As a PADI SCUBA Instructor, each new Bering Sea gold rush season has sends a few questions my way about the dredging they do. Specifically, how long they stay underwater before going past NDL (no decompression limit). So I thought I'd post the answer here for everyone who is interested.
It is common for dredgers to claim you don't have to pay attention to dive tables until you go past 10 meters. That is 100% incorrect. You cannot change physics no matter how much you may not like/comprehend physics. Physics tells us nature seeks equilibrium. So as soon as your body goes below the surface when SCUBA diving, you are absorbing excess nitrogen. Again, you can't change physics.
The confusion seems to arise due to dive tables starting at 10 meters. People assume any depth less than that and you don't have to worry about it. What they are forgetting (or never learned if they never got SCUBA certified) is that the tables tell you to use 10 meters for ANY depth 10 meters or less.
This is because when the Navy created the dive tables they could not distinguish any difference in nitrogen absorption between 0-10 meters. NOT that no nitrogen absorption takes place under 10 meters, just their measuring couldn't tell the difference in the amount of bubbles in the bloodstream. So you use the 10 meter dive table for any depth <= 10 meters.
How long do you have at 10 meters before you hit NDL? About 3 hours. So when you hear of someone on that show staying under for 6/7/8/9 hours, they have in gone into decompression mode. Not maybe, they are definitely in decompression mode. Again, not believing/not comprehending it doesn't change physics.
Does that change when using surface supplied air?
No, that does not change when using surface supplied air. For those who never heard of that diving method, instead of carrying an air tank you are connected via a hose to a compressor at the surface.
The reason is because the air is delivered at ambient pressure you are diving at when you inhale, regardless of the delivery method.
This does not apply to free diving where you hold your breath and dive. It only applies when breathing compressed air.