Is DCS possible under 20fsw, or 100% impossible?

[Water_is_my_blood]

Bubble formation and decompression sickness on direct ascent from shallow air saturation diving - PubMed

It seems that 8m is minimum depth where you can get DCS. And this is for saturation diving...

 

[lowwall]

Bubble formation and decompression sickness on direct ascent from shallow air saturation diving - PubMed

It seems that 8m is minimum depth where you can get DCS. And this is for saturation diving...

Exactly on point article. Here's the abstract:

To find the minimum supersaturation pressure for detectable bubble formation and for contraction of decompression sickness (DCS), three shallow air saturation dives at the depth of 6 m, 7 m, and 8 m were performed. The ultrasonic M-mode method was used for detecting bubbles. The exposure period was 3 d for all dives. Ten subjects were compressed to both 6 m and 7 m, and nine subjects were compressed to 8 m. One bubble streak was shown in the 6-m dive group. A small number of bubbles were seen in four subjects in the 7-m dive. All subjects in the 8-m dive presented various amounts of bubbles. DCS was not observed in the 6-m and 7-m dives. On the other hand, in the 8-m dive, four subjects suffered from DCS and required recompression treatment. The minimum depth for detectable bubble formation was assessed at around 6 m and the direct ascent from saturation at 8 m seems to have a high risk of DCS.

Note the dives were 3 days long. No DCS from subjects at 19.7' and 23'. Almost half the divers at 26.2' got bent. That's as definitive an answer as you are ever likely to get.

 

[boulderjohn]

This thread is quite informative about a topic that comes up from time to time. I believe ScubaBoard was once working on a knowledge back of such threads. If they are still at it, this thread should be a candidate.

 

[tursiops]

Bubble formation and decompression sickness on direct ascent from shallow air saturation diving - PubMed

It seems that 8m is minimum depth where you can get DCS. And this is for saturation diving...

This 1993 paper is effectively the same result as in a 2005 paper quoted in Is DCS possible under 20fsw, or 100% impossible?, in which the key info is: " <the> risk of DCS is zero at 20 fswg and 2.9% at 22 fswg. At 30 fswg the risk is 27%"
That 2005 paper does not cite the 1993 paper.

 

[lowwall]

Nitrox for shallow water artifact diving??

This is the study from which the quote was taken: https://www.researchgate.net/public...n_humans_DCS_risk_and_evidence_of_a_threshold

Here's the main data table from this study. I've cropped it to the key columns to make it easier to read. TDT is total decompression time, aka ascent time.

 

[tursiops]

I'm pretty comfortable, based on these studies, with using 6m/20fsw as a safe depth from which to ascend without decompressing from saturation on air, but 7m/23fsw is dicey, and 8m/26fsw is a no-no.
If one used 32% O2, the 6m/20fsw would become 8.6m/28fsw for a safe depth, 7m/23fsw would become 9.8m/32fsw, and 8m/26fsw would become 10.9m/35fsw.
Using 50% O2, the 6m/20fsw safe depth become 15.3m/50fsw for no DCS regardless of dive time.
This assumes Equivalent Air Depth formulae.

 

[Duke Dive Medicine]

I think part of the problem here is the framing of the question. In diving medicine, there are few questions for which "100% impossible" is an epistemically honest answer.

The theoretical models we have for questions like this are probabilistic by nature, and take a variety of input variables. Those variables and their values have been chosen over time, based on available evidence from a relatively small number of studies. Millions of people make millions of dives every year, some get bent, most don't. Of the ones who do get bent, a good chunk of them are following standard operating procedures that we consider to be "safe." This is important for all divers to understand and accept.

In the end, I choose to dive with a non-zero risk of the bends, and try to dive in such a way to keep that risk small, using the imperfect information available. I also choose every day to drive my car, and cross the street, and drink beer, and tie my shoes. Even though I cannot say for certain that these are risk-free activities, the reward outweighs the risk, so I do them.

All that being said, diving shallower than 20 ft seems to have a very small probability of causing DCS. I am not aware of any cases of this, and would be surprised to hear of one, especially given the information in the study John posted in #12. That study mentions that the 1999 US Navy Divers Manual allows for a direct ascent from 20 ft, even for fully saturated divers. I looked for a similar figure in the current USN manual (2017), and couldn't find it. Not sure if their position has changed, or if I just couldn't find it buried in the 1000-page document.

These haven't been published, but I have a friend out west who has treated several working divers for decompression sickness from roughly 20 feet. All involved vigorous work, the sawtooth profiles that @huwporter mentioned, and repetitive dives. It's rare, but definitely not impossible.

Best regards,
DDM

 

[lowwall]

These haven't been published, but I have a friend out west who has treated several working divers for decompression sickness from roughly 20 feet. All involved vigorous work, the sawtooth profiles that @huwporter mentioned, and repetitive dives. It's rare, but definitely not impossible.

Best regards,
DDM

The "roughly" modifier and the type of dives suggest that the true max depth of these dives is unknown.

If so, this doesn't invalidate the data suggesting there appears to be a threshold depth somewhere between 20' and 24' above which (i.e shallower than) you can't get DCS. What it does tell you is that if you are engaging in this type of activity that you need to either take extreme care not to exceed that threshold (perhaps with a platform?) or, more realistically, to do some sort of ad hoc safety stop and/or a very slow final ascent. Or use Nitrox to lower your equivalent air depth, but I guess most of these are surface supplied dives?

 

[Brett Hatch]

These haven't been published, but I have a friend out west who has treated several working divers for decompression sickness from roughly 20 feet. All involved vigorous work, the sawtooth profiles that @huwporter mentioned, and repetitive dives. It's rare, but definitely not impossible.

Best regards,
DDM

Thank you, this is helpful news to me. If your friend ever does publish this, I think we would be very interested to read about it. Or in the mean time, if they have a blog post or a YouTube video or something, that would be helpful as well.

 

[tursiops]

Thank you, this is helpful news to me. If your friend ever does publish this, I think we would be very interested to read about it. Or in the mean time, if they have a blog post or a YouTube video or something, that would be helpful as well.

Be aware that "working divers" can be a different breed, often not caring much about profiles and details. Yo-yo profiles are common, as are fast ascents. Recreatiional divers, following best practices, should be considerably better off....