Ommiting Nitrogen on Deco

[MonkSeal]

It doesn't matter what inert gas it is.

???

 

[nova]

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.

look at your profiles again. When you added HE to the deco gas the run time was 72 minutes.

The two bacis reasons to add helium to a breathing mix are,
1. normoxic, to releive the effects of nitrogen narcoses and,

2. hypoxic. to lower PPO2 levels to a safe range when diving deep. You can't add more N2 to hypoxic mix because of the added nitrogen narcosis factor.

Those are the reasons divers plan their mix around EAD (equivalent air depth) after finding the EAD of a mix ,you can use the deco tables for THAT EAD.

example: if your mix has an EAD of 130 feet. and you dive it to 180 for 30 minutes , your deco time is the same as is you were at 130 for 30 minutes. That's why I ask you to run the numbers with air for bottom gas. The catch is the exceptable range of O2 in the body, that has to be clocked also.

 

[PerroneFord]

Nova,

I think you're missing something here. The addition of Helium to the deco mix *IN MY CASE* was to increase the gradient for the nitrogen to get rid of it deeper. The profile I presented here shows a 72 minute runtime simply because I cannot change the gradient of helium at the shallow stops due to not having an 02 bottle availbe to maximize the gradient. So your two examples need an addendum.

3. Decompression, to maximize the offgas gradient of nitrogen at the deep stops.

I suppose I don't understand your logic for asking me to run the dive on air. I increased the EAD signifcantly over the normixic bottom gas but what does that prove?

 

[nova]

Nova,

I think you're missing something here. The addition of Helium to the deco mix *IN MY CASE* was to increase the gradient for the nitrogen to get rid of it deeper. The profile I presented here shows a 72 minute runtime simply because I cannot change the gradient of helium at the shallow stops due to not having an 02 bottle availbe to maximize the gradient. So your two examples need an addendum.

3. Decompression, to maximize the offgas gradient of nitrogen at the deep stops.

I suppose I don't understand your logic for asking me to run the dive on air. I increased the EAD signifcantly over the normixic bottom gas but what does that prove?

have you taken a decompression course? Because decompression is to offgas inert gas not just N2. Helium needs to be offgassed at a deeper depth because it come out of solution faster, and can bend you at shallow stops , that's all. It's faster in and faster out.

 

[PerroneFord]

No Nova, I have not taken a deco course, and I am trying to learn and make sense of the evidence I have been gathering. All evidence I have seen indicates that your statement about helium needing to come out deeper runs contrary to your statement.

I am not trying to discredit you here, I am just noting that what you are telling me seems to run counter to what I have seen from various sources I trust. Like the author of V-Planner. The profiles the program generates would also seem contradictory to your assertion that the helium needs to come out deep in the dive. Were that true, I would expect to see extended stops when decoing on the helium so that the stops at the top of the deco would be short. The opposite was true as was noted by the stop lengths with the 50/50.

 

[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.

OK, here is your two profiles. please look at them carefully. When you added helium to your deco mix ,you added 12 minutes to the run time RIGHT? why is that? Why would you want to add time to the dive when you don't have to? Because you are still on gassing helium at the deeper stops. this added time to the shallow stops.

Here's another clue for you. 50% O2 and 50% N2 the run time was 60 minutes, right?

yet 50%O2 and 50% HE the run time was 72 minutes right? This tells me that you're still ongassing HE at the deeper stops and now you need to offgas it at the shallow stops.

Have you concidered what size deco bottle you'll need for all the extra gas? and have you concidered dumping the ten foot stop and making your last stop at 20 feet? A ten foot stop in 5 foot swells can toggle the gradient to a dangerous point.

 

[ScubaSixString]

No Nova, I have not taken a deco course, and I am trying to learn and make sense of the evidence I have been gathering. All evidence I have seen indicates that your statement about helium needing to come out deeper runs contrary to your statement.

I am not trying to discredit you here, I am just noting that what you are telling me seems to run counter to what I have seen from various sources I trust. Like the author of V-Planner. The profiles the program generates would also seem contradictory to your assertion that the helium needs to come out deep in the dive. Were that true, I would expect to see extended stops when decoing on the helium so that the stops at the top of the deco would be short. The opposite was true as was noted by the stop lengths with the 50/50.

About helium, to my limited understanding. Nova's statement "Helium needs to be offgassed at a deeper depth because it come out of solution faster, and can bend you at shallow stops , that's all. It's faster in and faster out." is very important.

The purpose of deco stops is to simulate a very slow steady ascent (so slow that it would be impossible, or nearly so, to pull off) giving your body time to offgas the inert gases. Helium is a smaller molecule than N2. It ongasses and offgasses alot faster than does nitrogen. From a purely intuitive POV, if helium can come out of solution quicker, you're going to want to go SLOWER to prevent that. Does that make any sense? Anyone, am i way off base here?

 

[nova]

About helium, to my limited understanding. Nova's statement "Helium needs to be offgassed at a deeper depth because it come out of solution faster, and can bend you at shallow stops , that's all. It's faster in and faster out." is very important.

The purpose of deco stops is to simulate a very slow steady ascent (so slow that it would be impossible, or nearly so, to pull off) giving your body time to offgas the inert gases. Helium is a smaller molecule than N2. It ongasses and offgasses alot faster than does nitrogen. From a purely intuitive POV, if helium can come out of solution quicker, you're going to want to go SLOWER to prevent that. Does that make any sense? Anyone, am i way off base here?

your on the money , when perron added helium to the deco gas, he added time to the dive because of the faster properties of the helium.

 

[PerroneFord]

I think I see where you are going, but you are failing to see my point.

Yes, adding HE to the deco mix DID increase the run times, and yes it increased the time spent on the deep stops. The model is attempting to give added time to the HE to offgas in the lower depths. So this part of your contention IS correct. HOWEVER, in pratical use, the selection of adding HE to the deco mix PRESUPPOSES going to go o2 in the upper stops to clear the HE. In the case of the profile I posted, I DID NOT HAVE THE O2.

Yes, I have looked at the gas requirements for the dive. And yes, I am FULLY AWARE that one might not want to do a 10ft stop. But again, you seem to be ignoring what I have said from the beginning. I WOULD NOT USE the 50/50 WITHOUT OXYGEN for the 20ft stop. I was forced into that because I am not using a full version of V-Planner. I suppose I could modify the software to do and extended 20ft stop rather than a 10ft.

I would ask you given the posted profile, what gradient would be pushed to a dangerous level at the 10ft stop in 5 foot seas?

OK, here is your two profiles. please look at them carefully. When you added helium to your deco mix ,you added 12 minutes to the run time RIGHT? why is that? Why would you want to add time to the dive when you don't have to? Because you are still on gassing helium at the deeper stops. this added time to the shallow stops.

Here's another clue for you. 50% O2 and 50% N2 the run time was 60 minutes, right?

yet 50%O2 and 50% HE the run time was 72 minutes right? This tells me that you're still ongassing HE at the deeper stops and now you need to offgas it at the shallow stops.

Have you concidered what size deco bottle you'll need for all the extra gas? and have you concidered dumping the ten foot stop and making your last stop at 20 feet? A ten foot stop in 5 foot swells can toggle the gradient to a dangerous point.

 

[PerroneFord]

From a purely intuitive POV, if helium can come out of solution quicker, you're going to want to go SLOWER to prevent that. Does that make any sense? Anyone, am i way off base here?

At what point can you handle the helium coming out of solution shallow without it being dangerous? 20ft? 10ft? surface? We do the same with nitrogen all the time. Pop out of the water and still be subject to offgassing hours later.

Because Helium offgasses quicker, we "should" be able to get rid of more of it in the shallow stops than we could nitrogen. Which is the reason for using it in the first place.

I know the WKPP guys have played with this a bunch. I am trying to dig a bit deeper into the why's of it.