How close to no decompression limit

[Foxfish]

Thanks for the responses. I was wanting feedback from other divers on whether they thought this rule was a good idea and why or why not. Seems the general consensus is that the rule is overly conservative. Does anyone know if this is a general rule promoted by SSI ?

As noted about, the graphs of tissue saturation indicated at some stages of the dive some tissue exceeded no decompression limits even with the ten minute buffer. That was a concern. There is no provision on the dive charter boats for dealing with a diver who goes into decompression (ie. spare tanks in the water) and we surface with 50 bar (725 psi) in the tank.

I don't have a good operational understanding of what to do if you do exceed no decompression limits. This is not taught or discussed at this level of diving. I'm not even sure if my computer would provide guidance on decompression stops needed if I did exceed the no decompression limits. The rule is plainly - don't exceed no decompression limits.

 

[Foxfish]

Here is an extract from the dive profile indicating saturation limits were exceeded in some tissue.

 

[BDSC]

I'm not even sure if my computer would provide guidance on decompression stops needed if I did exceed the no decompression limits.

I imagine that if you have a fairly recent computer it would display the info but no doubt your manual will tell you.

 

[Thalassamania]

...
I don't have a good operational understanding of what to do if you do exceed no decompression limits. This is not taught or discussed at this level of diving. I'm not even sure if my computer would provide guidance on decompression stops needed if I did exceed the no decompression limits. The rule is plainly - don't exceed no decompression limits.

You do not need, "guidance on decompression stops needed." What you need is displayed in very simple terms: your ceiling. You must not go shallower that that ceiling, but you should ascend so that instead of continuing to take up more gas you will be able to offgass. So you ascent, the more you ascend the faster you off gas. BUT: you ascend no shallower than your displayed ceiling depth. The closer you get to that ceiling depth the faster your decompression status will be resolved. A good guideline is to ascend to 5 to 10 feet below you ceiling depth and wait to switch back to a no-D status.

 

[Foxfish]

In itself, this is not a bad thing provided you did it intentionally, knew what you were doing, and were adequately equipped for it.

The point being that at this level, none of these things are a given. Below is another plot showing ascent after tissue saturation limits were exceeded and indicating that ascent rates were too high in some situations. This dive was done observing the ten minute rule. Had this ascent rate occurred with a higher tissue saturation then the chance of DCS would have been greater.

(First jpeg is the one in question)

 

[Foxfish]

You do not need, "guidance on decompression stops needed." What you need is displayed in very simple terms: your ceiling. You must not go shallower that that ceiling, but you should ascend so that instead of continuing to take up more gas you will be able to offgass. So you ascent, the more you ascend the faster you off gas. BUT: you ascend no shallower than your displayed ceiling depth. The closer you get to that ceiling depth the faster your decompression status will be resolved. A good guideline is to ascend to 5 to 10 feet below you ceiling depth and wait to switch back to a no-D status.

Thanks for that.

 

[Dive-aholic]

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.

Even with only a few minutes of preparation time you should easily be able to plan a dive given the information you receive during your pre-dive briefing. 70 ft on air gives you a no decompression limit of 48 minutes (according to the table I'm using). You should also be calculating your time at depth based on your gas consumption. If you're respiratory minute volume is .7 you won't last even 35 minutes at that depth. You will likely begin your ascent 25 minutes into the dive. Even a .5 RMV will only give you 35 minutes at depth before your ascent begins. Simply to "jump in, swim around at some depth until 10 minute NDL is reached" is not appropriate. Your instructor is doing you a disservice by teaching that this is okay.

 

[Foxfish]

I'm diving in the ocean within limits for advanced recreational diving ie. < 30 m. The precise profile of the dive is largely dependent on the profile of the reef structures below which are unknown before the dive. I could refer to my computer dive planner which would give me a maximum time at a single depth but that is largely irrelevant because there is usually a large range in depth around the reef and the depths that I'll dive depend on what is interesting. If I need to go deeper, that just means less time at that depth. Providing you are within the NDL, why is that a problem? If I start getting low on air I head to the surface. The question here is how close is it safe to be to the NDL.

 

[Thalassamania]

The reality is that if you had enough air and trusted your algorithm, you could wait until a ceiling showed up and then begin your normal ascent. Odds are, by the time you reached the indicated ceiling depth, the ceiling would be gone as a result of out-gassing during your ascent.

 

[dumpsterDiver]

The point being that at this level, none of these things are a given. Below is another plot showing ascent after tissue saturation limits were exceeded and indicating that ascent rates were too high in some situations. This dive was done observing the ten minute rule. Had this ascent rate occurred with a higher tissue saturation then the chance of DCS would have been greater.

(First jpeg is the one in question)

I don't understand your graphs and your quuestions... How can you reach 100% of allowable no deco nitrogen absorption and simultaneously contiue to have a 10 minutes of remaining no-deco time?

At 100%, you should have zero time right?