Ask any group of tec divers what is the rule of thirds and you will probably get a number of different answers.  In all probability they will more than likely be close in concept and close to the actual rule but with a few variations in wording.  But what do they actually mean?

“1/3 in, 1/3 out and a 3rd in reserve”, “surface with a third of all your gas”, “save a third of your gas for emergencies” are typical statements. You might also hear divers say that they dive ‘to’ the rule of thirds but is that going to be the conservative option if you do need to utilise your reserve gas supply?

The rule of thirds is variously attributed to either the pioneers of cave diving in the UK and the founders of what was to become the Cave Diving Group (CDG) or to their slightly more modern counterparts exploring the springs of North Central Florida.

Regardless of origin, the rule was used by cave divers, allowing one third of the gas supply to be used on the inward journey, one third for the dive out and a third held in reserve in case things did not go according to plan.

In simple terms it seems to work.  Looked at more closely there are flaws, albeit only minor, which need to be addressed as part of your dive plan in order to stay safe if things go wrong.

First let us look at the common belief that we should dive ‘to’ the rule of thirds.  For this example we will assume identical breathing rates and identical cylinder sizes between two dive buddies.  Should one diver have a catastrophic loss of gas at the furthest point of penetration the other diver has a third of his gas ready to donate to the out of air diver.  However if the process of the first diver trying to deal with the gas lost in the first instance takes any time or the act of sharing air and sorting themselves out for the exit takes more time, the diver donating gas will have been using some of his or her exit gas already.  Those few minutes to get organised for the swim out could leave gas supplies perilously close to the limit or even being exhausted prior to the team reaching safety.

What if the air sharing swim out of the cave takes much longer just because the long hose has been deployed?  What if the two divers did not have identical breathing rates?  If the out of air diver has a measurably larger breathing rate then the donor diver’s reserve third might simply not be enough gas!

How can we plan to avoid these potential pitfalls? 

Firstly we can look at how we might apply the rule of thirds before we discuss other planning considerations.  When I said that many divers will dive to the rule of thirds the implication is that they will turn the dive when the hit the point of having used 1/3 of their gas.  But turning around takes time.  Making sure everyone on the team has seen the signal and they too have turned takes time.  All this is eating in to the gas for the swim out and as so in to your reserves as well.

So, why don’t we look ahead.  Rather than wait until you reach the exact turn pressure, consider turning just a few bar/psi earlier.  If your turn pressure is 140 bar consider turing when you are at 145 bar.  (For my non-metric friends a similar example might be a turn pressure of 2000 psi so perhaps turn at 2100 psi).  Let’s be honest, we aren’t really going to get that much further in for another 5 bar and we can always come back another day with more gas!

Think too about where you might turn the dive.  Right now might be a good time because if you swim another minute or so you and the team could be in a smaller cave passage making turning more difficult and if you keep going to where it opens out again it is almost certain you will have gone past your gas turn pressure.

So, rather than diving to the limit of the rule of thirds, we should try to dive within it.

Perhaps now would be a good time to have a look at how we go about calculating our turn pressure based on the rule of thirds.

Let’s say we have a starting pressure of 210 bar.  That one is nice and easy, 210 divided by 3 is 70 bar, so each third is 70 bar.  70 bar from our start pressure would mean the turn pressure is 140 bar.

(A simple imperial example would be a starting pressure of 3000psi.  Again, nice and easy, 3000 divided by 3 is 1000psi, so each third is 1000psi.  1000psi from our start pressure would mean the turn pressure is 2000psi).

What if the stat pressure is not a nice easy number to divide by 3?  I know a few divers who can seemingly instantly divide almost any starting pressure by 3 and calculate their turn pressure.  I have also seen diver then use this very precise pressure during the dive when they have digital readings or pressure.  However I have also noticed how this seems to entice those divers to go right to the limit of their gas.

Simpler and safer would be to err on the side of caution and make the maths easier.  Just for fun I will look at a couple of examples.

A starting pressure of 220 bar is not easy to divide by three in your head whilst floating in the head pool of a cave discussing the dive plan and turn pressures.  Far easier is to round the number down to the next lowest number that is easily divisible by 3.  In this case that would be 210 bar.  The usual turn pressure for 210 bar would be 140 bar (210 – 70) but in this case we are starting with 10 bar more, so very simply, calculate how big 1/3 is from our rounded down pressure (70 bar here) and subtract that from your actual starting pressure. 220 bar – 70 bar would give a turn pressure of 150 bar.  70 bar in and 70 bar out would leave the ‘largest 1/3’ as the reserve, 80 bar.

(Imperial – 3400psi start pressure, round down to 3300psi.  3300 divided by 3 is 1100.  1100 psi subtracted from your actual start pressure of 3400psi would give a turn pressure of 2300psi)

What about 200 bar start pressure.  Well 195 bar is easily divisible by 3.  Each 1/3 would be 65 bar.  65 bar subtracted from your actual start pressure would give a turn pressure of 135 bar.

For some that calculation might not be so easy but there is no reason why you can’t write a list of start pressures and turn pressures in your wet-notes so you don’t have to worry that your maths isn’t so good or that you might get it wrong.  

When you have calculated or looked up your turn pressure don’t just tell the team what your turn pressure is, tell them how much gas you have and then what your turn pressure will be.  That way you get a few other brains to check that you have got it right.  After all, it is their safety too that could be jeopardised if someone gets it all wrong.

Remember too that for wreck penetration dives, while still using the same approach with the rule of thirds, it is applied a little differently.  We have to take in to account the fact that having exited the wreck, in most cases, we still have an ascent to make and possibly decompression too.  The same might apply to a very deep cave dive. Perhaps that will be for a another post in the future.

What else can we do to to add conservatism and safety to how we use this rule?

There is something called gas matching which we can apply to make sure we don’t get caught short on our gas supplies.

How, when and where to gas match will be up soon.