Is my approach to diving with Nitrox logical?
- Thread starter Nitrox_DiverNY
- Start date
-
- Tags
- nitrox
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
Johnoly
Contributor
- Messages
- 4,015
- Reaction score
- 5,478
- # of dives
- 2500 - 4999
Like the many streets of politics, it depends on where & how you dive as to what your view point is. Most of my diving in about 100-110 ft on multiple dives. That 1.4 vs 1.6 therefore becomes an important choice if pushing a 36 mix and trying to avoid turning on the deco blinking light. In politics there is no single view point.......and not in scuba either.
dflaher
Contributor
Good article. A points that was mentioned more than once is that what matters is how long a diver stays at a certain elevated PPO2, not the PPO2 itself. For example 20 minutes at 1.4 could have higher risk than 5 minutes at 1.6. Is there a formula or calculation that normalized the 2 scenarios?
Reminds me of cooking meat. The FDA says `thou must cook chicken to 165F`, or whatever. But cooking it at a lower temp for longer is just as safe, and results in tastier chicken.
Reminds me of cooking meat. The FDA says `thou must cook chicken to 165F`, or whatever. But cooking it at a lower temp for longer is just as safe, and results in tastier chicken.
So far (I'm about halfway through) it just talks about not exceeding 1.6. Not a deco course so I don't expect they'll be much, if any, deco information. I've been looking for other limits but it's pretty clearly 1.6 for SDI.
ETA: Must be for bottom phase as the materials consistently say the MOD for EAN32 is 132ft; for PO2 of 1.4, it'd be 111ft.
Copy/pasted from the materials: "In addition to programming your computer with the information on the mixture you will be using, you will also need to program it with the maximum partial pressure to which you are willing to expose yourself. To get the maximum depth capability from EAN32 you need to program your computer for a partial pressure of oxygen of 1.6. If you want to be more conservative, you can set your dive computer for a PO2 of 1.5 or 1.4."
ETA: Must be for bottom phase as the materials consistently say the MOD for EAN32 is 132ft; for PO2 of 1.4, it'd be 111ft.
Copy/pasted from the materials: "In addition to programming your computer with the information on the mixture you will be using, you will also need to program it with the maximum partial pressure to which you are willing to expose yourself. To get the maximum depth capability from EAN32 you need to program your computer for a partial pressure of oxygen of 1.6. If you want to be more conservative, you can set your dive computer for a PO2 of 1.5 or 1.4."
This has been a wide-ranging but educational discussion.
I'm going to try to hijack the thread back to the original post, in which @Nitrox_DiverNY asked for opinions on his practice of getting the most enriched blend he can for his anticipated dives.
My response to his query would begin by asking the purpose of Nitrox diving for recreational divers; that is, what can Nitrox do and not do for recreational divers?
The takeaways a student should get from a basic Nitrox course are that:
--Nitrox will extend your NDL for a given dive to that of a depth between a quarter and a third less deep.
--Nitrox will shorten your required surface interval between repetitive dives.
--The cost of obtaining these benefits is that Nitrox caps your maximum depth at a level less than the depth limit for recreational diving. Going deeper than the modified depth limit incurs the risk of oxygen toxicity and death.
Let's apply these effects to the dives @Nitrox_DiverNY discussed.
Sometimes he plans dives with a maximum depth of 30-40 feet. If you look at the recreational dive planners, the NDL for a 40-foot dive is over 200 minutes. I don't know anyone who makes an AL80 last for three hours, so the limiting factor of a 40-foot dive is not NDL, which is a shorthand for the DCS risk the OP is trying to minimize. The limiting factor of these dives is the amount of breathing gas.
Because of that huge NDL, the surface interval before a subsequent dive to the same depth of 40 feet is also immaterial to the risk of DCS. The purpose of the surface interval in this circumstance isn't so much to offload nitrogen as to change tanks, re-hydrate, and rest a bit. By the time those tasks are completed, the NDL for the next 40-foot dive will again be longer than almost every diver can make the tank last.
So for single or repetitive dives to 40-feet, there is no benefit to using Nitrox at all, unless you are among the people who want "geezer gas" because they just feel more fresh and alert after breathing enriched air.
It is for dives between about 70 and 100 feet that Nitrox proves its worth to recreational divers. That's the depth range where the factor limiting the length of the dive shifts from available air to NDL.
The NDL for a 60-foot dive is a little less than an hour: a fair percentage of experienced divers can make a tank last that long at that depth. The NDL for a 100-foot dive is about 20 minutes; even more divers can make a tank last that long at that depth. For these divers, Nitrox flips the tables and again makes available gas the limiting factor.
Maximizing the richness of the Nitrox blend to minimize the risk of DCS for divers who obey the tables or their non-gun-decked computers takes a miniscule risk--and leaves it just as small. Therefore, my opinion is that the strategy does not improve the diver's safety and may distract him from obtaining the primary practical benefit that Nitrox affords the recreational diver.
I'm going to try to hijack the thread back to the original post, in which @Nitrox_DiverNY asked for opinions on his practice of getting the most enriched blend he can for his anticipated dives.
My response to his query would begin by asking the purpose of Nitrox diving for recreational divers; that is, what can Nitrox do and not do for recreational divers?
The takeaways a student should get from a basic Nitrox course are that:
--Nitrox will extend your NDL for a given dive to that of a depth between a quarter and a third less deep.
--Nitrox will shorten your required surface interval between repetitive dives.
--The cost of obtaining these benefits is that Nitrox caps your maximum depth at a level less than the depth limit for recreational diving. Going deeper than the modified depth limit incurs the risk of oxygen toxicity and death.
Let's apply these effects to the dives @Nitrox_DiverNY discussed.
Sometimes he plans dives with a maximum depth of 30-40 feet. If you look at the recreational dive planners, the NDL for a 40-foot dive is over 200 minutes. I don't know anyone who makes an AL80 last for three hours, so the limiting factor of a 40-foot dive is not NDL, which is a shorthand for the DCS risk the OP is trying to minimize. The limiting factor of these dives is the amount of breathing gas.
Because of that huge NDL, the surface interval before a subsequent dive to the same depth of 40 feet is also immaterial to the risk of DCS. The purpose of the surface interval in this circumstance isn't so much to offload nitrogen as to change tanks, re-hydrate, and rest a bit. By the time those tasks are completed, the NDL for the next 40-foot dive will again be longer than almost every diver can make the tank last.
So for single or repetitive dives to 40-feet, there is no benefit to using Nitrox at all, unless you are among the people who want "geezer gas" because they just feel more fresh and alert after breathing enriched air.
It is for dives between about 70 and 100 feet that Nitrox proves its worth to recreational divers. That's the depth range where the factor limiting the length of the dive shifts from available air to NDL.
The NDL for a 60-foot dive is a little less than an hour: a fair percentage of experienced divers can make a tank last that long at that depth. The NDL for a 100-foot dive is about 20 minutes; even more divers can make a tank last that long at that depth. For these divers, Nitrox flips the tables and again makes available gas the limiting factor.
Maximizing the richness of the Nitrox blend to minimize the risk of DCS for divers who obey the tables or their non-gun-decked computers takes a miniscule risk--and leaves it just as small. Therefore, my opinion is that the strategy does not improve the diver's safety and may distract him from obtaining the primary practical benefit that Nitrox affords the recreational diver.
halocline
Contributor
I apologize for only reading the first 5 or 6 pages, and I suspect that a few people have already basically said what I will. It's a great intellectual exercise to think about your blend, and O2 toxicity, and NDL risk, but what you are talking about has essentially zero practical application in a recreational dive to 30-40 ft. Honestly if you don't feel safe using air on a dive like that, you probably shouldn't be diving, or at least you are not seeing the larger picture of dive safety. There are all sorts of factors in DCS incidence, and your nitrox blend is only a tiny (and unproven) part of the risk equation. If you are initiating this conversation as a way of learning more, and not so much out of actual concern for your dive safety on a 40 ft recreational dive, then great. Learn everything you can about dive table and computer modeling, compartment theory, clinical cases of DCS and corrollated factors (dehydration, obesity, dive profile control, ascent rates, strenuous exercise immediately after diving, etc....) and then you will become a more informed student of dive safety in general. But if you believe that you will in any real-world way increase your safety on a single tank 40 ft dive by using a custom blend of nitrox, you are really barking up the wrong tree.
If you're talking about multiple daily dives, like for example on a trip to Bonaire where you can basically dive all day, tank after tank, at 30-40 ft, then using nitrox definitely has some real-world effect on your dive physiology, but at that point it becomes a balancing act between N2 and O2 exposure, so using a blend richer than the typical 32% may not have benefits.
As someone mentioned, we use 32% in cave dives in Mexico, which is often 4-5 hours per day of time spent at shallow depths similar to what you mentioned. Very very few people (nobody I know) ever uses custom blends for those dives. For planned deco dives, of course, but that's a different world.
There are just so many other FAR more important factors in avoiding DCS than your nitrox blend.
Similar threads
