Oxygen Exposure Limits

[djensen]

This is confusing me. I may get it, but I don't know if I get it. Because I use a computer, I suppose I don't really need to get it. But I want to get it. Hopefully somebody can help me get it. Get it?

When my chart says the Oxygen Exposure Limit for 1.4 PPO2 is 150 min for a single dive and 210 for a 24-hr period, does that mean I have maximum 150 minutes AT 1.4 PPO2? That seems really long to me.

For example: Does that mean - providing I took enough gas down - I could stay at 33M on EANx32 for 150 minutes? That's how I read it, but it doesn't feel right to me...

It's a theoretical problem for me. Not something I'm considering. :shocked2:

Thanks!

 

[scubadada]

That's exactly what it means though my NOAA table gives a 150 minute single exposure and a 180 minute 24 hour exposure at a pO2 of 1.4

The 24 hour exposure is generally more applicable to diving. As an example, my four 75 minute dives per day in Boynton Beach to about 60 feet using EAN 36 puts me right at the daily exposure limit of 5 hours at a pO2 of 1.0.

Good diving, Craig

 

[TreyR]

Pick up a IANTD CNS tracking chart.

At a PO2 of 1.4 you have a maximize run time of 150 mins/dive before your CNS is 100%. At 33m you would have significant deco obligation so run time would far exceed 150mins. But say on a shallow dive, yes theoretically you could breathe a gas with a po2 of 1.4 for 150mins. However that is not Conservative enough and is risky.

 

[DA Aquamaster]

One thing the charts do not account for is the decay or half life of the O2 exposure. Every one pretty much agrees it occurs, and that it occurs faster at higher PPO2s and loadings, but no one really knows how fast, so the tables just assume it does not exist.

 

[Orlando Eric]

What they said. -- O2 exposure is a concern when you begin diving repeatedly day after day. I personally had to bust out the charts and do some figuring one day to see if I wanted to go back out on the next boat in Jupiter. I dove two tanks at 80fsw the conditions were once in a year perfect. The next boat was a three tank dive. I was fairly loaded at the end but within reason (and I am still here) Unless your computer will allow you to plan the next two or three dives there will be times were being able to do some dive planning with a pencil will come in handy. Unless you get some software and carry a laptop with you.

 

[ajduplessis]

Yes, that is correct. You will however exceed your NDL at that depth. I just want to make sure you are clear on that.

 

[djensen]

Yes. Absolutely. Thanks gang. This is what I suspected, but wanted to know the math and theory behind it. I had a great conversation with a tri-mix blender and, with these remarks, it makes much more sense now.

 

[chriskinney18]

The 1.4 is the max for that single dive but the percent of exposure cant exceed 100% for the day i thought or am i wrong?

 

[scubadada]

The 1.4 is the max for that single dive but the percent of exposure cant exceed 100% for the day i thought or am i wrong?

There is a single exposure and a 24 hour exposure time limit for pO2s from 0.6 to 1.6. You may be thinking about the generally recommended pO2 exposure limit of 1.4

O2 Exposure (h)
pO2	Single	24 hours
0.6	12	12
0.7	9.5	9.5
0.8	7.5	7.5
0.9	6	6
1	5	5
1.1	4	4.5
1.2	3.5	4
1.3	3	3.5
1.4	2.5	3
1.5	2	3
1.6	0.75	2

 

[wedivebc]

It is highly unlikely a recreational open circuit diver could exceed the oxygen exposure single dive limits but for a rebreather diver who spends his entire dive at a constant PO2 the limits come into play frequently. When I do cave dives on a rebreather I reduce my set PO2 to 1.0 for that reason. Most tech divers try to stay below 80% of the CNS clock.