Ommiting Nitrogen on Deco

[PerroneFord]

Forgive me is this is the wrong place, wasn't sure where gas questions go!

In reading more about the various deco models, I noticed that some divers were ommitting Nitrogen nearly completely in their deco. I was curious if anyone here was doing this.

We all know nitrogen is bad bad bad for diving, so trying to get rid of it makes sense. Essentially, it seems to be used for much the same reason 100% is used. I guess the issue becomes how useful is helium as it ongases faster and the body would seem to take on more in the deep stops versus nitrogen.

Anyway, just wanted to toss this out and see if any of the big spenders out there were mixing helium in their deco tanks.

 

[Diver0001]

Forgive me is this is the wrong place, wasn't sure where gas questions go!

In reading more about the various deco models, I noticed that some divers were ommitting Nitrogen nearly completely in their deco. I was curious if anyone here was doing this.

We all know nitrogen is bad bad bad for diving, so trying to get rid of it makes sense. Essentially, it seems to be used for much the same reason 100% is used. I guess the issue becomes how useful is helium as it ongases faster and the body would seem to take on more in the deep stops versus nitrogen.

Anyway, just wanted to toss this out and see if any of the big spenders out there were mixing helium in their deco tanks.

I'm not a big spender but I do know some people who are. It seems to me that some people are putting a little HE in selected Nitrox mixes to avoid big N2 spikes that can cause problems with isobaric counter diffusion (in layman's terms thats a sort of weird-o inner ear bend that makes you very dizzy and nauseous). On the whole I would guess without having run the profiles that deco-ing on Heliox would result in longer run times than deco-ing on EAN50 and O2.

R..

 

[PerroneFord]

Two points.

1. I didn't mean ONLY decoing on Heliox. I meant decoing on Heliox in mid-water and o2 shallow.

2. Why would Heliox (with the same pp02) give longer run times than EAN50?

I'm prettty new at this, but it would see to me that EAN50 at 70ft would be pretty similar to 50/50 at 70ft, minus the nitrogen uptake you'd encounter at that depth. No?

 

[PerroneFord]

Actually, I just ran these two profiles and you were right. Now I need to understand why. Oxygen windows and stops depths were identical. Basically the profile I ran was 150ft, 30 minute dive, on 21/35. First stop was at 80ft. I selected between EAN50 and TriOx 50/50 for the deco gases. The shallow stops on the mix were much longer, and I have to assume that is because of the helium offgassing in shallow depths. I cannot run accelerated stops because I haven't registered the program yet and it won't let me use 80/20 or 100% until I do

Thoughts are welcome.

 

[nova]

Actually, I just ran these two profiles and you were right. Now I need to understand why. Oxygen windows and stops depths were identical. Basically the profile I ran was 150ft, 30 minute dive, on 21/35. First stop was at 80ft. I selected between EAN50 and TriOx 50/50 for the deco gases. The shallow stops on the mix were much longer, and I have to assume that is because of the helium offgassing in shallow depths. I cannot run accelerated stops because I haven't registered the program yet and it won't let me use 80/20 or 100% until I do

Thoughts are welcome.

You're scarin me man. Both N2 and helium are inert gasses, The object of decompresion is to de compress the inert gas. Normoxic mixes (18% O2 and higher )still have a large amount of inert gas in them, But helium will come out of solution faster because of the size of the molecule. This adds time at the deep stops.

 

[PerroneFord]

You're scarin me man. Both N2 and helium are inert gasses, The object of decompresion is to de compress the inert gas. Normoxic mixes (18% O2 and higher )still have a large amount of inert gas in them, But helium will come out of solution faster because of the size of the molecule. This adds time at the deep stops.

I understand they are both inert gases. I also understand that the idea of deco is to decompress the inert gases. What has me confused is that I *thought* helium would offgas more quickly than Nitrogen. Apparently the opposite is true.

Why are you talking about normoxic? The deco gas we are talking about is 50% oxygen or higher.

Here is the planned dive: (150ft, 30 minutes bottom, 21/50 bottomgas)

The profiles look like this:

Depth ft. stop (minutes) (runtime) Mix
150 27 (30) 21/50
80 0 (33) 21/50
70 1 (34) 50/50
60 1 (35) 50/50
50 3 (38) 50/50
40 4 (42) 50/50
30 5 (47) 50/50
20 9 (56) 50/50
10 16 (72) 50/50



Depth ft. stop (minutes) (runtime) Mix
150 27 (30) 21/50
80 0 (33) 21/50
70 1 (34) 50
60 1 (35) 50
50 2 (37) 50
40 2 (39) 50
30 4 (43) 50
20 6 (49) 50
10 11 (60) 50


So you can see that the He mix gives a much less aggressive profile, all else being equal. I don't know enough about Helium offgassing or properties to know why, so I am making guesses. I was just wondering if someone could offer insight.

Note there is no o2 in either profile, and both were run with the conservatism held steady.

 

[do it easy]

i'm not an expert, but i think that it has something to do with the fact that you don't have any O2 at the shallow stopsl- there is no He gradient to promote offgassing of He, so this slows you down.

i think when divers add helium to deco mixes, it's certain combinations of deco gasses. the theory behind using 50/50 (according to the v-planner decomyths webpage) is that yes you do continue to on-gas He, but you'll have a steeper N2 gradient, so you'll eliminate the N2 faster. Then because helium is a faster molecule, you can eliminate it as you ascend. Ideally, you will get clear of both gasses at the same time; in other words, you haven't added too much helium so that you slow down your deco, and you haven't added too little so that the nitrogen takes too long to clear.

 

[do it easy]

I think that 50/50 deco assumes O2 at the 20' stops as well. This is the missing piece that will offgas the He faster to accelerate the deco, so that the helium catches up to the nitrogen. it will also help clear up the nitrogen, but this has been offgassing since the 70' stop or 50/50 switch.

 

[nova]

I understand they are both inert gases. I also understand that the idea of deco is to decompress the inert gases. What has me confused is that I *thought* helium would offgas more quickly than Nitrogen. Apparently the opposite is true.

Why are you talking about normoxic? The deco gas we are talking about is 50% oxygen or higher.

Here is the planned dive: (150ft, 30 minutes bottom, 21/50 bottomgas)

The profiles look like this:

Depth ft. stop (minutes) (runtime) Mix
150 27 (30) 21/50
80 0 (33) 21/50
70 1 (34) 50/50
60 1 (35) 50/50
50 3 (38) 50/50
40 4 (42) 50/50
30 5 (47) 50/50
20 9 (56) 50/50
10 16 (72) 50/50



Depth ft. stop (minutes) (runtime) Mix
150 27 (30) 21/50
80 0 (33) 21/50
70 1 (34) 50
60 1 (35) 50
50 2 (37) 50
40 2 (39) 50
30 4 (43) 50
20 6 (49) 50
10 11 (60) 50


So you can see that the He mix gives a much less aggressive profile, all else being equal. I don't know enough about Helium offgassing or properties to know why, so I am making guesses. I was just wondering if someone could offer insight.

Note there is no o2 in either profile, and both were run with the conservatism held steady.

now run the whole plan using air for bottom gas and 50% o2 at 70 feet for deco gas.It should be the same (or close to) as the helium dive. That's because the inert gas in the mix is the same. It doesn't matter what inert gas it is . The only differance is that helium has longer deep stops to off gas HE. Remember, helium can bend you fast than N2

 

[PerroneFord]

Nova,

Given what I have seen, the idea with Helium is to offgas deep with ambient pressure (unless you are pushing the gradient) alone while trying to drop the slower offgassing nitrogen earlier in the deco process. Do it Easy was correct that the missing link in this is the 02 deco, which allows the helium to offgas quickly at the top of the dive so that you nearly completely offgas the inert gases at nearly the same.

Running air as a bottom gas would basically see us offgassing nitrogen slowly toward the first stop, and then accelerating the process as we maximize the gradient beyond that. The primarly difference being that we would have more nitrogen to offgas, thus increasing the run time.

... Ok, just ran it.. numbers look like this:

Depth ft. stop (minutes) (runtime) Mix
150 27 (30) 21
80 0 (33) 21
70 1 (34) 50
60 2 (36) 50
50 2 (38) 50
40 4 (42) 50
30 5 (47) 50
20 7 (54) 50
10 12 (66) 50

So I've basically extended my runtime 6 minutes to offgas the additional nitrogen taken on by diving air. So your assertion that "it doesn't matter what the inert gas is", is incorrect do the faster offgassing of Helium. Or so it would appear from this model.

Your statement that Helium can bend you more quickly than Nitrogen is true. However, given how the helium is likely to be used, if you omit the upper deco steps because of a problem, you have gotten rid of FAR more of it than you would have a comparable amount of nitrogen.