200' on air for 5 min bottom time?

[drrich2]

It seems there's debate at most any point on the 'slippery slope' after 130 feet with air. Given the degree of concern a number of folks have over the 130 - 140 foot bounce dives done at the Blue Hole off Belize (with a group, including at least one professional dive guide (dive master or instructor), warm water with excellent viz.), somebody doing a 1 or 2 diver 200 foot dive on air with only an 80 cf tank in conditions not yet shared...

is probably going to raise concern.

Richard.

 

[Steve_C]

Many good observations. Original question never said how much gas they had with them. That was not part of the question although as noted it is very important.

 

[TSandM]

I suspect the question may have arisen because of the death in Cozumel.

To the OP -- although I agree with everybody else that this is a very ill-advised dive, the short answer to your question about ceiling is that it is quite dependent on what decompression model you are using. If you are using a pure dissolved gas model (Buhlmann or modified Buhlmann) your ceiling will be quite shallow, and you will do a relatively long deco there. If you are using a bubble model or a hybrid model, your ceiling will be lower, and you will distribute your decompression between there and the surface.

Your descent and bottom time will give you an average depth of 150 (roughly) for that part of the dive, which is 5.5 ATA. That, for nine minutes, would give you 50 cubic feet of gas used at 1.0 cfm. At the average EXPERIENCED diver's consumption of .7 cfm, that's 35 cubic feet of gas. That is almost half of an aluminum 80. Although I won't list all the details, the system I use would call for 18 minutes of deco between 70 feet and the surface for that dive -- average depth 2ATA would be another 25 cf of gas for the deco. Now you're at 60 cubic feet out of your aluminum 80, and we haven't accounted for ascent time or deep stops which your model might call for.

The bottom line is that, even if you were willing to assume the narcosis risk involved in going that deep, and the risk of having no redundancy at all at that depth (which scares ME), you simply don't have enough gas to do the dive safely, even if you do it perfectly.

 

[Bob DBF]

I don't recall, but awap, do the USN tables expressly rely on the presence of a chamber or would the Navy use the same tables in a context of no ready access to one?

My understanding is that the Navy tables are for NDL except where they show decompression due to depth and bottom time. Except for Hard Hat diving and large operations, chambers are not necessarily present when divers are on SCUBA. The ships I was on in the 60's and early 70's had divers but no chamber. On the subs, the escape trunk could be used as a chamber but there is no lock for assistance to enter.

The idea is to know the limits.


Bob
-------------------
I may be old, but I'm not dead yet.

 

[oldschoolto]

I suspect the question may have arisen because of the death in Cozumel.

To the OP -- although I agree with everybody else that this is a very ill-advised dive, the short answer to your question about ceiling is that it is quite dependent on what decompression model you are using. If you are using a pure dissolved gas model (Buhlmann or modified Buhlmann) your ceiling will be quite shallow, and you will do a relatively long deco there. If you are using a bubble model or a hybrid model, your ceiling will be lower, and you will distribute your decompression between there and the surface.

Your descent and bottom time will give you an average depth of 150 (roughly) for that part of the dive, which is 5.5 ATA. That, for nine minutes, would give you 50 cubic feet of gas used at 1.0 cfm. At the average EXPERIENCED diver's consumption of .7 cfm, that's 35 cubic feet of gas. That is almost half of an aluminum 80. Although I won't list all the details, the system I use would call for 18 minutes of deco between 70 feet and the surface for that dive -- average depth 2ATA would be another 25 cf of gas for the deco. Now you're at 60 cubic feet out of your aluminum 80, and we haven't accounted for ascent time or deep stops which your model might call for.

The bottom line is that, even if you were willing to assume the narcosis risk involved in going that deep, and the risk of having no redundancy at all at that depth (which scares ME), you simply don't have enough gas to do the dive safely, even if you do it perfectly.

Nothing like a woman's voice of reason..... You really know how to be smooth ......

Jim....

 

[Tortuga68]

I assume 4 min at 50 fpm down then 5 min BT would put me into deco

Uh, yeah. Also most people measure BT from the start of the dive, so 4 mins descent plus 5 mins at max depth will give you 9 mins BT. If you use 9 min BT on tables or dive software your max stop depth is probably going to be around 27-30m with a first stop around 21m and total deco time in the 15-30 mins range as you do stops of various times at different depths. If your breathing is controlled you'll go through 1.5-2 AL80s worth of air. Using a technical dive computer or ratio deco (what TS&M was alluding to) will give you less deco since you'll get some credit for your descent (average depth). Using a recreational computer, you'll 'bend' it, following which it may give you some stupid amount of deco time at too shallow a depth, and you'll likely get bent as well.

As you've already been told, it's not a good idea to do a dive like this without appropriate training/experience/redundancy/gases/equipment etc. etc.

 

[ianr33]

At the moment I ascend what would my ceiling be?

Thanks

That really rather depends on what your ascent rate is.

 

[drrich2]

Steve_C:

Original question never said how much gas they had with them.

Sorry about that. My bad. If memory serves, I went through a thread with similar subject matter right before going through this one, and that 80 cf tank stuck in my head for some reason.

Richard.

 

[AfterDark]

IMO 200'+ isn't what I call "safe" depths unless you can breathe water. More than a few experienced, well trained, well equipped divers have not come back from those depths because of small things going wrong that may not have been fatal at shallower depths. Unless there is a reason to dive that deep why do it on any mix? I understand it some; I was a new diver and fascinated with what might be at deeper depths. I did dive deep air with experienced divers before tri-mix training was available or HE was available from anywhere except a welding supply store, before NITROX was available to sport divers. These gases were used by commercial or navy divers, sport divers (this was before rec / tech names) used air. I discovered at least here in New England if there isn’t a wreck at those depths there is nothing to see except mud and broken shells; a few anemones here and there but not a thing worth my life. The heavy detached feeling with air at those depths is something to contend with and no small thing. OP, think very hard about why you ask this question.

 

[drrich2]

Since this thread has content with overlap to the 'Diver lost at Tormento' thread, I'm posting this 'cross link' in case anyone sees this one but not that one. The other thread brought up the issue of a scenario where (allegedly) a buddy pair (both instructors) decided to aim for a 250 foot depth on 80 cf air tanks.

250 is > than 200 & CoopAir may be planning a larger tank than an 80 cf, or to dive side-mount or doubles, etc..., but I thought it was worth posting.

Richard.