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When The Rule of Thirds May Not Work: A look at the old rule as applied to Wreck Diving

When The Rule of Thirds May Not Work:

A look at the golden rule as applied to Wreck Diving

It should be noted that this article is designed to introduce people to ideas they may not have considered. I won’t be diving into exact gas usage formulas so as not to give anybody the false sense that this writing replaces actual training. If you’d like more info I’d suggest taking a technical course (preferably with East Coast Wreck Diving. Sorry I had to).

The Rule of Thirds. For those who aren’t familiar with this concept it’s quite simple. I use 1/3 of my gas on the traveling away from my exit, 1/3 on the way back and 1/3 remains as an emergency reserve. This rule mainly originated in cave diving where there’s no way out but the way you came in.

The problem is that we often see divers of all types blindly following this rule without considering the dive at hand.

In order to build new rules, however, we need to look at the purest example of the rule.

The rule of thirds used with zero conservatism assumes that:

  • Two divers with identical sized gas cylinders are diving an overhead environment.

  • Their entrance and exit will be at the same rate of speed and take the same amount of time.

  • There is no decompression requirement at the end of the dive that will increase exit time

  • They will be able to get both divers to the surface from the maximum point of penetration on one gas supply and use their last breath of gas as they break the surface.

In reality we recognize that real world application is quite different from on-paper planning. There needs to be conservatism built into this for many cases. This is when you’re going to have to use your brain and your ears. Talk to people who have done the types of dives you’re planning (whether it be an instructor or mentor). Emphasis on people and not one person. Just because a single person has gotten away with poor gas planning for years doesn’t mean it’s right.

Like airplane pilots we plan and train for events that rarely if ever happen. That doesn’t mean that they can’t though. The more you think and listen, the more you will come to a conclusion on your acceptable level of conservatism.

Let me rattle off a few cases where following the pure rule of thirds could actually not be conservative enough:

  • A siphon cave where the flow of water would result in a slower exit which uses more gas than the way in

  • A team member loses all of their gas at the maximum penetration, then silts out the cave/wreck resulting in a delayed exit

  • Two divers use different sized tanks. (Your third may be smaller or larger than their third)

  • Using a scooter (What if it stops working at the maximum penetration?)

  • CO2 hit (a CO2 hit may result in increased breathing rate)

  • Mandatory Decompression Requirements (1/3 in, 1/3 out, 1/3 reserve, 1/3 decompression= 4/3’s, hope you brought a fill station down with you)