Oxygen Exposure Limits

[burak]

you have to keep in mind that these percentages are just numbers. It's not like you're fine at 99% and you die when you reach 100%
I think many agencies advise that you have to calculate with toxicating above 75-80% of cns. On the other hand you could be fine with 200% as well.

It can be 150min at 1.4 bar to reach the 100% but at 1.6 bar its only 45 min. at 1.7 bar you have about 10 mins. At 1.8 bar it's like 4 mins or so. So you see the difference and how fast your time goes down if you have higher partial pressures.

if you have high ppo2 the chances are higher that you can get a instant cns toxification. In that case it's not important how high your cns percentage is. Under 1.6 bar it is considered to be stable.

 

[skankpile]

It can be hard to get CNS & OTU's straight in your head and their relationship/calculation. CNS is based off OTU's and EMPIRICAL HUMAN tests on what a persons nervous system can stand before nervous and respiratory sysmtpms appear. OTU's accumulate over time and both are limiting factors, but high CNS will get you symptoms quick and OTU's will build up and limit your dives on multiple days.

CNS is the CONVENTED part all agencies teach, and OTU's are what will burn out your lungs. CNS leads to high OTU, and with CNS tracking you are controlling your rate which you can tolerate OTU's. You MUST maintain an OTU buffer so you will be under the ceiling and be safe to take a chamber ride if needed... thus the repex method in the pdf I link.


(vPlanner will give the same, but this is how it is calculated)
say you have a dive air @:
100ft for 5 minutes
.846 PO2 is the P02 you have breathing air at 100ft ((100/33)+1).21=.846

There is a nasty formula for calculating OTU
otu= 5(.5/(.846-.5))exp-5/6 = 3.6 otu

What that means is that breathing 21% air for 5 minutes at 100ft was the same as breathing 100% air for 3.6 minutes at the surface; that is what you computer is calculating for OTS (ding ding.. your oxygen exposure), and during subsequent dives the limit you can sustain decreases. If you are doing one dive you can max out your CNS% and hammer the OTU's but on dives that span over a week you have to take it into consideration.

look at the NOAA tables for what is allowed on the CNS clock
cns= 5/405 (I split the difference on the table for 405)= 1.2%cns clock for that 5min dive

CNS has a half life so you can take a surface interval and then hammer it again at 80%, and for technical divers CNS and OTU is what really limits the dive times.. not the amount of air, scrubber duration, or decompression. When people see me with my CCR they always ask how long I can stay down and I say 4hrs, but its really a loaded answer because it depends so much on CNS & PO2.. so I just tell them the scrubber duration.

I dont know if I really explained it well as it will just click for you at some point.. check out this document:
http://www.oceanwreckdivers.com/images/tolerating_exposure_to_high_oxygen_levels.PDF

 

[wedivebc]

Actually OTUs are a gauge of whole body toxicity and a completely different animal than CNS toxicity which is what the OP was asking about.
If you stay within CNS limits you will never exceed OTUs limits even after days of diving.
OTUs are cumulative over several days, the CNS clock reduces by 1/2 for every 90 minutes at surface exposure (0.21atm)

It can be hard to get CNS & OTU's straight in your head and their relationship/calculation. CNS is based off OTU's and EMPIRICAL HUMAN tests on what a persons nervous system can stand before nervous and respiratory sysmtpms appear. OTU's accumulate over time and both are limiting factors, but high CNS will get you symptoms quick and OTU's will build up and limit your dives on multiple days.

CNS is the CONVENTED part all agencies teach, and OTU's are what will burn out your lungs. CNS leads to high OTU, and with CNS tracking you are controlling your rate which you can tolerate OTU's. You MUST maintain an OTU buffer so you will be under the ceiling and be safe to take a chamber ride if needed... thus the repex method in the pdf I link.


(vPlanner will give the same, but this is how it is calculated)
say you have a dive air @:
100ft for 5 minutes
.846 PO2 is the P02 you have breathing air at 100ft ((100/33)+1).21=.846

There is a nasty formula for calculating OTU
otu= 5(.5/(.846-.5))exp-5/6 = 3.6 otu

What that means is that breathing 21% air for 5 minutes at 100ft was the same as breathing 100% air for 3.6 minutes at the surface; that is what you computer is calculating for OTS (ding ding.. your oxygen exposure), and during subsequent dives the limit you can sustain decreases. If you are doing one dive you can max out your CNS% and hammer the OTU's but on dives that span over a week you have to take it into consideration.

look at the NOAA tables for what is allowed on the CNS clock
cns= 5/405 (I split the difference on the table for 405)= 1.2%cns clock for that 5min dive

CNS has a half life so you can take a surface interval and then hammer it again at 80%, and for technical divers CNS and OTU is what really limits the dive times.. not the amount of air, scrubber duration, or decompression. When people see me with my CCR they always ask how long I can stay down and I say 4hrs, but its really a loaded answer because it depends so much on CNS & PO2.. so I just tell them the scrubber duration.

I dont know if I really explained it well as it will just click for you at some point.. check out this document:
http://www.oceanwreckdivers.com/images/tolerating_exposure_to_high_oxygen_levels.PDF

 

[skankpile]

Yea Dave, I know you are not gong to push 1.2 at 210 minutes over days on end and jump back in the water for another 210 minutes as soon as you reach your minimum surface interval because you are a smart guy.

I see OTU's as the overall umbrella limit and CNS as a dive limit. If you are facing 10 days of diving you'll need to keep the OTU's 300-400 per day and not want to push your CNS limit so hard as your PO2 and exposure start impacting OTU's and your OTU vitality is steadily decreasing with the persistent hits. I don't think the OW nitrox guys generally make the connection.

 

[racerx_]

Ok I'm confused on the CNS-OTU thing.

1) Are you saying computer is tracking CNS but I need to calculate OTU for multi-day diving oxygen exposure?

2) Based on the computers calculations, I'm limiting overall o2 saturation to say 70-80% no matter how many days that stretches across. How is OTU affecting me in that case

 

[wedivebc]

CNS oxygen toxicity which is sometimes called the Paul Bert effect is the thing that causes convulsions in a diver and is what you are tracking when you are referring to NOAA chart posted above. It has a singe dive and multiple dive limit. The effects reduce during the surface interval at a rate of 1/2 of the oxygen clock every 90 minutes. In other words if you have a 80% CNS load after a single dive it will reduce to 40% after the first 90 minutes and 20% after 180 minutes at the surface.

OTUs track whole body toxicity also known as the Lorrain Smith effect. It causes inflammation of the lung tissue after long term exposure to high oxygen concentrations. The effect of whole body toxicity accumulates over days and does not reverse itself anywhere near as quickly as the CNS exposure. As I pointed out above if you observe CNS limits you will not exceed whole body limits so tracking both limits is not really practical. Furthermore, it would be very difficult for an open circuit diver observing the other limits of conventional diving such as max PO2 1.4 and NDL to come close to any other the oxygen exposure limits.

Ok I'm confused on the CNS-OTU thing.

1) Are you saying computer is tracking CNS but I need to calculate OTU for multi-day diving oxygen exposure?

2) Based on the computers calculations, I'm limiting overall o2 saturation to say 70-80% no matter how many days that stretches across. How is OTU affecting me in that case