Question about Oxygen Exposure

[DevonDiver]

That's how I can tell you weren't Army. No rational statements are EVER allowed to get in the way of a beer

We're talking about BUYING a beer.

(3 years...trooper... I'll have you know... then I went for an easier life )

 

[GrimSleeper]

I agree with everything said above, but show me a recreational diver who runs out of oxygen clock and I'll send you a warm beer.

<Hijiack> Actually, when I did my IDC Staff course the Course Director put together his own extra exam designed to test whether any of us had any actual diving knowledge, and one of the questions involved a series of recreational dives which did put the diver over the CNS limit. It was a while ago now and I can't remember the profiles, it's past 6 and I'm still at the shop, and I have cold beer in the fridge at home (so your warm beer is safe, fear not!), but I'll have a play with the tables and see if I can reconstruct it if I get bored of servicing regs and blending gas tomorrow...

And as for computers, yeah, what is it with Suunto? Absolutely nothing to do with the OP, but my D6 managed to put me at 185% of "Oxygen Limit Fraction" (whatever that means) on a dive where V-Planner (and, because I am sad, manual calculation) showed me at 55%. 185% involves an awful lot of annoying beeping. </Hijack>

 

[eternaljonah]

Recreational divers (as trained by PADI) don't dive at 1.5ppO2. Beer denied...

Naui beer regained !


superlyte27 gas was and mostly is 32%

 

[darushin]

34 meters on what gas? Their average depth is 34 meters for 35 minutes at 1.5? I don't care about your bad computer, at 1.5ppo2 your oxygen clock is good for more than 35 minutes.

---------- Post Merged at 10:55 PM ---------- Previous Post was at 10:53 PM ----------



Be more specific and I'll show you that the dives you are doing are either using the wrong gas, or not "recreational".

I would say this probably is a conservative setting in the computer and or that that computer uses truncation to display the ppo and rounding of the internal calculation points and inputs. It is possible that a computer will not calculate a 45min max o2 at 1.6 ppo because of rounding, formula, and general accuracy of computer data.

 

[GrimSleeper]

I agree with everything said above, but show me a recreational diver who runs out of oxygen clock and I'll send you a warm beer.

Caveat: Obviously, this entire exercise is a nonsense, taking profiles to extremes just to sort-of prove a point (and claim that all-important warm beer). But you can take a recreational diver off the CNS clock if you really try. Equally obviously, my wife's doubles regs have NOT been serviced this morning, and I'm in terrible trouble when she gets back in from the dive...

Using the DSAT/PADI EAN32 and EAN36 tables alongside the DSAT Oxygen Exposure Table, let's suppose (he said, going all PADI for a moment) that a diver is on a liveaboard, and that liveaboard offers free Nitrox (only to suitably certified divers, or we'd all be in terrible trouble).

First dive of the day is a wreck at 30m. (Sorry, I live in metricland, don't have a set of tables in feet).

29 minutes at 30m on EAN32. Diver surfaces in Pressure Group Q, 20% of allowable daily CNS O2 exposure.

The wreck's so nice, they wait 90 minutes, then do it again.

19 minutes at 30m on EAN32. Pressure group Q again, now 35% CNS.

The boat moves to a wall dive, SI before dive three is 120 minutes. Our diver wants to maximise bottom times on his fabulously expensive liveaboard trip, so switches to EAN36 and jumps in.

48 minutes at 24m on EAN36. Pressure group W, 65% CNS.

Again, beautiful dive, let's go again. Plus, we didn't really get the photo we wanted of those anemone fish. It's dive number four, though, so let's be a little more conservative, eh?

90 minutes at 16m on EAN36. (OK, not that much more conservative, but we've got good air consumption). PG V, 95% CNS.

Time for a nap.

Three hours later, the crew announce that we're at the night dive spot where we can photograph Mandarin fish mating. Let's rock! Oh, but hang on, going by the tables we have all of 15 minutes at 16m (the minimum depth on the EAN36 table) before we hit 100% CNS loading for the day...

Like I said, purely a (dumb) academic exercise, but it's the kind of diving a lot of people do on liveaboard trips.

Do I get my warm beer now, or are you going to pull some sort of real-world rule and point out that, especially with the wall dives and multi-levelling, the diver is actually nowhere near 100%?

 

[Superlyte27]

That's assuming entire dive time is at max depth. So, how often does what you described really happen? Almost never. Secondly, How often do you do your ENTIRE dive at maximum depth.

Average the depths, take the ppo2 average, use that for the o2 clock, and you'll see you were no where near timing out. Granted, I supposed what you described is possible, as is going and sitting on the bottom for the entire dive, but it's also possible for birds to start crapping gold nuggets. But I'm not going to hold my breath. Ultimately, beer denied.

 

[GrimSleeper]

That's assuming entire dive time is at max depth. So, how often does what you described really happen? Almost never. Secondly, How often do you do your ENTIRE dive at maximum depth.

Average the depths, take the ppo2 average, use that for the o2 clock, and you'll see you were no where near timing out. Granted, I supposed what you described is possible, as is going and sitting on the bottom for the entire dive, but it's also possible for birds to start crapping gold nuggets. But I'm not going to hold my breath. Ultimately, beer denied.

I did point out the whole exercise as I did it was nonsense... It was meant as a joke. Albeit a joke that involved me not servicing any regs and thus spending the next week in the doghouse.

I do actually agree that you'd have to really work at it to go off the clock recreationally. Which is why I don't bother checking my CNS loading when I do two-or three-tank trips on 30%. If you do find one of them nugget-crapping birds, can I have one of those, instead?

 

[ianr33]

Wouldn't take long to run out of O2 limit when you're doing deep altitude dives ;-) Shall I have my beer now or later

Sorry, no beer for you!

MODs for a specific nitrox mix are actually slightly deeper at depth. ( Surface pressure is less than 1 ATM, so total pressure at any depth of water is less than it is at sea level )

 

[darushin]

That's assuming entire dive time is at max depth. So, how often does what you described really happen? Almost never. Secondly, How often do you do your ENTIRE dive at maximum depth.

Average the depths, take the ppo2 average, use that for the o2 clock, and you'll see you were no where near timing out. Granted, I supposed what you described is possible, as is going and sitting on the bottom for the entire dive, but it's also possible for birds to start crapping gold nuggets. But I'm not going to hold my breath. Ultimately, beer denied.

Actually assuming a deeper depth probably would be a good counter estimate to offset for non air integrated computers to compute sat as I believe that sac rate does affect how quick someone one O2 sats. Not being a doctor, I can't say for sure and I am too lazy to look up the formulas that current gen computers use to calculate O2 sat.

Daru

 

[skankpile]

You are not going to hit the clock or really worry about this in a basic nitrox course because the recreational diver rarely sees the CNS clock get near 80% unless you are nitrox with multiple dives per day like on Franks liveaboard. In basic nitrox they give you an overview of CNS, talk about PO2 of 1.4 & 1.6 and give you a limit with no real explanation of what it is. Just learn to use the table and understand your CNS% limits and max PO2 for your gas max depth planning.

If you are technical then CNS clock and OTU's are a limiting factor.. and to truly understand it you'll need to understand that CNS is based off OTU's and the %CNS you calculate is actually based on your time/NOAA CNS limits. The NOAA CNS limits are based off sustained OTU and padded so you can still take a stage 6 chamger ride if you get a DCS hit.

the nasty of it:

say you do 100ft for 5 minutes.. here is how you calculate the numbers

100/33+1=4.03 --> 4.03*.21= .846 Po2 of air at 100ft
OTU= 5(.5/(.846-.5))exp-5/6 = 3.6 actual OTU's (an OTU is equivalent to breathing 100% O2 at 1ATA for 1 minute)

now you see that your 100ft dive was equivalent to breathing 100% O2 for 3.6 minutes on the surface.

now the CNS clock is based on the NOAA tables so you look up your Po2 on the table and you find that there is a 0.8 & 0.9..(360 & 450 respectively) split the difference and you see you max CNS clock is ~405 minutes for a PO2 of 0.846

5/405=1.2% If you run it in vplanner it will pad 2 minutes to travel to 100ft and back so it uses 7/405=1.7%cns