Question about depth / decompression?

[John C. Ratliff]

Decompression isn’t a consideration when snorkelling or freediving. However, if you’ve been diving early in the day there could be an issue because of micro bubbles.

Decompression is a consideration for breath-hold diving, as it is the accumulated time at depth that determines how much nitrogen is absorbed into the blood stream and tissues. Therefore, a competitive spear fisherman who dives repeatedly to 66 feet (3 atmospheres absolute, 20 meters depth) can over a day can accumulate enough nitrogen to get bent. Some goes for submarine escape breath-hold divers in a U.S. Naval training tank from greater than 33 feet; this has been documented.

Micro bubbles have nothing to do with the time of day to my knowledge; only the saturation of nitrogen in the blood. Please reply with references to this claim.

SeaRat

 

[John C. Ratliff]

Noob question, but I have my SDI scuba... When I went through that training, was told to come up / go down slowly. Long story short, I bought free diving fins, and wanted to swim down 20-30feet several times for photography (just by holding my breath).

I assume, since I'm not breathing and but rather holding my breath, I'm fine?

I guess what I'm asking is if I want to go snorkeling, and swim down 20-30 feet, then quickly come up (in one breath), am I good?

Okay, there are some mis-statements in some of the information above. First, if you look at the decompression tables, you’ll see that they really don’t apply until you dive deeper than 33 feet (one atmosphere, 10 meters). And, the deeper after that, the less time you have as bottom time in scuba diving. The same applies to free diving (breath-hold diving to depth). So in answer to your first question, swimming down to 20-30 feet should not put you into a decompression situation. But deeper freediving can, and has, resulted in decompression sickness to U.S. Navy divers, Japan’s Ama divers, and spear fishers.

Now, another point brought up by @Subcooled has to do with the ascent, and is called “Shallow Water Blackout.” This can happen when breath-hold diving for records underwater, or for depth. It has to do with hyperventilating, and thus removing CO2 from the bloodstream. CO2 is the chemical that signals the brain to breath, not lack of oxygen. Therefore, you can lower the CO2 in the system to a point where the oxygen is depleted and unconsciousness accrues. This is also associated with depth, as at depth the oxygen levels can be sufficient for consciousness but with ascent and decreasing pressure the oxygen actually can migrate from the blood back into the lungs, and blackout occurs at a point underwater but near the surface. Read up on Shallow Water Blackout, and its prevention.

Hypoxia in Breath-Hold Diving - Divers Alert Network

The final question about ascent rate coming up; the answer is that there is no jeopardy of lung Overpressure when you descend and ascend on the same breath. You cannot have more air in your lungs than you started with. Except…if you take a breath at depth from a scuba diver, then ascend you can have a lung overpressure situation. So don’t take a breath from a scuba diver at depth when free diving, and you should be fine.

SeaRat

 

[Edward3c]

Decompression is a consideration for breath-hold diving, as it is the accumulated time at depth that determines how much nitrogen is absorbed into the blood stream and tissues. Therefore, a competitive spear fisherman who dives repeatedly to 66 feet (3 atmospheres absolute, 20 meters depth) can over a day can accumulate enough nitrogen to get bent. Some goes for submarine escape breath-hold divers in a U.S. Naval training tank from greater than 33 feet; this has been documented.

Micro bubbles have nothing to do with the time of day to my knowledge; only the saturation of nitrogen in the blood. Please reply with references to this claim.

SeaRat

My response was aimed at the normal diver who also does some snorkelling/freediving whilst on a diving trip. Most will never get deeper then 10m.

 

[inquis]

Micro bubbles have nothing to do with the time of day to my knowledge; only the saturation of nitrogen in the blood

Pretty sure that response meant to warn of scuba diving "earlier than" or "prior to" freediving. Bubbles, if present, compress during the freedive descent. They may make their way to places they wouldn't have otherwise, which can be fatal.