Really stupid question... Why dive thirds with doubles?

[DA Aquamaster]

I don't need to drag an extra bottle that just would slow me down and pretend its the same as a 3rd diver. Even with the thinking that you can lose all your backgas all at once, the only time reserves are "razor thin" is at max penetration.

To each his own. Personally I like the person I dive with and in many cases prefer the speed, flexibility and ease of communication of that two person team on many dives. Properly rigged, the impact of a single stage on speed is minimal, and even if the point in the dive where the reserves are thin is minimal, I'd prefer to have the extra gas.

It becomes even more important if the team has a good SAC and the potential for long penetration on back gas and with the potential for significant deco.

Besides, you never know when some moron with a scooter deeper in the cave, an Intro class doing a lights out exit, etc, etc, etc, will blow the viz and slow the exit.

 

[PfcAJ]

In addition to gas loss durig the dive, you do need to consider the loss of at least one deco gas. Generally speaking cave divers we'll drop the deco gas in the cave rather than take it with them as in most cases they are coming out that way. Even with a traverse you will normally leave your deco gas near the exit on a set up dive.

In some cases, like Ginnie, it is not unknown for a deco bottle to go missing - sometimes removed from the cave by a well meaning but mostly stupid OW diver, or in some cases, just plain stolen. There is also always the potential for someone to screw up and take your deco bottle rather than thier own, but that is incredibly bad form and ie easily prevented with proper marking and attention to detail.

In any event, you want to ensure enough reserve gas to be able to do the much longer deco on backgas in the event you lose a deco gas.

We just bring 1.5x the deco gas we need to complete the decompression. If my buddy loses his bottle (which at Ginnie, you really need to bring it in the cave a bit to keep sticky fingers away), we have enough gas to do 1.5x the deco while sharing the bottle. We do this no matter what the deco is.

 

[PfcAJ]

If you want to take extra stuff for non existent problems, have at it.

And btw, if it takes you more than twice the time to get out in a siltout (which is always limited) than it took you to swim in, you should hang it up. This is baloney.

 

[DA Aquamaster]

Could you give an example of a recent dive where you carried a pony bottle in the cave like this? I'm just curious to hear in what cases people do this other than 2-3 stage scooter dives.

We recently did the grand reverse (grand traverse but from P1 to OG) and took a stage for an extra reserve.

On a normal backgas only dive in a low flow cave or siphon, one of us will take an AL 72 or AL 80 as the safety and leave it at about 1000' in the cave. I'll normally drop it prior to entering any of the smaller off shoot passages, or drop it just after the jump to those passages. It's never been needed, but it is one of those things that if you ever need it, you need it real bad.

Again, it is only something we do with a two person team.

 

[ScubaDocER]

Whats this "we" stuff.

I will use a bust disc rated for the actual fill pressure rather than the service pressure, for example, a burst disk intended for 3500 psi service in my cave filled 2400 psi tanks, or I may use a burst disc intended for 4350 psi tanks, but I won't double them.

Another acceptable option (in my opinion) is to use a stainless steel disc in place of a copper burst disc.

Doubling the disc is problematic as the burst disc assembly as a whole is not intended nor engineered to have 2 discs in it. The result, ironically enough, can be a burst disc failure. Just ask the folks at VA Scuba what can happen.

Realistically, if you use a burst disc assembly compatible with your actual fill pressures and change it at least every 5 years (when the tank is hydro tested works well) you will not have issues with burst disc failure. If that interval makes you nervous, change them every 2-3 years or use a burst disc rated for well over your fill pressure (for example a burst disc intended for 4350 service pressures).

That's an interesting point DA. When I was first taught to cave dive all of my mentors had doubled the burst discs in their valves. I wonder what the rest of the cave diving community does to address the issue of overfilling their LP tanks with regard to the burst disc.

 

[DA Aquamaster]

If you want to take extra stuff for non existent problems, have at it.

And btw, if it takes you more than twice the time to get out in a siltout (which is always limited) than it took you to swim in, you should hang it up. This is baloney.

Don't worry, I'm not asking you to come along. I find that the more I dive and the older and wiser I get, the more a little extra safety makes sense. The days when I was wiling to hang it out on the edge are long past I am am frankly happy to have survived some of that idiocy. If you're still in that phase, good luck.

Your implication that my skills in terms of exiting in a siltout are weak is just a cheap shot. Again, I'd rather have the extra gas than have a chain of events go south and end up wishing I had it.

 

[ucfdiver]

We recently did the grand reverse (grand traverse but from P1 to OG) and took a stage for an extra reserve.

On a normal backgas only dive in a low flow cave or siphon, one of us will take an AL 72 or AL 80 as the safety and leave it at about 1000' in the cave. I'll normally drop it prior to entering any of the smaller off shoot passages, or drop it just after the jump to those passages. It's never been needed, but it is one of those things that if you ever need it, you need it real bad.

Again, it is only something we do with a two person team.

Why not put that gas on your back so you have it when you really need it instead of on the line waiting to get stolen or messed with?

 

[Meng_Tze]

....
Stages go tits up more often than anything else because they are repressurized underwater, and thats why its a good idea to maintain your reserve in your backgas. The backgas can be shared, you keep it with you, has the most volume, and is more versatile as far as what failures can occur and you still can access it in its entirety.

Words of the wise.

 

[PfcAJ]

We recently did the grand reverse (grand traverse but from P1 to OG) and took a stage for an extra reserve.

On a normal backgas only dive in a low flow cave or siphon, one of us will take an AL 72 or AL 80 as the safety and leave it at about 1000' in the cave. I'll normally drop it prior to entering any of the smaller off shoot passages, or drop it just after the jump to those passages. It's never been needed, but it is one of those things that if you ever need it, you need it real bad.

Again, it is only something we do with a two person team.

There is a sinkhole every few hundred feet on that dive....The farthest you can get from a sinkhole is 800 or so feet. Thats 16 minutes. Something isn't right here. Either you aren't comfortable doing these dives or you aren't sure of your training and protocols.

Sounds like a time for option number one.

 

[DA Aquamaster]

There is a sinkhole every few hundred feet on that dive....The farthest you can get from a sinkhole is 800 or so feet. Thats 16 minutes. Something isn't right here. Either you aren't comfortable doing these dives or you aren't sure of your training and protocols.

Sounds like a time for option number one.

There you go with the assumptions and implications again. It's not rocket science, the extra gas just gives you more options and options are not a bad thing. I'd argue that training and protocol should open doors, not close them in regard to options.

I have the advantage in that I know what its like to be 20 something but, assuming you are not in fact a 40 something year old PFC, you have no idea what it is like to be 40 something. Let's revisit the issue in 20 years when you've basically doubled your adult experience.