No Stop Limit for Recreational Scuba Diving
No decompression diving (No stop diving) refers to "diving that you can always ascend directly to the surface without stopping, yet without significant risk of decompression sickness" (according to PADI Open Water Manual). A traditional way to plan a dive is to use a dive table (e.g., see the picture below). For example, if I have a 20-minute dive at 20 meters, then I'm in Pressure Group G, meaning that there's is G amount of nitrogen in my body, thus limiting the duration of my next dive.
However, my question is that, for example, if I want to do a 40-meter dive with air, why is my no decompression limit capped at 9 minutes? I would only have G amount of nitrogen in my body. However, if I dive at 20 meters for 45 minutes, I could have U amount of nitrogen! The amount of nitrogen in my body determines the No Decompression Limit, is that right? If so, why can't I dive til I have G amount of nitrogen in my body at 40 meters? Thanks everyone.
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As a rough rule of thumb, a dive at 45 metres is a ratio of 1:1 where every minute at that depth requires one minute of decompression stops. Therefore every dive to 45 metres is a decompression dive, which by definition is a technical dive and not a recreational dive, as covered by the RDP (Recreational Dive Planner).
To this one must add that dives to 40 metres (130 feet) and below are generally considered outside of recreational diving as you must have redundant gas supplies, be prepared to do decompression stops, normally using accelerated decompression techniques (e.g. oxygen rich mixes in stage bottles) and you must be able to handle the narcosis possibly using helium in your back-gas mix. Most importantly any uncontrolled or rapid ascent from >40 metres (130 feet) is almost certain to end in serious injury and a trip to the pot (recompression chamber).
The NDLs (No-Decompression Limits) are reached in a non-linear fashion where depth dramatically reduces the NDL. With the rough rule of thumb of 1:1 at 45 metres, it's 1:2 at 60 metres (e.g. 1 minute on the bottom gives 2 minutes of deco)
By the way, all dives below 30 metres (~100 feet) on the PADI RDP require a mandatory 3 minute decompression stop (which they call a "safety" stop).
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The amount of nitrogen in your body is only part of it. Nitrogen is absorbed quicker by your lipid, or fat, tissues at greater depths. The cut off point for a non-deep PADI qualified diver is 30m. There is a course to go deeper, down to 40m, as the absorption rate increases. This can cause various effects including nitrogen narcosis. I will add nitrogen narcosis can happen at any depth, it is specific to the diver.
I should also add that going down to 40m will result in a safety stop on the way up. You won't simply be able to drop down to 40m then ascend straight away. You'll need to have a 3-5 minute stop at 5m. Satety stops are considered manditory on any dive deeper than 30m by most agencies. Any dive computer you dive with will want you to do a safety stop on any dive deeper than 10m.
Also also, general rule of thumb is do your deepest dive first, when you have the least amount of nitrogen in your body.
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Answering the specific question: why does a shorter but deeper dive leave a lower residual nitrogen limit than a longer shallower dive.
Common gas absorption decompression models such as Buhlmann work on the principle of several gas "compartments" which absorb inert gasses (e.g. nitrogen, helium) at different rates and these represent the speed that gas is absorbed in different tissues. In the case of Buhlmann there's 16 compartments from fast to slow (roughly but not exactly equating to blood absorbing faster than, say, bone).
When you do a deep bounce dive, the fast compartments will on-gas very quickly, but the slower compartments won't. The deeper the dive, the greater the pressure, so there's a greater inert gas pressure which will "fill" the compartments. i.e. the fast compartments "fill" faster.
As the recreational dive planner is for "No Decompression", you cannot allow any of the compartments to "fill", otherwise you'll get gas bubbles when you ascend or will have to hold a decompression stop. With the PADI RDP and all other minimal/no decompression planners, the fast compartments will control the bottom time as they'll have too much inert gas to release during the 9 metre/minute ascent (safety stops aren't included in this calculation).
With a shallower dive, the fast compartments still 'fill' but not as much as with a deep dive as the pressure is lower. In this case they can off-gas during the standard ascent. However there's been a longer bottom time in which case the slower compartments will control the bottom time.
Your surface interval residual nitrogen (inert gas) timings are controlled by the slower compartments. In a shallower dive these are off-gassing on the surface but more slowly than the fast compartments, hence the greater residual nitrogen group.
A deep dive in comparison never allowed the slower compartments to fill, so there's only a small amount of residual nitrogen to off-gas.
Side note:
Technical divers regularly dive well beyond the limits of the RDP, for example 30 minutes at 40 metres. They're constantly on-gassing during the bottom phase of the dive and need to come up and stop for long durations according to the plan - over 60 minutes of decompression is required if diving to 40 metres on air, during which the diver cannot ascend to the surface without serious risk of DCS!
This can be considerably reduced if nitrox (e.g. 28%) is used for the bottom phase and oxygen is used for decompression (in this example reducing the decompression obligation to less than 20 minutes).
Thus technical divers need to be able to plan the dive (which will include consideration for equipment failures, narcosis, gas selection and requirements), have the right equipment (multiple cylinders containing adequate gas for the plan), and have the skills to dive that plan (e.g. following the plan and holding a decompression stop)
If you're interested in this topic, it's taught to all technical divers by all agencies and is explained very well in books such as Deco for Divers (Powell, Mark)
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