Calculating Search gas

[TSandM]

I think what Jean is saying, and what has come out of this discussion on other boards (and Jean, correct me if I am wrong) is that vague general statements regarding this issue don't cut it when you have to make decisions. Having a plan in place, that takes into consideration flow and position in the cave but can be FORMULATED and stated, means that under the stress of the actual problem, you will have something you can easily pull out and put to use. Trying to make decisions based on generalities, in the face of significant stress, is likely to lead to error -- either you'll exit the cave with tons of gas you could have used to search, or worse, you'll find your buddy without the resources to get both of you out of the cave. There should be some kind of rational calculation readily at hand.

 

[Rick Murchison]

...There should be some kind of rational calculation readily at hand.

Precisely. And, just as knowing your SCR is critical to prior proper planning, and keeping track of how much gas you have at all times is required for safe diving, having a very firm grasp of how much gas you need to get out of the cave from your current position given the conditions of the moment is key to knowing how much gas you need to reserve during a search. If I don't know, then I have to fall back on the "general rule" of thirds or something, which may or may not be even remotely reasonable given the circumstances.
But it isn't a matter of just one number... as an exampe:
I realize my buddy is missing at the point where I need 200 psi to exit (given the flow, visibility, etc., say the first T in JB on a high flow day). The first decision I have to make is whether to leave immediately and go for help/more gas, or try to find my buddy. In this case I'm gonna try to find my buddy. I decide the vis is good enough to use the X4 rule (double for my buddy; double for excitement), so I know I need to start collapsing my search area at 800 psi. If I have 1500 psi then I can either wait, or penetrate further until I'm at a point where my remaining gas is four times what I need to get out... (300/1200, for example) - either way, I must start collapsing the search at 4X my needed gas, but I do not need to exit at a normal pace... I can leave at a fourth my normal exit speed and keep sufficient reserves to get me and my buddy out at all times all the way out. (I leave the T with 800 psi - if I go half way out I'll have 700 psi, but only need 100 to get the rest of the way, so I have 300 to wait with... I could have retreated slower, so that I arrive at the hald-way point with 400, improving my buddy's chances of reaching me).
Again, setting a sound search/retreat gas plan is dependent on knowing yourself and your position well enough to know what you normally need to get out.
Rick

 

[DeepSeaExplorer]

The instant the decision is to go into search mode, my gas requirement calculations change, based on "what does it normally take me to get out from here?"
I then set my abort gas at somewhere between four and eight times that amount, depending on conditions.
For example, if I need 200 psi to make my normal exit, then I'm going to double that so that if I find my buddy as he's running out of gas I have enough for both of us - then I'm doubling that again for the excitement factor - that's four times. If the visibility is blown then I'm doubling it again, eight times.
Rick

So if you would normally need 800 PSI to exit from that point you would turn at 6,400 PSI?!

 

[Cave Diver]

So if you would normally need 800 PSI to exit from that point you would turn at 6,400 PSI?!

Several factors come into play here. Rule of 1/3 is the [-]most[/-] least conservative rule that cave divers use. Sometimes it's 1/6th, or something else depending on the dive.

You use 1/3 of your gas to penetrate, you reserve 1/3 for exit and 1/3 for emergency. Most caves are springs, meaning they have flow going towards the exit as opposed to syphon which has flow pulling you into the cave.

High flow systems like Ginnie, Jackson, Little River, etc. tend to take much more gas to penetrate against the flow than it does to come out. You can literally "ride the current" out.

In a situation like this, it may take you 800 psi to reach a certain point of penetration, but only 200 psi to return to the exit. However you dont plan your dive that way. You still plan as if it will take you 800 and that additional 600 difference just ends up being reserve. Plus the 800 you already have in reserve for an emergency.

Generally, if you anticipate a dive that is going to require larger gas reserves, they will bring stage bottles, or if your dive is going to incur substantial deco obligation, they bring deco bottles rather than relying strictly on backgas. Contingency for loss of any of these gasses is generally figured into the dive plan.

So, using Ricks example, if he had penetrated to a point requiring 800 psi to exit, odds are that there would be additional gas supply available. Secondly, returning from a penetration that far, hopefully both divers would have calmed down once they started sharing gas and exiting, which would lower the gas requirements from the 4x example he gave.

 

[Rick Murchison]

... most conservative...

"Least"... least, he meant least.
Rick

 

[Rick Murchison]

So if you would normally need 800 PSI to exit from that point you would turn at 6,400 PSI?!

Well, in the first place, I'm not going to be in that position, because I'm not going to penetrate that far. At least not without a couple of stages (which adds a level of complexity to search gas calculations not addressed here). But... if I were 800 (normal) psi from an exit, excited, with an excited OOA buddy and blown visibility, we'd better be mighty lucky and mighty calm and controlled to make it out on less than "6400 psi."
Rick

 

[Cave Diver]

Least... least, he meant least.
Rick

Thanks Rick!

 

[Rick Murchison]

An important point to consider here is that divers who've never had the opportunity to calculate their gas consumption rate with the dragon all the way up their back tend to (1) underestimate their ability to use gas in a hurry, and (2) overestimate their ability to get the dragon off.
(I'm reminded of the 100% certainty that everyone who's never been through it believes - I mean really believes - that they can endure waterboarding for at least twice as long as they really can)
Believe me, even a calm diver and a full dragon buddy can burn gas at over four times the calm diver's normal rate. Failing to consider that in your calculations for abort gas when in search mode can bite you.
Rick

 

[UCFKnightDiver]

Well, in the first place, I'm not going to be in that position, because I'm not going to penetrate that far. At least not without a couple of stages (which adds a level of complexity to search gas calculations not addressed here). But... if I were 800 (normal) psi from an exit, excited, with an excited OOA buddy and blown visibility, we'd better be mighty lucky and mighty calm and controlled to make it out on less than "6400 psi."
Rick

If you are in a low to no flow cave you could easily be 800psi or more from the exit...

 

[Juno]

Absolutely Rick. Hence my comments on the other thread for some of the folks that picked very low numbers.
I believe too many divers think way too much of their own abilities.

I have never seen a posthumous medal of honor for cave diving.

Though Parker Turner probably deserved one.