InDepth Article on Thirds

[inquis]

InDepth magazine has a recent article by Stratis Kas regarding the Rule of Thirds, and it makes the case it just won't work in no flow conditions when someone has a catastrophic loss of gas. The main reason for this post is to correct the general equations describing his suggestion for something better that he calls an Adaptive Reserve Gas Model. He gives both metric and imperial versions, but both yield different answers than his numeric example therein (which is correct, BTW). In general, I like the central message that one should be inflating the SAC on exit, and he gives his suggestions for how to do that. In keeping with the spirit of a "divisor", I've rearranged the corrected general expression to use a penetration factor:

D = 1 + (1+S_d) + (1+S_r)/(n-1)​

where S_d and S_r are the inflation factors for donor(s) and receiver, and n is the number of divers in the team. Intuitively, this can be viewed on a diver basis as

D = [entrance] + [inflated exit] + [portion provided to the OOG diver]​

The penetration pressure (metric folks) or volume (imperial folks) is simply the starting pressure (or volume) divided by D. His example of n=3, S_d=45% increase, and S_r=100% increase (i.e., the SAC doubles) results in D = 3.45, which is seen in Section 5 of each PDF.

 

[Norwegian Cave Diver]

I assume ? That this is for two tanks - Sidemount or Backmount doubles?

If there is a stage involved then WKPP in with a delay going back in your stage(s) exiting.

 

[inquis]

The article and general equation didn't consider stages, but the concept of consumption increasing on exit (and estimating that based on increments) would obviously still apply. His article assumed a catastrophic gas loss (so yes, a doubles scenario), but application to sidemount (without stages) would only need a slight adjustment in the last term of the penetration divisor I gave:

Sidemount: D = 1 + (1+S_d) + (1+S_r)/(n-1)/2​

[correction: ... requires an adjustment that reduces the demand on the two exit components (relative to the BM/total loss case):

Sidemount: D = 1 + ( (1+S_d) + (1+S_r)/(n-1) ) * (n-1)/(n-0.5)​

]

​

For the example in his walkthrough, Thirds would just be sufficient when assuming only half the gas of one diver was lost (and the inflation factors were accurate, of course).

For application to stages... I'll just say it becomes very analogous to planning a DPV dive and leave it at that.