Do you think Nitrox is a deep diving gas?

[HowardE]

There may be no good arguments for not using Nitrox responsibly, i.e. planning safe dives and diving the plan safely, however it does does place limits on what a diver can do safely.

As described in Lessons Learned, I was recently on a dive where another diver made an unannounced/unplanned/undiscussed descent to 130'. I was diving EAN32, which left me the chocie between aborting and diving below my MOD. Had I been diving air, I could have descended with him and persuaded him to retun to a moderate depth.

Nitrox is a trade-off. I personally take the benefits over the loss of flexibility in cases like this, however I would also accept as reasonable someone who wanted the flexibility. For example, a DM or instructor may routinely have to chase students back up to the planned depth.

32% Nitrox at 132 feet is 1.6 PPO2.

For how long would you have been at 130 feet? 2 minutes? 10 minutes? 30 minutes?

Exceeding the "mod" for a short period of time will not kill you. You need to factor in the TIME and EXPOSURE LEVEL.

In your case, I don't see why you wouldn't go get your "buddy" and tell them to come shallower, as the depth and PO2 made you feel uncomfortable.

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According to the widely accepted CNS Clock table... What is the maximum single dive exposure time at 1.6 PPO2? What % per minute do you rack up while at 1.6 PPO2? If you've been diving at 1.0 PPO2 for 30 minutes... for how long could you stay at a PPO2 of 1.6 while remaining within the CNS Toxicity limits of the table?

People who can't answer these questions... shouldn't be diving nitrox.

 

[Chatterton]

Forget knowledge and education. Nitrox will give you wings!!

 

[Reg Braithwaite]

According to the widely accepted CNS Clock table... What is the maximum single dive exposure time at 1.6 PPO2? What % per minute do you rack up while at 1.6 PPO2? If you've been diving at 1.0 PPO2 for 30 minutes... for how long could you stay at a PPO2 of 1.6 while remaining within the CNS Toxicity limits of the table?

People who can't answer these questions... shouldn't be diving nitrox.

Prior to making that dive, I decided that 1.4 was the maximum PPO2 I wished to tolerate and that I would not descend below 100' on that mix. I think it's reasonable for others to choose different limits and I will not argue with you if you take the same tank and choose to write "MOD 130'" on it.

However, it's my personal strategy to set a limit and stick to it, not modify the plan "on the fly" and decide that while 100' is a fine limit for most dives, 130' for two minutes is an acceptable limit for following someone. The issue here is not whether 130' is or is not safe, it's whether 90' during a dive going sideways is the right place to rewrite the limits I set for myself.

If you and I were diving together and you persuaded me that 130' on 32% was fine provided we observed the CNS exposure limits, I would have written MOD 130' on my tank before getting in the water. And had you descended to 160', I would let you go.

 

[HowardE]

No offense... but your answer is exactly why most basic nitrox courses don't give enough information to divers to make educated decisions about their gas mixtures.

 

[Reg Braithwaite]

No offense... but your answer is exactly why most basic nitrox courses don't give enough information to divers to make educated decisions about their gas mixtures.

I'm not offended, however we are arguing at cross purposes. My point is not about what constitutes safety for anything, it's about setting and sticking to limits even when under duress.

For example, I set myself limits around the amount of gas I must have when diving a single tank at various depths, the "minimum gas" or "rock bottom" amount. I don't try to make an educated adjustment to my limits during a dive, if I reach my minimum gas for my depth, I let my buddy know I've reached my limit and up we go. If there's a good argument that I could stay at that depth with even less gas, I prefer we work that out on the boat in advance of the next dive, not tear up the limit set for this dive.

To me the issue of whether one sets and sticks to limits is orthogonal to the issue of what those limits should be. Perhaps you prefer not to have limits but to work it all out on the fly based on circumstances and relative risks. Nothing wrong with that, but if that was my feeling about enriched air I wouldn't have written an MOD down.

Once the letters M, O, and D went on my tank my hands were tied.

 

[HowardE]

I don't think we're arguing at cross purposes.

MOD - is MAX OPERATING DEPTH. Not the brick wall to which we'll not run into, and therefore can not cross because it's written on the tank.

Understanding the concepts of Oxygen Toxicity, and the effects of higher partial pressures of Oxygen and exposure times, help divers to make educated decisions in the moment as opposed to sticking to an arbitrary guideline. Ask yourself - Why is 1.4 PPO2 the MOD for recreational divers? Why do technical divers use 1.6 ppo2? Why do decompression chambers use 2.0 PPO2? Why do most rebreather divers use 1.3 PPO2 as a setpoint?

Would you not rescue your buddy at 140 feet if you were using 32% nitrox? Would you just let them die? Or would you exceed the MOD to make the rescue?

Observing a gas management rule as opposed to MOD's are 2 different things. Gas management is ensuring that there is enough breathable gas available to complete the dive. MOD is really a guideline; as long as you know your exposure limits, and how much of your CNS clock you've used up for the day.

Once the letters M, O, and D went on my tank my hands were tied.

Your hands were tied? Who would chastise you? Instead of pushing your PPO2 to 1.58 PPO2, you chose to separate from your buddy? I just don't understand that choice, all because you wrote MOD on your tank?

While most divers make a plan, and do their best to stick to it. Things happen along the way that cause us to modify the plan on the fly. A good dive computer that tracks oxygen exposure as well as decompression obligations is really essential in my opinion.

I'm not saying for a moment that you made the wrong choice to stick to your plan,or MOD, since first and foremost, you completed the dive; but what I am saying is that I wouldn't have made that choice, and had I been at a point where I wasn't comfortable with the situation; I would have alerted my buddy that we need to ascend to a shallower depth.

 

[Reg Braithwaite]

MOD - is MAX OPERATING DEPTH. Not the brick wall to which we'll not run into, and therefore can not cross because it's written on the tank.

Understanding the concepts of Oxygen Toxicity, and the effects of higher partial pressures of Oxygen and exposure times, help divers to make educated decisions in the moment as opposed to sticking to an arbitrary guideline. Ask yourself - Why is 1.4 PPO2 the MOD for recreational divers? Why do technical divers use 1.6 ppo2? Why do decompression chambers use 2.0 PPO2? Why do most rebreather divers use 1.3 PPO2 as a setpoint?

Howard, I know that you can treat MOD as a guideline. I understand that the ox-tox is a statistical process and that the likelihood of getting a hit under the circumstances of that particular dive is insignificant, quite possibly less than the likelihood of suffering a malfunction that would require buddy assistance. I believe I know teh answer to the first two of your questions but not the rebreather question (but I'm happy to guess that the answer is that most rebreather divers dive a constant PPO2 which is an entirely different thing than a maximum PPO2).

Nevertheless... I tie my own hands when I choose limits for myself. I do not have buckets of experience, nor do I have much training... The basic nitrox course, a recreational trimix course, and reading a few books. So to a certain extent I do not consider myself a thinking diver yet.

Quite honestly, I don't trust myself to make a bunch of decisions on the fly while managing nitrogen narcosis under the stressof a dive that is going wrong. I believe that my brain is the weakest link here. This may change when I have a lot more dives and a lot more training. But until then... I choose to err on the side of following my plan slavishly rather than taking a chance on making a very wrong decision to change the plan.

I take your criticism as being leveled at the training. I think the training I received did not say that diving to 130' under those circumstances was verboten, nor did it rob me of the information that doing so is unlikely to be fatal. I think that if you want to criticize anything here, it should be my choice and not the training.

 

[BHB ScubaTroll]

There may be no good arguments for not using Nitrox responsibly, i.e. planning safe dives and diving the plan safely, however it does does place limits on what a diver can do safely.

As described in Lessons Learned, I was recently on a dive where another diver made an unannounced/unplanned/undiscussed descent to 130'. I was diving EAN32, which left me the chocie between aborting and diving below my MOD. Had I been diving air, I could have descended with him and persuaded him to retun to a moderate depth.

Nitrox is a trade-off. I personally take the benefits over the loss of flexibility in cases like this, however I would also accept as reasonable someone who wanted the flexibility. For example, a DM or instructor may routinely have to chase students back up to the planned depth.

Prior to making that dive, I decided that 1.4 was the maximum PPO2 I wished to tolerate and that I would not descend below 100' on that mix. I think it's reasonable for others to choose different limits and I will not argue with you if you take the same tank and choose to write "MOD 130'" on it.

However, it's my personal strategy to set a limit and stick to it, not modify the plan "on the fly" and decide that while 100' is a fine limit for most dives, 130' for two minutes is an acceptable limit for following someone. The issue here is not whether 130' is or is not safe, it's whether 90' during a dive going sideways is the right place to rewrite the limits I set for myself.

If you and I were diving together and you persuaded me that 130' on 32% was fine provided we observed the CNS exposure limits, I would have written MOD 130' on my tank before getting in the water. And had you descended to 160', I would let you go.

Wow!

Please understand that writing a number on your tank doesn't mean it is absolute... Especially in a danger situation where another diver's life (If you truly felt that way) was in danger.

If you actually know that diving to 130' is not past the absolute MOD, you can make this descent for the emergency situation whether it is written on your tank or not.

This part is truly an issue:
The issue here is not whether 130' is or is not safe, it's whether 90' during a dive going sideways is the right place to rewrite the limits I set for myself.

If you are not looking at contingency limits when dive planning, you are an accident waiting to happen.

I have to agree with HowardE if this is a typical example of how others are learning to use Nitrox gas mixtures - many basic Nitrox courses don't give enough information to divers to make educated decisions about their gas mixtures.

 

[Reg Braithwaite]

Wow!

Please understand that writing a number on your tank doesn't mean it is absolute... Especially in a danger situation where another diver's life (If you truly felt that way) was in danger.

I didn't feel the other diver's life was in danger. I still don't. I don't recall saying his life was in danger.

If you actually know that diving to 130' is not past the absolute MOD, you can make this descent for the emergency situation whether it is written on your tank or not.

I actually don't believe there is an absolute maximum, simply a statistical process where the likelihood of a seizure varies with a number of factors, of which PPO2 is a significant factor. I believe the things were are taught to calculate represent useful approximations and guidelines but do not necessarily match the underlying process. In this respect, I believe it is much like decompression sickness.

If you are not looking at contingency limits when dive planning, you are an accident waiting to happen.

Contingency planning is all well and good for handling certain types of emergencies. As stated above, I did not and do not believe there was any kind of emergency involved.

I have to agree with HowardE if this is a typical example of how others are learning to use Nitrox gas mixtures - many basic Nitrox courses don't give enough information to divers to make educated decisions about their gas mixtures.

I am not going to discuss how others are learning to use Nitrox. I learned the exact things Howard brought up in both of the courses I took discussing hyperoxic mixes. In each course, I was taught to use that information to formulate a plan based on my own personal tolerance for risk. In other words, to use my judgment.

I am prepared to suffer being publicly shamed for poor judgment, but I think it is asking too much of a course to teach good judgment. That is what mentors are for.

 

[HowardE]

I want to reinforce that I wasn't trying to shame you, or blame you.

I do blame (for the most part) the training (and Reg - maybe not yours specifically) in general with basic nitrox.

Most training agencies teach basic nitrox with the premise "don't exceed 1.6 ppo2 or you'll die"

I just wanted to point out again, and maybe a few newer divers will learn that nitrox and increased PO2 where the Pressure is greater than 1.0 ATA's of O2 PP has a risk of oxygen toxicity. In reality... you can spend several minutes (45 even) at 1.6 ppo2 and not be in the danger zone. Decompression chambers usually keep people at 2 ATA's of Oxygen Partial Pressure for 30 minutes at a time.

The key point for newer divers to remember and learn is that oxygen toxicity is due to a combination of: level of exposure (ATA's of Oxygen Partial Pressure) and time at that level.