dive computers and reverse profiles
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Why do you think this?
I am not a doctor and I am not a specialist in hyperbaric medicine. This reply is based on Deco for Divers. It is possible that I have misunderstood something. If so, please correct me.
My answer:
1. Because on ascent nitrogen gas can enter the microbubbles that naturally exist in your blood stream...
(any fast flowing liquid will have bubbles: Cavitation - Wikipedia)
2. And these microbubbles will grow not just on ascent but also by offgassing...
(the surface tension of the bubbles must be overcome, which means no bubble growth from offgassing will happen if your dive is 1.6 ATA or six meters or 18 feet deep only; the pressure change however has its effect, but that is not a problem, because excessive growth cannot happen)
3. And be filtered in the lungs...
4. But if they get re-compressed and small, they can avoid that, and get through, and remain in blood stream for longer (=simply more bubbles), and cause trouble somewhere (eg. block blood flow in the skin = skin problems or block blood flow in the brain = neurological DCI). This is why yo-yo-profiles are bad.
Hence, I believe that it is not only the dissolved gas that kills you. It's also your up-down-up-down-movement. There are actual scientific papers that list yo-yo -profiles as a risk (see figure 2):
https://www.dhmjournal.com/images/IndividArticles/49Dec/Lundell_DCIFinland_2019-42(564).pdf
ps. If the Buhlmann algorithm alone would be enough (supersaturation alone leads to trouble), then only the drop in pressure would be relevant, not any back-and-forth motion, because sinking would make gas dissolve and cancel bad things, would it not? Pehaps the bubble models just need work?
Well, I received several responses that mostly said the same thing, but this one seemed to be the most authoritative, so I will respond to the group here.
First of all, thank you for educating me. I now understand the issue. I was previously unaware of that actually having happened.
What I didn't see described in the responses was the original reason for the 1/4 turn back. Many older valve designs had packing on the valve stem, rather than an O-ring. On those valves, opening to a hard stop damages the packing & will result in a stem leak after a moderate number of uses. Some early o-ring stem valves had a similar issue, where a hard open would side load the o-ring & over a long enough period of time would also cause a stem leak, but this would take a lot longer to happen compared to a packed valve stem. A properly designed modern valve will have stop flanges on the stem assembly & not allow the o-ring to be impacted, so the issue of equipment damage from a hard open would then be be eliminated.
I am going to assume that everyone here will agree that the possibility of drowning will outweigh any concern over possible minor equipment damage, so the 1/4 turn practice should be a thing of the past.
This does leave me with one disturbing thought though. If you can't count on someone to know which way to turn a valve, they should not be checking anybody's equipment. A nitwit that doesn't know which way a valve gets turned can change it from fully on to fully off the same way that they can change it from almost fully on to almost fully off.
In my personal case, I do not expect that I would get myself into trouble over this in actual practice because I always take a couple of hard breaths off my reg while looking at my SPG before I jump into the water. If the valve is closed, the pressure reading will drop. If the valve is nearly closed, I will see the reading dip as I inhale. Also, a partially open valve will give me the same feeling as my old J-valves used to when the tank gets low, so I am already well practiced in how to deal with that issue when it happens. That being said, I am not recommending against fully opened valves. Please don't misunderstand me to be saying that. I was just describing some of my personal habits that are also a good back up against bad things happening.
That is a problem on dive boats where a dive guide/DM/boat hand "checks" that the valves are open. They have closed peoples valves to a quarter turn open and this has killed people. Look at the standards of most agencies for becoming a DM. You basically have to have a pulse. It appears that way at least.
I have found that on a full or even half full tank (haven't tried with nearly empty) that the only way to get the SPG needle to move was to barely have the cylinder open. Even a quarter turn open I couldn't get it to move. YMMV.
But if it is fully off, you are more likely to catch that before jumping. Less of a problem than only slightly on.
That is true. I don't like people randomly f***ing with my valves, and this is one reason why.
The other reason: I have dived a few times with one individual who is in the habit of setting up their rig, then turning off the valve, going to do other stuff, opening the valve again, and splashing. The idea here is to prevent any gas from leaking out while they are away (which I disagree with, but that's neither here nor there). Works OK when it works, but on two occasions I have seen this person get in the water with their valve off. Clearly their pre-dive check sequence is insufficient, let alone buddy checks. On one occasion, I set up my rig and then started eating. When I returned, I found that they had turned my valve off for the same reason. Luckily I caught it in my pre-dive checks, but I was pretty pissed off
.EDIT: By the way, it seems like these valve indicator knobs would more or less solve this problem. Do people use these things?
True, but that does not mean they are not out there. Two examples.
-Had this happen to me in Cancun. I caught it before a hot drop. Tank just cracked open. New crew member.
-Was picking up a tank of nitrox. Wanted to check my tank. Dive shop employee handed me an analyzer that he had just used on a nitrox tank. I said I wanted to calibrate. He just adjusted the analyzer to 20.8 and handed it to me. I took analyzer and recalibrated it myself.
They are out there.
While that is true, try sometime figuring out which way to turn the knob when you're standing on the wrong side of the diver and holding on to the railing with your dominant hand. See how long it takes.
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