Rule of Thirds February 1, 2019 by lukesheppard

I was once diving on Elphinstone in the Red Sea, it was a pleasant dive, nice and warm so I was only wearing a Rash Vest. I’d been down with my buddy around 50 minutes and we were winding down looking to make a final turn and head back to the boat. I crossed paths with a Dive Master from my Dive Centre at the time who was heading back to the boat, he glanced down to his gauges, looked at me and put a single finger up. He had only 10bar left.

This was a few years back and sticks in my head as an example of the complacency of divers and trust in their SPG. Recreational divers are taught to be at the surface with 50bar left in their cylinders and while I used to teach all of my students why, I figured maybe some Instructors don’t, and even if they do I hear and read some divers saying “well I like to use all of my tank, otherwise it goes to waste.”

In my career I’ve seen a few out of air situations, while some have been due to equipment malfunctions, most have actually been due to diver error. I remember being on a RIB in the Red Sea when a diver surfaced and panicked, shouting at the boat hand to help him because he couldn’t inflate his BCD. He had used up all his air and forgot to orally inflate his BCD. If he had planned his dive better and ascended a few minutes before he would have been fine.

50 Bar Rule

The 50Bar Rule, or 500psi for the imperial divers out there, is drilled into you in most diver courses and there’s a couple reasons for it.  On most SPGs you’ll see a red section below 50Bar to alert you visually when you’re getting close. Like all analogue gauges though; they aren’t perfect. Analogue gauges have a margin of error.

Analogue gauges often have a tolerance of ±10%. So while your gauge might read 50bar, you could be on 45.  If you’re relying on the accuracy on your gauge 100%, you may want to rethink that. Mechanical needles may stick, the bourdon tube may not be 100% accurate so I’d rather err on the side of caution and assume it’s 10% lower than what it says.

50bar is not an arbitrary number though, it was worked out that a pair of divers could last 5 minutes at 3 meters on around 50bar. That way, should the worst happen, you and your buddy can perform a safety stop and ascend on a single cylinder.

You have to remember that the last 50Bar isn’t actually for you alone, it’s there if anything should happen to your buddy and you both need to make a stop on the way up. If you only have 10 Bar left then you’re both going to be making an emergency ascent. Oh and remember that Dive Computers aren’t 100% tailored to your body’s decompression, they’re just a conservative guide so I wouldn’t like to push the limit and ascend any faster than it recommends.

Rule of Thirds

Cave Divers use a different Rule and that is the Rule of Thirds. 1/3 of the cylinder pressure to go out, 1/3 to get back to the exit and 1/3 as redundancy should something happen. They’re not thinking about the dive time they’re missing out on, they’re thinking about safety and everybody coming back. If they want to venture further then they take more gas.

Say you start your dive with 230Bar. You’ll want to turn the dive at around 155Bar and exit at 75Bar. Take into consideration any current, your buddies SAC Rate and anything else you can think of that may affect your dive. More on SAC Rates at the bottom.

This is mainly for overhead environments like caves and wrecks but it’s worth having this mentality for all of your diving. I’d rather have a shorter dive and make it back safely than push the limit too far.

Final Thoughts

Most divers use a single 12L tank but of course it’s worth considering the volume of your cylinders. 50Bar in a small 7L cylinder is almost half that of a big 15L.  If you’re diving with a gas guzzler then you may want to increase that 50Bar because nobody will deal with an out of air situation as calmly as they do in their Open Water Course.

Both of your breathing rates will be higher than normal. Bear in mind that even if you’re a calm, experienced diver looking after another diver who can’t control their buoyancy because they don’t have air in their cylinder can be strenuous so a little extra gas in the tank can be beneficial.

Diving on twin 12s I know that if I reach 50Bar I still have twice as much gas as a diver with a single 12. Now that doesn’t mean I can wait until 25Bar before ending the dive, I just have more redundancy, just in case.

It’s important to work out your SAC Rate and improve it as much as you can. Your SAC Rate is your Surface Air Consumption Rate and basically tells you how much air you’re breathing at sea level. A calm diver in easy conditions will have a low SAC Rate and make their air last longer. Working hard by swimming against current or due to drag from too much equipment will increase your SAC Rate, using your air faster.

Wireless Air Transmitters are useful for working out your SAC Rate, many work out your SAC Rate at each part of the dive for you that you can study after the dive. Another benefit of Transmitters is that they are always looking at your tank pressure and you can set alarms on most computers to let you know when you reach a certain reading. This doesn’t mean you can stop looking at your gauges but you can get distracted from time-to-time and it will let you know if you reach 50Bar.

On that note, check your gauges often. All manner of things can happen during a dive without your knowledge and if you don’t keep an eye on your gauges you might not notice a sudden increase in your breathing rate due to a leak or malfunction.  By checking your gauges every few minutes you can plan and adapt your dive profile accordingly. Going back to Transmitters most computers will have some kind of Air Time Remaining function where it will work out based on your SAC Rate and remaining gas how much time you have remaining in minutes so you get a better idea of how long your air will last at the current depth.

Always plan your dive with the end in mind, not just getting to the interesting part of the wreck but getting back to the surface if the worst should happen. If your plan doesn’t add up so you get to the good bit and back to the surface with enough redundancy, then you can’t do the dive, it’s just not worth it.