Suspected Nitrogen Narcosis... And Decompression.

[John C. Ratliff]

Okay, I'm going to do this the "old fashioned way," using the U.S. Navy Diving Manual, Rev. 6.
http://www.usu.edu/scuba/navy_manual6.pdf
Page 9-58 gives the worksheet, and the tables are on subsequent pages.

If you dived for 5m (20 feet on the tables) for 90 minutes, you end on Repetitive Group "E," even though there is no limit for the dive. After 90 minutes, you are at the New Group of "A."

For the second dive, there was a surface interval of 3.5 hours, which put him into Repetitive Group "B." On the next dive, that gives a Residual Nitrogen Time of 8 minutes, to be added to the bottom time. Now, I had to extrapolate the bottom time because the dive computer said that for this dive, he had five minutes of no decompression limit time. The U.S. Navy Table 9-9 shows that the no decompression time for that dive is a maximum of 25 minutes. So I took the RNT of 8 minutes, plus 5 minutes shown on the computer, to give a bottom time of 12 minutes to have that 5 minutes left of no decompression time. But for this dive, this table shows a total ascent time of 3:20, and no stops required (although it is customary to take a safety stop anyway). The REPET Group at the end of this dive would be "H."

One of the things I have continued to do is to keep track of my bottom time independent of the dive computer. That way, I can reference back to the tables if I need to.

By the way, can't you print out your dive profile from that dive computer of yours? That would make looking at the dive much easier.

'Not sure this helps, but it was a good exercise for me, and a refamiliarization with the U.S. Navy Diving Manual's table for Revision 6.

SeaRat

 

[Diver0001]

OK, I am trying to understand exactly what you said here. Please correct anything I say that is a misunderstanding.

1. You said your computer showed that your NDL was 5 minutes, and you started to ascend. That is very normal.

2. The you said that after you had been ascending a while, "it now shows 5 mins, small deep stop and ceiling." That part has me confused, partly because I don't know what kind of computer you are using. If you signaled the ascent when you saw that you had 5 minutes of NDL, ascended briefly and looked again, it is quite likely that you still had 5 minutes of NDL. As you ascend at a normal rate, your NDL will get larger or stop getting lower because you are getting shallower. It is possible that your computer is telling you that you have a ceiling that is perfectly normal at the safety stop depth; I can't tell without a more detailed description or a look at the computer. On the other hand, it is also possible, depending upon the computer, that the is telling you that you have a mandatory deep decompression stop. That is EXTREMELY unlikely--going from 5 minutes of NDL to a mandatory deep decompression stop requires a very long delay in your ascent--many minutes.

3. You then said you hung around for a minute and the dive time went up a minute and the NDL dropped to 4 minutes. That is exactly what should happen if you are holding depth for a minute.

4. The you started ascending and you said the number was ascending and you didn't know why. That is perfectly normal. As you ascend, you are getting more and more shallow, and at shallower depths, you get longer and longer NDLs. When you get into the really shallow depths, it might jump to 99 (which is as high as it goes) or it may show a - - sign. Then you do your safety stop.

In summary, if I understand what you wrote correctly, you just experienced something that should happen every time you dive within NDLs. The only thing that confuses me is what you wrote in the part I summarized in point #2..

John,

A computer showing a deep stop will show you at what depth it expects the stop to happen. Apparently the computer showed a 5 minute deep stop (highly unusual) and Tim read the deep stop depth as a ceiling.

My guess is that in the confusion he misread the NDL for a deep stop time. He had 5 min of NDL and he said he had a 5 min deep stop. Normally you have to be well OVER the NDL's to see a computer give a deep stop longer than 1 min.

R..

 

[Diver0001]

I’m with John on this one, without knowing what model of computer you were using we can only speculate. However, those that do deep stops are normally a bit more expensive than the norm. Doing deep stops on a computor not designed for them will increase your deco time whilst you do them.

On my last dive my computer went from 0 no-stop time to 3 at 25m then reasonably quickly to 5 then 6, not until I got above 15m did it start to decrease. By the time I was at 6m deco time had reduced to 4 minutes. My buddy was patient and waited with me. I still exited the water with 80bar after 65 minutes.

Not an unusual dive: me on a 300bar 12Lt and pony, my buddy on a rebreather. And I had planned up to 9 minutes of deco before going in.

Edward,

I think there is some confusion about what a deep stop is.

I agree with you completely that if you just put in random stops at random depths that your deco obligation can grow, sometimes by quite a lot.

However, a "deep stop", at least in technical diving, has a specific definition. Actually 2 definitions depending on which school of thought you prefer.

In both cases the deep stop depth is calculated in such a way that the pressure is reduced below the "on gassing" pressure of the controlling compartment (the compartment that the computer is displaying at the point when the ascent begins). A deep stop of a minute or two MAY result in a different compartment becoming controlling but the deco obligation will either remain the same or at most increase by a small margin.

In all of the dives I've made (I make about 50 deco dives a year) I've only ever seen decompression obligations begin to grow again during the ascent when the ascent speed was too slow. Deep stops don't seem to increase the deco obligation although they also do nothing to decrease them. As an aside, there seems to be some research to suggest that deep stops SHOULD result in longer deco times even though almost all computers do not do this.

What you said about deco obligations reducing only in the shallow zone (18m and above) that's generally what I notice during AdvEanX dives as well. Normally we'll stop accumulating deco in de deeper part of the dive and then start working it off during the shallow part of the dive. I would imagine that if you made biggish Trimix dives then you'll see a different effect here but I'll admit that I don't make a lot of dives deeper than about 50m.

As an example of the above, if we have 40 minutes of deco at 50m then we'll try to manage the ascent to not accumulate more time that that until 18m where we normally switch to 50% and then after the switch it will drop straight away to 30 min which is what we do between 18m and 5m.

During that phase of ascending from 50 to 18 we'll do 1 deep stop and the switch depth usually gets extended as well and in so doing we're not seeing any big jumps in deco obligations.

R..

 

[Edward3c]

Edward,

I think there is some confusion about what a deep stop is.
...

R..

My understanding is they are for the fish Richard Pile brought up so they could decompress.

Back to the OP, the important lesson is to plan your dive than dive YOUR plan.

 

[Diver0001]

My understanding is they are for the fish Richard Pile brought up so they could decompress.

Back to the OP, the important lesson is to plan your dive than dive YOUR plan.

Fish don't need deep stops but you're right. Pyle stumbled on to the idea when he was bringing fish up from depth and needed to pause to "vent" the swim bladders so they didn't explode.

Pyle didn't set out to develop deep stops and It was further refined from there. In fact if you read some of Pyle's stories about his diving then you'll realize that he wasn't really the guy you wanted to be listening too back in the day... LOL

That said, I definitely agree with your synopsis of the lessons for the OP.

R..

 

[timz]

It is difficult to know which numbers you are referring to without having seen the computer, but from your description I'd guess that you had gone into deco.

Besides the fact that you didn't plan your dive and were trying to remember what the DM had said while being possibly narked, that confusion could have also been increased by not understanding the concepts behind an ascent strategy for a dive like that and also the information on your computer? Had you seen it before going into deco?
Because according to your description you had a ceiling and saw a time related to it that did not decrease when you ascended a bit, but also at first it did not increase. Although, since you had only gone up a few meters and were still below 20, the deco time eventually went up again.

I am using Suunto D4i. So, the NDL and the Safety Stop is actually located around the same place.

I have been considered dense, but shouldn't your deco time increase while doing a deep stop as it would not be shallow enough to off-gas N2. Not a perfectly planned and executed dive which you calculated the deco including the stop, but say you overstayed the deep stop by 5 minutes, that would by my thinking increase the deco time of the dive.

I have never done deep stops, with the exception that I will hesitate at depth if I feel I'm ascending too fast for too long to keep the total rate on track.


Bob

Yes. Thanks. Will learn to plan my own dive next time.

John,

A computer showing a deep stop will show you at what depth it expects the stop to happen. Apparently the computer showed a 5 minute deep stop (highly unusual) and Tim read the deep stop depth as a ceiling.

My guess is that in the confusion he misread the NDL for a deep stop time. He had 5 min of NDL and he said he had a 5 min deep stop. Normally you have to be well OVER the NDL's to see a computer give a deep stop longer than 1 min.

R..

Actually it did. But I just missed it and only saw the NDL as Deep Stop time. That's why I suspect I was narked.

My understanding is they are for the fish Richard Pile brought up so they could decompress.

Back to the OP, the important lesson is to plan your dive than dive YOUR plan.

Thanks. I will from now.

Fish don't need deep stops but you're right. Pyle stumbled on to the idea when he was bringing fish up from depth and needed to pause to "vent" the swim bladders so they didn't explode.

Pyle didn't set out to develop deep stops and It was further refined from there. In fact if you read some of Pyle's stories about his diving then you'll realize that he wasn't really the guy you wanted to be listening too back in the day... LOL

That said, I definitely agree with your synopsis of the lessons for the OP.

R..

Yeah. Will learn to plan my dive better.

 

[timz]

OK, I am trying to understand exactly what you said here. Please correct anything I say that is a misunderstanding.

1. You said your computer showed that your NDL was 5 minutes, and you started to ascend. That is very normal.

2. The you said that after you had been ascending a while, "it now shows 5 mins, small deep stop and ceiling." That part has me confused, partly because I don't know what kind of computer you are using. If you signaled the ascent when you saw that you had 5 minutes of NDL, ascended briefly and looked again, it is quite likely that you still had 5 minutes of NDL. As you ascend at a normal rate, your NDL will get larger or stop getting lower because you are getting shallower. It is possible that your computer is telling you that you have a ceiling that is perfectly normal at the safety stop depth; I can't tell without a more detailed description or a look at the computer. On the other hand, it is also possible, depending upon the computer, that the is telling you that you have a mandatory deep decompression stop. That is EXTREMELY unlikely--going from 5 minutes of NDL to a mandatory deep decompression stop requires a very long delay in your ascent--many minutes.

3. You then said you hung around for a minute and the dive time went up a minute and the NDL dropped to 4 minutes. That is exactly what should happen if you are holding depth for a minute.

4. The you started ascending and you said the number was ascending and you didn't know why. That is perfectly normal. As you ascend, you are getting more and more shallow, and at shallower depths, you get longer and longer NDLs. When you get into the really shallow depths, it might jump to 99 (which is as high as it goes) or it may show a - - sign. Then you do your safety stop.

In summary, if I understand what you wrote correctly, you just experienced something that should happen every time you dive within NDLs. The only thing that confuses me is what you wrote in the part I summarized in point #2..

What happened was it did show Deep Stop and Ceiling. Partly I did not understood what Ceiling means. Which i now do.

I misread the NDL as Deepstop time which happens to be at the same place. And i should be more aware about other details showed in the Com, i just didn't think much or maybe it was the narcotic effect.

@timz - will you tell us what computer you were diving, please?

And what were the settings - conservative, liberal?

It's Suunto D4i. And according to friends which have experience with Suunto and other Dive Com, it should be very conservative.

 

[timz]

Okay, I'm going to do this the "old fashioned way," using the U.S. Navy Diving Manual, Rev. 6.
http://www.usu.edu/scuba/navy_manual6.pdf
Page 9-58 gives the worksheet, and the tables are on subsequent pages.

If you dived for 5m (20 feet on the tables) for 90 minutes, you end on Repetitive Group "E," even though there is no limit for the dive. After 90 minutes, you are at the New Group of "A."

For the second dive, there was a surface interval of 3.5 hours, which put him into Repetitive Group "B." On the next dive, that gives a Residual Nitrogen Time of 8 minutes, to be added to the bottom time. Now, I had to extrapolate the bottom time because the dive computer said that for this dive, he had five minutes of no decompression limit time. The U.S. Navy Table 9-9 shows that the no decompression time for that dive is a maximum of 25 minutes. So I took the RNT of 8 minutes, plus 5 minutes shown on the computer, to give a bottom time of 12 minutes to have that 5 minutes left of no decompression time. But for this dive, this table shows a total ascent time of 3:20, and no stops required (although it is customary to take a safety stop anyway). The REPET Group at the end of this dive would be "H."

One of the things I have continued to do is to keep track of my bottom time independent of the dive computer. That way, I can reference back to the tables if I need to.

By the way, can't you print out your dive profile from that dive computer of yours? That would make looking at the dive much easier.

'Not sure this helps, but it was a good exercise for me, and a refamiliarization with the U.S. Navy Diving Manual's table for Revision 6.

SeaRat

Sadly, I don't have the data cable to download my dive profile. But I will plot it manually on Excel and upload it 2moro.

 

[John C. Ratliff]

timz,

Don't do that--plotting it manually. Wait until you get a cable and download it then.

SeaRat

 

[BDSC]

The last time I read an article on deep stops, from what I remember, there didn't appear to be any real benefit in doing them. In speaking with the folks over at DAN, they indicated you'd be better off just slowing your accent time by a minute or two rather than doing a deep stop for a minute or so.