Shearwater Teric NDL?

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@Joneill

Understood.

No stop dives are just that and deco dives do not have free access to the surface,
Agreed - I would just add that "deco" dives (dives that accumulate a defined decompression stop schedule) can be more nuanced: as they should be, some are planned, require advanced gas planning/supply and a more rigid execution of stops before you can safely surface. Others, like the example I cited might not have been planned but can actually be managed without making defined stops or carrying extra gas.

I get that one should ideally know this is the plan in advance and preplan, but it does not always work that way. I'd normally have ascended before reacnhing NDL, but given the DM's acknowledgement of my NDL and gas, I felt no need to leave the group and ascend in ripping current in the middle of the pass.

Again - i'm not advocating for unplanned deco stop diving (bad idea) - just acknowledging that sh_t happens so helpful to understand the underlying science as that is key to staying safe.
 
Hi @Joneill

I don't necessarily disagree with you. I frequently dive off charters in SE FL. Several times each year someone asks me why their computer is not working normally. The most frequent answer is that they missed a deco stop and are locked out in violation gauge mode. This is most common with RGBM computers after repetitive dives. Understanding and following one's computer is far from universal. You have a good understanding of your computer.
 
I'm tired of hearing this "quote".....
Firstly, when and where did Powell say this? I can't find it in his book. What was the context of his statement (if he made it)? Was he being flippant? Colloquial? Joking? Rhetorical?
Secondly, in common usage "deco dive" is one with mandatory decompression stops, hence all dives are not "deco dives." Yes, all dives involve some decompression....but for an NDL dive that is done with ascent rates.
All dives involve decompression but not all dives are deco dives.
This has been discussed endlessly on SB; do we have to do it all over again?
Min 3:30
 
I'm tired of hearing this "quote".....
Firstly, when and where did Powell say this? I can't find it in his book. What was the context of his statement (if he made it)? Was he being flippant? Colloquial? Joking? Rhetorical?
Secondly, in common usage "deco dive" is one with mandatory decompression stops, hence all dives are not "deco dives." Yes, all dives involve some decompression....but for an NDL dive that is done with ascent rates.
All dives involve decompression but not all dives are deco dives.
This has been discussed endlessly on SB; do we have to do it all over again?
Check the video. The problem with separating NDL and deco dives is that the boundary is completely dependent on the conservatism you choose. The same dive (profile, gas, etc) can be recreational with GF 99/99 or deco dive with GF 50/60.
 
To me, it’s more important to understand what he lays out as the line can be blurred. I’ve been on dives that were supposed to be NDL, but went to required stops (so “deco” by your definition) and then went back to NDL before the stop depths were reached.

On those dives, the required deco stops cleared well before I ever ascended to the depths the DC had indicated that I needed to do stops at. To me that is a bit different in practical terms to dives where you plan to accumulate obligations and must execute timed stops at those specific depths.

We were not briefed on this pre-dive and when I saw my NDL dropping below 5, I showed the DM. He waved it off indicating not to worry about it. When I spoke to him after the dive, he told me that a “temporary deco obligation” was common on these profiles, but would clear well before needing to be done so was not anything to be bothered by. I was then prepared for this situation on the subsequent dives.

Agreed - I would just add that "deco" dives (dives that accumulate a defined decompression stop schedule) can be more nuanced: as they should be, some are planned, require advanced gas planning/supply and a more rigid execution of stops before you can safely surface. Others, like the example I cited might not have been planned but can actually be managed without making defined stops or carrying extra gas.

I get that one should ideally know this is the plan in advance and preplan, but it does not always work that way. I'd normally have ascended before reacnhing NDL, but given the DM's acknowledgement of my NDL and gas, I felt no need to leave the group and ascend in ripping current in the middle of the pass.

As @scubadada has already pointed out, the dives you are talking about were deco dives. Meaning, they were dives with mandatory deco stops. The specific depth at which you spent time satisfying your mandatory deco obligations is irrelevant. The fact that you did not ever stay at one specific depth for an extended period - at least not purposefully - is also irrelevant.

What you are really illustrating here is the concept of normalization of deviance.

The DM waved his hand and said "these are not the deco stops you're looking for", uhh, I mean "that was a temporary deco obligation", and you said okay and continued happily on your way.

Well... ALL mandatory deco stops are "temporary deco obligations". That's what deco stops ARE.

The reason divers should not incur "temporary deco obligations" without the proper training is because, if there is any emergency, you CANNOT ascend directly to the surface. At least, not without substantially elevated risk of DCS.

THAT is what is relevant. You were diving in a situation where you were not free to ascend directly to the surface. And, even more concerning is that you seem to think that that is perfectly fine just because "the DM said so" and "they do it all the time."

You did multiple deco dives and nothing bad happened. That is very fortunate.

I apologize for sounding harsh. I don't mean to sound like I'm scolding you. I'm just being blunt. Repesenting the Plain Talk Party, as it were. You are (presumably :) ) a grown-up and a certified diver and I am not the Scuba Police. I'm just trying to call a spade a spade.

You can dive how you want to dive. Let's just be accurate and honest in describing it.
 
I1As @scubadada has already pointed out, the dives you are talking about were deco dives. Meaning, they were dives with mandatory deco stops. The specific depth at which you spent time satisfying your mandatory deco obligations is irrelevant. The fact that you did not ever stay at one specific depth for an extended period - at least not purposefully - is also irrelevant.

What you are really illustrating here is the concept of normalization of deviance.

The DM waved his hand and said "these are not the deco stops you're looking for", uhh, I mean "that was a temporary deco obligation", and you said okay and continued happily on your way.

Well... ALL mandatory deco stops are "temporary deco obligations". That's what deco stops ARE.

The reason divers should not incur "temporary deco obligations" without the proper training is because, if there is any emergency, you CANNOT ascend directly to the surface. At least, not without substantially elevated risk of DCS.

THAT is what is relevant. You were diving in a situation where you were not free to ascend directly to the surface. And, even more concerning is that you seem to think that that is perfectly fine just because "the DM said so" and "they do it all the time."

You did multiple deco dives and nothing bad happened. That is very fortunate.

I apologize for sounding harsh. I don't mean to sound like I'm scolding you. I'm just being blunt. Repesenting the Plain Talk Party, as it were. You are (presumably :) ) a grown-up and a certified diver and I am not the Scuba Police. I'm just trying to call a spade a spade.

You can dive how you want to dive. Let's just be accurate and honest in describing it.
I appreciate the feedback, but do feel that you are misrepresenting what i was saying a bit. I was not trying to say these were not decompression dives - I was pointing out that they were not planned as such. We were not told prior to the first dive that this would be the case so I was concerned when I saw my NDL ticking down to zero at ~21 minutes into the dive and we were not yet ascending - which is why I let the DM know my state.

I also did not think it was "perfectly fine" - but what would you suggest I do?? Leave the group and ascend into a ripping current with limited viz (I'd have lost sight of the group for sure) in the middle of a pass with standing waves and boat traffic on the surface?? I did not think that was a good idea and I still had plenty of gas, so felt that the best choice at that point in the dive was to trust the DM who takes divers on these dive profiles many times each day and was indicating to me that it was OK. I had read "Deco for Divers" so understood basics of deco and what my computer was telling me and why - so I knew what was happening (we had stayed deep long on the front end). I also knew we would start ascending once we got through the pass and into shallower water. In any event, I would have headed for the stop depth if my gas supply had required it.

You also need to understand that these were not significant deco obligations: the first alert called for 1 min @ 9.8 ft. That increased to 3 min @9.8 ft, before shifting to 1 min @ 19.7 ft and then finally settling at 5 min @ 9.8 ft which counted down to 1 min and before going away and being replaced with the Safety Stop timer by the time I had ascended to 22 feet. So, the deco obligation that was accumulated would have been cleared with my safety stop and final ascent. Therefore, I was only concerned for a short period when it appeared to be increasing - once we started ascending 5 min later and I saw the deco stop diminishing, I had no real concern.

After the dive, I spoke to the DM about my concern and he went over the typical profiles and the fact that we might see these small deco obligations pop up. However, the deco obligation would be early in the dive when we had plenty of air to satisfy it if we needed to ascend to the surface for some reason and it would clear fully in the normal, planned profile otherwise. Once I knew that, I had no concern with that type/level of deco popping up on subsequent dives (had 1 or 2 min @ 9.8 ft pop up and clear well before safety stop depth was reached on 3 subsequent dives). I honestly see no reason for me to get trained for deco diving to manage this type of dive profile now - I know what I need to know. I'll add that I have not had that situation arise again in almost 200 dives since Rangiroa and would normally avoid it by shallowing up, if at all possible.

The last thing I'll say on the topic is that I still personally believe that there is a huge difference between my example of what I get were actual deco dives (albeit unplanned) with very small (safety stop like) deco obligations and planned deco dives where the stops (#/depth/time) can be complex/significant and require advanced planning to ensure sufficient gas supply/backup and possible surface support. I get that requires special training to do safely but I have no interest in ever doing that type of diving.
 
Check the video. The problem with separating NDL and deco dives is that the boundary is completely dependent on the conservatism you choose. The same dive (profile, gas, etc) can be recreational with GF 99/99 or deco dive with GF 50/60.
However, you are obliged to follow the computer and the settings that you chose to use. If the computer says you are in deco, then you are in deco. If you choose to violate the decompression stop(s) because you believe your computer is too conservative, with the majority of computers, you run the risk of being locked out of use for 24-48 hours. You may also be increasing your risk of DCS.

If you believe your NDLs are too short, change settings on your computer to make it more liberal or change computers.

I dive 2 computers with different liberal deco algorithms, an old Oceanic VT3 running DSAT and a Teric running 80/95. If it is a no stop dive, the more conservative computer controls the dive. If it is a light deco dive, I clear one or both of the computers that went into deco. I use SurfGF to control my surfacing GF from either no stop or deco dives.
 
I appreciate the feedback, but do feel that you are misrepresenting what i was saying a bit. I was not trying to say these were not decompression dives - I was pointing out that they were not planned as such. We were not told prior to the first dive that this would be the case so I was concerned when I saw my NDL ticking down to zero at ~21 minutes into the dive and we were not yet ascending - which is why I let the DM know my state.

First, let me apologize in advance because my online demeanor can come across as ... brusque. This may come across as really beating you up on this and I don't mean any of it with any rancor. But, I do think it is a very important subject that merits direct and clear communication. It may not change your mind at all, but I'm also thinking of other people who may come along later and read all this and I hope it will at least help some of them make the best decisions they can.

I also did not think it was "perfectly fine" - but what would you suggest I do??

First (on that), you said the DM explained things after the first dive and "I was then prepared for this situation on the subsequent dives." That sounds like you thought it was fine. You did more dives of the same nature, right?

As for what I would suggest: Use your best judgment. If it was better, on that first dive, to stay with the group than to ascend, well, there's a couple of things about that.

If you didn't have a buddy that would ascend with you, that is a problem.

If it was a group dive and the group would not have ascended with you, then that is a problem.

If you were diving where the Golden Rule was not in effect, that is a problem. "Anyone can thumb a dive, at any time, for any reason, with no repercussions." It sounds like you did not feel like you could thumb the dive. And, from what you've posted so far, I suspect that your training is "when your computer shows an NDL of 0, you should be ascending to the surface." So, you didn't follow your training. Notice I did not say "you should be ascending." I said "you should be ascending to the surface." Another problem.

Regardless of how you handled it during the first dive, my suggestion would be to not engage in any more of those type of dives after that, once you understand that they are taking you on dives that are beyond your training and putting you in danger (of which you are not even fully cognizant, because you don't know what you don't know and - worse - you think you DO know).

Leave the group and ascend into a ripping current with limited viz (I'd have lost sight of the group for sure) in the middle of a pass with standing waves and boat traffic on the surface?? I did not think that was a good idea and I still had plenty of gas, so felt that the best choice at that point in the dive was to trust the DM who takes divers on these dive profiles many times each day and was indicating to me that it was OK. I had read "Deco for Divers" so understood basics of deco and what my computer was telling me and why - so I knew what was happening (we had stayed deep long on the front end). I also knew we would start ascending once we got through the pass and into shallower water. In any event, I would have headed for the stop depth if my gas supply had required it.

You also need to understand that these were not significant deco obligations: the first alert called for 1 min @ 9.8 ft. That increased to 3 min @9.8 ft, before shifting to 1 min @ 19.7 ft and then finally settling at 5 min @ 9.8 ft which counted down to 1 min and before going away and being replaced with the Safety Stop timer by the time I had ascended to 22 feet. So, the deco obligation that was accumulated would have been cleared with my safety stop and final ascent. Therefore, I was only concerned for a short period when it appeared to be increasing - once we started ascending 5 min later and I saw the deco stop diminishing, I had no real concern.

After the dive, I spoke to the DM about my concern and he went over the typical profiles and the fact that we might see these small deco obligations pop up. However, the deco obligation would be early in the dive when we had plenty of air to satisfy it if we needed to ascend to the surface for some reason and it would clear fully in the normal, planned profile otherwise. Once I knew that, I had no concern with that type/level of deco popping up on subsequent dives (had 1 or 2 min @ 9.8 ft pop up and clear well before safety stop depth was reached on 3 subsequent dives).

Deco for Divers is a great book. But, it is only a foundation for understanding the training that comes in TDI Advanced Nitrox and Decompression Procedures (or equivalent). Understanding what that book teaches you is NOT a basis for thinking that you know how to safely execute decompression dives (i.e. dives with mandatory deco stops). And your statements make it clear that you actually do NOT know how to do that.

You described having a mandatory stop for 1 minute at 19.7 ft. You further described that as "not significant" as a deco obligation. The politest thing I can say is that I respectfully disagree.

I happened to do a deco dive yesterday where my first stop was at 20 feet. At the time that my computer clicked over from a 10 foot stop to having a stop for 1 minute at 20 feet, my SurfGF was 131%. Had I ascended directly to the surface right then, I would have gotten there with my leading compartment at 30% OVER the M-value! That is NOT "insignificant". Further, if your computer was set to less conservative settings than I was using yesterday (which, mine were fairly conservative), then your SurfGF would have been even higher.

I hope your understanding gleaned from Deco for Divers informs you on how significant a SurfGF of 131% is. I am COMPLETELY confident that, had I ascended directly to the surface just then, even at a normal ascent rate, I would have been very bent when I got there or soon after. I say that from actual experience, not just book learning. Unless you have your computer set on some ridiculous, unrealistic level of conservatism, once you have built up enough deco to have a stop at 20', you are well into SERIOUS deco territory. Not "you're going to die" territory (probably). But, "serious pain and a chamber ride" territory, anyway.


I honestly see no reason for me to get trained for deco diving to manage this type of dive profile now - I know what I need to know.

Again.... I respectfully disagree. Not based on anything you have posted, anyway.

I'll add that I have not had that situation arise again in almost 200 dives since Rangiroa and would normally avoid it by shallowing up, if at all possible.

The last thing I'll say on the topic is that I still personally believe that there is a huge difference between my example of what I get were actual deco dives (albeit unplanned) with very small (safety stop like) deco obligations and planned deco dives where the stops (#/depth/time) can be complex/significant and require advanced planning to ensure sufficient gas supply/backup and possible surface support. I get that requires special training to do safely but I have no interest in ever doing that type of diving.

You were DOING that type of diving. And despite thinking that you know what you need to know, you didn't even know you were doing that type of diving - much less how to do it safely. Reading Deco for Divers and an explanation from a Divemaster is not an adequate substitute for formal training on how to safely execute the exact kind of dives you were doing. "They do them all the time" is not an acceptable validation that diving like that is safe.

This is TEXTBOOK Normalization of Deviance. This kind of thinking is a signifcant factor in many diver deaths.

You do you, but I hope this discussion at least helps someone that reads it, some day, to avoid engaging in the same behaviors.


Anyway... I am bowing out of this now. Unless you ask me a specific question, I will just leave this alone now. I think I have killed the horse. :)

ps. Again, I apologize if that all came across as being a jerk or being overly harsh. If you read it and try to hear in your head a sincere and concerned tone of voice - not an angry tone of voice - you will maybe understand how I am trying to "speak".
 
First, let me apologize in advance because my online demeanor can come across as ... brusque. This may come across as really beating you up on this and I don't mean any of it with any rancor. But, I do think it is a very important subject that merits direct and clear communication. It may not change your mind at all, but I'm also thinking of other people who may come along later and read all this and I hope it will at least help some of them make the best decisions they can.
I have tough skin so not feeling beat up at all- this is how we learn.
First (on that), you said the DM explained things after the first dive and "I was then prepared for this situation on the subsequent dives." That sounds like you thought it was fine. You did more dives of the same nature, right?
Given that the profile we were diving accumulated less than 5 min (rest of dives were 1 or 2 minutes) of deco obligation at safety stop depths that cleared before we even got to SS depth and gas supply was not an issue , I felt the risk in doing these dives as executed was very low, accepted it and take responsibility for that.
As for what I would suggest: Use your best judgment. If it was better, on that first dive, to stay with the group than to ascend, well, there's a couple of things about that.

If you didn't have a buddy that would ascend with you, that is a problem.
No specific assigned buddy - I stayed close to the DM (very close once I had the deco stop message)
If it was a group dive and the group would not have ascended with you, then that is a problem.
Group dive

If you were diving where the Golden Rule was not in effect, that is a problem. "Anyone can thumb a dive, at any time, for any reason, with no repercussions." It sounds like you did not feel like you could thumb the dive. And, from what you've posted so far, I suspect that your training is "when your computer shows an NDL of 0, you should be ascending to the surface." So, you didn't follow your training. Notice I did not say "you should be ascending." I said "you should be ascending to the surface." Another problem.
Nope - my training was to keep your NDL above 5 and don't ever ride that NDL if you get there. If/when you see ever your NDL dropping below 5 (uncommon for me on typical dives) you shallow up to increase NDL so that you will not be NDL limited on the dive - you do NOT ascend to the surface!

That is how I've always generally dived - these 4 dives were the exception. I pay attention to NDL, don't ride low NDL's and shallow up to manage it. However, shallowing up was not really feasible on these dives without getting separated from the group (current would have swept me away) which I felt was a way higher risk at that point in the dive.
Regardless of how you handled it during the first dive, my suggestion would be to not engage in any more of those type of dives after that, once you understand that they are taking you on dives that are beyond your training and putting you in danger (of which you are not even fully cognizant, because you don't know what you don't know and - worse - you think you DO know).

Deco for Divers is a great book. But, it is only a foundation for understanding the training that comes in TDI Advanced Nitrox and Decompression Procedures (or equivalent). Understanding what that book teaches you is NOT a basis for thinking that you know how to safely execute decompression dives (i.e. dives with mandatory deco stops). And your statements make it clear that you actually do NOT know how to do that.

You described having a mandatory stop for 1 minute at 19.7 ft. You further described that as "not significant" as a deco obligation. The politest thing I can say is that I respectfully disagree.

I happened to do a deco dive yesterday where my first stop was at 20 feet. At the time that my computer clicked over from a 10 foot stop to having a stop for 1 minute at 20 feet, my SurfGF was 131%. Had I ascended directly to the surface right then, I would have gotten there with my leading compartment at 30% OVER the M-value! That is NOT "insignificant". Further, if your computer was set to less conservative settings than I was using yesterday (which, mine were fairly conservative), then your SurfGF would have been even higher.

I hope your understanding gleaned from Deco for Divers informs you on how significant a SurfGF of 131% is. I am COMPLETELY confident that, had I ascended directly to the surface just then, even at a normal ascent rate, I would have been very bent when I got there or soon after. I say that from actual experience, not just book learning. Unless you have your computer set on some ridiculous, unrealistic level of conservatism, once you have built up enough deco to have a stop at 20', you are well into SERIOUS deco territory. Not "you're going to die" territory (probably). But, "serious pain and a chamber ride" territory, anyway.

Again.... I respectfully disagree. Not based on anything you have posted, anyway.

You were DOING that type of diving. And despite thinking that you know what you need to know, you didn't even know you were doing that type of diving - much less how to do it safely. Reading Deco for Divers and an explanation from a Divemaster is not an adequate substitute for formal training on how to safely execute the exact kind of dives you were doing. "They do them all the time" is not an acceptable validation that diving like that is safe.

This is TEXTBOOK Normalization of Deviance. This kind of thinking is a signifcant factor in many diver deaths.

You do you, but I hope this discussion at least helps someone that reads it, some day, to avoid engaging in the same behaviors.

Anyway... I am bowing out of this now. Unless you ask me a specific question, I will just leave this alone now. I think I have killed the horse. :)

ps. Again, I apologize if that all came across as being a jerk or being overly harsh. If you read it and try to hear in your head a sincere and concerned tone of voice - not an angry tone of voice - you will maybe understand how I am trying to "speak".
I also monitor Surf GF these days after reading up on it when I saw posts from @scubadada on how he was using it to monitor/manage tissue loading/safe surfacing. I'll consider taking the deco training at some point but have zero interest in ever doing planned deco/technical diving. As stated, I have not done dive profiles like this since the 4 dives in Rangiroa back in 2019.
 
I have tough skin so not feeling beat up at all- this is how we learn.

Given that the profile we were diving accumulated less than 5 min (rest of dives were 1 or 2 minutes) of deco obligation at safety stop depths that cleared before we even got to SS depth and gas supply was not an issue , I felt the risk in doing these dives as executed was very low, accepted it and take responsibility for that.

No specific assigned buddy - I stayed close to the DM (very close once I had the deco stop message)

Group dive


Nope - my training was to keep your NDL above 5 and don't evert ride that NDL if you get there. If/when you see ever your NDL dropping below 5 (uncommon for me on typical dives) you shallow up to increase NDL so that you will not be NDL limited on the dive - you do NOT ascend to the surface!

That is how I've always generally dived - these 4 dives were the exception. I pay attention to NDL, don't ride low NDL's and shallow up to manage it. However, shallowing up was not really feasible on these dives without getting separated from the group (current would have swept me away) which I felt was a way higher risk at that point in the dive.

I also monitor Surf GF these days after reading up on it when I saw posts from @scubadada on how he was using it to monitor/manage tissue loading/safe surfacing. I'll consider taking the deco training at some point but have zero interest in ever doing planned deco/technical diving. As stated, I have not done dive profiles like this since the 4 dives in Rangiroa back in 2019.

Most of the training that is part of any "advanced" scuba course is not "how to do X". It is "how to handle all the various failures, etc. that can come up while you are doing X." Usually, actually doing X is the easier part.

When you had 1 minute deco at 20', you had a significant deco obligation. If you were trained for that, and had prepared according to your training, you would (almost certainly) have been able to deal with any of the numerous failure/emergency possibilities that *might* have occured - without getting bent.

Instead, if my "vision" of your situation is accurate, there are a variety of things that could have happened that (most likely) would have resulted in getting bent or maybe even drowning. Admittedly, those things were all pretty darn unlikely.

You may or may not have really been aware of ALL the risks involved in what you were doing. That is what happens in the formal training. You learn what all those risks are. Without the training, maybe you have figured them all out on your own (with your experience and Internet and other research). Or maybe you haven't.

I am sincerely glad that it all went well.
 
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