ppO2 for nitrox, why so low?
- Thread starter Bowers
- Start date
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
I've helped a diver out of the water that was twitching after a deco stop on a mix dive. He had cleared the deco but he had CNS symptoms.
I also dived with a couple of groups that had had divers take a CNS hit on the stop. The one I was briefed about, he was lucky, we used to deco on the trapeze, so he had assistance. But it was a close run thing.
One thing that was drummed into me on every course I've taken is don't push the max PO2. That's from Basic Nitrox through to Hypoxic Trimix, watch the PO2 limits. I've taken that bit of advice to heart!
Gareth
I also dived with a couple of groups that had had divers take a CNS hit on the stop. The one I was briefed about, he was lucky, we used to deco on the trapeze, so he had assistance. But it was a close run thing.
One thing that was drummed into me on every course I've taken is don't push the max PO2. That's from Basic Nitrox through to Hypoxic Trimix, watch the PO2 limits. I've taken that bit of advice to heart!
Gareth
I think what is important to remember is that CNS toxicity risk is not 1 dimensional.
It’s not about the PPO2 only...cumulative exposure time is a big hitter for the types of exposures most of us are subjected to.
2.0 with low exertion on a bounce dive? Probably going to be fine...but maybe not.
1.6 with low exertion within NDL?
Probably going to be fine.
Exertion or long exposure times to either of those PPO2s? Starting to gamble like betting 00 on a roulette wheel.
It’s not about the PPO2 only...cumulative exposure time is a big hitter for the types of exposures most of us are subjected to.
2.0 with low exertion on a bounce dive? Probably going to be fine...but maybe not.
1.6 with low exertion within NDL?
Probably going to be fine.
Exertion or long exposure times to either of those PPO2s? Starting to gamble like betting 00 on a roulette wheel.
CuzzA
Wetwork for Hire
Tom you hit the nail on the head.
I set my computer for 1.6 and plan to dive 1.4, but am willing to drop below the 1.4 floor if the situation calls for it and I don't want to hear my computer screaming at me for doing so.
I think that if you look for the most consistent motif in this thread, it is uncertainty.
In the most well known cases of oxygen toxicity, there is no question about it, as in breathing 100% O2 at 100 feet. In other cases, though, if you read carefully, O2 toxicity is offered as a possible explanation for the fatality, with nothing else presenting as a more likely cause. In some cases, O2 toxicity is not even the most likely cause but is considered a possibility.
I remember reading the full DAN article that cautioned about taking drugs like Sudafed before diving on Nitrox. Although the article does finally warn against it, the majority of the article is devoted to explaining why there really isn't enough clear evidence to support that warning--they are just acting in an abundance of caution. The few cases upon which the warning was based were not clearly O2 toxicity, and it is impossible to tell that if O2 toxicity were involved, it had anything to do with the Sudafed. The warning is actually based on vague symptoms in mice given a drug similar to Sudafed.
So the bottom line is we really don't know what constitutes a safe level for any individual diver on any individual dive, so most of use choose to err on the side of caution. If you make a mistake in the other direction, you probably won't be able to apply that lesson to future diving.
In the most well known cases of oxygen toxicity, there is no question about it, as in breathing 100% O2 at 100 feet. In other cases, though, if you read carefully, O2 toxicity is offered as a possible explanation for the fatality, with nothing else presenting as a more likely cause. In some cases, O2 toxicity is not even the most likely cause but is considered a possibility.
I remember reading the full DAN article that cautioned about taking drugs like Sudafed before diving on Nitrox. Although the article does finally warn against it, the majority of the article is devoted to explaining why there really isn't enough clear evidence to support that warning--they are just acting in an abundance of caution. The few cases upon which the warning was based were not clearly O2 toxicity, and it is impossible to tell that if O2 toxicity were involved, it had anything to do with the Sudafed. The warning is actually based on vague symptoms in mice given a drug similar to Sudafed.
So the bottom line is we really don't know what constitutes a safe level for any individual diver on any individual dive, so most of use choose to err on the side of caution. If you make a mistake in the other direction, you probably won't be able to apply that lesson to future diving.
TONY CHANEY
Contributor
TONY CHANEY
Contributor
NOAA’s table allows up to 45 minutes at 1.6.
NOAA CNS Oxygen Exposure limits: Table gives percentage of NOAA "allowable" limits for a single dive. At the 1.6 ATM exposure level the "CNS Clock" runs almost 4 times faster as at a PO2 exposure level of 1.4 atm. An exposure to a PO2 1.4 ATM is the maximum "recommended" limit for the working portion of any dive. Exposure levels above a PO2 of 1.4 ATM are shown for contingency planning and for calculations concerning the decompression (resting) portion of stage decompression dives.
The NOAA CNS Oxygen Exposure Limits chart is used for avoiding CNS Oxygen Toxicity problems while conducting multiple dives on one day. The first column represents the maximum oxygen partial pressure exposure in atmospheres. The second column gives the maximum duration, in minutes, a diver may safely remain in the water at that exposure during a single dive. The subsequent columns represent the percentage of the maximum exposure limit used during a single dive, in five minute intervals.
A diver having made the first enriched air nitrox dive of the day would find the CNS exposure percentage on the table. By subtracting that number from 100% they could determine the maximum exposure for their next dive. Following the second dive, the CNS exposure percentages from each dive would be added, then subtracted from 100% to determine the maximum allowable exposure for a third dive. While many authorities suggest that a 90 minute half-time is applicable for surface interval credit when considering oxygen exposures, such credit is seldom taken into account for planning recreational dives.
This table is commonly used by divers trained in the use of Enriched Air Nitrox and/or Trimix. Diver's who have not completed training in the use of these specialized gases should not attempt to execute dives using them. The NOAA CNS Oxygen Exposure Limit chart is provided for informational purposes only.
NOAA CNS Oxygen Exposure limits: Table gives percentage of NOAA "allowable" limits for a single dive. At the 1.6 ATM exposure level the "CNS Clock" runs almost 4 times faster as at a PO2 exposure level of 1.4 atm. An exposure to a PO2 1.4 ATM is the maximum "recommended" limit for the working portion of any dive. Exposure levels above a PO2 of 1.4 ATM are shown for contingency planning and for calculations concerning the decompression (resting) portion of stage decompression dives.
The NOAA CNS Oxygen Exposure Limits chart is used for avoiding CNS Oxygen Toxicity problems while conducting multiple dives on one day. The first column represents the maximum oxygen partial pressure exposure in atmospheres. The second column gives the maximum duration, in minutes, a diver may safely remain in the water at that exposure during a single dive. The subsequent columns represent the percentage of the maximum exposure limit used during a single dive, in five minute intervals.
A diver having made the first enriched air nitrox dive of the day would find the CNS exposure percentage on the table. By subtracting that number from 100% they could determine the maximum exposure for their next dive. Following the second dive, the CNS exposure percentages from each dive would be added, then subtracted from 100% to determine the maximum allowable exposure for a third dive. While many authorities suggest that a 90 minute half-time is applicable for surface interval credit when considering oxygen exposures, such credit is seldom taken into account for planning recreational dives.
This table is commonly used by divers trained in the use of Enriched Air Nitrox and/or Trimix. Diver's who have not completed training in the use of these specialized gases should not attempt to execute dives using them. The NOAA CNS Oxygen Exposure Limit chart is provided for informational purposes only.
More like 20 years ago. Max PO2 was 1.4ish, she toxed on Hill 400. CO2 and exertion probably played a huge role in it too.
Similar threads
