How close to no decompression limit

[Foxfish]

Oh that explains a lot. Maybe the instructor wants you to not exceed the no-deco limit by more than 10 minutes then..

Also the advice of hitting your safety stop at a time when your computer gives you just 10 more minutes of no deco time...... , would probably require that you do a 15 or 20 hour long dive, (assuming the SS stop was around 20 feet).

This was a general directive to all divers.

 

[Wookie]

I think that's a pretty sad rule of thumb, especially for someone with between 200 and 500 dives. By that level of experience you should be looking way more closely at deco theory, different algorithms, and the effects of your personal physiology on decompression. If your instructor is giving you a silly thumb rule like that, it may be time to find an instructor who is ready to bring you along to the level that your experience suggests you should have. Especially for a relatively cool water West coast diver such as yourself.

 

[Foxfish]

Deleted

 

[DiveNav]

Hi,
As it was mentioned already in this post, a "10 minutes rule" per se, without indicating depth doesn't make too much sense as the NDL changes continuosly during the multi-level dive/time profile.
You might want to play with some dive planning software (such as our divePAL) to see how Nitrogen Loading is affected by multilevel dive profiles.

I use the Suunto D4 with the default personal settings. The Dive Manager software which allows you to view the dive profile shows the tissue saturation bar graphs. The levels in medium density tissue did exceed the saturation limit toward the end of one of my dives but fell to just below the limit after doing the safety stop.

Would you mind sharing the profile on divePAL so we can have a look at it?

​

Alberto (aka eDiver)

 

[Foxfish]

There's so much more to it than just how long you were in the water. Hopefully your instructor covered things like water temperature, ascent rate, saw-tooth profiles, exertion, repetitive dives, hydration status, etc.

These things can affect you likelihood of getting DCS. Not aware of anyone who has generated a practical quantitative method for calculating NDL's with reference to these factors. I suspect that the reason for the 10 minute limit is to account for some of these factors.

 

[Dive-aholic]

If you plan your dives properly before getting in the water then you don't have to watch the computer to see when you have 10 minutes left. Plan your dive based on your depth(s) and dive your plan. It's that simple.

 

[Foxfish]

If you plan your dives properly before getting in the water then you don't have to watch the computer to see when you have 10 minutes left. Plan your dive based on your depth(s) and dive your plan. It's that simple.

The diving is done from a charter boat and comprises two dives. Details of dive depths are known a few minutes before jumping in the water. First dive to a maximum depth of over 65 ft. Second dive the same but within no decompression limits. Dive plan (second dive) is - jump in, swim around at some depth until 10 minute NDL is reached, reduce depth as required to increase NDL until air gets low, rise to level for safety stop and then get out. Dive charters often avoid a deep second dive so this is not a problem.

 

[dumpsterDiver]

The diving is done from a charter boat and comprises two dives. Details of dive depths are known a few minutes before jumping in the water. First dive to a maximum depth of over 65 ft. Second dive the same but within no decompression limits. Dive plan (second dive) is - jump in, swim around at some depth until 10 minute NDL is reached, reduce depth as required to increase NDL until air gets low, rise to level for safety stop and then get out. Dive charters often avoid a deep second dive so this is not a problem.

If you are diving to a max depth of 65 feet, beginning your ascent when you have 10 minutes of no-deco time left is a reasonable and simple direction to follow. But you see, when diving.. the time/depth relationship is kinda important so you can't make blanket statements about starting the ascent when you have 10 minutes of no-deco time left unless you specify the depth.

 

[DevonDiver]

It's a very generic rule and not suitable for any sort of "very strong" advice/recommendation.

Firstly, 10 minutes from NDL at 25m is <em>not</em> the same as 10 minutes from NDL at 10m. A 10 minute NDL from a single deep dive is not the same as a 10 minute NDL after multiple deep dives. NDL is just a point where the nitrogen saturated in your body won't clear under a normal (according to ascent speed required) ascent to the surface.

On any given dive, depending on depth, duration and previous dives - the saturated nitrogen will be loading different theoretical tissue compartments to varying degrees. Which tissue compartment controls the dive (sets the NDL) depends on the factors mentioned. <

Secondly, as others have mentioned, such a rule would be useless for deep diving - especially where the entire NDL may not be more than 10 minutes.

Thirdly, Setting a statuatory and self-imposed limit is merely an issue of adding conservatism to your profile. Adding conservatism is a good idea... but it's still just an 'added' safety precaution. Respecting your initial NDL is a safety factor. Keeping further from it is better. Basically, you're setting an NDL inside an NDL. Oh... that's getting a bit 'Inception' isn't it?

Fourthly, Maintaining a 'boundary' from your NDL is great... but it shouldn't allow you to disrespect other factors carte-blanc. All the 'extra-NDL' in the world isn't much benefit if your buoyancy is rotten and you're yo-yoing around on the dive creating a zig-zag profile... or ascending to quickly.... or rushing to the surface in a few seconds from your safety stop. Good dive profiling, slowwww ascents and extended safety stops are the first things you should be doing to add safety. Setting 'Inception' NDLs is well down that list.

 

[drbill]

I've now been diving for 50 years. In the old days we didn't have computers (used tables). No idea how close to no decompression limits I got back then because I always did multi-level dives, not square profiles. I only started using a computer about15 years ago. I remember the first time I exceeded my NDL... I was a little concerned, but my long dive profiles always involve sigfinicant time in the shallows (as a videographer this is easy to accept). Once I got over the psychological concern about going "into deco," I had little trouble with it afterward. However, one must realize that by going into deco (at least based on your computer), you have added an extra level of concern... an unseen ceiling.

For about a decade MOST of my dives were "deco" dives. Only rarely did I encounter any problems due to the added safety margin I had based on my multilevel profiles, deep stops and fairly long stays in the shallows (above 30 ft) to film.

I do NOT recommend that newly trained divers take this to heed! Only that they understand with proper training and experience deco diving (with appropriate redundancy) can be done sagely (IMHO).