dive tables

[Charlie99]

diverbrian once bubbled...
Yes, the time is a factor. That is shown by the "CNS clock" and is normally reflected on repetitive dives similar to residual nitrogen except the excess oxygen does not off-gas at the same rate. For normal recreational diving, one can do the math and see that unless you are on a live-aboard diving rich nitrox mixes, you probably won't have a problem with it. But it is something to look at.

So don't underestimate either component in avoiding an oxygen toxicity hit. .

Perhaps you are confusing CNS and OTUs. You can easily exceed your CNS clock on a single dive, while you won't exceed the pulmonary/OTU limits with a single tank.

As an aside, most nitrox computers will gladly let you far exceed the NOAA CNS 24 hour limits. This includes the rather conservative Suunto computers. This comes about because Suunto and other computer manufacturers treat O2 CNS exposure as something that decays with a 60 minute (Suunto) or 90 minute (Oceanic) haltime during the surface interval, while NOAA simply totals CNS exposure for a 24 hour period.

The halftime for pulmonary effects (tracked by OTUs) is very long (a few days?) since the damaged tissue needs to heal.

 

[diverbrian]

I use the old navy nuc rule for that one. I will go down and risk the short term exposure if I have to for a rescue. I would easily go to 1.6- possibly 1.8 in that case. But I must say if MY buddy is down to 1.6, I would hope that he has good reason, because I won't be there under anything resembling normal conditions.

 

[Charlie99]

diverbrian once bubbled...
IANTD preaches 1.4 in cold water and actually swimming/working.
A PO2 of 1.6 is acceptable for decompression hangs when you aren't working. The IANTD standards recommend no higher than a working PO2 of 1.5 regardless of water conditions. Seeing that I live in Michigan, you can guess that I use the 1.4 and 1.6. I use the 1.5 in the Caribbean if I want to take pictures and the fish happens to want to hide just below my 1.4 MOD. I am typically just floating waiting for Mr./Ms. Fishie to come out and give me a good shot anyways in that case.

I differentiate between a limit and a target.

My limit is 1.6, unless it is a life and death emergency. The target/expected level is more like 1.2. In other words, I won't spend significant time higher than 1.2ata ppO2, but will do _short_ excursions to 1.6.

My point is that the risk is determined by how far your run up the CNS clock, not by the maximum instantaneous ppO2.

 

[Charlie99]

diverbrian once bubbled...
I use the old navy nuc rule for that one. I will go down and risk the short term exposure if I have to for a rescue. I would easily go to 1.6- possibly 1.8 in that case. But I must say if MY buddy is down to 1.6, I would hope that he has good reason, because I won't be there under anything resembling normal conditions.

Your numbers are far more conservative than the NOAA "exceptional exposure" limits, which are intended for significant, but not necessarily life threatening situations.

I don't have a copy of that handy, but it allows over 5 minutes at 2.0, IIRC.

Edited to add in NOAA table info.

ppO2 in ata: 1 dive, 24 hr, exceptional exposures in minues

1.4: 150, 180, 180
1.5: 120, 180, 150
1.6: 45, 150, 120
1.8: xxx, xxx, 60
2.0: xxx, xxx, 30

In other words, 30 minutes max at 2.0ppO2 is max limit on NOAA exceptional O2 exposure limit.

 

[diverbrian]

I agree with you that my target is normally in the 1.3/1.4 range with excursions a little higher than that. If I am planning a dive where the CNS clock may be an issue (normally a deco dive or one of several repetitive dives), I figure it out long hand on a dive planner with pencil and paper and back up the calculations with my Voyager software. My VyTec does NOT figure into that equation. I don't trust it that far. I don't like to push any of the limits if I don't need to. Need to is defined by life or death emergency.

Then I think of it like I did radiation exposure in the Navy. I would have taken massive amounts in a short excursion to save a life or several lives, but I wasn't going to go high at all under normal circumstances.

My target is normally at the limit in this case because 1.4 is pretty conservative by itself and the less nitrogen that I take in, the happier that I am.

I just don't want to encourage the pushing of 1.6 PO2's in normal diving as these limits have been established for a reason and much of the time is by other peoples mistakes.

 

[cornfed]

scubasean once bubbled...


I've operated on the premise that more info is generally better than less, especially when my life may depend on the info being shared.

They don't need to know what your tank previously contained. If you had a nitrox tank that you used all but a 1/4 of and ask to have it filled with AIR the only thing you need to tell them is, "here's my tank... fill it with air." Oh sorry, they might also need to know what fill pressure you want.

How is telling them the gas you last dove going the help in this situation?

Cornfed

Ps Too much information is just as bad as too little.

 

[cornfed]

Genesis once bubbled...
would have trouble using my tanks, as they're DIN-valved and most of the people I dive with are yoke-type people.

Nonetheless, I might kill MYSELF if I don't label my own tanks, and that's at least as bad!

Any tank of mine without a label is treated as having both an unknown pressure and FO2; until both are verified it cannot be used. That includes tanks of air; if it doesn't have a contents label, it may as well be full of CO from my point of view.

OK here's the scenario...

I dive nitrox (32% let's say) and have 800 psi left. I take the tank to a shop that only does air fills (if they had nitrox that's what I'd get and we wouldn't be having this discussion). Since the shop doesn't do nitrox they don't have a O2 meter. Now I have a full tank with an unknown F02! Better call off the next dive, right? Wrong. I strap the tank on, dive my old MOD and use my air tables!

There are times when I won't do this, ie the tank is going to sit in the garage and I might forget, but if I'm going to dive the tank right away I don't see anything wrong with this.

Cornfed

 

[cornfed]

Chris,

The link uses Navy Tables, PADI uses (I think) Navy Tables that have been doppler corrected/modifed and you're Mosquito uses RGBM.

Not only are you comparing apples and oranges, but you've thrown pears in too.

chrpai once bubbled...


Sorry Charlie, ( lol no pun intended ) but I wouldn't trust that table you quoted farther then I could throw it.

First they use 1.6 for MOD calcs instead of 1.4 and then it has very liberal calculations for No-D time.

For example it says MOD for 36% is 113'. Now lets say you want to dive a Max 1.4 so thats 95'. The chart you quote will give you an EAD of 70' and an NDL of 50 minutes.

Now PADI AIR RDP will only give me 40 minutes at 70' and the Nitrox 36 tables planning for 94 ( rounded to 100 ) will only give me 35 minutes. My Suunto Mosquito will only give me 38 minutes at 94' on 36%.

Doesn't seem like good diving to me....

 

[Genesis]

I dive nitrox (32% let's say) and have 800 psi left. I take the tank to a shop that only does air fills (if they had nitrox that's what I'd get and we wouldn't be having this discussion). Since the shop doesn't do nitrox they don't have a O2 meter. Now I have a full tank with an unknown F02! Better call off the next dive, right? Wrong. I strap the tank on, dive my old MOD and use my air tables!

There are times when I won't do this, ie the tank is going to sit in the garage and I might forget, but if I'm going to dive the tank right away I don't see anything wrong with this.

It only takes once that the tank goes out with you on the boat and for whatever reason you don't do the dive you intended - you have a gear malfunction, you can't clear on the first dive and decide not to try again that day, you just feel like crap, etc.

Now the tank goes into the garage with no contents label, and sometime later gets grabbed and you think it has AIR in it.

That could be bad. Its not terribly likely to be bad, but it could be.

I have a contents label on ALL of my tanks, or they are treated as "unknowns" and not diveable until I can verify their contents. If I don't have the O2 meter with me, then the tank is useless on that trip.

It only takes one mistake to nail you with a CNS O2 hit.

 

[Manfred]

Any Nirtox tank should be marked - period -
My nitrox tank is also O2 clean so filling it at a station that's not set up for it means I have to get it cleaned again.

If there's only 800 psi left just drain it and get air, that way you know what you're diving with.