How close to no decompression limit

[Foxfish]

I recently did the advanced adventurer course through SSI and was advised in very strong terms that you should never get less than ten minutes to the no decompression limit on your computer. What do you think?

 

[herbdb]

Seems excessively conservative to me. I am not a really experienced diver, but have never heard this before. I was advised to be near the up line as I approached a limit be it gas or NDL, so I was in position to start up when I was up against a limit.

I am interested in hearing the views of some of the more experienced divers on the board.

 

[RJP]

I recently did the advanced adventurer course through SSI and was advised in very strong terms that you should never get less than ten minutes to the no decompression limit on your computer. What do you think?

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.

At the end of the day, your body can't read tables or your computer. Lots of people get bent well within NDLs. Lot's of people violate NDLs without a problem. Getting out of the water 10min prior to NDL will surely give you a greater margin of safety but, as was cited above, sounds unnecessarily conservative for a typical recreational dive. (Hell, on some computers you might not even make it to the bottom if you had to be out 10min prior to NDL!)

 

[John Larsen]

My compuer has 2 modes i can set either conservative or non conservitave...I did a dive in consrvative mode and got to within 3-4 minutes of a NDCL and when I compared dives with my buddy who uses a different less consevative computer he was no wheres near a NDCL and we dove the same proflie, So I would say it all depends on your computer. I now dive with my computer set to non conservative

 

[Bob DBF]

If you have no experience or training in gas management or emergency deco, its a good plan to stay well within NDL. The computer might walk you through deco but if you do not have enough gas to complete it, the results are a crap shoot.

Know how conservative your computer is and reserve gas for that contingency then you can sit on NDL as long as you wish, it is your choice. Be advised that should you be deep and overstay the deco adds up pretty quickly and you are using a lot of air with each breath which you may need later.



Bob
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I may be old, but I’m not dead yet.

 

[dumpsterDiver]

10 minutes??? What a load of crap. As mentioned, that would preclude you from doing any diving much past 110 feet.

Wait a minute? Are you over 450 lbs and this was a special recommendation the instructor gave you?

 

[GrumpyOldGuy]

The 10 minute rule would be a sign of someone who does not understand decompression or perhaps you misunderstood what the instructor said.

First, the minutes remaining are relative to the nature of the computer and its settings and most importantly depth.

If you dive deep at all, you will run right up to the NDL at depth (often within a few minutes or so) because the fast tissues compartments typically control NDL. Most dives are not a square profile and as soon as you start ascending to shallower depths, the NDL time will "magically" increase because different compartments start becoming the critical ones. Often referred to as riding the computer. Now if the instructor said ending the dive with at least 10min NDL or hitting you s/s with 10, it makes a little more sense, at least until you becoming proficient and confident in gas planning and have redundant gas.

 

[redacted]

That rule is simple and easy to use but excessively and unnecessarily limiting if you are diving deeper than 60 ft or so. The way to add a bit of conservatism to your computer is to limit your nitrogen loading as a percentage of the maximum allowable loading. Some computers have tissue loading bar graphs which are displaying the tissue loading as a percentage. If you dont have this feature, you can have a similar effect by limiting your NDL to a percentage of the allowable NDL. That means you have to plan your dive a bit closer or you have to have remember a approximate NDL for each depth. I believe a course rule of thumb is NDL + depth = 120. So, at 80 feet, NDL is about 40 minutes. If you want to be a bit more conservative, 10% is 4 minutes so don't go less than 4 minutes NDL at 80 ft. At 100 ft, you would leave a 2 minute cushion. And at 60 ft, a 6 minute cushion. A bit crude but not as limiting as a mindless 10 minute rule.

 

[Foxfish]

Some computers have tissue loading bar graphs which are displaying the tissue loading as a percentage. If you dont have this feature, you can have a similar effect by limiting your NDL to a percentage of the allowable NDL.

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.

 

[dumpsterDiver]

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.

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).