Suspected Nitrogen Narcosis... And Decompression.

[boulderjohn]

Can you link to that thread?

There have been several threads. The first of these is the most contentious.

Deep Stops Increases DCS
Are People backing away from VPM?

 

[Storker]

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A ScubaBoard Staff Message...

A number of posts discussing the merits of deep stops and the NEDU study have been moved to the thread Deep Stops Increases DCS

Storker
SB Moderator

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[KJ4GXU]

What I am about to say are questions, not criticism. I am a brand new diver and not qualified to criticize anyone. I am hoping to understand the experience you had and take some knowledge away from it.

1st question, during training we were taught to always start with the deepest depth and move to shallower depths on subsequent dives or later in the same dive. Is this one of those things that's taught, not practiced? Seems this dive profile was the reverse.

2nd question, immediately on descending you were separated from your DM, were there other divers in addition to you 3? What did they do, return to surface, break off on their own or other? Did the DM discuss what to do if separated prior to the dive or did you just have to wing it when it happened?

Had I been in that situation I'd have followed what we were taught as the standard of search for 1 minute then reunite on surface but generally I don't feel qualified to make any other decision in that scenario. Your diving experience, DM briefing and other factors likely affected your decisions in ways I'm not considering.

Thanks
Eric

I was on a weekend dive trip last Saturday. Upon arrival, we had our first shore dive at 5m depth for 1 1/2 hour. Then we had 3 1/2 hours surface interval before we went for a wreck dive with 28m depth.

So, this was what happened. We descend through a buoy line and reached the wreck at 28. And we separated from the DM within a minute due to poor visibility (3m). 3 of us however, decided to continue diving with all our torches on. Saw some nudibraches and tunicates, and we did some shots(photography). Swimming around the wreck, looking at the Dive Com, the NDL says 5 mins and I suggested an ascent. So, we did.

 

[dmaziuk]

1st question, during training we were taught to always start with the deepest depth and move to shallower depths on subsequent dives or later in the same dive. Is this one of those things that's taught, not practiced? Seems this dive profile was the reverse.

5m is above the no-limit depth so I doubt it counts as "the reverse", but even if it did: yes, deep-to-shallow is safer but with a computer tracking your loading it's not that critical.

2nd question, immediately on descending you were separated from your DM, were there other divers in addition to you 3? What did they do, return to surface, break off on their own or other? Did the DM discuss what to do if separated prior to the dive or did you just have to wing it when it happened?

IME DMs generally won't stop you from doing your own thing as long as you stick to buddy system and get back on board X minutes later or with Y gas left in your tank, whichever comes first. There are places where they'll advise you to stay close because of something or other, and in many places they'll see and point out critters you will miss so you'd want to stay close for that, but there's no universal requirement to thumb the dive if you lost sight of the DM.

 

[boulderjohn]

1st question, during training we were taught to always start with the deepest depth and move to shallower depths on subsequent dives or later in the same dive. Is this one of those things that's taught, not practiced? Seems this dive profile was the reverse.

These are really two different issues.

Going to shallower depths on subsequent dives: A workshop examined this issue about 15 years ago, and it found that no one was truly sure where the rule came from or why it existed. The earliest mention of it in any written form (1972) was a suggestion with no explanation for the reason. Over the years, that suggestion became a rule, again with no explanation. Back then, diving was done with the U.S. Navy tables, and the reason for that suggestion was very likely to minimize the required surface interval. The U.S. Navy tables used an algorithm that led to very, very long intervals between dives. When using the tables, if you were planning two dives to different depths, your required surface interval was MUCH shorter if you did the deeper dive first. There is no known safety reason involved. As long as you have had enough of a surface interval to do the planned second dive, whether you are using tables or computers, you can do that dive.

Going to shallower depths during a dive: This is what just about everyone does, and it is a generally a good practice. As you get shallower, your body will begin to get rid of its excess nitrogen that was absorbed into your faster tissues. If you are using a computer to measure that multi-level dive, doing this will positively impact your no decompression limits for the dive and your surface interval before the next dive. Once again, though, you can go deeper later in the dive safely as long as you understand and accept the impact on decompression limits and surface intervals.

 

[KJ4GXU]

Thank you for the info.

5m is above the no-limit depth so I doubt it counts as "the reverse", but even if it did: yes, deep-to-shallow is safer but with a computer tracking your loading it's not that critical.



IME DMs generally won't stop you from doing your own thing as long as you stick to buddy system and get back on board X minutes later or with Y gas left in your tank, whichever comes first. There are places where they'll advise you to stay close because of something or other, and in many places they'll see and point out critters you will miss so you'd want to stay close for that, but there's no universal requirement to thumb the dive if you lost sight of the DM.

 

[KJ4GXU]

Thank you for the info, that makes sense.

These are really two different issues.

Going to shallower depths on subsequent dives: A workshop examined this issue about 15 years ago, and it found that no one was truly sure where the rule came from or why it existed. The earliest mention of it in any written form (1972) was a suggestion with no explanation for the reason. Over the years, that suggestion became a rule, again with no explanation. Back then, diving was done with the U.S. Navy tables, and the reason for that suggestion was very likely to minimize the required surface interval. The U.S. Navy tables used an algorithm that led to very, very long intervals between dives. When using the tables, if you were planning two dives to different depths, your required surface interval was MUCH shorter if you did the deeper dive first. There is no known safety reason involved. As long as you have had enough of a surface interval to do the planned second dive, whether you are using tables or computers, you can do that dive.

Going to shallower depths during a dive: This is what just about everyone does, and it is a generally a good practice. As you get shallower, your body will begin to get rid of its excess nitrogen that was absorbed into your faster tissues. If you are using a computer to measure that multi-level dive, doing this will positively impact your no decompression limits for the dive and your surface interval before the next dive. Once again, though, you can go deeper later in the dive safely as long as you understand and accept the impact on decompression limits and surface intervals.

 

[dmaziuk]

Thank you for the info.

Not that by deep-to-shallow is safer but with a computer tracking your loading it's not that critical I meant strictly in terms of DCS risk. What John said about shorter NDLs & longer SIs still applies: the computer will get you up safely but if it's unhappy with your profile and/or length of your surface interval, it'll penalize you on the next dive.

 

[timz]

What I am about to say are questions, not criticism. I am a brand new diver and not qualified to criticize anyone. I am hoping to understand the experience you had and take some knowledge away from it.

1st question, during training we were taught to always start with the deepest depth and move to shallower depths on subsequent dives or later in the same dive. Is this one of those things that's taught, not practiced? Seems this dive profile was the reverse.

2nd question, immediately on descending you were separated from your DM, were there other divers in addition to you 3? What did they do, return to surface, break off on their own or other? Did the DM discuss what to do if separated prior to the dive or did you just have to wing it when it happened?

Had I been in that situation I'd have followed what we were taught as the standard of search for 1 minute then reunite on surface but generally I don't feel qualified to make any other decision in that scenario. Your diving experience, DM briefing and other factors likely affected your decisions in ways I'm not considering.

Thanks
Eric

Like everyone stated above. If you are using a dive computer, u just need to follow ur dive computer and it will do the calculation for you live.

2nd question. In case of separation, search for 1 minute and surface if u don't find ur buddy. It's recommended ifnu separate from your buddy.

Most of the photographers got separated from the DM and the group. As long as ur buddy is with you, u can continue to dive. Make sure u have the necessary tool with you to make u safe while separated and while ascending. SMB is one thing that can avoid boat hitting you.

 

[EFX]

Going to shallower depths on subsequent dives: A workshop examined this issue about 15 years ago, and it found that no one was truly sure where the rule came from or why it existed. The earliest mention of it in any written form (1972) was a suggestion with no explanation for the reason. Over the years, that suggestion became a rule, again with no explanation. Back then, diving was done with the U.S. Navy tables, and the reason for that suggestion was very likely to minimize the required surface interval. The U.S. Navy tables used an algorithm that led to very, very long intervals between dives. When using the tables, if you were planning two dives to different depths, your required surface interval was MUCH shorter if you did the deeper dive first. There is no known safety reason involved. As long as you have had enough of a surface interval to do the planned second dive, whether you are using tables or computers, you can do that dive.

Perhaps the most important reason to avoid reverse dive profiles which later became almost a golden rule is efficiency. Spending time at the shallower depth first will give you less NDL at the deeper depth than a forward profile would give you.

The workshop John cites was hosted by the American Academy of Underwater Sciences (AAUS) of which the dive data did not point to any problems with reverse dive profiles. The group sought to retract the warnings regarding these dives. However, David Yount and a few other bubble deco model researchers had reservations. Their bubble models showed there could be problems. The basic problem is that bubbles that grew at the shallower depth may grow beyond the critical size at the deeper depth creating serious problems on ascent. Bruce Weinke, the developer of the RGBM (a bubble model), argued successfully for a maximum depth difference of 12m/40ft between repetitive dives. This limitation was agreed upon by the committee.