Nitrox on boat with air refill

[kelemvor]

Were you really diving mixtures so rich that 3 tanks would make exposure an actual concern? I mean, were you sitting at 1.4 or higher for any length of time?

BTW, there's a great article about 02 exposure limits here: https://decodoppler.wordpress.com/2009/02/10/daily-limits-for-cns-oxygen-toxicity/


Take a look at some of Doppler's example calculations. That's well above "recreational" mixes, and even include deco at 1.6. Yet they still don't exceed the prescribed limits.

 

[IVC]

Thanks for all the great responses. I am glad to see that the advice from more experienced divers is to do it properly instead of pretending that the problem doesn't exist. My main concern was that the issue of O2 loading wasn't addressed at all, not that it was likely to cause any harm *in this case.*

 

[stuartv]

The simple, conservative answer is, for the 2nd and 3rd dives, set your computer to air and manually calculate your O2 exposure based on your original EANx blend. I.e. if you started the 1st dive with EAN32, then calculate O2 exposure for dives 2 and 3 based on 32%, even though you know it's actually less.

The result will be a shorter NDL (than necessary) on your computer and a higher number calculated for O2 exposure than you actually have. But, the expectation is that your calculated O2 exposure will still work out to be within safe limits, so you would then "know" that you are safe (as much you can be) on O2 exposure and also safe (as much as you can be, thanks to your computer) on N2 tissue loading.

If your calculated O2 exposure WERE to come out higher than "allowed", then you could do the math for your start and end pressures to get a more accurate (and lower) estimate of O2 exposure. But, when you calculate worst case and it's still acceptable, there is no reason to do all that extra work.

 

[IVC]

I keep an O2 Exposure slate from IANTD in my wetnotes.

Having notes/tables is something I'm realizing more and more is a must. Dive computers are nice and handy for coral reefs, but once you start deviating from the standards, you suddenly find yourself having to work out solutions instead of following an established protocol. The only way to make this "working out solutions on the fly" safe is to have both the understanding of the problem and the data with which to calculate. I'm beginning to really appreciate the way tech divers think.

I've ordered Nitrox tables. They provide cumulative CNS O2 exposure based on time-depth so I can quickly see if I'm anywhere near the limits.

---------- Post added November 6th, 2015 at 10:22 AM ----------

The simple, conservative answer is, for the 2nd and 3rd dives, set your computer to air and manually calculate your O2 exposure based on your original EANx blend. I.e. if you started the 1st dive with EAN32, then calculate O2 exposure for dives 2 and 3 based on 32%, even though you know it's actually less.

This is what I *wanted* to do, but I didn't have tables for O2 exposure and my computer (I'm just starting so I'm using Mares Puck Pro) won't do the CNS calculation the way it will do, e.g., MOD. It will keep *calculating* O2 exposure, but won't let me use it in plan mode where I can get a simple depth-time limit the way I would get limit for no-deco time (N2 exposure.)

---------- Post added November 6th, 2015 at 10:29 AM ----------

Were you really diving mixtures so rich that 3 tanks would make exposure an actual concern? I mean, were you sitting at 1.4 or higher for any length of time?

Heh, I wouldn't know unless I calculated it and I didn't have the tools to calculate it because I didn't have any model or data off of which to base my O2 exposure, nor did I have any prior experience with Nitrox. Remember, this was my first post-OW dive and my first dive using Nitrox. All I had to work off of was the little "CNS bar" on my computer. I didn't know anything about the exposure model the computer was using or how quickly would that bar move.

In retrospect, Nitrox class should spend more time on models behind the calculations and simple rules of thumb for estimating O2 exposure.

---------- Post added November 6th, 2015 at 10:37 AM ----------

If your tank was O2 clean when you got on the dive boat, you've got a bigger concern because it wasn't O2 clean once you got the first fill on the boat. And it wouldn't have mattered if the boat did nitrox fills or not

It was a rental and it wasn't clean, but it's an important thing to keep in mind. However, that's another confusing part of scuba diving, where every single agency *outside* the scuba diving requires O2 clean equipment for anything above around 25% O2, while in scuba diving the limit for having to be clean is 40%.

 

[Lorenzoid]

. . .
I've ordered Nitrox tables. They provide cumulative CNS O2 exposure based on time-depth so I can quickly see if I'm anywhere near the limits.

. . .

Heh, I wouldn't know unless I calculated it and I didn't have the tools to calculate it because I didn't have any model or data off of which to base my O2 exposure, nor did I have any prior experience with Nitrox. Remember, this was my first post-OW dive and my first dive using Nitrox. All I had to work off of was the little "CNS bar" on my computer. I didn't know anything about the exposure model the computer was using or how quickly would that bar move. . . .

Well, at least you're giving the cumulative O2 exposure limits some thought, which is more than I suspect they teach in most recreational Nitrox classes. From what I recall in my PADI Nitrox class, they taught that we need only be concerned about not exceeding the MOD on any given dive, and that cumulative exposure is something a diver using only 32% max on single-tank dives to recreational depths need not be concerned about. From the examples in the article by Doppler (Steve Lewis) that kelemvor linked to above, it indeed seems virtually impossible. But it's certainly worth understanding.

 

[stuartv]

This is what I *wanted* to do, but I didn't have tables for O2 exposure and my computer ...

Heh, I wouldn't know unless I calculated it and I didn't have the tools to calculate it because I didn't have any model or data off of which to base my O2 exposure, nor did I have any prior experience with Nitrox.

Given all that, I would say the answer gets even simpler: Let the rest of the gas out of your tank to get it down to "empty", then fill it with the air on the boat. Set your computer for diving air and done.

Diving with only OW and Computer Nitrox (or whatever PADI calls it - that's what SDI calls it) training does (potentially, unless your instructor exceeds standards) leave you with some limitations compared to what you would be trained to be able to do with a course like TDI Nitrox.

---------- Post added November 6th, 2015 at 02:37 PM ----------

Having notes/tables is something I'm realizing more and more is a must.

Or two computers (for redundancy) and an O2 analyzer, and accepting that you might have to sit out if both computers crap out....

 

[wedivebc]

The problem here is what many experienced divers do without thinking, ie. knowing that as long as you treat it as air for deco purposes you won't exceed your O2 exposure limit. This will appear as a cavalier attitude to an new diver but as you can see from the above posts it's not really much of a concern. I would have hoped a CD or any experienced diver would have taken the time to explain why we can be a little lax about the mix under certain conditions but unfortunately that didn't happen.
Asking questions like this demonstrates a very healthy attitude toward diving and you should question things that seem out of place.
Get a cheap nitrox analyzer and feel secure in the knowledge that you always check what you breathe.

 

[geoff w]

Having notes/tables is something I'm realizing more and more is a must. Dive computers are nice and handy for coral reefs, but once you start deviating from the standards, you suddenly find yourself having to work out solutions instead of following an established protocol. The only way to make this "working out solutions on the fly" safe is to have both the understanding of the problem and the data with which to calculate. I'm beginning to really appreciate the way tech divers think.

I've ordered Nitrox tables. They provide cumulative CNS O2 exposure based on time-depth so I can quickly see if I'm anywhere near the limits.

No offense, but as a relatively new diver, it sounds like you're way overthinking the issue and losing sight of diving for fun (maybe that comes from having a physics background)! If you do the math, to exceed the daily recommended O2 CNS limit, you would be doing several long, deep dives. In general, stay within your NDL times (which is hard to exceed on most one day So Cal boat dives) and enjoy the diving. It's really easy to get wrapped up in all the technical intricacies reading posts on Scubaboard, and before you know it you won't be able to do a single simple dive without dual tanks, redundancy buoyancy, long hoses and enriched breathing mixes, even if you're doing just one dive off the beach to 25 feet for an hour. Whatever floats your boat!

It was a rental and it wasn't clean, but it's an important thing to keep in mind. However, that's another confusing part of scuba diving, where every single agency *outside* the scuba diving requires O2 clean equipment for anything above around 25% O2, while in scuba diving the limit for having to be clean is 40%.

NOAA and OSHA generally follow the guideline that enriched air up to 40 percent oxygen is treated the same as air, and above 40 percent oxygen, gases become more dangerous to handle. See the NOAA Scientific Diving Standards and Safety Manual at http://www.ndc.noaa.gov/pdfs/NSDSSM_rev1.pdf for one set of guidelines. If you're not seriously into tech diving, O2 clean is not worth it for recreational diving in my opinion (being O2 clean means you can put up to 100% O2 in your tank or through your regulators, and is important if a dive shop you frequent for fills does partial pressure blending for nitrox, as opposed to filling nitrox with a membrane system, a nitrox stick, or having it already in high pressure banks). But it is important to understand and should have been covered in your nitrox class

 

[rx7diver]

IVC,

The NOAA CNS oxygen exposure table is simple and easy to memorize. I recommend that you go ahead and memorize these oxygen exposure limits (along with the associated couple of rules). Then, if you have these oxygen limits and rules memorized, and you have an oxygen analyzer and a set of somebody's (e.g., U.S. Navy, NAUI, DCIEM, PADI, etc.) submersible air tables, you'll always be prepared for planning not only your initial dive, but your repetitive dive as well. Do your calculations using your slate and pencil during your surface interval—if you didn't already do them *before* your initial dive.

Safe Diving,

rx7diver

 

[The Laconic]

Why would you even need a calculator? Surely you can get close enough in your head.

For example, if you were diving EAN32, and you had 1000 psi left, and topped off with air to 3000 psi, then the pct02 would be 32*1/3 + 21*2/3 which is close enough to EAN25. Which you might as well treat as air anyway for deco planning.

If you had more than 1000 psi left, you should have stayed down longer...