How to Properly Apply The Rule of Third's

[Kevrumbo]

Here's a comprehensive Open Circuit gas plan/turn pressure exercise for a planned wreck penetration at 100fsw depth (4ATA) for 30 to 40 minutes max bottom time on Nitrox32 and O2 deco. In this case, the plan is complicated by dissimilar tank sizes and unequal breathing rates between the Buddy Pair.

Given Diver A:
AL80 twinset (total 154 cf); 3000 psi fill; nominal SCR (Surface Consumption Rate; also known as SAC rate/RMV) of 0.6 cf/min;

Diver B:
HP100 twinset (total 200 cf); 3600 psi fill; nominal SCR of 0.8 cf/min.

Note: You cannot apply Rule of One-Third's at this point because of unequal SAC Rates and dissimilar tanks.

To see this, take one-third of Diver A's total volume of 154cf and one-third of Diver B's total volume of 200cf, which are 51cf and 66cf respectively.

Diver A consumes 51cf in 21min (51cf divided-by 0.6cf/min, divided by 4ata is 21 minutes); Diver B consumes 66cf in the same 21min (51cf divided-by 0.8cf/min divided by 4ata is 20.8 minutes).

They both need a combined sum of 51cf plus 66cf equals 117cf total needed to exit in an emergency gas sharing contingency at the turnaround: Check -->Does Diver A with the lesser total amount of available gas have enough usable reserve volume to get him and Diver B out of the wreck?

Diver A: 154cf available minus 51cf consumed equals 103cf (117cf is needed -No, they will both be out-of-gas inside the wreck). . .

 

[Kevrumbo]

(Cont.)

Given Diver A:
AL80 twinset (total 154 cf); 3000 psi fill; nominal SCR of 0.6 cf/min;

Diver B:
HP100 twinset (total 200 cf); 3600 psi fill; nominal SCR of 0.8 cf/min.

Solution:
First start with Minimum Gas Reserve (MGR) Volume -a "Rock Bottom" Calculation Sum from 100fsw or 4ATA depth- (assume emergency stressed 1 cf/min SCR for both divers with a 30ft/min ascent rate), to reach an Open Water O2 Deco Stop at 20fsw, after exiting wreck:

4.0 ATA x 2 min x 1 cf/min = 8 cf. [Two minutes to retrieve O2 deco bottles];
3.7 ATA x 0.3 min x 1 cf/min= 1.1 cf
3.4 ATA x 0.3 min x 1 cf/min = 1.0 cf
3.1 ATA x 0.3 min x 1 cf/min = .93 cf
2.8 ATA x 0.3 min x 1 cf/min= .84 cf
2.5 ATA x 0.3 min x 1 cf/min = .75 cf
2.2 ATA x 0.3 min x 1 cf/min = .66 cf
1.9 ATA x 0.3 min x 1 cf/min= .57 cf
1.6 ATA x 2 min x 1 cf/min= 3.2 cf. [Two minutes to switch to O2 Deco bottles].

Total cubic feet: 17 cf for one diver.

Total MGR backgas volume for 2 Divers: 17 x 2 = 34 cf;

Diver A Pressure MGR reading: 34 cf divided-by 154 cf multiplied by 3000 psi = 662 psi; Round this up to and use 700 psi actual reading instead on the SPG for conservatism.

Diver B Pressure MGR reading: 34 cf divided-by 200 cf multiplied by 3600 psi = 612 psi;
Round this up to and use 700 psi actual reading instead on the SPG for conservatism.

 

[Kevrumbo]

(Cont.)

Let's assume Diver A consumes 200psi and Diver B consumes 300psi already just getting to the entrance of the wreck. Available remaining gas for Diver A is 3000psi full fill minus 200psi equals 2800psi; and Diver B is 3600psi minus 300psi equals 3300psi.

So Diver A Usable Gas: 2800psi available minus 700psi MGR equals 2100 psi. So 2100 psi x (154cf/3000psi) = 107 cf usable;

Diver B Usable Gas: 3300psi - 700psi = 2600 psi. So 2600 psi x (200cf/3600psi) = 144 cf usable.

Now Apply Proper Limiting Rule of Thirds:

Tank Match Starting Reference is to Diver A since he has the lowest and limiting amount of usable gas (107 cf). One-Third of 107 cf is 35 cf which divided by his tank factor of 154cf/3000psi is 681psi for an actual SPG Turn Pressure reading of 2119 psi for Diver A (2800 - 681 = 2119 psi).

Calculate Diver B turn pressure: now apply limiting Third's turn volume of 35 cf to Diver B, divided-by his tank factor, of 200cf/3600psi. So (35/200) x 3600 = 630psi. Actual SPG Turn Pressure reading for Diver B is 3300 - 630 = 2670 psi.

 

[Kevrumbo]

(Cont.)

Diver B will reach the turn volume of 35cf in 11min; (35cf divided-by 0.8cf/min Sac Rate, and then divided-by 4ata equals 11 minutes.). Diver A in that 11min will only use 26cf (11min x 0.6cf/min Sac Rate, and then multiplied by 4ata = 26cf).

Another way of figuring this: Because of having a lesser SCR that is 75% of Diver B, Diver A uses only three-quarters (0.6/0.8 which is the same as 3/4) of his allotted 35cf turn volume in 11min, which is 26cf.

They both need a combined sum of 35cf plus 26cf equals 61cf total needed to exit in an emergency gas sharing contingency at the turnaround: Check -->Do they have enough usable turnaround reserve gas volume?

Diver A: 107cf usable minus 26cf consumed equals 81cf available for an emergency gas share (61cf is needed, check -yes).

Diver B: 144cf usable minus 35cf consumed equals 109cf available for an emergency gas share (61cf is needed, check -yes).

The point is, to properly apply the Rule of Third's, you have to take one-third of the diver with the lowest volume of usable gas, and use that amount as the Team Turn Volume. The teammate with the highest SAC Rate will reach this Turn Volume first and calls for the turnaround.