Understanding Decompression Sickness

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Think back to your Nitrox class where pulmonary oxygen toxicity was said to be a negligible concern for rec diving. You're starting to encroach on that danger, so don't overdo it. The daily limit if breathing oxygen on the surface is 5 hrs.

Since your computer almost certainly won't track surface off-gassing on O2, you wouldn't realize a shorter SI or longer NDL for a given conservatism setting as a practical matter. Even if you could, though, the gains would be minor. For instance, after a 60 ft NDL dive on air (44 min @ GF x/85) and an hour SI, the second NDL time would be 38 minutes. If you breathed O2 for 10 of those 60 minutes, your repetitive NDL time would be 40 mins. Enduring the odd glances of your fellow divers while breathing O2 for the entire SI results in a 53 minute NDL time. But again, I know of no dive computers that would make such a computation.

ETA: those gains neglect any inefficiencies introduced by mucus buildup in the lungs. Our bodies don't actually like breathing O2, and tech divers take breaks during longer O2 deco stops to recover a bit.
wow, I never thought about computer calculations in my response. hm.

I'd think the only way O2 would factor in with deco calculations is either using 50 and/or 100% O2 on stops, or using nitrox due to the lower nitrogen percentage. sure, I think nitrox would decrease your SI (depending on how close you were to the nitrox NDL and all) but I don't think computers have a setting for pure O2 and SIs.

if they breathed it on the surface though (not for too long, maybe like 15 minutes), would it help off-gassing go faster separate of the required surface interval? oh, I just thought of something- other divers may think the diver breathing O2 is bent or there's a possibility, so it may make someone think there's a danger where there isn't one
 
I'd think the only way O2 would factor in with deco calculations is either using 50 and/or 100% O2 on stops
Breathing O2 instead of air on a 5 minute safety stop after a 60 ft NDL dive (GF x/85) gives you 1 more minute of NDL after an hour SI. That is just not worth the hassle of dragging that bottle around and risk of seizure/death should you have difficulty with depth control.
 
Breathing O2 instead of air on a 5 minute safety stop gives you 1 more minute of NDL after an hour SI. That is just not worth the hassle of dragging that bottle around and risk of seizure/death should you have difficulty with depth control.
Crap, I forgot to specify that I meant decompression stops, not safety. yeah, if someone took a safety stop at, for example, 15 feet and they couldn't stay at/above that- it wouldn't be good.
And I think the chance of toxicity increases when you exert yourself more, which if you're struggling to keep a certain depth I'd think there'd be extra work involved(?)
But no, I was just curious if breathing O2 on the surface actually helps with anything regarding off gassing, or if there's not really a point to it. Eta- I don't really think there is unless you have a lot of residual nitrogen honestly but that could be off
 
Crap, I forgot to specify that I meant decompression stops, not safety. yeah, if someone took a safety stop at, for example, 15 feet and they couldn't stay at/above that- it wouldn't be good.
And I think the chance of toxicity increases when you exert yourself more, which if you're struggling to keep a certain depth I'd think there'd be extra work involved(?)
But no, I was just curious if breathing O2 on the surface actually helps with anything regarding off gassing, or if there's not really a point to it. Eta- I don't really think there is unless you have a lot of residual nitrogen honestly but that could be off
Well, the emergency treatment for symptoms of DCS at the surface is 100% O2, so it has to do something?
 
Well, the emergency treatment for symptoms of DCS at the surface is 100% O2, so it has to do something?
Something, yeah...just not sure how much it'd help with a surface interval time. eta- I do think it would help off-gas, yeah, but as inquis said, computers don't calculate it
 
The answer to #1 is, it depends. This fits in nicely with your other question about why your tank doesn't shrink - recommend you go back and read that to refresh the physics in your mind. Recall that under water, the regulator will deliver air at whatever the surrounding pressure is, which means that the air pressure in the diver's lungs is the same as the surrounding water pressure. It's still air though, which as others stated above is 78% (roughly 80%) nitrogen.

As @tbone1004 stated above, Dalton's Law of Partial Pressures says that the total pressure of a gas mix is the sum of the partial pressures of the gases in that mix.

Air is a gas mix that's made up of 78% nitrogen, 21% oxygen, and 1% other gases, mainly CO2 and argon. So, in air, 78% of the total pressure is caused by nitrogen, 21% is caused by oxygen, and 1% is caused by those other gases. At 1 atmosphere absolute (sea level), the partial pressure of nitrogen in air is 78% of 1, or 0.78 atmospheres absolute. At 33 feet, the total pressure is 2 atmospheres absolute, so the pressure in the diver's lungs will be the same - 2 atmospheres absolute. Now, the partial pressure of nitrogen in the diver's lungs is 78% of 2 atmospheres absolute, or 1.56 ATA. The percentage of gases in the mix (air) does not change, but the partial pressure does. This increases linearly as the diver goes deeper: at 66 feet/3 ATA, the partial pressure of nitrogen in the lungs is 78% of 3, or 2.34 ATA. At 99 feet, it's 78% of 4 ATA, or 3.12 ATA. If you know the percentage of gas in the mix, you can calculate the partial pressure of that gas in the diver's lungs at any depth.

This is where Henry's Law starts to work. Henry's Law states that the amount of gas that will dissolve in a liquid is directly proportional to the partial pressure of that gas on the liquid. In @OTF 's soda bottle example, the gas/liquid interface is at the surface of the soda, and the force that keeps the CO2 dissolved is the partial pressure of CO2 acting directly on the surface of the soda.

In a diver's body, the gas/liquid interface is in the lungs. There are two very thin layers of tissue between the gas in the lungs and the blood in the body, and gas molecules can pass freely across those layers.

Our bodies are in equilibrium with the surrounding air right now, i.e. because there's 0.78 ATA of nitrogen pressure in our lungs, there's roughly 0.78 ATA of nitrogen dissolved in our bodies. As the pressure increases in the lungs with diving (remember what diving gear does for you), the partial pressures of the gases in the lungs increase, which creates a gradient between the gas pressures in our lungs and the gas that's dissolved in our bodies. The nitrogen will follow that gradient and begin to dissolve into the body via the lungs - first into the blood stream, then into the tissues supplied by the blood. This doesn't happen instantaneously - it's dependent on the pressure gradient and the rate at which the different tissues absorb and release nitrogen. The higher the partial pressure of nitrogen in the lungs (i.e. the deeper the diver goes) and the longer the diver stays down, the more nitrogen will become dissolved in the body via the lungs. So, the first part of question 1 is, on descent and at the bottom, the excess nitrogen is in the lungs. This also speaks to question 2 - how does this excess nitrogen form?

When a diver begins to ascend at the end of a dive, the pressure of gas in the lungs decreases. Critical to understanding decompression (and this is slightly simplified) is that at some point on ascent, the pressure of the dissolved nitrogen in the body will be greater than the partial pressure of nitrogen in the lungs. At that point, the pressure gradient reverses. Instead of going from the lungs into the body, it's going from the body into the lungs. The art and science of decompression involves controlling that pressure gradient so that the diver doesn't develop symptomatic bubbles (i.e. decompression sickness). That's the other half of the answer to question 1 - on ascent, the excess nitrogen is dissolved in the blood and body tissues, and it's our task as divers to release that excess nitrogen safely via our lungs - in a recreational diver's case, by staying within the no-decompression limits.

If the pressure gradient gets too high or if something else goes awry with the diver's decompression, it's like suddenly popping the cap on @OTF 's soda bottle - the pressure inside the bottle goes from high to low very quickly, and the CO2 that was dissolved in the soda forms bubbles. So it is with divers - bubbles of nitrogen can form in the blood and body tissues. Those bubbles can cause symptoms by directly blocking the circulation and by activating the inflammatory pathways in the body. This is decompression sickness.

Best regards,
DDM

Thank you very much for this thorough explanation.

In the online class it says this:

"As you ascend, nitrogen absorption slows, so the remaining no stop time will increase."

I thought nitrogen absorption (into body tissues) occurs only when you're descending, and that when you're ascending, the nitrogen comes out of your body tissues? Can absorption also occur when you're ascending?
 
Thank you very much for this thorough explanation.

In the online class it says this:

"As you ascend, nitrogen absorption slows, so the remaining no stop time will increase."

I thought nitrogen absorption (into body tissues) occurs only when you're descending, and that when you're ascending, the nitrogen comes out of your body tissues? Can absorption also occur when you're ascending?
it can, but it's not a concern in recreational diving. for recreational, when you ascend, you're always off-gassing the excess N2. but you're still going to be supersaturated relative to your normal tissue N2 pressure at the surface. this mainly affects your NDLs, which has been discussed further up in the thread.

when you're carrying out decompression stop dives, if you do stops that are too deep, or don't produce an efficient pressure gradient, the "slower tissues" (ones with a longer N2/He on and off gassing halftime) will absorb nitrogen. or helium too if you're using trimix. which is why you set your gradient factors so that your first stop isn't dangerously above your bottom depth (giving a tissue tension/pressure gradient that's too high), but not too close to it either.
 
Thank you very much for this thorough explanation.

In the online class it says this:

"As you ascend, nitrogen absorption slows, so the remaining no stop time will increase."

I thought nitrogen absorption (into body tissues) occurs only when you're descending, and that when you're ascending, the nitrogen comes out of your body tissues? Can absorption also occur when you're ascending?
Great question. I don't know the context of this statement, but in a basic diving curriculum it's probably referring to a diver who is early into a multilevel no-stop dive. During the deeper part of the dive, the diver is absorbing nitrogen more quickly, and as the diver ascends to shallower depths during the dive and before ascending, nitrogen absorption slows.

The diver can also absorb nitrogen during the ascent phase if there is a delay in ascent (depending on depth).

Best regards,
DDM
 
it can, but it's not a concern in recreational diving. for recreational, when you ascend, you're always off-gassing the excess N2.
Hi @kaylee_ann , that's not necessarily the case - too slow an ascent rate or a deep delay in ascent can cause a diver to continue to on-gas.

Best regards,
DDM
 

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