Dive Comp for OW+AOW

[reaver]

Not to break the ongoing debate, but thanks for the responses! I agree with the points on learning to use the table and being comfortable with it while taking the course instead of relying too much on the computer Thanks guys!

 

[PerroneFord]

I'm sorry we derailed your thread. Hopefully, you got what you needed, and are following what we are saying.

What it boils down to, is that if you dive to the NDL on the tables, with an AL80, you might not have enough air left to get you and your buddy safely to the surface. A computer will allow you even more dive time than the tables in nearly all cases, thus allownig you to dig yourself a deeper hole in terms of getting to the surface.

Dive safely, and don't blow a wad on a computer. I have a Suunto vyper that occasionaly I dive as a computer, but mostly as a bottom timer.

 

[mjatkins]

Hey, now we are getting somewhere!

Yes, this discussion only deals with a single tank. I do not, nor would I ever enter a cave with a single tank unless it is a dire emergency. So yes, we are talknig about non-overhead environments real or virtual.


In any event, when you look at what it would ACTUALLY take to get two divers sharing an AL80, to the surface from what is considered a mundane dive, it becomes clear that the AL80 is hugely inadequate for what we ask it to do. Now consider the problem that:

There are several points on which we agree, and several on which I don't think we do. But hey, such is life. I have run some math and believe that even with this worst case scenario that we have discussed, and AL80 would get both you and your buddy out of the water with a full 1/3 of a tank left when you account for lower consumption at lesser pressure. Sounds more than adequate.

I am still curious though about that "rule of thirds" thing. I was under the impression that it is the common rule for cave divers. As a cave diver would you mind either confirming that or correcting me. And what are the "dire emergency" situations that would lure you into a cave with what you believe to be inadequate equiptment?

Thanks,
Matthew

 

[reaver]

I'm sorry we derailed your thread. Hopefully, you got what you needed, and are following what we are saying.

What it boils down to, is that if you dive to the NDL on the tables, with an AL80, you might not have enough air left to get you and your buddy safely to the surface. A computer will allow you even more dive time than the tables in nearly all cases, thus allownig you to dig yourself a deeper hole in terms of getting to the surface.

Dive safely, and don't blow a wad on a computer. I have a Suunto vyper that occasionaly I dive as a computer, but mostly as a bottom timer.

No worries, an exchange of ideas is always welcome. This thread has been really helpful. I'll be doing my course on Saturday and I'll be taking my trusty casio along with me. I'll consider acquiring a dive comp after I get more dives under my belt.

 

[PerroneFord]

There are several points on which we agree, and several on which I don't think we do. But hey, such is life. I have run some math and believe that even with this worst case scenario that we have discussed, and AL80 would get both you and your buddy out of the water with a full 1/3 of a tank left when you account for lower consumption at lesser pressure. Sounds more than adequate.

I am still curious though about that "rule of thirds" thing. I was under the impression that it is the common rule for cave divers. As a cave diver would you mind either confirming that or correcting me. And what are the "dire emergency" situations that would lure you into a cave with what you believe to be inadequate equiptment?

Thanks,
Matthew

Let's assume that two experienced and fit divers are doing a wreck dive to 80ft. They breathe much better than I do. Instead of 90psi per minute at depth, they breath 40psi per minute at depth. Lets say they are doing a normal AOW dive like the Spiegel Grove. They are 200ft from the ascent line when one loses an o-ring at the regulator. He's in a bubble shower and trying to shut it down, his buddy realizes he has a problem, and assists in shutting the valve down, and gives his buddy the octopus on a 40" hose. This is a day with a bit of current so they go to find the anchor line. Let's say the donating diver began the donation with 1200psi. To go 200ft to the ascent line is approximately 50fpm swimming. This is pretty hard with only a 40" hose between them, but it's a standard speed for cavers to swim so we'll leave it at that. So when they reach the ascent line its been 4 minutes. From 80ft to 50ft takes a minute. From 50-20 is another minute, they do a 3 minute safety stop. They take another half a minute to get ther fins dealt with and hit the surface.

I count a 6 minutes before they get to their safety stop, and 3.5 shallow. Their combined breathing rate at depth was 80psi per minute. In 4 minutes getting to the line they breathed 320psi of the 1200 available. So they have 880 when they hit the line. They lose another 80 or so on the first minute of ascent. So now 720. They are coming shallow so lets say they only burn 50 coming to toward the stop, so that's 670psi. Lets say that now they are breathing at 15psi per minute each instead of the 40 at depth. So that is a combined 30psi for the 3 minute safety stop or another 90psi total on the safety stop. They have 580psi available to surface with.

This is a sufficient amount to surface with. What would have happened if they had decided to turn the dive at 800 psi? Or 750, like so many AOW do? I hear of VERY few divers who think it's necessary to turn the dive with more than 1/3 of their tanks left. How many of AOW divers even DO gas management scenarios to figure this stuff out?

As to the rule of thirds in cave, it is merely a guide. Several features unique to caves make it feasible. But as you learn VERY quickly in diving real overheads, the rule of thirds is often inadequate which is why diving on thirds is considered a MINIMUM amount to reserve for EXPERIENCED cave divers. New cave divers actually dive sixths.

As for what dire emergency I might take a single into a cave? Maybe if my wife was going to drown if I didn't go. That Grim reaper sign stands a the cave entrance for a very good reason. Being a trained cave diver doesn't sufficiently mitigate that risk if you go in without proper cave gear.

 

[SparticleBrane]

HAH I just found a new emoticon here on scubaboard.
In honor of this thread: "Read the @(*&(*&ing manual"
:rtfm:

 

[Blackwood]

Since the thread has been hijacked already...

Hopefully, you'll be learning to dive using the tables, not a dive computer.

I go back and forth on this point.

"You have to learn to crawl before you learn to walk"

While I agree that have an understanding of fundamentals is important for any activity, I can't agree that learning to use tables provides this understanding any more so than does a computer (which is really just an electronic table).

Really... the ability to lay a finger on a set of numbers, slide it down/over/wherever and read a subsequent number is a far cry from understanding decompression mechanics/theory.

It's hard for me to say one is necessarily better than the other (with the minor point that a table can't run out of batteries).

Often people say: if a computer fails, your tables back you up. But really, how many recreational computer divers keep enough mental notes about their profile to switch from a computer to tables? I'd say closer to 0% than 1%, because the few people with that kind awareness generally don't use computers.

 

[PerroneFord]

Ever watch someone at a fast food place stumble when you buy something for $1.97 and you give them $2.02?

Computers are electronic versions of tables. And yes, using one is the same as using the other. It's when we use the computer and LOSE the underlying theory, that they become dangerous.

Given that this was a class scenario, I think it is prudent to leave the computer alone, and work on learning the underlying theory.

But that is just my opinion.

 

[Blackwood]

Computers are electronic versions of tables. And yes, using one is the same as using the other. It's when we use the computer and LOSE the underlying theory, that they become dangerous.

I agree, but that statement assumes that "we" have the underlying theory in the first place. That's my point.

Most recreational divers don't have the underlying theory TO lose.

 

[PerroneFord]

Point taken.