Deep Dive and rule of thirds

[mikedarmody]

Hypothetical dive plan: Go to 120 feet for wreck photo opportunity. Don't need to stay long at depth. With experienced deep divers. If I follow strict rule of thirds, I was wondering is the first third enough? How could I calculate? What are the parameters? Assume southern california in winter with one AL80 at 3000psi and no reserve.

A modification to plan: Bring reserve air for ascent. What is the best minimal equipment configuration? How much air? What are the parameters to calculate?

 

[mech]

I'm not completely sold on the rule of thirds.
One thing on a deep dive plan is to allow for your buddy as well.
Would your rule of thirds cover that?
Would not a rock bottom time plan cover it better?
Andy

 

[matt_unique]

Hypothetical dive plan: Go to 120 feet for wreck photo opportunity. Don't need to stay long at depth. With experienced deep divers. If I follow strict rule of thirds, I was wondering is the first third enough? How could I calculate? What are the parameters? Assume southern california in winter with one AL80 at 3000psi and no reserve.

A modifiaction to plan: Bring reserve air for ascent. What is the best minimal equipment configuration? How much air? What are the parameters to calculate?

I think the rule of thirds is a good guideline. 1/3 in, 1/3 out, 1/3 left in the tank when you surface unless something comes up. The point is to give you reserve gas in the event of something unexpected happening. Depending upon your air consumption rate (SAC or RMV) you can easily complete a non-deco dive to 120' and surface with more than 1000psi.

If you decide you are prepared to make this dive, make sure you follow strict turn pressure rules and a no-decompression profile. At 1000 psi head back. If the first part of the dive was drifting in current then obviously you need to turn back before 1000psi was reached to adhere to the rule of thirds. There are courses that specifically address breathing rate and deep dive preparation.

--Matt

 

[Uncle Pug]

The rule of thirds isn't for the dive you are suggesting.

If you are making a dive to 120 (no penetration of the wreck) and you can ascend at any time and in any place then you only need concern yourself with having enough gas held in reserve to allow both you and your buddy to make the ascent in a controlled manner with a safety stop of 3 minutes at 15'. It is prudent to calculate this reserve using 1 for your SAC.

If, however, you must make it back to an upline for your ascent then you might want to consider the rule of thirds PLUS the reserve mention above. The first third is for your excursion away from the upline, the second third is for your return to the upline and the last third is for the worse case scenario where you must supply both yourself and your buddy with gas back to the upline. At that point your reserve should be adequate for both you and your buddy to ascend. The rule of thirds without the reserve would not allow for such an ascent in this case.

Another scenario that is quite common is the desirability of returning to the entry point but not the necessity of doing so. In this case a *rule of halves* might be followed in which the reserve amount mentioned in my first paragraph is calculated and then subtracted from the total gas supply. The remaining gas is then divided into halves... the first half for the excursion and the second half for the return. If something goes awry and gas is lost then an ascent is made from that point.

BTW ~ figuring thirds and halves requires that you know your buddy's SAC as well as tank size. You turn at the lowest common denominator.

 

[theskull]

Hypothetical dive plan: Go to 120 feet for wreck photo opportunity. Don't need to stay long at depth. With experienced deep divers. If I follow strict rule of thirds, I was wondering is the first third enough? How could I calculate? What are the parameters? Assume southern california in winter with one AL80 at 3000psi and no reserve.

A modifiaction to plan: Bring reserve air for ascent. What is the best minimal equipment configuration? How much air? What are the parameters to calculate?

Rule of thirds is based on overhead environments--1/3 for first half of dive, 1/3 for second half of dive, 1/3 reserve in case buddy needs to share at furthest point of dive.

A simple deep dive with no penetration and no deco obligation doesn't necessarily have a "second half of dive". You might descend, swim around for awhile within sight or easy navigation of the descent line, and then ascend. Therefore your "reserve 1/3" only needs to be enough to return to the ascent line and ascend while sharing air with your buddy--and this will not necessarily require a true 1/3 of your air supply.

***If you followed that, you can see that dive planning, buddy familiarity, and gas management are all very important to diving safely. Rule of thirds is usually overkill for recreational, but it does make the dive planning easy if you don't mind giving up a few extra minutes of bottom time.***

theskull

 

[mikedarmody]

So, if there is no penetration and I stay close to wreck/line (as theskull suggests) , I could theoretically use the "half tank" of 1500 as a planning number, ie, turn back point. Then the remaining 1500 should be adequate for ascent, 3 min safety stop and (if necessary) share air with buddy on ascent.

btw: I am just starting to learn how to enter a dive plan in my computer, but I started thinking about the scenario. This computer has "simulate" mode so I can "what if" the scenario and then enter as a dive plan. (Well that's my theory, I'm still learning how to use all the features.)

 

[CNTdiver]

Uncle Pug.....I agree with you.....there is no reason for having him do a 1/3 dive when there is no penetration into the wreck. Even though it is deep for a recreational dive, that is not needed. My only words of advice are to make sure you watch your SPG all the time and do it more frequently the deeper you go. But you probably already know this. Happy diving!!!!!

 

[Uncle Pug]

Here is something that you need to consider... actually several things:

1) Do you really need to go all the way to 120' to get your photographs? I would suggest backing that off 10' if possible, especially on air.

2) How much time should you spend taking photographs at 110'~120'? I would suggest limiting it to a few minutes and then start working your way back up shallower, especially on air.

3) Your required reserve (rock bottom) changes as you get shallower.

 

[MXGratefulDiver]

Seems to me that if you're planning a direct ascent to the surface all you'd really need to calculate is your rock bottom ... and begin your ascent when you reach it.

If you don't know how to calculate rock bottom, do a search on the term. There's some excellent info in here. You'll need to know both your SAC rate and your buddy's ...

... Bob (Grateful Diver)

 

[pt40fathoms]

Here's a little point you may not have though of.

At 120 feet to a wreck, diving a profile that is nearly square in descent and ascent, your bottom time may not be governed by how much air you have. Unless you have an extremely high SAC rate, you will most likely run out of NDL time before you reach your turn pressure.