The Rule of Thirds.

[Aquamaniac]

I can think of a few.

1. REAL heavy surface currents and surge.

2. You have SO many crayfish in your catchbag that your going to need 1/3 to fill the lift bags to get em up.


3. Alot of boating Traffic in the general area, remember you flag is on the BOAT. (reason for anchor return , not thirds)

Dave

 

[AquaTec]

staying with a recreational frame of mind. i cant think of any dive that requires a rule of thirds, or especialy greater.

i use the old surface with 500 psi.

so on a simple out and back dive i would subtract 500 psi then use half of whats left out and the other half back,surfacing with that 500 psi

this may be an outdated principle, but i am an old man, and maybe slightly outdated.

 

[Aquamaniac]

Added to my last posts.
There are also some "rec"sites were we dive on fringe and barrier reef, the suface has heavy surf. You need to anchor a fair way off, and aproach the wall from the bottom. We try to dive "thirds" in that location, because a premature surface can be real ugly.

Dave

 

[Uncle Pug]

AquaTec once bubbled...
staying with a recreational frame of mind. i cant think of any dive that requires a rule of thirds, or especialy greater.

Dave mentioned some of them... dives where it is important to surface at the point of entry.

Should an NDL open water OOA occur at the furthest point the option to ascend directly exists but could prove dangerous.

If the rule of thirds plus rock bottom has been followed there is nothing to prevent the safe movement of the buddy team back to the exit point while air sharing.

The idea that an ascent must be immediately initiated just because one diver is OOA is commonly taught... but I am suggesting that it is not necessarily the best course of action if ascent would put the divers in harms way.

Proper planning and the use of Rock Bottom gas reserve with the rule of thirds gives the divers the ability to egress to the point of safe ascent.

This can only be done if Rock Bottom is held in reserve.... just the rule of thirds will not accomplish this.

Doug, semantics aside, surely there are *recreational* dives that you can imagine that would call for exit at point of entry as the best course of action.

Dave has already made some good suggestions... I will add one more... a current dive in Deception Pass.

Having enough gas for the trip in and enough for both divers’ trip back out plus the gas necessary to ascend safely is good planning.

Half in and half back with just 500 for ascent does not make for happy campers if things go sidewise. Not only does one have to contend with boat traffic... the walls on the inside are shear and prevent egress.

Another thing I would like to address... 500 psi is not enough for the average rec. diver and buddy to make a safe ascent from all depths. What amount would you figure for rock bottom from 100'?

 

[detroit diver]

9 cuft/person

1 to sort things out at 100 ft.
1 for ascent to 70 ft
1 for 1 min at 70 ft
1 for 1 min at 60 ft
1 for 1 min at 50 ft
1 for 1 min at 40 ft
1 for 1 min at 30 ft
1 for 1 min at 20 ft
1 for ascent to surface

 

[Divesherpa]

Very nicely stated, Pug.
The rule of 1/3's is not advocable on 30 foot reef dives. Nor is the rule of 1/3's necessary for "non-deco" single tank diving in most scenarios. Remember OW guys that this stuff is great fun. Don't read too much into it. You don't have to dive like you were planning a 15,000 foot cave penetration. Oh, wait, I accidentally bashed DIR.

Sorry Pug, I can't help myself.
Cheers

 

[O-ring]

a current dive in Deception Pass

Isn't this where they do OW checkout dives? I heard the water is like glass and the vis is upwards of 100'..
:hiding:

 

[Uncle Pug]

Divesherpa once bubbled...
Sorry Pug, I can't help myself.

I use that excuse myself...

 

[Uncle Pug]

Even though there are some ow rec. dives where it is prudent to take enough gas for thirds plus rock bottom....

It is not prudent to take more than your plan calls for just because you read in a book or on the internet that cavers use the rule of thirds.

If your OW dive really calls for the rule of thirds then it probably calls for more.... the ascent gas as well (rock bottom.)

Now what is really needed for most recreational dives?

The rule of halves if coming back to the point of entry is desirable.
OR
Just Rock Bottom if ascent can be made anywhere, such as during a drift dive using a live boat.

But even in the above cases ascent gas adequate for two divers must be held in reserve. The old standard of reserving 500psi or even 700psi may or may not be enough.

For an NDL dive you should either spend 3 minutes at a 15' safety stop or better yet 1 minute each at 30', 20', 10'.... with that in mind you should calculate the gas you need for ascent from the deepest planned portion of the dive using a realistic SAC and then double that amount (for your buddy.)

We have several standard Rock Bottoms to cover a range of planned depths. 500psi is one of them.... but we also have others... this way we don't have to actually figure it at the beginning of each dive... and don't figure it out exactly to something like 728 psi... always round up to the nearest 100 psi.

Now if there are other factors that call for the addition of extra gas for a safety margin... by all means add that in... but it should be for a reason!

 

[Uncle Pug]

detroit diver once bubbled...
9 cuft/person
1 to sort things out at 100 ft.
1 for ascent to 70 ft
1 for 1 min at 70 ft
1 for 1 min at 60 ft
1 for 1 min at 50 ft
1 for 1 min at 40 ft
1 for 1 min at 30 ft
1 for 1 min at 20 ft
1 for ascent to surface

You make a good point DD that the Rock Bottom needs to be figured in cf but then that needs to be converted to psi for actual use specific to whatever tank size you are using.

Questions:
Why stops at 70,60,50,40 for an NDL recreational dive?
Why the same amount of gas used at 70' and 20'?
How are you figuring the cf required?