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

[huwporter]

The question was about DCS, not DCI. It has already been established you can get DCI from a 4 ft dive, holding your breath during ascent.

You are right, I used DCI when I meant DCS. I've corrected my post.

 

[Brett Hatch]

Considering a side job scrubbing boat hulls?

No thanks, just trying to put 2 and 2 together. There's a lot of facets to this -- good studies, bad studies, more reliable and less reliable anecdotes, theoretical models running the gamut between useful and debunked, millions of divers all around the world doing lots of different types of diving, some getting bent and others not. It's kind of a friggin' mess, honestly. But I will take what I can get. I want to learn more about the statistics, the physiology, the physics, the psychology. For my own safety of course, but let's be honest here, also because geeking out on hard multi-variable problems is fun for certain sickos in the world. I'm an engineer, after all

... 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.

The image of nitrogen buckets eventually filling up at any given depth, sounds to me a lot like the "compartments" in Haldane's deco model. Where the body is composed of various tissues, and because each tissue is connected to the bloodstream, the gases dissolved into them will asymptotically approach the gas in our lungs. Lungs -> blood -> tissues, all eventually in equilibrium. Different tissues absorb at different rates, which can be modeled with a number of "compartments" in the body, each with a different halftime.

To my ear, these "bucket" folks on FB are more or less on the right track. Just to be clear, I'm not making a semantic argument here, I think you probably have a real point, it must have just gone over my head. Have I misunderstood what you meant?

 

[Blackcrusader]

I found this thread to be a good read. As I am not diving much with not being able to travel internationally to Philippines Thailand or Indonesia I am left to dive planning exercises.

On an AL80 at 7m or less with my usual sac rate at 8L/min I could do a 3 hour dive and still only use around 2300 liters of air. If at 10L/min could still do two and a half hours dive time. Probably still do a safety stop at 3m depth for 5 minutes. However I have never attempted to do such a shallow dive for that length of time. If it was a great muck dive on a sunny day with no current I might run my camera out of battery first. My other issue is finding a dive buddy that would even consider a two hour dive as some tend to get bored.

I like letting my mind do it's own thing when it gets bored. I find diving to be very relaxing. Cant wait to start doing my regular 45 dives in two weeks on vacations again. I might try to find a muck dive that is less than 7m or 20ft and see what I can get timewise from an AL80 in a no current dive.

 

[lowwall]

The image of nitrogen buckets eventually filling up at any given depth, sounds to me a lot like the "compartments" in Haldane's deco model. Where the body is composed of various tissues, and because each tissue is connected to the bloodstream, the gases dissolved into them will asymptotically approach the gas in our lungs. Lungs -> blood -> tissues, all eventually in equilibrium. Different tissues absorb at different rates, which can be modeled with a number of "compartments" in the body, each with a different halftime.

To my ear, these "bucket" folks on FB are more or less on the right track. Just to be clear, I'm not making a semantic argument here, I think you probably have a real point, it must have just gone over my head. Have I misunderstood what you meant?

The problem is the bucket approach ignores the fact that (at a constant temperature) the amount of gas that can dissolve into a liquid is directly proportional to the pressure of the gas, i.e. Henry's Law.

It also hides the most important point that the gasses are normally dissolved in the tissues and your goal is to keep them that way. You understand that, but when you start talking about buckets of gas, I suspect most people think of the gas in terms of blowing up a tire.

I'm going to walk the dog and see if I can come to with a better metaphor than buckets.

Edit: Nope. Can't think of a shortcut way to describe it. Solutions of gasses under varying pressure is just too far outside our everyday experience. Bubbles when you open a Coke illustrate what goes happens if it all goes wrong, but doesn't cover the mechanics of gas diffusion into and out of our tissues.

 

[Brett Hatch]

The problem is the bucket approach ignores the fact that (at a constant temperature) the amount of gas that can dissolve into a liquid is directly proportional to the pressure of the gas, i.e. Henry's Law.

It also hides the most important point that the gasses are normally dissolved in the tissues and your goal is to keep them that way. You understand that, but when you start talking about buckets of gas, I suspect most people think of the gas in terms of blowing up a tire.

I'm going to walk the dog and see if I can come to with a better metaphor than buckets.

Edit: Nope. Can't think of a shortcut way to describe it. Solutions of gasses under varying pressure is just too far outside our everyday experience. Bubbles when you open a Coke illustrate what goes happens if it all goes wrong, but doesn't cover the mechanics of gas diffusion into and out of our tissues.

OK, thank you for explaining, I think I see the issue with the buckets. If you can accept that the "buckets" grow with respect to depth, then we're talking about something pretty close to Haldane's compartments. But of course, rigid containers like buckets don't do that. And if you start thinking about flexible containers like balloons, well hell, they do the opposite! I can't think of a better container for the visual either, because I cannot think of a container which grows as you bring it deeper, only ones which shrink or stay the same.

Comparing buckets to balloons is helpful when trying to illustrate Boyle's law, and to explain something like a lung embolism. But it may be hopelessly misleading for explaining Haldane's compartments, since the concept forces you to imagine the container growing with respect to depth, which is just kinda physically bonkers. So any analogy where the key variable is volume is always gonna be wrong. Instead one needs to think about it in terms of density or partial pressure. Maybe there is an everyday object that would more clearly illustrates this in those terms, but I can't think of one off the top of my head.

 

[dmaziuk]

The image of nitrogen buckets eventually filling up at any given depth, sounds to me a lot like the "compartments" in Haldane's deco model.

They aren't. Tissue compartments are slices of the integral, there to approximate an unknown curve of gas loading with a sum of "known" rectangles. (Except we don't need an infinite number of infinitely thin slices, we just need enough to get out of the water not bent.) The whole thing is about rates and ratios, the idea of "eventually filling buckets" is about what you'd expect from failbook.

 

[Duke Dive Medicine]

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?

There probably is a threshold, but that threshold would vary between individuals and in the same individual from day to day, so I don't think one can draw solid conclusions.

Re dive depth of the individual dives I'm talking about, the max depth was pretty consistent. Still, these are just a few data points that indicate that DCS can happen from that depth given a certain set of circumstances.

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.

I get the part about saturation not being the point of the thread. However, if a diver "can't" get bent when coming straight to the surface after being saturated at 20 fsw but can get bent when following NDLs at 25 ffw, equivalent to 24 fsw, what conclusions can we draw?

I would remind you that this is a learning zone. There is absolutely no reason to belittle these divers.

Best regards,
DDM

 

[tursiops]

However, if a diver "can't" get bent when coming straight to the surface after being saturated at 20 fsw but can get bent when following NDLs at 25 ffw, equivalent to 24 fsw, what conclusions can we draw?

24 is deeper than 20, and the body knows it.

 

[Duke Dive Medicine]

24 is deeper than 20, and the body knows it.

Of course. And, saturation is different than repetitive shallow sawtooth diving within NDLs. There is no absolute, correct answer here. It's a continuum. You could start at the extreme: can I get bent after being saturated, such as it is, in a pool at 3 feet? I'd agree that's impossible. What about 12? 18? 20? 24? You start slowly eeking toward "possible", but again, it's very dependent on individual physiology and diving conditions.

 

[boulderjohn]

The phrase "filling the bucket" implies a bucket of a specific size, and as it was used in the thread I described, once the diver was underwater, the bucket will start filling, and will keep filling until it is fill. If the diver goes to 99 feet, the bucket will fill twice as fast as it would at 33 feet, but it will fill either way.

That is, of course, not remotely true, but I think most divers believe that.