When thirds are not enough....

[Lobzilla]

Very interesting thread.

I have never set a fin in a cave and I need plenty of skills and experience before having to worry about thirds in overhead environments. However, similar questions apply to the (dumb) rule of "on the boat with 500" or "rule of halfs", etc.

Rules are not a replacement for individual, critical analysis. That analysis should start with the question of what the rule is supposed to accomplish. If I understand correctly, the rule of thirds is to assure the safe retreat of two divers even if one diver suffers complete gas loss at maximum penetration. Anytime that is not probable because of flow, silt, increased RMV, etc. the rule is pointless since it does not meet the original intent and needs to be superseded by a better plan.

Example from an environment I am more familiar with: The FAA requires fuel reserves of at least 30 minutes past destination for VFR (nice weather) flights and 45 minutes for IFR (crappy, cant see sh!t weather). There are many scenarios where that approach to gas planning is suicidal.

Rather than getting hung up on the rule I make my own worst case assumption for additional fuel consumption due to headwind, rerouting, missed aproaches, etc. If the calculated fuel reserve is less than the FAA rule, I put in the tanks what the 'rule' requires. If it is more, I do not care about a rule that clearly does not apply to the scenario in front of me and put in the tank what I feel necessary.

As in diving there are plans where the tanks are not big enough to safely conduct the flight. Then you either need bigger tanks or you have to stage or forget the whole idea. Because:

Thou shalt be ever mindful of thy fuel lest there be nothing
in thy tank to sustain thee upon the air and thy days be made short

PS: For diving, I currently plan on GUE "Minimum Gas" (minimum deco ascent for 2 divers on smallest gas supply assuming increased RMV) plus halfs or thirds. Subsequently, my thirds are padded by the generous rounding for the ascent. Thanks to this thread I realize that thirds by themselves can get awefully tight.

 

[DA Aquamaster]

The problem with this idea is that it places a large reserve in the exact place you DON'T need it, and thats in the front of the cave. Having your reserve with you (in a sharable format) gives you much more options than a bottle that could be out of reach. There are at least two fatalities that come to mind where the diver did not reach their reserve gas in time.

Sucking it up and taking 1 more bottle and using it as a stage gives you that reserve in your backgas. You might be using the same total gas (drop stage at ~half, continue on backgas, turn when you complete your intended dive), but there is MUCH more gas on your person at max penetration. Plus, swimming a heavy bottle has got to be one of the most inefficient things in this entire discussion. Its even worse if you're trying to make an expeditious exit from the cave.

If you both carried a bottle and dropped it (like a safety bottle), then I could potentially see your point, but that's even a stretch on the dives you're talking about.

Then you are saying you are clearly on the "be conservative" side of the fence and are willing to accept (sucking it up) the added work of hauling a stage on the entire dive. In this case even on a dive that is right at "thirds" without the stage.

I understand your concern that the buddy bottle is not at the furthest point of penetration, but on a dive to "thirds" with the stage not figured in the planthat is not really where you need it most. People don't run out of gas at max P, they run out closer to the exit and that would be the case here where you pushed thirds, had a failure and found it was not enough.

Unless you really screwed it up, reaching the stage 1500 feet from the exit should be enough. If it's not then you clearly needed to both be diving stages, and there is enough difference in those dives to tell that before you start, or at least before you reach the planned turn pressure.

I partially agree with the heavy bottle thing - except I sidemount and top clip the bottle. As such it stream lines very well (on top of the left side primary tank) and I have not noted any trim issues doing it. In that context it stream lines better than an empty stage, even using Edd's system of rigging the stage.

Another aspect to consider is that we may also not want to take a stage into a tight sidemount tunnel that is already snug with just the priamry SM bottles. In that case it is still comforting to have the stage as far in the cave as possible. Although in that case, if the cave is unknown or the push is significant in terms of reduced reserve gas, I am then also far more likely to stage the dive and drop the stages as far in the cave as possible, preserving as much primary gas as possible for the skinny sections of cave.

What matters is not hard and fast rules but rather ensuring you look at the whole picture and adopt a plan that makes the most sense and achieves a workable comprimise between hurcluean efforts to set up the dive and a generous reserve to ensure you can do the dive safely.

 

[DA Aquamaster]

it does not meet the original intent and needs to be superseded by a better plan.

Example from an environment I am more familiar with: The FAA requires fuel reserves of at least 30 minutes past destination for VFR (nice weather) flights and 45 minutes for IFR (crappy, cant see sh!t weather). There are many scenarios where that approach to gas planning is suicidal.

Rather than getting hung up on the rule I make my own worst case assumption for additional fuel consumption due to headwind, rerouting, missed aproaches, etc. If the calculated fuel reserve is less than the FAA rule, I put in the tanks what the 'rule' requires. If it is more, I do not care about a rule that clearly does not apply to the scenario in front of me and put in the tank what I feel necessary.

As in diving there are plans where the tanks are not big enough to safely conduct the flight. Then you either need bigger tanks or you have to stage or forget the whole idea.

I used to fly out west in the wide open space states. With few airports, it was often possible for the weather to sock in all your potenital alternates, and with widely dispersed weather observation stations (many of them automated and only looking straight up) it was possible for this to start happening while in flight, so you found yourself obsessing on in flight weather reports.

And even under "good" IFR conditions, it was not uncommon for your only viable alternate to be the airport you were departing from, so if the weather was going south when you left the odds of it staying usable as an alternate were poor.

Discretion, realistic planning and good situational awareness were essential to staying alive. The same applies to cave diving.

 

[PfcAJ]

No, thats not what I'm saying. For starters, we're using different terminology and its leading to confusion on both parts. For clarity, I will define some terms so we're on the same page.

Stage bottle - a bottle that is breathed and then dropped
Safety bottle - a bottle that is dropped at a predetermined point that is not breathed unless needed.
Thirds - your pressure at which the thumb comes out and we head home.

I am NOT suggesting that you need to carry an extra tank with you through the entire dive. What I am saying is that diving a stage (breathing it to roughly half, then dropping it) is better than dropping a single safety bottle at x point in the dive.

Let's build a dive plan. We want to go see some cool room 2300' into the cave. Its a little tight leading up to it, but not "titties and tanks" small, or anything approaching that. We have a few options:

a) Do the whole thing on backgas, but it puts us real close to "thirds".
b) We could do it on backgas, but drag a safety to the halfway point, and still dive "thirds' out of our backgas
c) Dive a stage, drop it at half plus a bit, and dive to the cool room on backgas with a modified "thirds" turn pressure.

If I'm interpreting your posts correctly, option B is how you would approach this dive, while C is how I would do it. We both recognize that option A doesn't give enough wriggle room.

Diving a stage gives you WAY more than enough gas to complete your dive (with 104s, you get 365cuft to play with vs 288). That excess translates into extra reserve PAST your "two thirds" for exit . If you were going to carry the bottle anyways (in the form of a safety), you might as well breath it, carry it a shorter distance, and end up with extra reserves WITH YOU farther in the cave (in the cool 2300' room). You were already going to be there with your 104s (or whatever youre using), but by diving a stage, your tanks have more gas in them, anyways. Gas in the front doesn't do you any good if you need it in the back.

As far as swimming an extra (full) bottle, you not only content with the added bulk of the tank, but the added gas in your wing needed to compensate for the gas in the bottle. Its more than just a streamlining issue, too, as you're physically having to push more mass through the water.

You also can't share a single safety bottle. A single safety bottle greatly reduces your options compared to a proper stage, or even a pair of safeties.

If your dive plan puts you close to not having enough gas to complete the dive, use your resources in a manner than gives you the MOST options in the water. Filling and carrying one...more...tank... isn't really an issue. If it is, save your nickels and hit the gym

 

[Superlyte27]

What real life situation have you been in involving a completely OOA situation?

Which Time?



*edit...
I'll give you two.

Last year, in January in 30 degree water... I was several hundred feet into a pipe putting a flow meter in when I had a total loss of gas.

Two months later, inflating a bag in a sewage pipe, regulator froze into a solid block of ice and depleted all my gas in about 2 minutes, I was in a penetration on the wrong side of the bag.

Yup, commercial diving is no where near as safe as cave diving.

 

[SuPrBuGmAn]

Who's talking about commercial diving?

 

[Lobzilla]

Which Time?



*edit...
I'll give you two.

Last year, in January in 30 degree water... I was several hundred feet into a pipe putting a flow meter in when I had a total loss of gas.

Two months later, inflating a bag in a sewage pipe, regulator froze into a solid block of ice and depleted all my gas in about 2 minutes, I was in a penetration on the wrong side of the bag.

Yup, commercial diving is no where near as safe as cave diving.

Even when every penny is tight in order to turn a profit I would use technical means (i.e manifolded doubles) to turn this into: "I was several hundred feet into a pipe putting a flow meter in when I had a loss of some of my gas" and "one of my regulators (the one I inflated the bag with) froze into a solid block of ice and depleted some of my gas for about 1 minute (until I shut it down)"

Which relates to this thread about "thirds" as both are about having redundacy for your next breath when things go south.

 

[Superlyte27]

Who's talking about commercial diving?

We were talking about not panicking...
I'll type it slower if you are having a problem keeping up.

 

[LiteHedded]

we were talking about not panicking...
I'll type it slower if you are having a problem keeping up.

lmao!

 

[Cave Diver]

Which Time?



*edit...
I'll give you two.

Last year, in January in 30 degree water... I was several hundred feet into a pipe putting a flow meter in when I had a total loss of gas.

Two months later, inflating a bag in a sewage pipe, regulator froze into a solid block of ice and depleted all my gas in about 2 minutes, I was in a penetration on the wrong side of the bag.

Yup, commercial diving is no where near as safe as cave diving.

So which trick did you use? Skip breathing, or recycling the air from your BC?