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

[PfcAJ]

Ok, we could go through any number of situations that could potentially invalidate that, sure.

However, don't be fooled into thinking thirds will get you out of a no flow cave if someone loses all their gas at max pen, because it won't. If you are at Hole in the Wall, and just as you thumb the dive, you lose all your gas, you just aren't going to survive that, plain and simple. Thirds doesn't cut it.

If you are planning for a diver losing all their gas at max pen, you need something other than thirds. Funny how we'll say rock bottom assumes a stressed sac rate, but the same thought process doesn't exist in mainstream cave gas planning.

That being said, I do not dive thirds because I plan to get my buddy out if he loses all his gas at max penetration (never happens), I dive it because it allows me twice the gas needed to exit. I would take twice the gas needed to exit a syphon, too.

 

[SkimFisher]

The way I understand it is that diving thirds is not the same as calculating for Rock Bottom. You would calculate your Rock Bottom and then dive thirds on your breathable gas - effectively never using your Rock Bottom gas.

 

[DA Aquamaster]

Ok, we could go through any number of situations that could potentially invalidate that, sure.

However, don't be fooled into thinking thirds will get you out of a no flow cave if someone loses all their gas at max pen, because it won't. If you are at Hole in the Wall, and just as you thumb the dive, you lose all your gas, you just aren't going to survive that, plain and simple. Thirds doesn't cut it.

If you are planning for a diver losing all their gas at max pen, you need something other than thirds. Funny how we'll say rock bottom assumes a stressed sac rate, but the same thought process doesn't exist in mainstream cave gas planning.

That being said, I do not dive thirds because I plan to get my buddy out if he loses all his gas at max penetration (never happens), I dive it because it allows me twice the gas needed to exit. I would take twice the gas needed to exit a syphon, too.

Absolutely. The major danger is with a two person team where the reserve is razor thin.

When diving a two person team on back gas only, we will carry a stage at least part of the way in to increase the available reserve and in essence add the reserve we would have gotten from a third team member.

When diving stages, the potential to lose all your gas is much more remote, but the potential to lose access to at least some of it increases, so thirds is still pretty much the standard - even though there is some debate whether to carry all the reserve on your back or spread it btween back gas and individual stages.

 

[ucfdiver]

The way I understand it is that diving thirds is not the same as calculating for Rock Bottom. You would calculate your Rock Bottom and then dive thirds on your breathable gas - effectively never using your Rock Bottom gas.

I think that's for a deco cave dive, where you need enough gas to share to the entrance, AND through your deco stops until you hit your first deco bottle. Andrew Ainslie described a method for calculating Rock Bottom (double the gas to exit) within a cave over on cavediver.net. It was very highly debated, but a great exercise to make you think.

 

[Blackwood]

I have a better question.

Why not dive 1/3's?

Because it may be inadequate

 

[DA Aquamaster]

burst disk is much more likely to go at the beginning of the dive than at max penetration. but we plug or double those anyway.

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).

 

[PfcAJ]

I wouldn't carry a pony in a cave no matter what. A safety is a decent idea when you need more gas reserves, but that generally isn't encountered till you're at 4+ stages and scootering.

With manifolded doubles and a dash of training and common sense, you just can't lose all your backgas. Go through the 9 manifold failure modes and their management. All of these are solved by shutting down the post or isolating. You save the vast majority of your gas.

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.

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.

 

[DA Aquamaster]

I think that's for a deco cave dive, where you need enough gas to share to the entrance, AND through your deco stops until you hit your first deco bottle. Andrew Ainslie described a method for calculating Rock Bottom (double the gas to exit) within a cave over on cavediver.net. It was very highly debated, but a great exercise to make you think.

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.

 

[Blackwood]

Thanks everyone. To summarize, when diving doubles, conservative gas management plans like diving thirds do account for the unlikely possibility of a buddy losing all gas, but they also allow for managing other predicaments that would require additional gas to get home safely like becoming lost or getting silted out.

Great stuff.

It's not a "how many cylinders do I have?" or "how much gas am I carrying?"

It's simple logic based upon how long it takes to swim a given distance at a constant rate.

Assuming no flow and nothing slows me down (may be bad assumptions), it will take me just as long to get out as it took me to get in. With a few more (possibly bad) assumptions, I could say it follows that it will take me the same amount of gas to get out as it took me to get in. This is true with a single, with doubles, quads, or whatever configuration you're using.

Now you have to answer the question 'does my remaining 1/3 cover YOUR exit?', and I have to answer the complimentary question.

I think that's for a deco cave dive, where you need enough gas to share to the entrance, AND through your deco stops until you hit your first deco bottle.

Or wreck diving where you have to exit and then ascend.

 

[ucfdiver]

When diving a two person team on back gas only, we will carry a stage at least part of the way in to increase the available reserve and in essence add the reserve we would have gotten from a third team member.

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.