Why To Not Violate The Rule of Thirds

Half + 200?

In 1975 when I became certified as an NACD cave instructor we used a gas management rule in high flow caves such as Little River and Devil's Eye/Ear known as half + 200. The rationale was since we were swimming against high flow on the way in and being pushed out by that same flow it was acceptable to use this gas management rule.

The way it worked was, for example if we started with 3000 psi we could penetrate until we hit 1700 psi. (3000/2)+200=1700 psi was turn pressure.

Why is this an unsafe procedure?
Lets do some accident analysis on a fatality at Little River.

Some Background on the victim
  • This victim was a solo diver and drove up from the Tampa area
  • Victim had commented that the rule of 1/3rds was restricting his dive times and he was exiting with too much gas
  • Victim altered the gas management rules he was taught and wanted to increase his bottom time
  • The Dive, the Plan and the Tragic Outcome

    Since this was a solo dive the exact plan is unknown, however, we do know that the diver had stated previously that he intended to alter the gas management rules to extend his bottom times. The recovery divers were able to reconstruct some of the routes and jumps the victim made. The recovery team concluded that the victim made a navigational error during his exit, likely due to a siltout that caused zero, or near zero visibility.

    The recovery team also concluded that the victim was eventually able to correct his navigational mistake and was able to head toward the exit after considerable time being lost in the cave.

    The victim was found only a couple of hundred feet from the exit.

    The Conclusions
  • The victim used a gas management rule that exceeded 1/3 of his gas supply
  • The victim would have survived the dive had he used a more conservative gas management rule, such as 1/3
  • Cave Divers should use NO MORE than 1/3 of their gas supply for penetration into the cave.
  • Cave Divers should consider using even more conservative gas management rules under certain circumstances.

  • Jim@cavediveflorida.com

    Call or text Jim at 352-363-0013