USN decompression 30fsw/9m with 100% O2

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Tom Agneessens

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Hello all,

What are your thoughts, remarks on the fact that in the (latest) USN decompression tables, you find, unless I'm wrong of course, a decompression with 100% O2 beginning at 30fsw/9m? How about the 1.9 ppm?
If I correctly read the manual it states, however, that deco is preferably dealt with surface sourced air using, for instance, a face mask instead of our regular 2nd stages.
Again according to the manual, unless I'm wrong, the max time to use deco 100% O2 is 30min followed by 5min of air, and then again continue with 100% air for 30min and so forth...

I'm not from the Navy. I'm an average sporty guy. Is this translatable to 'normal life' decompression calculations?
I know of PTRD using the Navy tables, but I don't know up to what extend.

Kind regards
 
Which manual and which tables are you referring to? Can you be more specific?
 
@Tom Agneessens I haven't read the current manual cover to cover, and it's been a few years since I have read the old one, but that sound like something that would be in there.

Please remember, the US Navy Divers are comprised of some of the finest physical specimens humanity has to offer. They are also diving with recompression chambers on board, safety divers, all sorts of surface support etc. Many of their practices are not considered best practice for sport diving because their paradigm is different.

I did flip to that page and it appears they are calling for surface O2 which means they are pulling them up from the water, undressing them *they're wearing helmets, not half masks*, stuffing them in a chamber, then blowing them back down. 30fsw equivalent pressure in a dry chamber is standard.
 
@Tom Agneessens I haven't read the current manual cover to cover, and it's been a few years since I have read the old one, but that sound like something that would be in there.

Please remember, the US Navy Divers are comprised of some of the finest physical specimens humanity has to offer. They are also diving with recompression chambers on board, safety divers, all sorts of surface support etc. Many of their practices are not considered best practice for sport diving because their paradigm is different.
I agree and understand, yet they seem to be used in tables of different agencies like PTRD, NOAA and I guess others. But to what extent?
With what would you replace the 30fsw/9m deco if you would like to stick to oxygen (and oxygen @ 20fsw/6m)?
 
I agree and understand, yet they seem to be used in tables of different agencies like PTRD, NOAA and I guess others. But to what extent?
With what would you replace the 30fsw/9m deco if you would like to stick to oxygen (and oxygen @ 20fsw/6m)?
they have the "air" minutes for that depth, so you could use that for backgas.
NOAA is also a government agency and are bound by the same rules for decompression chambers IIRC. @Duke Dive Medicine was a Navy Diver and would have better answers for that.
 
Hello all,

What are your thoughts, remarks on the fact that in the (latest) USN decompression tables, you find, unless I'm wrong of course, a decompression with 100% O2 beginning at 30fsw/9m? How about the 1.9 ppm?
If I correctly read the manual it states, however, that deco is preferably dealt with surface sourced air using, for instance, a face mask instead of our regular 2nd stages.
Again according to the manual, unless I'm wrong, the max time to use deco 100% O2 is 30min followed by 5min of air, and then again continue with 100% air for 30min and so forth...

I'm not from the Navy. I'm an average sporty guy. Is this translatable to 'normal life' decompression calculations?
I know of PTRD using the Navy tables, but I don't know up to what extend.

Kind regards

@tbone1004 thanks for the add.

@Tom Agneessens the air decompression tables in the US Navy Diving Manual are designed for surface-supplied helmet diving. The 1.9 ATA inspired partial pressure of O2 at the 30-foot O2 stops is slightly higher than the recommended maximum for technical diving, but the advantage is that the divers have a full helmet on, which significantly reduces the risk of drowning in the event of a seizure. As you noted, there 5-minute air breaks every 30 minutes of breathing O2. If I read your question correctly, I think you're asking what to do if you want to use the US Navy tables but only breathe O2 at 20 feet vs. 30. The only answer to that would be to use different decompression tables, or use the air part of the decompression tables as @tbone1004 recommended. Modifying the tables yourself could lead to modified results.

Best regards,
DDM
 
With what would you replace the 30fsw/9m deco if you would like to stick to oxygen (and oxygen @ 20fsw/6m)?
Perhaps back up a step. Why are you trying to adapt USN tables which use surface supplied helmets or 2ata dry chambers for O2 deco to plan your dives in 2021? We have tools specifically for sport diving nowadays.
 
Tangential question for the hardhat divers / medical folks - I know that humans seem to inexplicably tolerate far higher PO2's in dry chambers than on wet dives. Is that also true for hardhat dives?
 
@OTF, humans don't inexplicably tolerate higher PPO2s in dry chambers than they do in the water. Rather, the changes in allowable PPO2 are based upon probabilities and risk mitigation. 1.4, 1.6, & 1.9 ATA aren't hard limits. In other words, you don't seize as soon as you hit 1.61 ATA on deco, for example. The higher your PPO2 goes (and the longer you stay there), the more likely you are to have a seizure.

In the water, a reg (or loop) needs to be held in the diver's mouth with a seal adequate to prevent the aspiration of water. This requires the diver to be conscious, i.e. not having a seizure. Because the consequences of a seizure in this situation are dire, the "allowable" PPO2 is decreased in order to decrease the probability of having a seizure and subsequently drowning.

Hard hat divers can use a higher PPO2, because the diver is less likely to drown if he/she has a seizure (because their gas supply does not require them to be conscious in order to maintain it). In other words, you can tolerate the increased risk of a seizure if the consequences of having that seizure are less dire.

Chambers can use a still higher PPO2, because even if the person inside does seize, it's pretty tough to drown when there's no water around...
 
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