kworkman wrote:Most of this stuff is over my head but it is a good read at 4:48am instead of doing my job. I have no intentions of doing cave or open water technical diving but interesting stuff. I wanted to get into vintage style diving because of the simplicity and tried the whole DIR style set up with a single tank and hated it. I always ask about the old time divers who did long deep dives with doubles and single outlet manifolds and never had any issues. Isn't this what the Navy does? I thought I had more to say but my eyes are starting to roll to the back of my head and I still have 2 hours to go before quitting time.
One thing I have learned from being mentored by a lot of divers with crazy amounts of experience, like Captain Tom here, Bryan, Rob, Greg, Roger, Bill, and others is that deep deco dives were done back in the day without a lot of redundancy. Typically, you and I would each strap on a set of doubles with a J-valve and a single regulator, dive deep, do deco on back gas, and then be done diving. The redundancy was in your buddy and maybe a hang bottle at 15 feet.
I have done deco dives this way. One thing to keep in mind is that unless you had access to fancy mixed gases and special tanks like the Cousteau team did, then your deco time was limited by the gas you could carry.
As an example, let's say we used double 72s and a Mistral to dive to a wreck at 150 feet of seawater for 20 minutes, using Buhlmann tables with gradient factors of 50/70 low/high which is a common set of factors supported by modern decompression research. I cut these tables using iDeco:
-150 feet for 20 minutes is 83.2 cubic feet of air.
-ascent to 120 feet is 3.5 ft3
-ascent to 90 feet is 2.8 ft3
-ascent to 60 feet is 2.1 ft3
-stop at 50 feet for 1 minute is 1.5 ft3
-stop at 40 feet for 3 minutes is 4.0 ft3
-stop at 30 feet for 4 minutes is is 4.6 ft3
-stop at 20 feet for 37 minutes is 35.7 ft3
This profile, if you and I dive it Keith, is 137.4 cubic feet of gas from splash to surface with a SAC rate of .75 CFM for dive and .6 CFM for deco. In this profile, neither one of us have any redundancy in our regulators other than buddy breathing, and no redundancy to prevent a total loss of gas from a tank valve o-ring extrusion, a burst disc failure underwater, and no positive way to manage our gas other than estimation if we chose a j-valve over an SPG. We have no gas in reserve, we are violating the rule of 1/3rds, and many of the tenets of " A Good Diver's Main Objective is to Live", which looks like this:
A Good Diver’s Main Objective Is To Live
□ Required volumes for all gases determined.
□ Required reserves for all gases determined.
□ Actual volume in all cylinders calculated and compared to required volumes.
□ All gases personally analyzed by diver immediately before dive.
□ All cylinders properly marked.
□ Gases compatible among team mates.
□ All valves and regulators tested.
□ Turn pressure(s) and gas matching (if required) determined.
□ Gas loss or failure contingency plans
Now, that's an extreme example. Many people, based upon books that I've read and people I've spoken to over the years, didn't do crazy deco dives with a double hose and double 72s. They did 150 feet for 10 minutes, with 9 minutes of deco at 15 feet. If they had a problem with their gear, then they buddy breathed for their 1 deco stop at 15 feet and then stopped diving. If they had a gear problem, then they buddy breathed to get to a hang bottle or their deco stop and buddy breathed at the deco stop.
Again, I think Luis was 100% correct when he said that ultimately the person is the arbiter of what his or her own appetite for risk is. In warm water, with a wetsuit on, I have done 10 minute decos with a single stop at 15 feet with just a doublehose and double 72s with a good buddy. I've actually done that with a few of the other VDH members here. What I personally would not do is a 150 deco dive with a single doublehose and double 72s in Puget Sound in a dry suit in 49 degree water where you cannot hang a bottle in a current. That I would do in backmounted isolated manifolded doubles with 2 first stages, 2 second stages, etc.
At the end of the day it's all about risk. You and I have been diving in just our shorts at Portage with a doublehose, a tank, no BC, and full foot fins. Super fun in Portage, but that would be super risky here. At the end of the day, it's all about stacking the deck in your favor to not drown or get bent, so you can drink beer and lie about how awesome your dives are over a steak.
Also, never be leery of participating in conversations like these. It's one of the ways you learn things.
The impossible missions are the only ones which succeed. -JYC