“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 may hear when talking about rule of thirds.
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.
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.
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 bouncing around on choppy seas 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.
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.
At Davy Jones Tech all this information and more is covered during training, so you fully understand and you can put it into practice.
Contact us now if you want to further develop and expand your diving.