USN decompression 30fsw/9m with 100% O2

[Angelo Farina]

@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...

I second this entirely.
When I did my first OW course, in 1975, most of the training was done using the ARO, a pure-oxygen CC rebreather (currently yet in use by military corps such as Italian Comsubin and US Navy Seals).
In the diving manual there were two "safety curves", relating depth with max time.
One curve was for "professional/military" use, meaning a rebreather equipped with a full face mask, which protects the diver from drowning in case of seizure.
The other curve was for "sport/recreational" divers, referring to rebreathers equipped with a normal mouthpiece.
The first curve had a maximum depth of 18 meters, 10 minutes.
The second one, instead, had a max depth of 10 meters, 10 minutes.
As my certification is recreational, my card says max 10m with the ARO (and max 50m with the ARA, that is open-circuit air).

 

[Joris Vd]

I'm not aware of any agency that does not advocate 100% deco gas at 6m and certainly not one that advocates 80%. 70-80% gases are only used by cheap divers that don't want to buy a booster *note that filling 70-80% is convenient because an O2 bank bottle comes in at 2000ish psi and if you top it off to 3000ish psi with air then the mix is in the 70-80% range. It has nothing to do with decompression efficiency.

My local CMAS-organisation advocates 100% with a max depth of 4m.

CMAS does some wacky things sometimes within the organisation. For example the Flemish CMAS does not recognize any other organisations ADV EANX and deco procedures and you have to do part of the course again just to prove you are 'worthy'.

Below I added some of the examples from the Belgian CMAS adv eanx course that were kinda 'different' in my opinion.

-100% oxygen use limited to 13 feet, so 4 meters, not 6.
- Throughout the entire course materials you'll see a person using a 7l steel tank, they don't mention the 53 of 40 cuft in the course at all, and these are readily available all over Europe. The 80 cuft is also mentionned as an inadequate stage tank
-The second stage should be color coded. (this is stressed multiple times throughout the course)
-The inflator hose is mandatory on the stage rigging.

I guess everyone can do their thing and that's ok,
but I feel like one of the issues especially with CMAS is that they want to do things, ever so slightly different. The PTRD-organisation OP mentions is technically CMAS-approved, so if they are enforcing oxygen use below 4 meters, they wouldn't be following cmas standards.

CMAS only allows a ppo2 of 1.6 on deco when you are NOT doing accelerated deco.
Otherwise it's 1.4 (below screenshot of the cmas standards as mentioned on their website)