91 replies to this topic

[PerroneFord]

Why don't you just start a new thread on the topic you want to discuss?

Because that won't clean up the mess we've already made of this thread. Why are you grinding axes with me? Did I do something to you?

[PerroneFord]

Cool... I want to dive Scappa (as well as Truk) but the problem is getting together with a group of like minded divers. Just let me know when you start to firm up the details.

I also hope to take a trip to the Keys this winter over a long weekend...do you have any time off?

It's is HIGHLY doubtful this SCAPA trip will be remotely DIR. I will do my best to make it as safe as possible, but it will be catch as catch can, likely with a mix of CCR and OC divers.

Not sure about a keys trip. When are you thinking of going?

[WreckWench]

Why don't you just start a new thread on the topic you want to discuss?

Because that won't clean up the mess we've already made of this thread. Why are you grinding axes with me? Did I do something to you?

I doubt he is grinding any axes...just suggesting you could start a new thread. He did not however take into consideration that many of these posts are the fuel for that new discussion(s).

If the thread is not split shortly I'll do it!

And NO AXE GRINDING FROM ME NOR ANYONE ALLOWED!!!

[pir8]

Why don't you just start a new thread on the topic you want to discuss?

Because that won't clean up the mess we've already made of this thread. Why are you grinding axes with me? Did I do something to you?

I doubt he is grinding any axes...just suggesting you could start a new thread. He did not however take into consideration that many of these posts are the fuel for that new discussion(s).

If the thread is not split shortly I'll do it!

And NO AXE GRINDING FROM ME NOR ANYONE ALLOWED!!!

What the Wench said.
I'm not even sharpening any knifes let alone grinding any axes.
I just figgered its just as easy to start another thread and leave the other one alone.
It really wouldn't bother me to leave unrelated posts in the original thread.
But thats just me.

Edited by pir8, 13 January 2008 - 01:49 PM.

[ScubaDrew]

Hi Perrone, ......sometimes I think you need to get out more.... " do they mandate you dive with a computer at all? " ...um, not just 'yes'..but "H*LL YES !!!'

I have been around a bit, not nearly as much as some, but still, I have not once ever seen anywhere that a computer was a requirement to dive, and that without one you would not be allowed to dive.

I like my computer. Sometimes I let it tell me what to do, sometimes I use it to validate my work on the tables. But if my computer fails I will dive without it. I don't need it, and being told I had to have one or I couldn't dive would make me look elsewhere for a dive op, even if I planned to do every dive with my computer.

[ScubaDrew]

I can see dive ops having a no deco policy if they don't want to sit on a site for the extra time those dives require and most others are NDL profiles. However unless you have to hang for 20 minutes or more longer than everyone else I see no reason to force that diver to sit out any dives if the SI says that they can get back in. Hell, for that matter you can violate your computer, hang, and still be out of the water before some other divers on the boat who don't trigger their computers.

See here's the problem.

Lets say I want to dive wreck XYZ on a charter. Most of the divers on the boat are diving XYZ to 60ft, and I am diving XYZ to 125ft but I am taking a 50% deco mix along. I intend to spend 20 minutes at 125ft. Now I will likely be back on the boat before the shallow divers, I will certainly have violated a recreational computer, but I will leave the water with less nitrogen loading than the divers diving shallow.

So Capt. Skippy examines my dive computer and says, "you're done for 24 hours"? I don't think so, and that's not a boat I would dive on.

I think it's imperative the divers dive within their training limits and the limits of safety. However, that line is quite variable based on diver education. And though some may not like to acknowledge it, there is diving beyond 60ft/55m and beyond 130ft. And some of it is really a lot of fun! I also don't think it's taboo to talk about diving beyond recreational limits in a forum of mostly recreational divers, because I would hope that discussion brings about some understanding of the very careful consideration that goes into making those dives.

One of the hardest things for me to do, is to go on just basic recreational dives with friends sometimes because they will want to do no planning and just fall in the water for an hour. Oddly, I'll do this with my tech buddies without problem because I KNOW they are going to multi-level, or build in stops during the dive. I did an 86 minute dive like this in December where the max depth was 100ft. I never even violated my recreational computer.

I don't think we are disagreeing with each other at all...

[VADiver]

Cool... I want to dive Scappa (as well as Truk) but the problem is getting together with a group of like minded divers. Just let me know when you start to firm up the details.

I also hope to take a trip to the Keys this winter over a long weekend...do you have any time off?

It's is HIGHLY doubtful this SCAPA trip will be remotely DIR. I will do my best to make it as safe as possible, but it will be catch as catch can, likely with a mix of CCR and OC divers.

Not sure about a keys trip. When are you thinking of going?

I'll definately be in Fla in May for a class, but I hope to make it down in Feb or sooner if possible.

[BubbleBoy]

Kind of a philosophical thought about the topic.

One of the frustrating things for me about diving is the fact that there is no way to feel how close to getting bent I really I am. With other sports it’s easy to know when I’m reaching my physical limit. If I’m skiing for example, I know when I’m getting tired or close to falling, and can ease back on my speed and slope difficulty. Some days I can run the moguls ten times and feel great. Other days I seem to get winded riding the chair lift, and stick to the medium slopes. But, the point is I’m constantly getting feedback about my physical condition so I can adjust.

I suspect the variation in any individuals physical response to diving is similarly ranging around some norm on any given day, just like it would be in other physical activities. Unfortunately, without being able to get physical feedback on where we stand (unless we get bent) we have to always assume the worst conditions. It’s like we’re always diving as if we’re skiing and getting winded on the chairlift, even though we may be physically capable of pushing much farther that day.

One thing for sure, a single mathematical algorithm built into a computer can at best only be an estimate of reality for the diving population. It may be the best we have, but, that doesn’t imply precision.

Which kind of brings to mind an old joke:

A man is standing under a lamp post at night looking around at the ground beneath it.

A stranger walks by, notices the man’s behavior and asks, “What are you looking for?”

The man under the lamp post says, “I dropped a quarter next to that tree over there and I’m trying to find it.”

Puzzled the stranger asks, “If you dropped a quarter over by that tree, why are you looking for it here?”

“Because you idiot.”, the man responds, “Under this light I can see”.

Edited by BubbleBoy, 14 January 2008 - 09:53 AM.

[shadragon]

From everything I have read while taking my DM course, I have to say that the NDL is hardly arbitrary. It has been established through established scientific method starting with Dr. Haldane and being continuously refined by many agencies, scientists and doctors. The modern tables we use today have been refined over many years based on scientific research and observation.

Regardless of which dive table or computer algorithm you use, the NDL is the defining limit of whether you can do a direct ascent to the surface or not. If you are under the NDL then you can theoretically (note the emphasis) do a direct ascent to the surface and be fine. If you are over the NDL then you need to satisfy DECO requirements and off gas in stages before you can safely ascend to the surface. I am talking about a controlled ascent in these cases, of course.

You can exceed the NDL as many people do daily, but you require special knowledge and training to do so. Owning a DECO capable computer is not enough. Recreational divers are taught to respect the NDL like drivers are taught to respect the speed limit; because of safety. You can exceed the limit and be fine, but eventually the odds will catch up to you and you are going to get into trouble.

If you are a recreational diver and find yourself in DECO situations, then you are not respecting your training limitations. Technical divers support each other in these situations by design. They carry enough gas to cover their dive plans with suitable reserves for emergencies. Recreational divers may not have the gas requirements to satisfy DECO. Either get the proper DECO training from a qualified instructor using appropriate equipment, or stay within recreational limits. If you go beyond NDL without the proper training or equipment then you are being irresponsible and are risking paralysis, injury or death.

Remember that all dive tables, algorithms and computers use theoretical models that do not necessarily conform to what is actually happening to your body. By staying well within the NDL limits you will minimize the dangers of DCI to yourself. Note I said minimize. If you put compressed air on your back and go diving there is always a risk regardless of your depth or duration.

[netmage]

Regardless of which dive table or computer algorithm you use, the NDL is the defining limit of whether you can do a direct ascent to the surface or not. If you are under the NDL then you can theoretically (note the emphasis) do a direct ascent to the surface and be fine. If you are over the NDL then you need to satisfy DECO requirements and off gas in stages before you can safely ascend to the surface. I am talking about a controlled ascent in these cases, of course.

I think there is a nugget in here worth commenting on.... The entire concept of the NDL is based upon the dated backdrop of Haldanean dissolved gas decompression models and I think provides the foundation for part of what Perrone is finding issue with....., that is dissolved gas models. The idea that there exists some line in the time/depth/gas equation, when you cross it bad things happen. OW divers are tought to fear the NDL, the entire concept of understanding the NDL is set aside, almost as if I were to propose the world is flat, and if you attempt to circumnavigate, you will fall off the side of the earth... I mean, I'm overexagurating, but it really does come across like that in OW training...

When you apply dissolved gas models to decompression diving, you end up what some folks like to refer as a 'bend and treat' dive profile. Couple this with traditional OW dive practices, i.e. 30'/min ascent rates (heck, it USED to be 60'/min!) and you end up with a situation where the dive is effectively bent at depth and during ascent, the deco stop 'treats' them...

The tech community first started changing practices by taking normal haldanean tables and introducing deep stops into them... People started feeling better. Now people are fundamentally looking at decompression through different glasses... At the heart of all decompression sickness is/are bubbles. Rather than examining dissolved gases, lets examine the bubble and see if we can prevent it from forming in the first place. If the bubble never forms and we manage the transition from dissolved in tissues to liquid and finally to gas, then we can prevent them from accumulating. It is through this approach that you start seeing NDL times actually decrease from Haldanean tables, and people started to adopt the concept that "all dives are decompression dives". When you come to terms with this idea, the NDL becomes as Perrone is writing, a meaningless line in the sand somewhere.....

-Tim

Edited by netmage, 14 January 2008 - 03:14 PM.

[pir8]

When Tech diving and carrying deco bottles this is all well and good. BUT when the majority of your diving is recreational single tank no deco it is much easier to observe the NDL Limits. Of course even this is no guarentee but your odds are considerably better.

[PerroneFord]

When Tech diving and carrying deco bottles this is all well and good. BUT when the majority of your diving is recreational single tank no deco it is much easier to observe the NDL Limits. Of course even this is no guarentee but your odds are considerably better.

I conform to the same "ascent profile shape" regardless how many tanks I am wearing. That is the point. The idea of NDL can essentially be tossed out the window as long as the ascent shape is proper for the dive. However, when combining proper ascent profile shape, and proper gas management, a diver couldn't even come close to the traditional NDL. It's more conservative, by a long shot, to dive this way. My ascents on single tank dives generally begin at 2000psi, and I am usually exiting the water with 400-800psi depending on conditions and factors.

This is VASTLY different to what you might see in a common recreational profile.

[secretsea18]

When Tech diving and carrying deco bottles this is all well and good. BUT when the majority of your diving is recreational single tank no deco it is much easier to observe the NDL Limits. Of course even this is no guarentee but your odds are considerably better.

I conform to the same "ascent profile shape" regardless how many tanks I am wearing. That is the point. The idea of NDL can essentially be tossed out the window as long as the ascent shape is proper for the dive. However, when combining proper ascent profile shape, and proper gas management, a diver couldn't even come close to the traditional NDL. It's more conservative, by a long shot, to dive this way. My ascents on single tank dives generally begin at 2000psi, and I am usually exiting the water with 400-800psi depending on conditions and factors.

This is VASTLY different to what you might see in a common recreational profile.

I'm no tech diver... but this ascent sounds a heck of a lot like ... multi-level diving. That thing that we recreational divers do.

[PerroneFord]

I'm no tech diver... but this ascent sounds a heck of a lot like ... multi-level diving. That thing that we recreational divers do.

It is very much like multi-level diving. But, done with a specific purpose and to particular constraints. And when done correctly, creates a very specific shape of ascent. Diving to 80ft for 10 minutes and coming up 10ft every 5 minutes will not produce this shape.

[gcbryan]

I'm no tech diver... but this ascent sounds a heck of a lot like ... multi-level diving. That thing that we recreational divers do.

It is very much like multi-level diving. But, done with a specific purpose and to particular constraints. And when done correctly, creates a very specific shape of ascent. Diving to 80ft for 10 minutes and coming up 10ft every 5 minutes will not produce this shape.

However, most recreational divers don't necessarily dive that way. It's setting up a strawman argument (potentially) to suggest that recreational divers "ride" their computers up slope and your way is different. Most recreational divers that I know also essentially dive a check mark shaped dive profile.To the extent that some don't you are correct, it's not a good idea.

One reason that more experienced recreational divers may end up with this profile even if they aren't doing it based on knowledge is as a result of managing their air. After doing a 20 minute dive to 100 fsw for example air would start to be limited and therefore the rest of their dive would be spent shallow. There wouldn't be enough air to "ride" their computer in the first place. For newer divers they could very well do as you suggest.

Also, keep in mind that among experience recreational divers there are many who are knowledgeable regarding what is taught in technical training but who just aren't interested in going that route.

The real contrast among diving is new vs experienced not rec vs tech.

Edited by gcbryan, 14 January 2008 - 05:41 PM.