Understanding Decompression Sickness

[TrimixToo]

The difference between US Navy and Padi tables is that the first always assume a square profile, given by max depth and total time. Padi made an extensive study on multi-level dives, developing an advanced table system known as RDP or simply "the wheel".
I was trained with the US Navy tables, which results in deco dives absolutely conservative and safe. As we were trained to deco procedures since the first OW course, this is not a big deal, it just takes more time for deco stops (and more air, of course).
I know only a small number of divers (3, actually, all are instructors) who understood the RDP and can use the Padi wheel for performing multi-level dives "riding the NDL".
So yes, it is possible using the Padi method for multi-level dives, but it is not easy, and I am not qualified nor trained for attempting this approach.
For all other divers, they use the computer.

I was also originally trained on the USN tables, that being all there was in 1972. That OW class included training on decompression diving, as did yours.

I don't agree that the Navy tables should be considered "safe" for recreational diving. They were constructed with the notion that (mostly) young, very fit divers would have what the USN regards as an acceptable percentage of DCS incidents. The Navy's OC diving requires, IIRC, a deco chamber on deck per the USN Diving Manual. So, DCS incidents rarely result in actual casualties or lasting disabilities. It might be the case that the bigger concern is having enough space in the chamber! Reading the USN DM, I was amazed and impressed at just how conservative the Navy is for OC diving, but that does not make using their tables safe absent the other things they do to improve safety in a recreational setting that rarely if ever includes them. I would hope the RSTC-accepted tables are better for recreational diving.

The USN DM is very much worth reading, for those who have not.

 

[Angelo Farina]

I was also originally trained on the USN tables, that being all there was in 1972. That OW class included training on decompression diving, as did yours.

I don't agree that the Navy tables should be considered "safe" for recreational diving. They were constructed with the notion that (mostly) young, very fit divers would have what the USN regards as an acceptable percentage of DCS incidents. The Navy's OC diving requires, IIRC, a deco chamber on deck per the USN Diving Manual. So, DCS incidents rarely result in actual casualties or lasting disabilities. It might be the case that the bigger concern is having enough space in the chamber! Reading the USN DM, I was amazed and impressed at just how conservative the Navy is for OC diving, but that does not make using their tables safe absent the other things they do to improve safety in a recreational setting that rarely if ever includes them. I would hope the RSTC-accepted tables are better for recreational diving.

The USN DM is very much worth reading, for those who have not.

All correct. But Cmas adopted two relevant modifications to the procedures described in the US Navy manual, for adapting to recreational deco diving:
1) the max ascent speed was set to 10 m/min instead of 18.
2) the ascent time is included in the dive time, which automatically allows to ascend as slowly as wanted.
With these two modifications the US Navy tables became "conservative enough" for the Cmas certified divers of the seventies.
Which of course were much more physically fit than today's vacation divers, as for being certified one had to swim 25m with underwater frog (no fins, no mask), swim 100 meters with crawl style in less than one minute and 30 s. Keep afloat a weight of 5 kg for 3 minutes. Make a static apnea of 3 minutes.
And perform a number of difficult exercises free diving, using the ARO or a twin tank.
A certified Cmas OW diver, in 1975, was not very far from a Navy Seal...

 

[TrimixToo]

All correct. But Cmas adopted two relevant modifications to the procedures described in the US Navy manual, for adapting to recreational deco diving:
1) the max ascent speed was set to 10 m/min instead of 18.
2) the ascent time is included in the dive time, which automatically allows to ascend as slowly as wanted.
With these two modifications the US Navy tables became "conservative enough" for the Cmas certified divers of the seventies.
Which of course were much more physically fit than today's vacation divers, as for being certified one had to swim 25m with underwater frog (no fins, no mask), swim 100 meters with crawl style in less than one minute and 30 s. Keep afloat a weight of 5 kg for 3 minutes. Make a static apnea of 3 minutes.
And perform a number of difficult exercises free diving, using the ARO or a twin tank.
A certified Cmas OW diver, in 1975, was not very far from a Navy Seal...

Ah, that does make a difference. The YMCA program used the tables as-is, with a 60FPM ascent. Other than breathholding, though, the requirements sound about the same, other than needing to swim 75' underwater (smile).