Nitrox on boat with air refill

[Iralub]

@ RX7diver, you must be joking
@iralub: why empty the bottle. No need.

Already 6 pages on a non existing problem. Some guys are wonderful

I agree there is no need because there isn't actually a problem unless the OP was diving very deep (and even then not really)

But OP perceived there was a problem and emptying the bottle seemed like an easier solution to the perceived problem than complex computation.



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[IVC]

But OP perceived there was a problem and...

No, OP perceived there might have been a problem, so he wanted to do the calculation to determine whether there was a problem.

The real question is: "How do you know there is no problem?" If the answer is anything along the lines of "if you do the calculation" or "if you look up the tables," then you shouldn't pretend it's wrong to actually do the calculation or look up the tables since without such a confirmation one cannot say it's safe. In other words, I would love to hear an argument why it's silly to determine one's O2 exposure that DOESN'T include calculation or table lookup.

---------- Post added November 8th, 2015 at 11:23 PM ----------

I agree there is no need because there isn't actually a problem ...

Great argument - because YOU did the calculation and determined "there isn't actually a problem," there is "no need" for anyone else to do the same calculation and determine there is no problem.

I assume when you took the Nitrox class your instructor said: "Here are some problems, but you don't have to do them because I know the answers and you'll be fine." Then you said: "You're right, there is no need to do these problems because there isn't actually a problem and I'll be fine." So now you're telling me that there is no problem and that I don't need to do anything because YOU say it's not the problem. So, how do you know there is no problem unless you calculated it?

---------- Post added November 8th, 2015 at 11:30 PM ----------

It will be a fun exercise, and, besides, I'll get a chance to use the new iPhone HP 15C calculator app (reverse Polish notation) I recently purchased from Hewlett Packard!

I grew up with 15C and if I could get one today I'd literally buy a dozen to last me a lifetime. It's the RPN thing and the lack of "=".

You made my day by letting me know I can get an app for that. Can't wait to get it.

 

[Freewillow]

@IVC.

I really love your argument.

The "problem" comes from the fact that the boat is making a refill with air in a tank that has a nitrox left over.

Now consider this.

1. Good practice demand that the second dive is a shallower dive that the first one. Correct?

2. The O2 content of the tank will be anywhere between the original Nitrox mix % and air. Right, you do not need a calculator to find this out

So the advise on the first page of this subject was: put your computor on air and do not worry about the nitrox content.

As long as your max depth is lower than in the first dive and your NDL is calculated on air, there is NO PROBLEM.


I hope that you got it, this time .

 

[packrat12]

Here is my thinking:

1. First of all, for these *recreational* dives, I would be completely comfortable using a P02 of 1.5 ata. So, I would be completely comfortable using NOAA Nitrox II at 100 fsw. (Moreover, this is completely consistent with what was taught in the IANTD Nitrox course I took in 1993 or so.)

2. For *planning* these 100 fsw recreational dives, I would compute using a max depth of 110 fsw (rather than 100 fsw), for which EAN36 has a corresponding PO2 of 1.6 ata, which has a NOAA CNS oxygen limit of 45 min.

Keep in mind, I am assuming you have only someone's (US Navy in my case) standard air tables, an oxygen analyzer, and a pencil and slate (and, maybe, a calculator).

Freewillow, Centrals, et al, does this additional detail help? If I can find some extra free time, I'll actually run through the hand calculations. It will be a fun exercise, and, besides, I'll get a chance to use the new iPhone HP 15C calculator app (reverse Polish notation) I recently purchased from Hewlett Packard!

Safe Diving,

rx7diver

Use a program like Multi-Deco to calculate the actual information. The tables are not granular enough. To state that you will use a depth of 110’ but your example was 100’ is just going to give results for diving at 110’ with a rounded PPO2 of 1.6, not the dive you proposed. This is more equivalent to doing 100% O2 deco at 20’. No agency even remotely recommends this PPO2 for anything but deco due to risks. We are talking recreational diving here. I would call any dive above 1.4 PPO2 high for a recreational dive, although I do excursions with higher PPO2, I will never plan one for a deep recreational dive. Most recreational computers will be alarming through the whole excursion. If we look at the higher elevation of 1.5 PPO2 at 100’, we will find that even with 2 dives to NDL, we will be at 23% at the end of the dive and well below 11% (1/2 @ 90 Min) before the 3^rd dive. These are real calculations using actual depth for the square profile. You also need to account for the descent and ascent portions, which take about 5 minutes.

Using your mix, I will normally write a MOD of 110’ but dive at a max depth of 95’. I would never plan a square dive below 95’ on that mix. If the depth was below it, I would remix.
I can easily create a 3 tank dive that will be in deco on the 2^nd dive but yet be a NDL dive for all 3 dives when using a computer. This is more like real dives done every day. I do not have to twist and bend to make it work. To try to make your example work, you had to jam your tank on the first dive and then fill to max psi on subsequent dives to give the highest mix possible. You also needed a square profile at a PPO2 higher than any agency recommends for the ‘work’ porting of the dive to try to make it work. It still came nowhere near exceeding CNS. Lots of dives for many days should be carefully monitored for CNS and OTU, but a simple recreational dive day will not exceed either unless there are unique circumstances which will be readily apparent.

I have done back-to-back deco dives with a short surface interval and doing 100% O2 deco and still did not reach my CNS. To exceed CNS O2, you must work at it in a recreational dive!

In the end, get an O2 analyzer and know the mix.

 

[rx7diver]

I grew up with 15C and if I could get one today I'd literally buy a dozen to last me a lifetime. It's the RPN thing and the lack of "=".

You made my day by letting me know I can get an app for that. Can't wait to get it.

[HIJACK]
Make sure you get the right one: https://itunes.apple.com/us/app/hewlett-packard-15c-scientific/id503720774?mt=8. A bit pricey ($15), but my understanding is, the underlying code is authentic. And so far it's been a blast to use!
[/HIJACK]

 

[Hatul]

How about this formula:

Final FiO2= (init pressure in tank * your Nitrox FiO2 + (final pressure- init pressure)* .21)/final pressure

so let's say you had 800 psi with 32 Nitrox left in your tank and it's filled to 3000 with air

final %= (800*32 + 2200*21)/3000 = 24 %

it's like a weighted average equation.

 

[IVC]

As long as your max depth is lower than in the first dive and your NDL is calculated on air, there is NO PROBLEM.

There are TWO aspects of O2 exposure. NDL is just one. The question was about the OTHER one. (NDL part is trivial.)

I like the "don't worry about the other one" responses (and I have acknowledged them) as long as they are supported by facts and actual analysis. To that extent, the best advice was to get the tables (and/or apps, calculator, online white papers, etc.) and do the math to be sure the numbers add up. In the process, several interesting concepts also came up, such as the role of oxygen sensor vs. calculations, etc.

 

[packrat12]

IVC - The point is that in recreational diving, it is highly unlikely CNS or OTU will be a factor in single day diving. Even at the richest of mixes, you would need significant amount of dives to go over either. CNS is 1/2 in 90 minutes. Even near 80% CNS, in an 90 minutes, you will be at 40%, in 180 minutes you will be at 20%. If you are pushing PPO2 higher then 1.4, it adds up quick but so does your NDL times as you are going deeper. In this specific scenario, it was adding air after a nitrox dive without being able to calculate it the new mix. As is often said in this thread, I know what I am expecting in my mix. The analyzer is to verify. When topping off, I will often give a basic order not a mix, e.g. add 400 psi helium, add remaining air to 2450. I will then verify with an analyzer what I am looking at. As for quick in the head calculations, if I use approx. 1/3 of gas and then top off the 1/3 with air I will do something like 36*2/3 - 21*1/3 = 31%. If I was to dive it without an analyzer, I would dive it as air with a MOD of 138' but no deeper than 115'. I have dove a number of times with my nitrox set to air. Again as a recreational diver, unless I am pushing something, CNS and OTU are not a risk.

What is pushing it? High PPO2, Long rich mix dives, Many dives in a day, Many dives in a short number of days.

Oh and NDL is not about O2 exposure. With O2 exposure, you are concerned about Central Nervous Syndrome (CNS) and Pulmonary O-tox (OTU). O2 is just used to remove Nitrogen which is the NDL issue in this case. I could just as easily use Helium and remove Nitrogen while holding the O2 at 21%. Other issues come to play but this is good enough for this example.

 

[JohnnyC]

There are TWO aspects of O2 exposure. NDL is just one. The question was about the OTHER one. (NDL part is trivial.)

O2 exposure has nothing to do with NDL. O2 exposure is dose/time. In terms of OTU's and CNS tracking, you're looking at PO2 over the time of exposure. NDL is irrelevant. You could have a theoretical NDL of infinity, but still have issues with oxygen exposure. The inert gas is what will affect the NDL, which is why nitrox has longer NDL's.

Example: A PO2 of 1.4, considered the nominal working max for an open circuit dive, is 1.63 OTU's/min, 0.67%/min for tracking purposes (per IANTD C-3201). None of that is relevant in terms of NDL.

You are tracking your oxygen exposure based on your dive time, and whether you remain within your NDL's or exceed it and have a decompression obligation makes no difference. Your oxygen exposure and decompression obligation are separate issues, however your required decompression obligation can, and likely will, most certainly have O2 exposure implications.

 

[IVC]

O2 exposure has nothing to do with NDL. O2 exposure is dose/time. In terms of OTU's and CNS tracking, you're looking at PO2 over the time of exposure. NDL is irrelevant. The inert gas is what will affect the NDL, why nitrox has longer NDL's.

Yes, yes - sorry, I was rushing to answer and what I wrote is clearly incorrect.

What I meant was that "There are two aspects of diving with enriched air" which is what the whole thread is about. I appreciate the example, though, because that is precisely the answer I was looking for - a well established protocol and calculation that can be used to get a meaningful (and safe) answer, but which unfortunately is not part of the basic Nitrox class.