Do not under estimate your own short coming!
I have never suggested that shallow dive has no NDL!!!! About 240 mins for 24ft dive. Period.
You are not the only participant in this thread. I was replying to AfterDark, who said repeatedly that there was no NDL at those depths. You joined him in his attack on me, so you became part of that response.
I hope none of your tec or rec student is reading the last couple of sentences!
Do you understand the meaning of saturation and supersaturation at all?
Incredible.
I do understand, but evidently you do not. You are evidently not interested in learning it, either, since I have already explained the process several times. What follows is a primer for others who might be interested in learning.
1. Tissue on-gassing is not like filling a bucket of a certain size. When we breathe, nitrogen (and other gases) enters our lungs and blood and passes through the tissues. As the blood travels, some nitrogen leaves the blood and enters the tissues, and some nitrogen leaves the tissues and enters the blood. It reaches the lungs and is expelled through exhalation. On the surface before a dive, we have usually been at that pressure long enough for this process to be in perfect balance. Just as much nitrogen enters our tissues as leaves it. Our tissues are in equilibrium. They are saturated at that pressure,
and they cannot take on any more nitrogen.
2. When the diver descends, our lungs take on more air with each breath, so they have more nitrogen.Therefore, the blood circulating through the body has more. The deeper we are, the more nitrogen in the blood. The blood circulating through the body thus causes more nitrogen to enter the tissues than leave it. The diver is on-gassing in all tissues. Each tissue, though, takes on the nitrogen at different rates, depending upon tissue composition and the amount of blood flow (perfusion).
3. During the dive, the faster-absorbing tissues will reach equilibrium (saturation), meaning that once again the amount of nitrogen entering the tissues is the same as the amount leaving them. Again, it is not like filling a bucket. If the diver goes deeper, those tissues will start on-gassing again. The slower tissues will still be on-gassing throughout the dive.
4. As the diver ascends, the tissues that are saturated will now have more nitrogen than the air being breathed, and they will begin to lose nitrogen faster than they are gaining it. They are supersaturated. They are off-gassing. The slower tissues will still be below ambient pressure and will still be on-gassing.
5. Each tissue has a slightly different ability to withstand supersaturation without getting into DCS trouble. If that were not true, we could never ascend. On a typical NDL dive, those faster tissues should be able to off-gas fast enough to stay within that safe level of supersaturation all the way to the surface. Research indicates that a safe level of supersaturation for surfacing will be at the pressure of about 20 feet, so a safety stop above that depth is recommended to ensure that the faster tissues will off-gas sufficiently to reach that safe level of supersaturation and allow the diver to go to the surface.
6. At that safety stop depth, the slowest tissues will still be on-gassing, but because on-gassing is not like filling a bucket,
they cannot reach a pressure greater than the pressure of their current depth. That means they can never exceed the tissue pressure deemed to be safe for surfacing. Once the faster tissues have off-gassed sufficiently, the diver can surface, because the on-gassing the the slowest tissues is of no concern.
7. If a diver spent the entire dive shallower than the safe pressure, then no tissues can possibly reach a supersaturation level which will create a risk of DCS.
8. Once on the surface after a typical recreational dive, all tissues will be supersaturated, hopefully at a safe level. All tissues will therefore be off-gassing. That is the reason for a surface interval prior to a subsequent dive.
