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

[Reg Braithwaite]

So I get that diving thirds or sixths or rock bottom or any similar gas management strategy is designed to make sure you have enough gas for you and a buddy to resolve an emergency and then get home together.

The basic assumption is that you are planning for your buddy to lose all of his gas at the worst possible moment. This makes lots of sense on singles. But why do this with manifolded and isolated doubles? Is it really possible for him to have a catastrophic loss of all of his back gas?

 

[MXGratefulDiver]

So I get that diving thirds or sixths or rock bottom or any similar gas management strategy is designed to make sure you have enough gas for you and a buddy to resolve an emergency and then get home together.

The basic assumption is that you are planning for your buddy to lose all of his gas at the worst possible moment. This makes lots of sense on singles. But why do this with manifolded and isolated doubles? Is it really possible for him to have a catastrophic loss of all of his back gas?

It depends on the dive. In an overhead situation, thirds makes sense because it gives you (in theory) adequate gas to get both you and your buddy out of the overhead in a worst-case situation.

For OW diving, using rock bottom usually makes more sense ... dependent on terrain and conditions. And keeping in mind that if you're accumulating a deco obligation you will want adequate reserves to deco on in the event you lose access to your deco gas ...

... Bob (Grateful Diver)

 

[JeffG]

Is it really possible for him to have a catastrophic loss of all of his back gas?

Possible. Yes (think isolation valve failure). Probability. low. Price of failure. Deaths



For non-overhead diving, 1/3's is overkill. Its quick to go straight up.

 

[Cave Diver]

So I get that diving thirds or sixths or rock bottom or any similar gas management strategy is designed to make sure you have enough gas for you and a buddy to resolve an emergency and then get home together.

The basic assumption is that you are planning for your buddy to lose all of his gas at the worst possible moment. This makes lots of sense on singles. But why do this with manifolded and isolated doubles? Is it really possible for him to have a catastrophic loss of all of his back gas?

One reason is taking in account the loss of a buddy.

If you're diving thirds in an O/E and you become separated/lost from your buddy, then you have a reserve to try and locate him and still have adequate supply to get you out from point of maximum penetration.

And yes, the whole "you shouldn't have gotten separated in the first place" argument nonwithstanding.

 

[timle]

In my opinion from a cave diving prospective, it is very unlikely you or your buddy loses all gas at max penetration. One would need a catastrophic failure of the manifold causing leaks from both sides for this to happen "all at once". However realize when a failure happens a lot of psychological issues crop up - these will tend to make up breath faster and use up your gas. So the more likely situation becomes - your buddy loses half his gas at max penetration this psychological stress shoots his breathe through the roof and he consumes the remaining half before the return to the surface is complete - you have plenty of gas for the both of you to complete the trip.

If you do lose all gas at max penetration in a low/no flow cave - you better stay calm and make an quick exit - because in theory you have "just enough".

 

[PfcAJ]

I personally do not think of "complete loss of backgas" as a reason to dive thirds. Its WAY down on the list. I would have to lose a post, misidentify it, and fail to isolate or shut it down in a timely manner. In reality, if your buddy loses it all at max pen and you only have 1/3 left, you die.

However, situations could cause you to have a delayed exit, and diving thirds means that you can take twice as long to get out as it took you to get in. Now, if you didn't add in a rock bottom calculation to the mix, you're still in trouble. When we do deeper cave dives with a lengthy ascent to the first gas switch, we always add in some buffer.

I will modify the gas plan so that my usable gas is up to or including 1/3 of starting volume, but not more.

 

[JeffG]

I haven't ever heard of anyone dying because they carried too much gas. The alternative sucks though.

 

[Tortuga68]

In reality, if your buddy loses it all at max pen and you only have 1/3 left, you die

What?

My understanding of the rule of thirds is that you turn at 1/3 not 2/3

1/3 in, 1/3 out, 1/3 for your buddy = 3/3

That's the whole idea right? That your buddy can lose it all at max pen and you both get out

 

[djcheburashka]

if your buddy loses it all at max pen and you only have 1/3 left, you die.

...

I will modify the gas plan so that my usable gas is up to or including 1/3 of starting volume, but not more.

According to thirds, you should have 2/3 of your gas left at max penetration (really, at whatever point it will take longest to surface from whether that's maximum penetration or not).

Doesn't "thirds" really mean not 1/3 of total gas, but that total gas required = (what you need to get to max penetration) + (what you need to get back from max penetration, possibly assuming deco gas loss depending on your deco bottle gas management strategy) + (what your buddy would need to get back from max penetration, with the same lost-deco-gas assumption) + 500 psi.

 

[PfcAJ]

Yea I meant 2/3, but regardless, now you only have 1/3 and your buddy has the other 1/3. Dead.
You aren't going to have a zen like sac rate right after you lose ALL your gas. Its just not realistic. \

Luckily, losing all your gas at max pen doesn't happen.