O2 exposure

[LittleFish]

I have a question about diving nitrox that I thought I knew, but am not so sure now. When you calculate O2 exposure do you only take into account only your exposure within the last 24 hours? Or do you continue to count any accrued exposure unless it was more than 24 hours ago. I was taught that you continue counting until you have had a 24 hour break from elevated PPO2 levels. I have not seen a definitive source that clarifies this point.

Example:
Sat 8:00am in – 8:45am out = 15%
Sat 10:00am in – 10:45am out = 20% (35% total)
Sun 9:00am in – 9:45am out = 15% (50% total or 35% total)
Sun 11:00am in – 11:45am out = 25% (75% total or 40% total)

Thank you for your assistance,
>(LF))>

 

[rottielover]

Excellent question, I was thinking the same thing myself and was planning on asking my instructor at class tonight. I'll be sure to stop back by and let you know what he says!

 

[MikeFerrara]

How they teach it depends on the class.

If you use a computer, most computers that I've seen do give surface credit for CNS exposure. I think most use a 90 minute half time but I have one that uses a 60 minute half time. When decay is calculated in half times the curve is pretty close to zero after 6 half times.

In advanced courses IANTD and TDI both teach (or did when I was taught) to use a 90 minute half time.

For those who are interested you can calculate residual CNS% with the equation

%CNS X (0.5 to the power of (number of minutes/90))

I don't know of a entry level class that teaches to take surface credit for %CNS. All the texts that I have state simply that the exposure in any 24 hour period should not exceed 100%. To me that means counting all dives in the last 24 hours. Do what your instructor says but applying surface interval credit, a good nights sleep should leave you clean and ready to go for the next days diving.

 

[Rick Murchison]

If you are using the % oxygen clock method it is a sliding 24 hour window. If it's 1500 today I count all elevated exposures since 1500 yesterday and drop the ones before that.
If diving for several days or weeks at high PO2, you need to have a look at the Repex tables in the NOAA diving manual as well, and restrict your OTUs accordingly.
I personally don't care much for the "half time" approach to oxygen exposure, as oxygen isn't an inert gas and doesn't behave as an inert gas... the gas itself is scavenged very effectively by the blood and tissue levels return to normal very quickly after elevated exposures. But the damage to tissues done by oxygen must heal... I have this little voice that tells me the half-time approach may be less than optimum, and a break of several days time every week or so is probably not a bad idea. But it's just a feeling.
Rick

 

[rottielover]

Example:
Sat 8:00am in – 8:45am out = 15%
Sat 10:00am in – 10:45am out = 20% (35% total)
Sun 9:00am in – 9:45am out = 15% (50% total or 35% total)
Sun 11:00am in – 11:45am out = 25% (75% total or 40% total)

Ok I asked my instructor this question last night, the official answer is a sliding 24 hour window...

So in your example

Sat you've used 35% of your total, by sunday 9 am the 24 hour window from 8:45am the prior day has passed so you add your 20% from Sat 10AM and Sun 9AM for 35%. after sundays 9am dive.

By sunday at 11AM the 24 hour window has passed both Sat dives therefore you add your 15% from sunday morning too your 25% for your 11AM dive and get 40%

At least that's the official PADI standing on it.

OTOH, I've heard (but have not seen) that DAN and/or NOAA have documents out there pretty much stating that if your not diving Enriched Air more than a few days in a row that you could "reset your O2 clock" each night, similarly to how you "reset" your RNT to 0 after a full nights sleep.

So in your weekend dive example, notice how using either the rolling 24 hours or resetting each night gives similar results?

If you "reset your clock" at night in your example after Sat. your ending O2 exposure is 35% (15% + 20%) , and after sunday it's 40% (15% + 25%). Notice that on sunday the results are going to be the same since the 24 hour window has passed.

Think of the 24 hour window as a first in, first out, bucket, but it can only hold onto something for 24 hours, no longer. So at 9:45 Sat. morning your 15% go's into the bucket, by 9:46am on Sunday, the bucket cannot hold that 15% and it get's dropped out.

I hope that helps!

 

[H2Andy]

i've always understood it as a sliding 24 hour window, with dives "dropping off"
the clock once they are over 24 hours old.

i'm glad that was confirmed by the experts

excellent question

 

[LittleFish]

It sounds to me like it is really a combination of the two. The 24 hour window rolls, but a break every week or so should also be done. This sits nice in my head based on what I learned about O2 exposure in my class and what I was thinking.

So far this has not been an issue because I have not done any multi-day diving since getting nitrox certified. I hope to fix that problem this summer and get more diving in.

Thanks Guys

 

[Charlie99]

If you are using the % oxygen clock method it is a sliding 24 hour window. If it's 1500 today I count all elevated exposures since 1500 yesterday and drop the ones before that.

I personally don't care much for the "half time" approach to oxygen exposure, as oxygen isn't an inert gas and doesn't behave as an inert gas...

.........the gas itself is scavenged very effectively by the blood and tissue levels return to normal very quickly after elevated exposures. But the damage to tissues done by oxygen must heal... I have this little voice that tells me the half-time approach may be less than optimum, and a break of several days time every week or so is probably not a bad idea. But it's just a feeling.
Rick

It sounds like you are kind of mixing pulmonary/whole body toxicity in with the CNS toxicity. The pulmonary effect have much longer time constants than does the CNS effects.

There are lots of things in this world that have an exponential decay or "half time" approach. Whether or not oxygen is an inert gas doesn't have much relevance. Half time is used for radioactive decay and oxygen isn't radioactive, either.

---------------

The sliding 24 hour window method of accounting has O2 the assumed effects of O2exposure continuing with the exactly the same effect for a full 24 hours, and then suddenly, magically dropping to zero instantly at the end of 24 hours. That's a pretty weird phenomena, and clearly was done like that just to ease calculations. Don't confuse a simplified tracking method with what is actually going on.

One may argue with a single 90 minute halftime, since in real life many things are a combination of several halftimes of different strengths, but IMO the exponential decay is an awful lot closer to real life than "full effect for 24 hours, then sudden drop to zero".

Even if you assume that the halftime is 2 or 3 hours, the effect of yesterday's dives are significantly reduced the following day. That isn't much difference than saying that a good nights rest lets you do a lot of healing.

Perhaps a better analogy is recovery from drinking alcohol the previous night. It's a gradual recovery rather than a sudden instantaneous recovery at some specific time after you finish drinking.

 

[H2Andy]

a break every week or so should also be done.

i wish i dove enough for this to be an issue

 

[H2Andy]

The sliding 24 hour window method of accounting has O2 the
assumed effects of O2exposure continuing with the exactly the same effect for a
full 24 hours, and then suddenly, magically dropping to zero instantly at the end of
24 hours. That's a pretty weird phenomena, and clearly was done like that just to
ease calculations.

how's that different from "using up" all your time in one dive, and then having
to wait 24 hours to dive again?