Deep Diving - Safe bottom air pressures

[drbill]

If you are indeed planning to do deep dives, I strongly suggest that you spend some time acclimating to increasing depth and determining your reaction to narcosis. Before I started doing my deep dives, my maximum depth was on the order of 140-150 fsw. When I decided I wanted to hit 200 fsw (to film certain deep critters), I spent about two months gradually increasing my depth and assessing my response to narcosis. This involved on the order of 30-40 dives gradually increasing my depth in 5-10 foot increments.

At that time I was doing about 300-350 dives per year. Currently I'm only doing about 100 dives a year with the deepest one over the last year being 150 fsw. Because I have not been conditioning myself to higher nitrogen loading, I find I get narc'ed as shallow as 105 fsw and would not consider 180-200 fsw dives without reconditioning myself over time.

I should add that this is strictly based on my personal experience and should not be considered as a specific guideline for other divers. Your physiology, conditioning and other factors are different than mine (and hopefully better since I'm a bit of a geezer after 48 years of diving). I would never recommend specifics regarding such deep dives to anyone else since I am not familiar with their physiological state and skills. However, the general procedures (acclimitization over time to depth) seem to be worth considering for anyone who is interested in trying deep dives.

 

[muddiver]

Yesterday I did a wreck dive with a single tank (and 13 cu-ft pony) with about 140 cu-ft in it at 3000 psi. Dropped in swam down to about 140 and then cruised for a few minutes as we drifted toward a wreck. As we got to the wreck, my buddy shot a large fish and drifted off in the 2 kt current.

I saw another fish, grabed onto the wreck at the deck at 160 ft, stalked a fish for maybe a minute, speared it, fought it a little, allowed myself to sink down into the ship to the bottom where I was out of the current (at about 190 ft). I then pulled out my knofe and killed the fish (at that point I remebered I forgot my ascent reel), I then removed fish from the shaft, placed it in my bag, re-loaded the spear, strung the line and loaded one band of gun for sharks. I remember feeling pretty narced and a little out of breath, when wrestling with the fish (I've not been over 140 feet for 8 months probably). I remember breathing faster than I should and could detect a little resistance in the reg, so i must have been sucking pretty hard.

I then ascended at a reasonable pace (my computer had died on the descent), which i did not realize until I was at a depth of about 100 feet. When I first realized the computer was dead, i checked the time and it was 8:25 am.

I hate ascending without a computer (or any depth guage) but I came up following my little bubbles and kinda hurried toward the surface because I wanted to send up a smb, which only had 25 of line on it, so I didn't stop deeper. My watch read 9:36 when I reached 25 feet. I did 12 minutes of hang and hoped that was a reasonable time and ascended.

I still had half my air left (so i used about 70 cu-ft).

Man! You are soooo non-DIR

 

[muddiver]

Thanks, Bwerb, Mikemill, Teamcasa, Thalassamania & Elan for the quick and detailed replies. I am planning such a deep dive with a buddy and was trying to work out what is the safest cylinder pressure at 50m and 60m when we should be ascending. However, the dive site where we intend to do the deep dive has only AL80 cylinders and the usual volume is 200bar (2900psi). How much extra air can an AL80 cylinder hold safely?

At 3000 psi an aluminum 80 does not hold 80 cubic feet of air. It is closer to 77.4 cu ft.

At 2900 psi you will have even less than that. Sorry I don't have the equasion on hand but you can use an average size of 678 cubic inches of space and your 2900 psi to work it out.

 

[Web Monkey]

Man! You are soooo non-DIR

dumpsterDiver:

I think he's trying to compliment you!

Terry

 

[Blackwood]

At 3000 psi an aluminum 80 does not hold 80 cubic feet of air. It is closer to 77.4 cu ft.

At 2900 psi you will have even less than that. Sorry I don't have the equasion on hand

Um, you posted the equation in the line above.

77.4/3000 = V/2900

 

[Anti-Hero]

Thanks, Bwerb, Mikemill, Teamcasa, Thalassamania & Elan for the quick and detailed replies. I am planning such a deep dive with a buddy and was trying to work out what is the safest cylinder pressure at 50m and 60m when we should be ascending. However, the dive site where we intend to do the deep dive has only AL80 cylinders and the usual volume is 200bar (2900psi). How much extra air can an AL80 cylinder hold safely?

Good luck finding someone to overfill an Aluminum 80. I'd also strongly reconsider what you're thinking about doing. Typically someone with the training and experience to execute a deep dive doesn't need to ask these types of questions.

 

[muddiver]

Um, you posted the equation in the line above.

77.4/3000 = V/2900

Just cus I'z an inganear, not mean I'z passed fizzicks.

 

[Web Monkey]

Thanks, Bwerb, Mikemill, Teamcasa, Thalassamania & Elan for the quick and detailed replies. I am planning such a deep dive with a buddy and was trying to work out what is the safest cylinder pressure at 50m and 60m when we should be ascending.

Without trying to be rude, if both of you can't figure all this out independently, you shouldn't be doing the dive.

Terry

 

[Doc Intrepid]

Would it be possible to use travel bands or stage rig an additional 80 to double your gas supply?

+1

Yeohcheeweng,

1. You're trying to learn deep air diving on the internet. Not smart.

2. The guys above have outlined why, unless you're doing some bounce dive (which still has potential issues involving air that deep), at which you have zero experience, trying to do a dive to those depths on a single 80 is ill advised.

3. You have zero margin for error. Slinging another single 80 (assuming from your post that single 80s are your only option at this specific site) gives you at least some total volume that provides you with adequate gas for planning. But only if you understand how to plan the dive.

4. See #1 above.

Before you and your buddy head down to 60m - 197 fsw - on a single 80, please consider getting some additional training.

You really are gambling with your life trying to do air dives to that depth going off "what some guys told me on the internet".



Doc

 

[Thalassamania]

I'd suggest this as a minimum workup schedule for deep air dives. With a buddy who is quite experienced on air at the depth you are going to, make a minimum of 12 dives in each depth bracket, completing each bracket before progressing to the next:
0 - 30
30 - 60
60 - 100
100 - 130
130 - 150
150 - 190