Deep followed by shallow

[NitrogenSweater]

Hello,

I know that in today's world, reverse profiles are no longer shunned as they once were.

My question is as follows, in regards to diving deep first, and shallower second.

Is it not only acceptable, but actually BENEFICIAL in regards to nitrogen off gassing, to complete a second, shallow dive after a first dive, say to 80ft (both on nitrox)

Essentially, will an 80ft NDL dive followed by a 15ft Dive for 30 minutes actually reduce overall gas tension in a diver compared to the single 80ft dive.

My thinking is that the second dive increases ambient pressure while allowing nitrogen to off-gassed. Normally, you would also reduce the rate of off-gassing by doing this second dive because the supersaturation ratio will decrease. However, if you used an appropriate nitrox mix, you could maintain the surface rate of deco on air.

In this case, the second dive would be not only permissible, but beneficial, no?

 

[Brett Hatch]

I'm not a doctor or a diving physiology expert. But, I think the short answer is: no.

Once you have arrived safely at the surface, it's true that re-entering the water will slow your decompression down (and some faster tissues will begin to recompress), and if you were right on the edge of a DCS hit, this may theoretically prevent it. But, the first dive to 80ft was an NDL dive. By definition, ascending to the surface should already be within a reasonable margin of DCS safety for most divers under most conditions. So, it being an NDL dive should mean that you are not right on the edge of a DCS hit, and therefore not be helpful.

If you were very close to taking a DCS hit when you arrived at the surface, the last thing you would want to do would be to reenter the water. DCS hits are much more manageable and survivable on the surface, because you won't drown if you lose consciousness or lose control of your breathing.

By the way, if you want to learn more about this, what you are describing is similar to the technique called "in-water recompression". If you search around SB or the internet for this, you will find examples of scenarios where highly trained divers with just the right equipment on-hand, and who are sufficiently far from a hyperbaric chamber may attempt this. But, in the vast majority of cases (and certainly all NDL cases), in-water recompression brings in more risks than it reduces them.

If you're interested in this topic, I'd recommend reading Deco For Divers by Mark Powell. It's a good book, and is pretty readable even for folks without a background in tech diving or human physiology. I learned a lot reading it.

 

[BlueTrin]

A lot of caveats:

• It’s not obvious that having a second dive after a surface interval can assimilated to in water recompression (my understanding is that for in water recompression time is of the essence)
• Once you surface, do a surface interval and redo a dive, it’s unclear to me that you could extrapolate that it is beneficial vs staying at the surface. I doubt you could quantify much either way if you stay within NDL for both dives tbh.

Unless you find studies that tested more or less your hypothesis, IMHO it’s hard to extrapolate …

 

[inquis]

Short answer is no.

You asked in regards to nitrogen off-gassing. The partial pressure of nitrogen when breathing air on the surface is 0.79 atm. At 15 ft you'd need EAN55 or higher to have a lower inspired PN2 and therefore a higher off-gas rate. However, what would be even better is breathing that same EAN on the surface.