Diving with EN50 to 6m (max) for 40 minutes or less - no nitrogen saturation?

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Scuba-Lad

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I'm curious on opinions about this thought experiment, both in theory and in practice.

Ignoring oxygen toxicity and CNS exposure for now, if we look at the partial pressure of N2 when diving EN50, we can see that the partial pressure of N2 reaches 0.8 bar at a depth of approximately 6m (19.6 ft for all the imperial people out there) - which is the equivalent of the N2 pressure at sea level. If one were to conduct a dive to a maximum depth of 6m (with a hard floor preventing further descent), but with the majority of the dive being less than this, around 3-4m, for around 40 minutes or so, in theory you would surface supersaturated with O2, but with less N2 than you entered with since you've been off-gassing nitrogen for the whole dive.

In essence a very mild chamber ride? I'm sure the rebreather crowd can chime in on this, since a shallow dive on the rebreather (or even an oxygen only rebreather) would be a very similar scenario. The reason to only dive to this depth isn't relevant, I'm more interested in the theory that this scenario would return a 0% chance of DCS, since there is no N2 supersaturation occurring, rather the opposite?
 

Fishdip

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boulderjohn

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Yes, breathing high O2 percentages at shallow depths will indeed give you lower tissue nitrogen than you started the dive with. That is common for tech divers using oxygen for final decompression.

Studies of shallow saturation divers showed they could safely go directly to the surface if saturated to depths shallower than 20 feet, and that is on air. Although some argue that DCS at depths that shallow is possible with sawtooth profiles and rapid ascents, I have never seen anything looking like research on that claim. The research I have seen says that you pretty much cannot get DCS diving air on a dive shallower than 20 feet, no matter how long you stay.

That is the magic of the safety stop. The longer you stay there, the more the supersaturated tissues will be able to drop to safe levels, and the slow tissues that are still on-gassing cannot reach dangerous levels.
 
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