Rule of Thirds & Shallow Rec diving

[KeithG]

Hey guys,
I'd love to hear you opine about this rule in regards to "vacation divers." If you're under 50 feet do you really want to be getting back on the boat with 1000psi left in your tank?
I understand in tech diving it makes sense as it is an overhead environment, but this seems overly cautious when the surface is a cesa away. I'm not advocating running it empty, but when you dive tropical beginner level dives what psi do you want to return with?

It all depends upon your SAC. Most experienced divers with a low SAC would have a hard time consuming enough air to actually worry about running out when following a "vacation dive" profile and getting back to the boat within the ops time limit.

My dive would be 25 minutes at around 45 feet followed by 35+ minutes above 25 feet. Little current, good visibility and not much swimming makes it real hard to do much damage to the contents of an al80.

All of my "vacation dives" are dive op (or self imposed) time limited. Not gas or NDL limited.


Deeper depths and marathon swims will alter that significantly.

 

[boat sju]

you don't have to recalculate... or at least i don't... i pre calculated a range of depths, they are easy to memorize for your most common depths and easier to put on a slate...

in that way you know your air limit for the depth you are currently at, my table looks like:

Depth	Rock Bottom PSI
<40	500​
<60	800​
<80	1100​
<100	1400​
<110	1700​
<130	2300​

There's a similar rule of thumb mentioned in the RB discussions: (depth x 10) + 300 which pretty much agrees with the above and probably would suit the OP's needs at this point. Also, max depth not to exceed the volume of the tank, i.e. 80 cft tank equals 80 ft max.

 

[Foxfish]

The rule of thirds is really not appropriate to most recreational divers doing the kind of dives you are describing. Yes, you want to have a reserve, but 1,000 PSI is not necessary.

How you determine your needs for the reserve depends upon the kind of diving you will be doing. The rule of thirds is the least used in basic recreational dives.

What John said. Some of the tech divers are out of touch with OW recreational diving. Go with what you learned in your OW course which is to surface with 50 b (converts roughly to 725 psi).

When should you 'turn the dive' or start your ascent? The answer is simply that you should have enough air so that you surface with 50 b. Our dive plan is typically, 'jump in, swim around, get back to the surface with 50 b, hopefully somewhere near the boat'. I'm normally starting to head back to the boat, 'turning the dive', with around 70 b. If I'm a long way for the boat I may turn the dive with 100 b. If I surface with 50 b and I'm a long way from the boat, I'll dive back down to a few metres to avoid surface current and swim back to the boat. I don't see the problem with drawing the air down below 50 b in these circumstances. Assuming a 12 L tank, on shallower dives around say 15 m I normally start my ascent with 60 b. If the dive is deeper, say 30 m, I start with 70 b. If things don't go quite to plan on the ascent you'll draw you air down below 50 b. No problem. That is why you have the reserve. With 50 b in my 15 L tank I've got nearly half an hour of air on a shallow dive.

I've wondered at times whether the 50 b rule is overly conservative particularly in shallower diving. I think it is intentionally so. Last year we had three divers on one weekend over here who ran out of air. I heard they all ended up in the chamber and at least one died.

 

[MXGratefulDiver]

I usually want to get back on the boat with at least 500psi. As others have already stated there are a lot of variables that might cause you to pad that a bit too. Also take into consideration that some of these people do not wear snorkels, because none of the cool kids do, so they may need more air at the surface to swim back to the boat.

I don't normally need either a snorkel or a regulator if I'm swimming on the surface ...

... Bob (Grateful Diver)

 

[TSandM]

Go with what you learned in your OW course which is to surface with 50 b (converts roughly to 725 psi).

A lot of the answers to the OP have to do with ways of figuring out how to end up on the surface with 50 bar (or perhaps a bit more). If you have a plan ahead of time, you're not going to be surprised when you're suddenly much lower on gas than you want to be, and the swim to shore, or the anchor line, or wherever, takes more gas than you thought.

This has NOTHING to do with technical diving, or being out of touch . . . it has to do with giving recreational divers the tools they need to stay out of trouble.

 

[NAM001]

With the volumn of ageincies i, ins ect nvolved in any aspect, it is no wonder that a one rule fits all is used. To the defence of such a rule i will say that 1000 psi when hitting the surface can be a good thing. Floating in line for 20 min on the surface, waiting your turn on the ladder can use a lot of air depending on sea state or the sudden finding of a leaking bc vent valve.

 

[DevonDiver]

Go with what you learned in your OW course which is to surface with 50 b (converts roughly to 725 psi).

The point being: HOW does a diver ensure they can achieve this?

That's a rhetorical question; they can either guess (and often get it wrong), or they can apply some rudimentary gas management and planning.

You attribute rudimentary gas planning and management as some form of 'technical diving over-complication' - and yet it is the ONLY answer to the question asked above.

I have this fun with recreational dive instructors.

I: "Surface with 50bar/500psi"
S: "How?"
I: "Errmmm...."

 

[Captain Bil]

With the volumn of ageincies i, ins ect nvolved in any aspect, it is no wonder that a one rule fits all is used. To the defence of such a rule i will say that 1000 psi when hitting the surface can be a good thing. Floating in line for 20 min on the surface, waiting your turn on the ladder can use a lot of air depending on sea state or the sudden finding of a leaking bc vent valve.

I teach floating in line on the surface is done with a snorkel in your mouth. The regulator is for underwater. A slight leaking BC can be manually inflated. A catastrophically leaking BC should result in ditching weight and the other folks on the line getting out of your way. These are basic skills that should be mastered in the pool. No need for the regulator in any of this. I contend that one should surface with some air in the tank for several other reasons, but none that you mentioned.

Sent from my XT907 using Tapatalk 2

 

[Foxfish]

A lot of the answers to the OP have to do with ways of figuring out how to end up on the surface with 50 bar (or perhaps a bit more). If you have a plan ahead of time, you're not going to be surprised when you're suddenly much lower on gas than you want to be, and the swim to shore, or the anchor line, or wherever, takes more gas than you thought.

This has NOTHING to do with technical diving, or being out of touch . . . it has to do with giving recreational divers the tools they need to stay out of trouble.

Agreed that it is good to plan ahead. My plan was listed in the last response in case you missed it. That is more detailed than most people I've dived with.

There are some very skilful divers on this forum, but sometimes the advice given is out of whack with what you'd expect on a recreational dive. The OP gives one example. If it puts your mind at ease I wasn't thinking of anyone in particular on this thread. Relax. :kiss2:

 

[TSandM]

Foxfish, have you read NW Grateful Diver's article on gas management? Or Lamont Grandquist's? Or my husband's? All of them will give a little more specific tools for figuring out how to manage your gas than, "If I'm a long way from the boat, I may turn the dive on 100 bar."

None of this is "tech diving". It's rational gas management, with a strategy that's appropriate for the kind of dive one is doing.