Deadly Down Current

[DandyDon]

Re 700psi - does he mean you should burn the extra gas doing as long a stop as you can?

Or does he mean dont thumb the dive unless you burn your air or have a huge problem because people pay money for this?

Because I consider "I dont want to dive anymore" a good enough reason.

I think the skipper who said back on the boat with 700# meant to use that as safety reserve - unless there is an emergency need to use it. To that, I would object! "500# is the industry standard! You should have told us that when we booked." Some skippers do that just to rush divers on time. Safety margins are good, but excessive margins are not.

 

[z3r0c00l]

But I dont get pressuring (lol no pun) people into staying down just because the skipper or buddy will be pissed might be a dangerous atmosphere (arg).

 

[LarryHinDC]

But I dont get pressuring (lol no pun) people into staying down just because the skipper or buddy will be pissed might be a dangerous atmosphere (arg).

I think what was meant was to come back with not less than 700 psi.

 

[z3r0c00l]

Oh not less than 700 is pretty conservative. Ive found that when people know there is no pressure and they can turn around at any time, they are MORE likely to be calm and want to stay. Its all a mental thing.

 

[b1gcountry]

So, a buddy team starts their ascent from a 100' wreck using AL80s with enough air for both of them to do a 30fpm ascent, and hold a 15' stop for 3min. Assuming a pretty decent SAC rate of 0.5, this would mean they both have 715psi in their tanks when they start their ascent.

Now assume one of the buddies has a problem with their first stage, and is suddenly OOG. He grabs the buddy's backup, and they head to the surface, but they are nervous, and their SAC doubles to 1.0 cfm. Their ascent will take 3:20 at an average pressure of 2ata, and the SS is 3:00 at 1.5ata. Doing the math, that takes 22.4 cf of gas, which equates to 861psi on a single AL80. The regs they are breathing from will probably start breathing pretty hard by 350psi, and stop completely at 250psi, so the team starting their ascent with 715psi will be about 400psi short of making it to the surface, and will have to make an abbreviated ascent.

This is all for problems that happen at the upline, and the divers head to the surface immediately. If they are deeper, or it takes longer to start their ascent, they will come up substantially shorter on air.

So, in other words, just because you are back on the boat with 500psi does not mean you are safe. To hell with industry standards, do the math yourself.

 

[b1gcountry]

PS: My quote from the captain was "if you come up with less than 700 psi there should be a good reason for it". A good reason would be:
a. My buddy had a free flow, and we did a share-air ascent, or
b. We got back to the boat with a lot of air, and dicked around at 15' under the boat for 20min, so 500psi was enough to get us back to the surface at all times.
Tom

 

[DandyDon]

Well, I apologize for getting this thread off track with my remarks, but then I avoid Ops that can't furnish 100 cf tanks (the 80 cf standard does suck) and I carry a 19 cf bail-out pony on dives planned beyond 50-60 ft.

Again, sorry...

 

[b1gcountry]

AL80, HP130, or AL63 does not matter at all. The point is you need to think about how much gas you need to be able to get back to the boat with you and your buddy. Look up Rock Bottom on this board for more info, it has been discussed plenty.

Tom

 

[Ayisha]

...Assuming a pretty decent SAC rate of 0.5, this would mean they both have 715psi in their tanks when they start their ascent...

Considering that the average SAC is apparently around 0.7, and some people breathe higher than 1.0 when stressed, it could likely be even worse than that.

I agree that leaving only 700 psi as an emergency reserve is not enough even for all recreational depths. It may be enough for shallow depths if the s#!t hits the fan, but it is not a well thought out plan for deeper dives or dives where you must exit where you entered. BTW, I used to like Lamont's old version of Rock Bottom (which I still have saved). His newer version is much more complicated and pretty much requires that I not even begin the dive with my AL 63, even though I have excellent gas consumption.

Some people have lived to tell about their experiences with downcurrents because it happened early in the dive or they had a sufficient gas reserve in case of emergency. When a downcurrent whips you 30 or 50 feet lower than planned, stressing you, causing you to expend energy to get yourself out of it, requiring increased ascent time and extra safety/decompression stops, you will use a lot of unplanned gas to bring you safely to the surface. My friends that were caught in the downcurrent in Cozumel were lucky that it happened early in the dive for exactly that reason. The husband told me that he used 1000 psi just keeping himself and his wife together and trying to get out the downcurrent, not knowing what to do. They also did a deep stop and 2 safety stops since they had enough gas left. If they did not have the extra 1000 psi that was used to tackle the problem, things may have turned out differently.

 

[DandyDon]

AL80, HP130, or AL63 does not matter at all. The point is you need to think about how much gas you need to be able to get back to the boat with you and your buddy. Look up Rock Bottom on this board for more info, it has been discussed plenty.

Tom

Yes, I do. I have screwed up and learned, but I manage my air and keep the pony slung in case anyone around me or I run into an air problem. I'll dive a 100 cf while most dive 80s as I just use a lot. Otherwise, I'll be back on the boat with a full pony and enough in my backtank that you won't need to viz it.

You said earlier: "The regs they are breathing from will probably start breathing pretty hard by 350psi, and stop completely at 250psi..." Have you ever done that, even as a test - to see if it's true?