I'm reading an old cave diving pamphlet by Sheck Exley. At one point it says to mark your SPG with a grease pencil for your rule of thirds. It got me thinking, do any dive computers have this type of function available where you could manually set an alarm (audible and or visual) to alert you. While I understand that if you can't do something like this in your head, you probably shouldn't be cave diving, it wouldn't be a bad piece of backup insurance.
Do any computers have a 1/3 rule function?
- Thread starter Malpaso
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Very little cave/technical diving is done with computers that monitor your air consumption.
Basking Ridge Diver
Contributor
One more thing to turn off on my PDC to save battery life. 
dallaskincaid
Contributor
You can certainly get dive computers that will alert you at a preset amount of air. However, as another commenter observed, it can't plan for your SAC rate.
scubastingray
Contributor
I don't think anyone commented that....
I assume OP is talking about for technical/overhead diving as that is one of the few places the rule of thirds is appropriate. In that case, you better be able to track your air. Counting on an alarm is silly; you need to be self-aware. What happens if you don't hear it or it dies? "Oh it will warn me, I can keep going!"
That and most tech configurations use a bare SPG, and no AI.
AfterDark
Contributor
Been diving that configuration for >30 years! That was before tech diving had a name think.
The Atomic Cobalt also lets you set an low-gas alarm pressure and knows the cylinder size.
I assume that any air integrated computer that can set off an alarm at a preset level can be preset to halves, thirds, or any fraction you want. As tursiops said, it is not common practice in cave diving because cave divers rarely use air integrated computers.
You should also understand that the rule of thirds is really more of a general category for planning and is not absolute. Most people use it as a maximum limit for an ideal situation--going into flow on the way in and with the flow on the way out. If the situation is not idea--as in entering with the flow and exiting against it, you need to make a decision on a turning point well before you reach thirds. I read a story about a guy who believed that going with the flow going in allowed him to go beyond thirds, since he would be exiting with the flow and would need less air. It is one of the stories in the book Diver Down. Unfortunately, he was unable to tell anyone why that didn't work on his last dive.
You should also understand that the rule of thirds is really more of a general category for planning and is not absolute. Most people use it as a maximum limit for an ideal situation--going into flow on the way in and with the flow on the way out. If the situation is not idea--as in entering with the flow and exiting against it, you need to make a decision on a turning point well before you reach thirds. I read a story about a guy who believed that going with the flow going in allowed him to go beyond thirds, since he would be exiting with the flow and would need less air. It is one of the stories in the book Diver Down. Unfortunately, he was unable to tell anyone why that didn't work on his last dive.
Very few divers use or understand the Rule of Thirds in what has become known as recreational diving. And as well, it cannot be universally applied, for examples:
1. A shore dive, 1/3 out, 1/3 back, one third still in the tank back on the beach. Typical Rule of Thirds scenario.
But;
2. A drift dive from a charter boat in WPB, the boat is staying nearby, the group(s) or diver(s) go with the current, there is no out and back and there really is not needed to crawl up the ladder with 1000 psi still in your tank, that would be awfully limiting.
BTW, technical diving is generally not computer based and has to accommodate overhead (soft or hard) and in the example you gave, a cave dive, there is an out and back or in and out and there is overhead in the form of rock and possibly soft as well with deco obligations.
The little SDI Solo course I just finished was all about the Rule of Thirds but the instructor sort of diminished it's application because the scenario we planned was a set of drift dives where again, even solo, having 1000 in the tanks (assuming 3,000 psi tanks) and a pony bottle of 19 to 30 cf, is a little overkill and I agreed.
Another application of the Rule of Thirds concerns the pony. I was quizzed on why I chose a 19 cf bottle instead of the 30 cf. I explained it as the result of application of the Rule of Thirds. I typically perform most of my solo dives as beach/shore dives, and my most common and favorite tanks are either an aluminum 63 or a steel 72 (about 66 cf at 2,250). I like the small tanks for their light weight and small dimensions and it is enough for me to do hour long dives at 60 feet even following the Rule of Thirds (or close enough). Therefore 1/3 of 63 cf is roughly 20 cf or closer to 19 than 30 so my pony would have the same amount of gas as my back tank would have had, even if a LP event lost all of my gas. If it was enough to begin with, it is enough in a pony. In other words, I size my redundant air supply to my main by closest capacity to 1/3 of the back gas. The exception is twin ponies, back mounted, lol, i.e. independent doubles. But here, independent doubles, the second tank is not sacrosanct but is utilized as primary gas, a true pony tank is NEVER used except in an emergency.
N
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