Rule of Thirds & Shallow Rec diving

[boulderjohn]

It really isn't all that useful to tell someone a general rule to apply to all dives. "Turning a dive" usually implies heading in one direction for a while and then turning around and coming back the way you came. that is different from a dive to a site (say in Key Largo or on a wreck) where you may swim around for a while and then ascend. It is different from a drift dive in which you head in one direction for a while and then ascend. In all three, you have a minimum gas reserve, but what you do before that is what differs.

In general, there are three types of plans, and what you do in terms of gas planning is different for each. Here is a simplified explanation:

1. All usable: I can use all but the gas needed for the reserve before I ascend.
2. Rule of halves: I am heading one direction for a while, turning the dive, and heading back. If something goes wrong and I don't get back, it's not the best, but it's still OK.
3. Rule of thirds: I will head out and back, but it is really imperative that I get back to the starting point before my ascent.

 

[sheeper]

for a shallow rec dive, I dive my plan (as I do for EVERY dive!). my shallow plan does not change. for a shallow (20 foot) beach dive i'll plan on coming back with 200psi. Generally I end up with more. for a shallow (50 foot or so) boat dive with all things being equal (nice day, minimal current, etc), I plan on being on the surface with 500psi and generally end up with more.

 

[mala]

most pressure gauges have a red bit.
its not for fashion its a hint as to whats good and whats not.

the only thing worse than having a needle in the red is having your only needle in the red.

if you want to dive longer then either get better on air or take down more gas.

 

[TSandM]

Using rule of thirds on shallow recreational dives is like pounding a finishing nail with a sledge. It'll get the job done, but it's a very crude method.

There are tools to allow the calculation of reasonable gas reserves, using the dive analysis that boulderjohn describes above. Basically, you always want enough gas to get you and a buddy to the surface, doing what you consider to be an acceptable ascent in the event of a gas emergency. If you are likely to encounter difficult surface conditions, you might want to reserve more, in case you have to do some surface swimming, or the water is very rough. (I am NOT a believer in the utility of a snorkel in rough water on the surface.)

I'd rather see people taught rule of thirds for recreational diving than see them not taught ANY concept of gas reserve. But it's not that hard to learn a more precise way to figure what you need and plan your dive a little more reasonably.

 

[phoenix31tt]

I use rock bottom as i find it's more suitable to rec diving and can be easily adjusted according to depth etc.

the only time i deviate from rb is if the guide has a higher reserve

 

[Jerry_]

also i would consider the conditions too... strong current, rough seas, -- i'd turn around with 1000 or so left -- otherwise i'd do 500 or whatever the min dictated by the boat since some have rules on that. but isn't psi a poor indicator of how much cu ft of gas you have left since it's dependent on tank size?

For gauges marked in Bar I have always seen the red zone starting at 50Bar (i.e. 725psi), i.e. lower than the 1000psi mentioned here

That being mentioned, the point is to plan to get out of the water with sufficient backup gas for a problem, including gas sharing with an excited buddy and still making it safe to the surface. Usually, in recreational dives, as mentioned by the OP, this is when the needle starts getting into the red.

However I also share the point of iluvtheocean: if in a single group the type of bottle sizes varies a lot then you might want to allow for a lower psi for the one with a large size tank and be more restrictive on users of small size tanks - i.e. the content would be a more accurate measurement then the pressure. However I do think that this rarely applies for groups of recreational divers, as most of them would rent (single sized) bottles, or have followed a recommendation for a standard size tank when buying.

 

[scubastingray]

I turn the dive at 1/2 plus 200. It also depends on the site and what I am doing. I dont think there is a rule that works great for every situation.

Ah okay. I missed the turning the dive part. I thought you were climbing the ladder with 1700 psi.

I guess the type of diving i have in mind isn't point a->b turn around then get on the boat. A lot of the rec sites are more of "let's explore that way" and you don't always travel linearly. For instance, I've been trying to create maps of the sites I'm planning based off of online descriptions and coordinates but I'm not looking to swim a fixed pattern. If i see something sweet underwater you can bet my buddy and I will be checking it out!

It seems logical that in calm weather you can make the swim back on the surface, but half of your air to get back after swimming in an S seems overly cautious also (not that the goal shouldn't be safety and caution in diving).

 

[iamrushman]

i never let my tank get lower than 500# for fear of getting sea water in my tank...

 

[boulderjohn]

i never let my tank get lower than 500# for fear of getting sea water in my tank...

Well, you are certainly safe in that regard. You would also be pretty safe in that regard if you got the tank down near zero, despite what some shops will tell you when they charge you for an unnecessary visual inspection because you brought the tank back with a low PSI.

 

[scubastingray]

i never let my tank get lower than 500# for fear of getting sea water in my tank...

That might be a bit much for that reason alone, but it's probably good to have at least that amount for cleaning gear later, blowing off the reg as you disconnect it, etc.

I have a much better consumption rate than my buddy, so I'll probably end up somewhere in the 1000# range when she hits 500.