Bent. I guess it really can happen to me.

[HIGHwing]

mmadiver:

So, what do you do? Halve your NDL? Add a pressure group, or reduce your depth to move up one? Knock 25% off your NDL? A 5 min. safety stop instead of a 3? Do a deep stop? Stay on the boat if you can't run a mile in 10 minutes (inside joke from another thread)?

What's 'good enough' conservatism?

Richard.

That's the precise point I was trying to make in the other thread. On easy dives I exit the water at 85% of the M-Value, on deep, cold, (insert variable) dives I come out at 70% of the maximum M-Value. You'll notice earlier in thread we looked at some quotes from the manual. Everyone closely reviewed the disclaimers, but what's more interesting is the conservative GF settings available in Tec mode.

If you look at the data available, and you're worried about decompression stress, perhaps running 30/70 or 30/75 makes a lot of sense. In fact, Shearwater ships computers set at 30/70 out of the box, but that 70 will certainly make you unfriendly with your other dive buddies as it will negatively impact your NDLs versus a recreational dive computer significantly. The good news is you can learn about the model, look at the success statistics and then make choices. If you want to run bubble model, you can add VPM if you like via unlock code. Whatever makes you happy.

 

[TechDeep]

That's the precise point I was trying to make in the other thread. On easy dives I exit the water at 85% of the M-Value, on deep, cold, (insert variable) dives I come out at 70% of the maximum M-Value. You'll notice earlier in thread we looked at some quotes from the manual. Everyone closely reviewed the disclaimers, but what's more interesting is the conservative GF settings available in Tec mode.

If you look at the data available, and you're worried about decompression stress, perhaps running 30/70 or 30/75 makes a lot of sense. In fact, Shearwater ships computers set at 30/70 out of the box, but that 70 will certainly make you unfriendly with your other dive buddies as it will negatively impact your NDLs versus a recreational dive computer significantly. The good news is you can learn about the model, look at the success statistics and then make choices. If you want to run bubble model, you can add VPM if you like via unlock code. Whatever makes you happy.

HIGHwing, I'm actually the OP with the lynx, but that can get confusing in a 14 page thread. Liquivision has revised the manual, as there have been several revisions of firmware since the original manual was published. You can read it here.
http://liquivision.com/docs/Lynx_User_Manual.pdf
They've watered down the disclaimer somewhat, but they still advise using backup gear, which I always do anyway. I have found that if it uses batteries and you put it in water it will fail someday, no matter who makes it. I appreciate your information regarding the decompression models, and I'm always up for learning something new. I mean, I tell my 6 year old every day as he goes off to school to "learn something new" so I need to practice what I preach.

I know you live in Virginia, and you go to Lake Rawlings. Sure would like to talk live sometime at the lake. Thanks.

 

[redacted]

That's the precise point I was trying to make in the other thread. On easy dives I exit the water at 85% of the M-Value, on deep, cold, (insert variable) dives I come out at 70% of the maximum M-Value. You'll notice earlier in thread we looked at some quotes from the manual. Everyone closely reviewed the disclaimers, but what's more interesting is the conservative GF settings available in Tec mode.

If you look at the data available, and you're worried about decompression stress, perhaps running 30/70 or 30/75 makes a lot of sense. In fact, Shearwater ships computers set at 30/70 out of the box, but that 70 will certainly make you unfriendly with your other dive buddies as it will negatively impact your NDLs versus a recreational dive computer significantly. The good news is you can learn about the model, look at the success statistics and then make choices. If you want to run bubble model, you can add VPM if you like via unlock code. Whatever makes you happy.

Since M-values are already dependent on depth, why add depth in as a variable a 2nd time? I think I understand cold and workload but confused why you would adjust twice for depth. That must really become quite limiting on repeated deep (recreational) dives.

 

[mathauck0814]

mmadiver:

Chatterton's article you linked to was quite interesting, though I contest the idea that you 'earned' the hit if you get DCS, because you 'came up too soon,' made in the style of a blanket statement (I'm not using a direct quote, but reading the article, I believe I've summarized the sentiment).

Because people don't have any way to know for sure just how much decompression to do on the dive. Turning to greater conservatism to reduce DCS is an issue of diminishing returns. You could just stay on the boat. Or you could dive, but use tables and do your multi-level dive as though the whole dive was done at the deepest depth. Or dive a Suunto on the grounds it's said to be more conservative than some competitors, or an Oceanic on its more conservative setting. Or dive nitrox on an air dive profile. Then there are the arguments about how long to wait before flying.

My point is, it's not precisely known at what point diminishing conservatism becomes recklessness, and cause to 'blame' somebody or say he/she 'earned' the hit. Taken to an extreme, if you chose to get in the water, you asked for it, eh?

A small excerpt from Chatterton's article:


Yes, that's all true. But how do you modify anything based on your interpretation? For example, let's say you plan to do a pair of 1 tank boat dives on air, on a charter boat. I'm not going to drag out tables, but let's stay for sake of argument they will be square profile wreck dives, fairly deep, with large tanks that can push NDLs, and these dives come out to:

Dive 1 - NDL 40 minutes.

Dive 2 (1 hour later) NDL - 20 minutes.

So, how much NDL time do you knock off, or how much do you add to your 15' safety stop, for account for any one of these factors:

1.) You're fat - 150% ideal body weight.

2.) You're old - 55.

3.) You're 'out of shape' - not terribly so, but an office worker who doesn't do any fitness regimen, mid. 40's, in decent health but you get winded a little easily.

4.) Water's cold - 45 degrees. You're in a 7 mm wet suit.

5.) There's modest current and so you'll exert moderately swimming against it part of the dive.

6.) Low viz. muck dive & keeping up with your buddy is mentally stressful.

7.) You stayed up late and didn't get much sleep.

8.) You're a little jet-lagged from yesterday's flight.

9.) You might be a little dehydrated because you don't want to pee in your wet suit on the boat trip.

So, what do you do? Halve your NDL? Add a pressure group, or reduce your depth to move up one? Knock 25% off your NDL? A 5 min. safety stop instead of a 3? Do a deep stop? Stay on the boat if you can't run a mile in 10 minutes (inside joke from another thread)?

What's 'good enough' conservatism?

Richard.

TDI and IANTD address this in higher levels of training - perhaps it would be better to introduce it earlier. You add conservatism as "imperfections" in the plan come up. Cold water? High current? Big waves? etc. all get factored into your dive planning.

People have different ways of doing this, but I believe a commonly taught way that PADI incorporates into some of its more basic courses is to plan your dive as though it were deeper/longer to accommodate cold water. If you're diving to 50 feet in cold water, plan it as though you were going to 60 (and pad the safety stop by a minute or two). There are logical ways to accommodate variables that deviate from the ideal dive.

Chatterton's article is addressing a different kind of diving that I don't feel is very applicable to this conversation. Divers undertaking very deep dives with complex gas plans, etc. are constantly evaluating themselves (before, during and after the dive) for any small difference and have a level of awareness that isn't present in recreational divers. I don't think his statements have a lot to add to the discussion of the OP's hit.

 

[Storker]

I know I'm late to the thread, but I really have a problem with some of the replies here.

The profile - Dive 1-55.5 ft. 42:49. Surface interval 43:55 Dive 2-55.5 ft. 44:21.

My PADI table is metric, but 55.5 feet equals 17 meters. Shouldn't make a difference, right? *checking table* Nope, doesn't look good to me.

Those numbers alone is WAY more conservative than my normal THREE dive day, repetetive days.

:shocked2:

you could use the PADI tables and go group Q after the first dive to group E after the SI

Um, no. 43 min bottom time @18m puts you in PADI group Q and a 44 minute SI puts you in group H. THat gives you an NDL of 32 minutes at 18m for your second dive. Which is more than a little in excess of the 44 minutes' duration of the second dive...

If the OP is correct on his profiles, the hit is undeserved Planned or Not:

Straight from V-Planner: Square Profile on Air no Safety Stop!

[...]

Using Suunto's Dive Planner (yes I know Suunto's algorithm is quite conservative even with a conservatism setting of 0), assuming square profiles, entering the actual depths, subtracting 4 minutes bottom time for ascent and safety stop time to make total time submerged equal to the OP's data, the planner puts you in deco 31 minutes into the second dive. If you continue to 40 minutes bottom time, Suunto prescribes 11 minutes of deco at 3m. Even if I enter the OP's average depths - eyeballed from the profiles to 50ft/15m, Suunto tells me I'd be just one minute from deco on the second dive. Doing the same thing with the PADI table, I end up with an NDL time of 49 minutes on the second dive.

So what's with the discrepancies between what I find from my tables and simulation - that the OP was really walking the NDL line - and the replies in this thread - that the DCS hit was unexpected?

 

[lowviz]

...//... the OP was really walking the NDL line ...//...

Storker, I share your concerns. "Walking the NDL line" means that you will, if you believe in statistics, sooner or later take a hit.

Quoting from the NOAA Diving Manual: "Should a diver realize that he has exceeded the no-decompression limits prior to reaching the surface, and he does not have access to U.S.N. Decompression Tabls to determine required in-water decompression time, the diver should take the following precautions:

Stop at 10 to 15 fsw for a minimum of 15 minutes or until he reaches 300 psi in their cylinder, whichever comes first."

This should be taught at the OW level, along with enforcing a really slow ascent from the last 10 ft...

 

[Tigerman]

I know I'm late to the thread, but I really have a problem with some of the replies here.


My PADI table is metric, but 55.5 feet equals 17 meters. Shouldn't make a difference, right? *checking table* Nope, doesn't look good to me.


:shocked2:
Um, no. 43 min bottom time @18m puts you in PADI group Q and a 44 minute SI puts you in group H. THat gives you an NDL of 32 minutes at 18m for your second dive. Which is more than a little in excess of the 44 minutes' duration of the second dive...


Using Suunto's Dive Planner (yes I know Suunto's algorithm is quite conservative even with a conservatism setting of 0), assuming square profiles, entering the actual depths, subtracting 4 minutes bottom time for ascent and safety stop time to make total time submerged equal to the OP's data, the planner puts you in deco 31 minutes into the second dive. If you continue to 40 minutes bottom time, Suunto prescribes 11 minutes of deco at 3m. Even if I enter the OP's average depths - eyeballed from the profiles to 50ft/15m, Suunto tells me I'd be just one minute from deco on the second dive. Doing the same thing with the PADI table, I end up with an NDL time of 49 minutes on the second dive.

So what's with the discrepancies between what I find from my tables and simulation - that the OP was really walking the NDL line - and the replies in this thread - that the DCS hit was unexpected?

nvm brainfart

Still dont change the fact that the dive is technically shorter than the total dive time according to the tables due to the fact that youre supposed to not include ascent and SS time and the fact that the depth isnt quite the max depth for the duration of the dive.
It ALSO dont change the fact that thousands of profiles similar to this is done every day without incident in resorts all over the world..

 

[phoenix31tt]

It ALSO dont change the fact that thousands of profiles similar to this is done every day without incident in resorts all over the world..

I guess that's the problem with not being conservative... The longer you do dives that are close to NDLs the greater your odds are for a hit...

I won't be surprised if many of these resort divers that follow similar profiles could be getting minor hits and not noticing... But then again, it wouldn't surprise me if most of them were doing these profiles without taking a hit, as physiology plays a big part in whether you get a hit and how its manifested



Sent from my Nokia Lumia using Tapatalk

 

[Storker]

nvm brainfart

My error. I didn't use the actual PADI table, I used the Dive Planner app on my Android phone. When I installed the app, I checked that it gives the same PGs as the PADI table for the first dive in a series. But apparently the app is more conservative than the PADI table WRT surface time credit, and I hadn't checked that. You were right, I was wrong

But the simulations still hold, though, after I double-checked them. I used Suunto Dive Planner v. 1.0.0.3, bundled with the Suunto Dive Manager v3; Suunto's RBGM algorithm with deep stops. So part of my argument stands: That according to at least one deco model, the OP was closer to the NDL than I'm generally comfortable with.

 

[mathauck0814]

I won't be surprised if many of these resort divers that follow similar profiles could be getting minor hits and not noticing... But then again, it wouldn't surprise me if most of them were doing these profiles without taking a hit, as physiology plays a big part in whether you get a hit and how its manifested

I agree. I see it a lot on day boats in CA. Folks get off the boat with a lot of enthusiasm and energy and run their relatively deep profiles up to the limits of their computer, board the boat immediately crawl into a rack and sleep for the rest of the trip. I think people have those subclinical type of symptoms and fatigue frequently and chalk it up to having been "exercising" in the water on a dive.