O2 Exposure/Toxicity

[TheGoast]

When diving with enriched air, O2 become a relavant factor to be awear of.
I have found meany tables and formula to calculate how much exposure you gain during each dive, usualy a percentage of a max of 100.
Each dive you add the percentage to the last, making sure you don't exceed a total of 100.

Can anyone tel me if there is a formula that works out the rate at which the 02 percentage exposure dissipates?

How long do you have to wait before you are back to 0% O2 Exposure?

 

[FIXXERVI6]

When diving with enriched air, O2 become a relavant factor to be awear of.
I have found meany tables and formula to calculate how much exposure you gain during each dive, usualy a percentage of a max of 100.
Each dive you add the percentage to the last, making sure you don't exceed a total of 100.

Can anyone tel me if there is a formula that works out the rate at which the 02 percentage exposure dissipates?

How long do you have to wait before you are back to 0% O2 Exposure?

If your refering to the NOAA CNS clock it has a halflife of 90 minutes. There are a great number of poeple that question this clock, many people blow the clock by a great deal and manage CNS with gas breaks when long oxygen decompression is used.

Some of those computers use some kind of funky calculation that combines CNS with OTU and gives you a bar or other useless read out.

I beleive the TDI tables are meant for a 24 hour period, I don't remember as I haven't used them for o2 tracking in a LONG time. If you go by the 24 hour period it would be 24 hours from the last exposure not for "the day" if that makes any sense.

 

[DA Aquamaster]

It's not part of the published portion of the NOAA based exposure tables but some dive computers give you credit for surface intervals.

As noted above a half life of 90 minutes is assumed, so after about 450 minutes you are pretty much down to near zero levels (3%).

Realistically though unless you are diving a bottom mix over 1.4 (a bad idea), are using accellerated O2 decompression along with high PO2s on the bottom, or are using high percentage nitrox mixes at really shallow depths where NDL's would not be a factor anyway, you really have to work at it to exceed the exposure limits.

 

[SkimFisher]

I've done long, repeated shallow dives on mixes in the low 40's and never even come close to reaching my exposure limit. I think it becomes a bigger factor during deco dives and/or pushing pO2's to the high end.

 

[TheGoast]

Thankyou all, I was trying to work out the max amount of diving I could do in a week using Enriched air (32% or 36%) without having to spend one of the days on the surface.
Looking at what you say, if I take a minimum of 90 minute surface interval each dive then I can half my previous O2 exposure percentage.
Providing my next dive is not going to give me an exposure of more than 50%, then I should never reach 100.

 

[tregrrr]

While working on a liveabord, we did up to 5 dives a day when short handed. Staff always used EANx when certified for it, and no one I know ever maxed out their CNS 0₂ clock

that worked out to 27 dives in a week without any negative effects other than fatigue and irritability

 

[Bronson]

Your question on formulaes is based on using Dalton's law to calculate the PO2's at he relevant depth/gas switches on a dive. Suggest you consider a book called "Technical Diving from the bottom up" by Kevin Gurr which has an appendix with all the calculations in a table. Not a simple formula but it works the same.

 

[ianr33]

Thankyou all, I was trying to work out the max amount of diving I could do in a week using Enriched air (32% or 36%) without having to spend one of the days on the surface.
Looking at what you say, if I take a minimum of 90 minute surface interval each dive then I can half my previous O2 exposure percentage.
Providing my next dive is not going to give me an exposure of more than 50%, then I should never reach 100.

I took a close look at this in V planner a while back. Came to an interesting conclusion.

If you dive 32%. Do not go below 111 ft (1.4 pO2) and do not go into deco then IT IS NOT POSSIBLE to exceed O2 limits. That applies no matter how many dives you do in a day. Within the above parameters deco is always the limiting factor.

I dont suggest you just take my word for it but personally I wouldnt worry about it.

It may well be possible to exceed the limits on a computer as most computers keep O2 exposure for 24 hours then it instantly drops to zero which obviously is very conservative but can not mirror reality.

On another note has there EVER been a case of O2 toxicity when diving within deco limits and 1.4pO2 ?