How much cushion do you leave..

[acteg]

3-5 minutes. My computer is pretty conservative.

 

[MXGratefulDiver]

Those are two things that make me want to leave a cushion.

Number A) The computer knows nothing about me (obviously, its a computer ;p )
The formula's are based upon an "Idealized" persons absorption.
I havent done too many dives, so maybe im hanging with the wrong crowd, but I have had maybe 3 or 4 divers that seem to be "Ideal" physique-wise .
I remember hearing - may or may not be true- two things.
One was that fat absorbs nitrogen at a higher rate than muscle
And the second was servicemen were used as the ''test subjects'' for analyzing nitrogen absorption (something along those lines)

If that is the case, and the algorithims are based upon those studies (I don't know if they are or are not), than the non ''ideal'' diver (most of us here in the USA) would have different rates [higher because of fat- fat that was not as high in concentration in military people] of absorption.

And the slim possibility of having a PFO

Losing 15 minutes of my dive seems better than losing many hours, and dollars in a chamber

All of this is generally true ... and it shows a very nice, safe attitude about your diving. So I don't want to discourage it. But I would like to point out that there are way more variables than just physique involved. Hydration, exertion, and water temperature can play a huge role ... as can the amount of sleep you got the night before the dive. But probably the biggest factor is ascent rate ... PARTICULARLY that last 15 feet after you've completed your safety stop. That should take a minimum of 30 seconds ... and most divers exceed it by huge amounts, often ending up on the surface 5 to 10 seconds after completing their safety stop. This is where (in my opinion) a lot of "undeserved" hits occur.

Servicemen were indeed used as data points for creating the Navy tables ... and many commonly used algorithms are based on those tables. But you have to also be aware that these tables were not adopted strictly in creating those algorithms. For example, Navy tables are based on an ascent rate of 60 feet per minute. Most computer algorithms are based on an assumed rate of 30 feet per minute. And across the board, dive computer NDL's are generally lower than Navy table NDL's. This is because companies that sell these computers want to minimize their liability, and so increase the level of conservatism applied to the Navy tables as a whole.

Decompression is far from an exact science. You can't really apply a number and say that staying within that number is "safe". All you can really do is decide how much risk is acceptable for you. And you are correct that general fitness plays a key role in that decision. For people who are less fit, staying well inside your no-decompression limit is a prudent way to dive. It might shorten your profiles a bit, but generally speaking the more conservative you make your dives, the less risk you will have of ever taking a ride in a chamber.

... Bob (Grateful Diver)

 

[redacted]

All of this is generally true ... and it shows a very nice, safe attitude about your diving. So I don't want to discourage it. But I would like to point out that there are way more variables than just physique involved....

I am not sure to what degree "physique" is involved or exactly what variables of physique might come into play. Sure soft tissues absorbs quicker than harder tissues and fat is one of those soft tissues (as is the brain). But the tissues that absorb faster also off-gas faster - it is a 2 way street. The dive planning tool that I referenced in post #5 will allow you to monitor the on and off gassing of 16 tissue compartments through your planned dive. If you contact the thread starter (DiveNav) he may even be able to add your computer to the simulation.

 

[D_B]

It varies , I use a 10 minute remaining warning, but thats to back up myself information wise ... I don't use it as a time to ascend indicator
I use more of a what's this dive like/how am I feeling today to decide that


Also, it's a good idea to know just what your computer does if you accidentally go over NDL ... you should not be so distracted that you do go over, but it does happen and that's not the time to try to figure out what it's telling you

you also bring up something else, your comment about progressing to nitrox and not having a nitrox capable computer is why you see recommendations here to get a nitrox capable one as your first computer

 

[Akimbo]

You can also add cushion at the other end. If you use the last second of bottom time for a no-D dive and have air left, fool around at 10' and enjoy the mid-water experience. No point in hauling all that unused air back to the beach when you could enjoy it and increase your peace of mind.

 

[blackvans1234]

Bottom line-
Its time for EANx

Well in this scenario yesterday I was outbreathing someone with nitrox and didnt have my SMB, we had a capt on the boot following us around.
Current was also moderate-strong so I really didnt want to deal with the stress

Its a learning process though, and I'm learning (can you smell the smoke?)

 

[blackvans1234]

you also bring up something else, your comment about progressing to nitrox and not having a nitrox capable computer is why you see recommendations here to get a nitrox capable one as your first computer

Yeah, you're definitely right about that, however, three months ago I did not foresee my diving 'career' going this well

 

[Dirty-Dog]

I don't have any qualms about diving to within a couple minutes of my NDL. My primary computer is a Suunto, which are known to be pretty conservative. My backup computer is an Aeris, which are slightly less conservative.
Frankly, we tend to avoid the issue on OW reef dives by planning our dives as multi level, on sites where this is practical. Gas (mine, since my SAC rate of about .5-.6 is higher than hers) is generally our limiting factor.

 

[rhwestfall]

adding to Bob's thoughts - the "old" Navy tables also showed 60 minutes at 60 feet, and 25 minutes at 100 feet. The "new" recreational limits have been reduced from that now as an increase in conservatism which likely show up in your computer's programming. Saving a couple minutes more is good for the "what if" scenario, or if you are planning a intense number of dives over a compressed schedule, but the amount you are talking is robbing you of diving....

 

[redacted]

I don't mind the idea of pushing NDL times but I do not plan my dives to do that. I do plan to take on some fairly heavy nitrogen loads on some dives but those type dives include a slow ascent with a couple 3 to 8 minute stops on the way up to off-gas the more loaded compartments.

I really don't mind the idea of exceeding NDLs and taking on a "small" deco obligation. Such small obligations would most likely be worked off in the course of my normal planned ascent. But it is not the kind of dive I would plan and so far I have never incurred a deco obligation.