitely not cold, and 36m is the seafloor which I didn't actually touch. The highest depth my computer logged was 35, which is a ppo2 of 1.45, and I stayed there for less than a minute. There was no current either, so I wasn't exerting at all either.

All this being said, I'm sure I read in some thread in scubaboard about how PPO2 isn't as much of a hard limit as it is a factor in how fast your CNS toxicity"clock" is running, and that a long dive at 1.4 would be significantly more risky than a short one at 1.6.

 

[BlueTrin]

I can’t remember the source but I think it’s because if you are on OC, you won’t be at 1.4 all the time so if you upper limit is 1.4 it’s likely to be less on average.

1.4 is conservative in the sense that it is rare to have had oxtox convulsions at this depth.

However, to give you an idea, on CCR where you have constant target PPO2 people dive 1.3 or 1.2 because it is constantly at this level.

Also when this was tested in chambers, it seemed that there was a huge variability between people and even with the same person the level at which they’d oxtox would change depending of when they’d test it.

So it’s up to you and your risk appetite for seeing a few fishes/wrecks.

Anyone with better sources please post.

 

[L13]

Good to know all these risk factors. Water was actually 24° according to my peregrine, definitely not cold, and 36m is the seafloor which I didn't actually touch. The highest depth my computer logged was 35, which is a ppo2 of 1.45, and I stayed there for less than a minute. There was no current either, so I wasn't exerting at all either.

All this being said, I'm sure I read in some thread in scubaboard about how PPO2 isn't as much of a hard limit as it is a factor in how fast your CNS toxicity"clock" is running, and that a long dive at 1.4 would be significantly more risky than a short one at 1.6.

There are two kinds of risk from O2, a relatively short term risk of seizure, and and a longer term risk of lung function. People routinely confound the terminology and risk factors between the two. The "Oxygen Clock" is relevant to the long term, not the short term risk.

Seizures can occur relatively quickly at elevated PO2 especially in the presence of High CO2. Risk in this case is probably better though of as a relatively constant risk per minute of exposure, rather than a growing risk from exposure over time. Think of it like speeding on the highway. 1 min at 90 mph to pass a car has has about the same risk as any 1 min of a 1 hour drive at 90 mph, but the total risk in that hour is greater.

ETA:
@OctopusLover , I feel like your are a little too complacent about the high PO2 risk from short term exposure. The Seizure risk is not a "how fast your clock is running" type risk.

 

[OctopusLover]

There are two kinds of risk from O2, a relatively short term risk of seizure, and and a longer term risk of lung function. People routinely confound the terminology and risk factors between the two. The "Oxygen Clock" is relevant to the long term, not the short term risk.

Seizures can occur relatively quickly at elevated PO2 especially in the presence of High CO2. Risk in this case is probably better though of as a relatively constant risk per minute of exposure, rather than a growing risk from exposure over time. Think of it like speeding on the highway. 1 min at 90 mph to pass a car has has about the same risk as any 1 min of a 1 hour drive at 90 mph, but the total risk in that hour is greater.

That's interesting, and I was definitely not aware of this. Do you have any links where I can read more about this? Especially the chances of seizures for a given PPO2 and why 1.4 was chosen as an acceptable risk vs 1.5 and so on.

To be clear, it's not that I'm trying to squeeze meters out of the mix but rather that this shop doesn't give you a choice, they offer 32% and that's it. Am I supposed to choose air over 32% because the dive site floor is at 36m?

 

[BlueTrin]

That's interesting, and I was definitely not aware of this. Do you have any links where I can read more about this? Especially the chances of seizures for a given PPO2 and why 1.4 was chosen as an acceptable risk vs 1.5 and so on.

To be clear, it's not that I'm trying to squeeze meters out of the mix but rather that this shop doesn't give you a choice, they offer 32% and that's it. Am I supposed to choose air over 32% because the dive site floor is at 36m?

You should try to find a shop that can do partial pressure for these dives.

 

[L13]

That's interesting, and I was definitely not aware of this. Do you have any links where I can read more about this? Especially the chances of seizures for a given PPO2 and why 1.4 was chosen as an acceptable risk vs 1.5 and so on.

To be clear, it's not that I'm trying to squeeze meters out of the mix but rather that this shop doesn't give you a choice, they offer 32% and that's it. Am I supposed to choose air over 32% because the dive site floor is at 36m?

Your nitrox course should have covered this. I don't have links handy, maybe some else on this thread does?

With a hard floor at 36m, I would dive 32% as well, as long as I wasn't planning to exert myself below 27m or spend more than a couple minutes below 33m.

 

[lowwall]

To be clear, it's not that I'm trying to squeeze meters out of the mix but rather that this shop doesn't give you a choice, they offer 32% and that's it. Am I supposed to choose air over 32% because the dive site floor is at 36m?

What you are supposed to do is stay at or above the MOD you have calculated for the mix you are using. For 32% and 1.4, you get a MOD of 33.75m.

That's where I would stay. Although I could see dropping down to the bottom for 30 seconds to pick up something or get a quick photo.

Since the likely outcome of convulsions at that depth is death, IMO it's just not worth pushing the envelope.

 

[OctopusLover]

Your nitrox course should have covered this. I don't have links handy, maybe some else on this thread does?

With a hard floor at 36m, I would dive 32% as well, as long as I wasn't planning to exert myself below 27m or spend more than a couple minutes below 33m.

I don't think the Nitrox course went into this level of detail. In any case, from Wikipedia:

"The risk of seizure appears to be a function of dose – a cumulative combination of partial pressure and duration."

This seems to contradict what you said above.

 

[BlueTrin]

I don't think the Nitrox course went into this level of detail. In any case, from Wikipedia:

"The risk of seizure appears to be a function of dose – a cumulative combination of partial pressure and duration."

This seems to contradict what you said above.

That only contradicts it if you only know linear functions …

PADI Nitrox material mentions seizures

Tidbits here:

Oxygen Seizures at PO2 ≤ 1.6 Bar: How Rare?

Learn about the signs and symptoms of central nervous system (CNS) oxygen toxicity and the likelihood of oxygen seizures.

www.shearwater.com