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[Akimbo]

Maybe @Akimbo could shed some light on this question.

I would defer to @Duke Dive Medicine on the "why".

In practice, the Navy does very little decompression diving in normal operations. They occasionally get recovery and salvage ops that put them in planned decompression but they have decompression chambers on site and primarily do Sur-D-O2 — which is also the norm for commercial surface-supplied decompression diving.

Notice that Table 9 9, Air Decompression Table, combines in-water decompression on air, in-water decompression on air and oxygen, and surface decompression on oxygen (Sur-D-O2). Nearly all the Sur-D-O2 tables I have seen have their last in-water stop on O2 at 20'. The primary purpose is to get divers out of the water as quickly as possible for comfort and safety and to free up the dive station for the next team of divers.

Most of the company-proprietary Sur-D-O2 tables I have seen are closer to a treatment Table 5. They typically allow five minutes from leaving the 20' water stop to be at 60' on O2 in the chamber.

 

[Bob DBF]

Notice that Table 9 9, Air Decompression Table, combines in-water decompression on air, in-water decompression on air and oxygen, and surface decompression on oxygen (Sur-D-O2).

I was wondering about that. Does that mean they just go/stay on O2 once on the surface?

I do know a little bit about decompression in a chamber rather than in the water from my reading about surface supplied divers.

 

[Akimbo]

I was wondering about that. Does that mean they just go/stay on O2 once on the surface?

There wouldn't be time to do anything but stay on O2. Remember that there is 300-600' of 3/8" hose full of O2 and 20' to the surface is 40 seconds (usually less). Divers could turn the free-flow valve on the hat wide open and wouldn't get the hoses purged before tenders have the hat off, you are almost down to your birthday suit, and you are being hustled to the chamber.

Just like recreational diving you want the diver on max O2 to reduce decompression time. The risk of a convulsion in a surface-supplied lightweight hat is far less than a Scuba diver. You aren't going to drown if you convulse and they can hear that you are having a problem. All they have to do is yank in hose to reduce the PPO2. You can go on O2 at 30' with Table 9-9... way too aggressive for a recreational Scuba divers.

 

[Bob DBF]

There wouldn't be time to do anything but stay on O2. Remember that there is 300-600' of 3/8" hose full of O2 and 20' to the surface is 40 seconds (usually less). Divers could turn the free-flow valve on the hat wide open and wouldn't get the hoses purged before tenders have the hat off, you almost down to your birthday suit, and you are being hustled to the chamber.

I saw a couple of hard hat dives off of a diving barge in San Francisco Bay, the transfer to the chamber on board as a drill rather than a necessity. As I remember, they were keeping up on their quals. It was quite a dance as far as I was concerned.

I was a line handler on the barge, for the tug that took the barge out. The divers and barge were attached to COMSUBGRU San Francisco, as was I as an MAA and general gofer.

Interesting -- have they been updated? I think it says 63 minutes on page 493 (Table 9-7) here:

I suspect these have been updated. I learned on Navy tables in 1985.

It pays to read. Look at table 9-7 NDL tables. It seem that we were reading the decompression tables, and assuming that the NDL range were the NDL tables. I don't believe the Navy does deco on scuba unless something goes very wrong.

 

[Akimbo]

Navy does deco on scuba unless something goes very wrong.

Open-circuit Scuba for sure. I have heard stories of planned decompression using mark 16 (mixed gas) rebreathers out of subs by SEALs but they have their own playbook that navy divers don't get to use.

EOD (Explosive Ordinance Disposal) "might" be allowed on rare occasions. @Duke Dive Medicine would have more insight there as well.

 

[Couv]

and you are being hustled to the chamber.

...never-the-less, when I was a tender some of the divers did their best to grab a few quick drags off of a cigarette on their way into the chamber. Some of the divers did not consider you a good tender if you did not smoke the same brand as they did. As a non-smoker, I was never a "good" tender.

 

[MrVegas]

Thanks to everyone for the additional background and history -- always interesting. Maybe the OP should include links to the NOAA dive tables:

https://www.omao.noaa.gov/sites/default/files/documents/NOAA No-Decompression Table_032316.pdf

https://www.omao.noaa.gov/sites/default/files/documents/1.4 Nitrox Tables New ver 7 Oct 2016.pdf

and the Navy Dive Manual (most recent version??):

https://www.navsea.navy.mil/Portals/103/Documents/SUPSALV/Diving/Dive Manual Rev 7 Change A.pdf

The Navy Dive Manual actually has lots of interesting reading -- the history section was fascinating to me. For the surface decompression, was I correct in reading that the divers are supposed to make it from the 40 foot stop to inside the surface chamber in less than 5 minutes? (With a 30ft/second ascent rate.) That seems like it would be quite the fire drill.

 

[Pressurehead]

Thanks for those links, very good read.

 

[Couv]

was I correct in reading that the divers are supposed to make it from the 40 foot stop to inside the surface chamber in less than 5 minutes?

If I remember correctly it's 5 minutes to get from the last water stop to the chamber stop. I.e. the diver leaves his 30 foot stop, is hauled to the surface, goes into the chamber and is blown down to 40 feet sw pressure in no more than 5 minutes.

 

[Duke Dive Medicine]

Open-circuit Scuba for sure. I have heard stories of planned decompression using mark 16 (mixed gas) rebreathers out of subs by SEALs but they have their own playbook that navy divers don't get to use.

EOD (Explosive Ordinance Disposal) "might" be allowed on rare occasions. @Duke Dive Medicine would have more insight there as well.

EOD divers go by the US Navy Diving Manual. The verbiage in there is that Commanding Officer or Officer-in-Charge approval is required for any planned decompression dives on open-circuit scuba. Interpretations of this vary, but I believe it tightened up a bit after the Aberdeen accident in 2013. For MK-16, it says that CO approval is required for HeO2 dives deeper than 150 fsw.