Rule of thirds in open water?

[Campana]

I'm taking a trimix class where we have to manually plan deep, multigas dives. When calculating gasses needed, and if using the rule of thirds, the Ft3 needed add up to some very, very large numbers.

I'm curious; you open water tech divers, do you typically use a rule of thirds? Do you calculate thirds from descent until alloted bottom time is used up? In other words, do you plan on having three times the gas needed to complete bottom time and reserve one third for ascent and one third for emergencies? This seems conservative, but I am naive in the ways of non cave divers.

I'm not taking into consideration any deco gasses you carry, just your back gas.

Or, do you use some more "liberal" rule like halves?

After all, I don't have any 250 ft3 tanks.......

 

[Uncle Pug]

If we must come back to the upline/anchor line to start our ascent we subtract the amount of gas it will take for two divers to make the ascent (or the ascent to the first deco stop) from the gas supply before we figure thirds.

If we do not need but would prefer to come back to the upline/anchor line then we subtract the ascent gas and figure halves.

If we do not need nor prefer to come back to the entry point (drift dive with live boat p/u) then we back out the gas needed for the ascent and dive to that.

 

[Norda]

UP,

Here are the rules for gas management as I think you and I discussed for an open water environment:

If no ascent constraints: Rock Bottom only

If ascent constraints (identified 3 conditions as follows):

1) Directional Constraints (of any kind): Apply Rule of Halves.

2) Mandatory Egress at Point of Entry: Apply Rule of Thirds.

3) Different Egress than Point of Entry: Apply Rule of Halves + Contingency (contingency depends on situation/conditions).

Calculations as follows:

Rock Bottom: Gas needed for you and your OOA buddy to make it to the surface doing all stops and with safe ascent rate. Based on a SAC of 1 CF per minute per diver and planned depth.

Rule of Halves: (Cylinder CF - Rock Bottom CF) / 2.

Rule of Thirds: (Cylinder CF - Rock Bottom CF) / 3.

 

[padiscubapro]

Campana once bubbled...
I'm taking a trimix class where we have to manually plan deep, multigas dives. When calculating gasses needed, and if using the rule of thirds, the Ft3 needed add up to some very, very large numbers.

I'm curious; you open water tech divers, do you typically use a rule of thirds? Do you calculate thirds from descent until alloted bottom time is used up? In other words, do you plan on having three times the gas needed to complete bottom time and reserve one third for ascent and one third for emergencies? This seems conservative, but I am naive in the ways of non cave divers.

I'm not taking into consideration any deco gasses you carry, just your back gas.

Or, do you use some more "liberal" rule like halves?

After all, I don't have any 250 ft3 tanks.......

You don't understand the rule of thirds..

the rule of thirds is, 1/3 of gas out, 1/3 of gas back, 1/3 for your buddy...

at worse case (not allowing for lost deco gas)
you decend on your bottom gas, when you get to 1/3 your supply you turn around... you head back to you r starting point and begin your ascent.. you should actually have a more than 1/3 left since you will be switching to a deco gas.

enormous supplies are needed unless you are a hoover.. twin 125s will cover very deep dives for most divers.. If I dive oc my twin 95s are more than enough for 250ft plus dives... being concervative a set of twin 125s can get me 30 minutes plus more than 1/3 reserves, overfilling a set of 95s to about 3200, will cover the gas.. If I'm in warm water and in wet suit, I'll carry twin 80s and a travel bottle to save my bottom gas (plus of course required deco gas).. My travel mix will depend on what depth I plan to go to.. but 32% down to 130 gives me at least 5-10 mins extra on bottom (for 200-250) without violating rule of thirds

Calculating how much gas you need is simply figuring how much YOU need for the anticipated run time then multiply it by 1.5..

Even rule of thirds may not be concervative enough if the buddy team has vastly differend RMV rates and tank sizes.. In this event volumes must be matched..
This should be covered by your instructor..

Personally I now usually dive a CCR so I just need to carry enough bailout gas... bottom times of an hour are easily done at 200-250 fsw...

to do a simple 200fsw for 30 minutes I only need about 40cf of 50/50 bailout and 20 cuft of bottom gas (assuming total loop loss). This assumes I'm not in a HARD overhead..

 

[Divesherpa]

padiscubapro once bubbled...

You don't understand the rule of thirds..

the rule of thirds is, 1/3 of gas out, 1/3 of gas back, 1/3 for your buddy...

In a cave environment, this method won't work if you have a deco gas loss. The rule of 1/3's works on the premise of no deco obligation or no lost deco gas. Cave trimix and OW trimix are not the same animal

 

[padiscubapro]

Divesherpa once bubbled...


In a cave environment, this method won't work if you have a deco gas loss. The rule of 1/3's works on the premise of no deco obligation or no lost deco gas. Cave trimix and OW trimix are not the same animal

as I said earlier.. worse case, not accounting for lost deco gas ect.. some contingency has to be done.. besides deco on a trimix backgas is pretty impractical anyway, there is no way you could carry enough gas for many of the deco algs out there with a hypoxic mix. Its more practical for the dive team to each carry extra deco gas and have enough bottom gas to get you to support..

 

[DIR Tec Diver]

The rule of 1/3 does most certainly apply to cave diving, this is where the rule of 1/3 originated from. Remember Shek Exley?

1/3 gas usage penetration, 1/3 out, 1/3 contingency. If you lose your deco bottles (how would you do this if you know what you are doing, especially if they are attached to you) there should be no problem, but that is why for all stage/deco bottles you apply the rule of 1/3s as well. Your buddy has enough gas for your deco. If you don't plan it this way, you are asking for trouble. If you use your entire bottle for deco, then you made a huge mistake. If you read the rule book, you should carry another stage/deco bottle if you will comprimise the rule of 1/3rds

 

[Campana]

DIR Tech Diver is correct. In a cave, we apply thirds to all bottles. If using a stage, then we use 1/3 of the stage and then go on back gas. When 1/3 of the back gas is used, we turn the dive. Picking up the stage on the way out, we go back on the stage and breath the second 1/3. That means we breathed 2/3 of the stage, 1/3 in and 1/3 out, and 2/3 of the back gas, 1/3 in and 1/3 out. 1/3 of each gas supply is for contingencies.

What I didn't understand is how this applies in open water. I believe now that I understand that 1/3 is used for descent and swimming around, 1/3 is used to get back to the ascent point and ascending.

I am not considering any deco gases, as they are, of course, used for deco and must be planned separately.

Padiscubapro also appears to make a valid point, in that since you will go on deco gas at some point, you won't be using back gas all the way to the top, you should have more than 1/3 left of the back gas.

Uncle Pug seems to be right because, well, he's Uncle Pug.

Thank you all for this glimmer of enlightenment to my feeble existance.

Dave

 

[Divesherpa]

DIR Tec Diver once bubbled...
The rule of 1/3 does most certainly apply to cave diving, this is where the rule of 1/3 originated from. Remember Shek Exley?

1/3 gas usage penetration, 1/3 out, 1/3 contingency. If you lose your deco bottles (how would you do this if you know what you are doing, especially if they are attached to you) there should be no problem, but that is why for all stage/deco bottles you apply the rule of 1/3s as well. Your buddy has enough gas for your deco. If you don't plan it this way, you are asking for trouble. If you use your entire bottle for deco, then you made a huge mistake. If you read the rule book, you should carry another stage/deco bottle if you will comprimise the rule of 1/3rds

How are the deep caves in Finland? I would love some pictures.