Tank configuration

[swimmer_spe]

The problem with simple questions is they attract difficult answers

(snip)

Happy diving!

Thank you for the information. Thank you for not bashing me. I now see why the odd looks from other divers when I showed up in my OW single tank set up. I also know that Cavern diving is about all I want to do. I have no desire to go and try to "fit" into places.

 

[Chilly_Diver]

The reason some prefer SM over backmount, and others prefer BM over SM really comes down to personal preference. They each have pros and cons. Some people believe BM is the best route for the majority of cave diving, others view SM as the best route for the majority of cave diving.

However, regardless of the differences between SM and BM, based on the things you have written in this thread, you are not mentally prepared to engage in cave diving. Period.

There's a thing called stress, which can lead to perceptual narrowing, which can lead to panic.

Stressors are like the layers of an onion. Things like a wet breathing regulator, being in an overhead environment, monitoring your air, monitoring your time, maintaining trim, being in unfamiliar gear, a leaking mask, having a regulator tugged out of your mouth by the wall of a cave, all add stress and build on top of each other. When you get to a point where the cumulative toll of the stressors is enough that you start focusing on those stressors -- let's use the need to clear a regulator, for example -- then you begin losing your awareness of everything else around you. You may lose attention to the guideline, you may lose awareness of which way is out, you may get separated from your buddy, you may cause a silt-out. When you start having perceptual narrowing problems, and then the fit hits the shan, you may very well wind up in full bore panic.

Once you're in full blown panic, there's no coming back. Your thinking brain shuts down and your lizard brain takes over. You become a hazard to yourself and the people you are diving with.

This really happens. I've seen people in full out panic. It's not a pretty sight.

In technical diving (and really, it should be all diving), we have the golden rule: Anyone can call any dive at any time whatsoever with no repercussions. If something as basic as taking a second stage out of your mouth, putting it back in, and clearing it causes you stress, then I strongly suggest you use the golden rule to avoid going into the overhead environment entirely. Focus on diving in the openwater and perfecting the basics.

Fact: Water filled caves can be some of the most unforgiving places. The cave doesn't care if you live or die, it is just there.

Fact: When you have a problem in a water filled cave, you cannot go straight to the surface. You have to deal with the problem in the cave and make your way out.

Fact: Remaining calm in a cave environment is MANDATORY. The first or second thing I teach brand new openwater divers is reg removal and clearing, if the idea of doing this causes you mental discomfort and stress then you have ZERO business cave diving.

Look, I'm really trying to help you here. I'll even be happy to mail you a free copy of Sheck Exley's Blueprint for Survival which outlines several of the risks and hazards in cave diving by looking at a half-dozen fatalities in detail if you PM me an address.

I couldn’t help but hear that entire post in the voice of Dwight Schrute from The Office...

 

[tbone1004]

Thank you for the information. Thank you for not bashing me. I now see why the odd looks from other divers when I showed up in my OW single tank set up. I also know that Cavern diving is about all I want to do. I have no desire to go and try to "fit" into places.

you say that now, just wait until you get in there! one of the common misconceptions about cave diving is that we are all squeezing into things. While many of us do, myself included, most of the caves are quite large. One of the "restrictions" *pun intended* of cave training is that you are not allowed to take divers through any restrictions. The definition of a restriction is where two divers can't pass through a passage together, that is either side by side, or one on top of the other, with side by side being most common. Most of these passages are quite large, so trying to "fit" into places is something that you may never have to do.

Either way, it doesn't change the fact that going to those dives does go beyond your level of training, so even if you only want to stay in the cavern zone, you have to go another level of training.

 

[Dan_P]

On consistency, scalability, redundancy and compartmentalization - and how manifolds (high-pressure and low-pressure) fit into the equation:

 

[halocline]

On the first dive, I should come up on half the gas I went down with. With the second dive, do I still use the 1/6 or do I do the 1/3s?

Here's how it works with diving to 1/6 pressure. As an example, use AL80s filled to 3000 PSI. First dive, you can penetrate 500 PSI, because that is 1/6 your pressure. You should exit with 2/3 your gas, not 1/2. (If you had a true emergency which caused your use of the additional gas, you're probably done for the day)

Second dive, starting with 2000. You can still penetrate 500 (1/6 your original pressure), and you should exit with 1000 PSI. You still have 1000PSI as a reserve, which is the lowest allowed reserve for those tanks. (Assuming everyone on your team is using them)

The reason for the 1/6 rule is not to give you more than 1000 PSI final reserve, it's to keep you from penetrating too deeply on any dive. It ensures that if you have an emergency, you are no more than 1/6 of your tank capacity from the surface.

After reading this thread, I believe that you have received some excellent (but sometime blunt) advice. There are other issues aside from your personal gas usage in any overhead environment. Generally speaking, the further in you go, the more dangerous it is for a variety of reasons, and additional gas allows you to do exactly that. Using cave gear to do cenote diving does NOT make it safer on it's own; it's the additional training that does that. I'm sure you understand that.

The best way to get the advice you are seeking on SB is to be crystal clear about what it is you are trying to do. You did not specify that you wanted to limit yourself to guided cenote dives until pretty late in this thread. For that purpose, I would just advise you to go with a larger single tank. You might start by switching your hoses to include a long-hose primary/bungee necklace alternate. Get used to that first. Doubles, either in BM or SM, have a learning curve and you can certainly learn to use them, but it would be way better to do so in OW as part of an intro to tech course or something like that. And they will not extend your guided cenote dives, if the guide is really doing his/her job. This is because the generally accepted rule is 1/3 on single tank, 1/6 on doubles. It's the same 26 cuft (AL80s).

Sorry for the long post, hopefully it clarifies some things for you.

 

[WetRocks]

Second dive, starting with 2000. You can still penetrate 500 (1/6 your original pressure), and you should exit with 1000 PSI.

Um, no. You don't base your usable gas for a dive on the amount of gas you started your last dive with.

I agree with most of your last post but absolutely not this.

 

[halocline]

Um, no. You don't base your usable gas for a dive on the amount of gas you started your last dive with.

I agree with most of your last post but absolutely not this.

I understand the problem with calculating gas planning on gas that you don't actually have

But in my intro class, that's how the restriction to 1/6 was explained to me, and that's what we used throughout the course. It's a restriction not based on gas planning as much as on limiting penetration for students on the cavern/intro level. Hence the ability for those same students to use thirds on a single tank.

 

[Dan_P]

Are we still talking cavern or overhead penetration here?

The whole "guided cave diving"-thing is actually a thing, unfortunately.

 

[kensuf]

I understand the problem with calculating gas planning on gas that you don't actually have

But in my intro class, that's how the restriction to 1/6 was explained to me, and that's what we used throughout the course. It's a restriction not based on gas planning as much as on limiting penetration for students on the cavern/intro level. Hence the ability for those same students to use thirds on a single tank.

This is a gross misrepresentation of the rule of 1/6ths. Please consider calling the agency you were certified through and having a talk with their training or quality assurance department.

 

[swimmer_spe]

Are we still talking cavern or overhead penetration here?

The whole "guided cave diving"-thing is actually a thing, unfortunately.

OP here.
I got all I needed several pages back.