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

[lowwall]

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.

Considering a side job scrubbing boat hulls?

 

[Jeremy Williams]

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.

I’m not saying that you’re wrong, but I think this is too far. I think it seems ‘mostly harmless’ to think about this since most of us will never reach saturation at any depth ever. But the idea that any SCUBA diver will maintain a +- 3ft depth precision in shallow water over saturation length timelines to avoid decompression seems quite implausible.

 

[boulderjohn]

But the idea that any SCUBA diver will maintain a +- 3ft depth precision in shallow water over saturation length timelines to avoid decompression seems quite implausible.

I am not understanding your point here. The only practical way to achieve full saturation at that depth is to stay in some kind of habitat.

If we are talking about actual scuba diving, his point goes to the general idea of this thread--can I get DCS doing long dives at shallow depths? There is another thread going on right now in which someone asks about diving in a river with a maximum depth of something like 17 feet. The nearest dive site to me right now has a maximum depth of 21 feet. For dives like that, those are useful guidelines.

 

[Jeremy Williams]

I am not understanding your point here. The only practical way to achieve full saturation at that depth is to stay in some kind of habitat.

If we are talking about actual scuba diving, his point goes to the general idea of this thread--can I get DCS doing long dives at shallow depths? There is another thread going on right now in which someone asks about diving in a river with a maximum depth of something like 17 feet. The nearest dive site to me right now has a maximum depth of 21 feet. For dives like that, those are useful guidelines.

My point is that this is all hypothetical for almost everyone who reads it. It’s Olympic level Internet diving. And that anybody that has any business actually being close to any of these situations wouldn’t use advice like this to plan their dives because the parameters are a bit silly…

 

[ginti]

My point is that this is all hypothetical for almost everyone who reads it. It’s Olympic level Internet diving. And that anybody that has any business actually being close to any of these situations wouldn’t use advice like this to plan their dives because the parameters are a bit silly…

Well, the discussion evolved because people here want to understand a bit more about decompression physiology. It isn't silly at all since it has implications for other kinds of dives.

For instance, I understand that this depth limit is related to the M-values and, consequently, to the ascent speed and depth stops during deco dives. Please correct me if I am wrong.

 

[tursiops]

I’m not saying that you’re wrong, but I think this is too far. I think it seems ‘mostly harmless’ to think about this since most of us will never reach saturation at any depth ever. But the idea that any SCUBA diver will maintain a +- 3ft depth precision in shallow water over saturation length timelines to avoid decompression seems quite implausible.

Maybe you being too literal.
If you can't get bent from a 20fsw saturation dive, then you can't get bent from a 1h dive to 20fsw. THAT is the point.

But even though 23fsw, or worse 26fsw, are only a little deeper, you are past the tipping point, and you CAN have problems if you are saturated.
So, can you get bent from a 1h dive to 26fsw? Yes, says the Navy, using their tables 9-7 and 9-6.
The ascending from saturation is not irrelevant info, it is just a limiting case.

 

[Duke Dive Medicine]

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?

These were all scuba dives with a max depth of 25 feet. All were within no-stop limits, so nowhere close to the saturation that's been discussed in the thread.

Best regards,
DDM

 

[tursiops]

These were all scuba dives with a max depth of 25 feet. All were within no-stop limits, so nowhere close to the saturation that's been discussed in the thread.

Best regards,
DDM

The THREAD is about DCS on shallow NDL dives. The discussion of saturation arose because there is data and to provide a limiting case; if DCS does not happen on a shallow saturation dive, it won't happen on one of lesser duration.

Added: we are assuming recreational dives performed using best practices, not idiotic working dives with divers zooming up and down repeatedly.

 

[lowwall]

These were all scuba dives with a max depth of 25 feet. All were within no-stop limits, so nowhere close to the saturation that's been discussed in the thread.

Best regards,
DDM

The question at hand is about 20'. None of the quoted studies, at least one of which is a meta-study, found DCS cases on dives shallower than that depth.

What the data suggest is that there is a physiological threshold, presumably caused by the physics of bubble formation in living tissue, where DCS cannot occur. The threshold seems to be somewhere deeper than 20', but shallower than 23'.

25' is deeper than the apparent threshold, so it is not surprising that scuba dives to that depth with extreme risk factors are occasionally going to result in DCS.

Were any of the dives resulting in DCS under shallower than 23'? If so, how clean is the depth data?

 

[boulderjohn]

About a year or so ago, the very question of this thread came up in a FaceBook dive group with which I was participating. That thread showed two important things:

1. Why the FaceBook format totally sucks for this kind of discussion.
2. The degree to which DCS misinformation exists in the scuba community.

In that thread a handful of knowledgeable people tried to provide facts in evidence, but those scattered voices were drowned in the chaos of misinformation. On FaceBook, there is no effective way to respond to people, and people don't read any of the responses anyway. The thread was instead filled with people saying that as soon as you get under water, your tissues start to fill up with nitrogen, as if it were coming from a spigot into a barrel, and if you stay at any depth long enough, those tissues will get filled up. Some depths fill the tissues fast, but eventually, even at the shallowest depths, those tissues will be filled--again, as if they were buckets.

I believe what I just wrote in my last paragraph describes what a lot of divers--perhaps the majority--believe. At least, that is what I got from the flood of posts in that thread. A discussion like this one, at least, allows some of that thinking to be corrected.