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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)

  • Mixed Team Diving (one 7’0” Rebreather diver and one 5’0” Open Circuit Diver are going to have very different exit gas requirements)

  • Solo Diving***

These are a just a few of the cases where the rule of 1/3's may not be the best approach, if you sit down and think i'm sure you can come up with more.

***Solo Diving sidetrack: Welcome to the solo diving gas management rant. I’ve put this separate so those who are adamantly against it wont have to sully their eyes by reading a logical opinion on it. If you can’t handle it I suggest covering your eyes and screaming for a friend, family member, or complete stranger, to come and scroll down to the next section for you.

!!!DON’T SUE ME DISCLAIMER!!! It should be noted that I neither condemn nor condone solo diving and that everyone has the right to accept the level of risk they deem fit. Until legal regulations tell me otherwise I’ll maintain that opinion.

Now then, very often I see people tell me that a manifolded set of doubles with an isolator is a good idea for solo divers. While I agree that it’s better than a single tank, most people don’t understand how to plan gas for this and will default to the rule of thirds. That’s fine in some cases but what you cannot forget using doubles as a solo diver is that…

You must be able to complete your dive with whatever

gas supply remains in any single cylinder while also considering that there exists the unlikely but possible scenario of losing your entire gas supply.

Sounds simple but actually a lot of people neglect to consider it like this. Again, I’m not telling you what to do either way, however if you do decide to solo then at least think about this. It may also be prudent to carry a ‘bailout bottle’ if solo diving, similar to what a rebreather diver does. Now back to our scheduled programming.*** END RANT

Airing to the side of caution is the side of the coin that we hear most. However, what I’m going to tell you next may shock and or horrify you. There are times when thirds may be too conservative.


All right, calm down and hear me out for a minute. On dives where our maximum penetration will require less than 1/3 our gas to exit we can start rationally thinking through whether or not thirds may be too conservative. I have to mention, however, that once you start reading this section please read it to the end. This means I don't want emails from people saying that I said "thirds is stupid and always too conservative."

Lets use a real world situation to illustrate my point. I’m diving off our charter boat Tempest, located on the beautiful South Shore of Long Island (shameless plug but hey, I’m writing the article here). We typically anchor on every wreck dive we do; by typically I mean we’ve never not anchored on a wreck dive. Lets say I figure out, based upon the depth, that both my partner and I have a 60-minute total gas supply in our respective double sets (not thinking about ascent gas or decompression for this exercise). Using the pure rule of thirds we could swim 20 minutes away from the anchor, 20 minutes back and still have a 20-minute reserve in case someone loses their gas.

Now typical diver swim speed is about 50 feet per minute. That would mean that, in theory, I could swim 1000’ from the anchor line and back. We don’t have anything that long. Even the USS Oriskany (an aircraft carrier) is only 888’. What that means is we are more likely to look around, dig for artifacts, take photos, hunt for food, etc, at a distance closer to our exit point than the maximum distance thirds allows us. In this case it may be more prudent to base our turn pressure off of how long it would take us to reach the anchor line from our furthest point away while keeping a reserve supply that will allow me and my buddy to get to a point of safety on one gas supply, whether it be the surface or your first decompression cylinder, while completing our required ascent and decompression.

For instance, if I go down the anchor line with 3000 psi, spend my dive taking photos 2’ from the anchor while accruing ascent/decompression requirements that my buddy and me can complete in 200psi, would it make sense to start my ascent at 2000psi? You’re now talking about having 1800psi extra reserved. Now listen carefully here. If this is the level of conservatism you feel you need to have then that’s fine. If you want to do a 10 hour boat ride to the Doria and do a 3 minute bottom time, that is fine! Want to dive the rule of 1/10’s? I’m not judging. Diving is about enjoyment. You can be as conservative as you deem personally necessary (as long as it’s still safe). I have immense respect for the diver who truly knows their limits and wont bow to peer pressure. What I’m trying to get at is that there may be times where turning your dive at 2/3’s remaining pressure is not necessary. Likewise there may even be cases where diving pure thirds could be an easy and conservative method of deciding when to turn the dive.

It's important for me to note here that if you're going to start bending rules to be less conservative, you'd better know what the limit is and be very clear about what you're doing. If you don't have proper training, can't calculate gas requirements, and can't work this problem through, then I implore you, do not engage in penetration or decompression until you've remedied that lack of knowledge and training. Just becaus there are times where diving thirds may not be necessary doesn't mean to be reckless and never use them again.

While I use an extreme example it is also important to remember that the anchor line is what I call a ‘quasi-mandatory exit point’ and it’s good practice to plan enough reserve gas for both divers to return to it (similar to a cave dive).

Is it the end of the world if you don’t come up the anchor line? Generally not. Things happen and sometimes it’s more important to ascend than to swim back. We learn how to free ascend from our most entry-level training. We’ve had some really great divers have to free ascend in the past (myself included but we wont talk about that). Drowning isn’t worth not bruising your pride; we don’t live by the code of the samurai, people. I’ll never criticize someone who had to do what they had to do to get home safely.

Is it advisable to plan to prevent it? Simply put, when diving off an anchored vessel it’s generally best practice to return to the boat rather than the boat returning to you.

So what should you take away from this? Diving is a thinking person's endeavor. Sometimes it helps us to hear different ideas and ways to approach what we do. We must constantly learn and grow. If we can question the dogma from a rational standpoint and still come away saying, “there is a better way,” then maybe there are exceptions to the rule. Until next time.

Live Here. Dive Here.

-Capt Tom McCarthy

East Coast Wreck Diving/ Long Island Dive Boat Tempest

IANTD Technical Diving Instructor

rEvo Rebreather Instructor

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