diving thirds vs rock bottom

[Kevrumbo]

all,
i want to thank everyone for contributing to this thread.
some very good information was shared by different divers who all dive looking thru different windows.
it seems that there is more than one way to skin the ole cat.
it seems most divers want to have fun but safety is the one thing we all agree. (somewhat)
thanks for y'alls thoughts.

Don't let anyone tell you that these concepts are "too complicated" for the basic scuba forum ((shame on you to those two condescending lazy moderators --that was the real misinformation and disservice coming from a moderator who is an "Educator", and another who was a Math major undergrad, both "DIR" divers).

Rock Bottom & Rule of Thirds are vital concepts and when asked about should be given an honest explanation and discussion (including the arithmetic and algebra involved).

("You teach best what you need most to learn". --Got that Boulderjohn. . . ???)

 

[stevensamler]

I think you're referring to that IANTD SAC Ratio Factor Table, but I'd rather do the full analysis exercise and just use straight thirds of the volume amount of usable gas left after first subtracting out the Rock Bottom Reserve. If dissimilar tanks are used, take one-third of the smaller tank volume after first subtracting out Rock Bottom, and then match that amount to the remaining tanks of the team by their respective metric cylinder ratings. For conservatism, figure out Rock Bottom using the teammate with the higher Surface Consumption Rate (SCR).

Example:
28 litres/min SCR, with 18 meters (2.8 ATA) depth NDL with one minute stops every 3 meters to surface:

2.8 x 28 x 1 = 78.4
2.5 x 28 x 1 = 70
2.2 x 28 x 1 = 61.6
1.9 x 28 x 1 = 53.2
1.6 x 28 x 1 = 44.8
1.3 x 28 x 1 = 36.4
1.0 x 28 x 1 = 28

Sum Total: 372.4 litres gas needed to ascend to surface for an emergency contingency.

Suppose you have two divers with different cylinder ratings:

In a 12 litre/bar tank, 372.4 litres is 31 bar (divide 372.4 by 12); in a 15 litre/bar tank, 372.4 is 24 bar pressure.
For these two divers sharing gas in an out-of-gas contingency, multiply the respective pressure readings by 2 for actual Rock Bottom:

12 litre/bar tank: 62 bar; 15 litre/bar tank: 48 bar.

The 12 litre/bar tank has a starting pressure of 180 bar; subtract out Rock Bottom of 62 bar yields 118 bar usable.
The 15 litre/bar tank has a starting pressure of 150 bar; subtract out Rock Bottom of 48 bar yields 102 bar usable.

Which of the two tanks above has the smaller remaining volume of usable gas left?
12 litre/bar x 118 bar = 1416 litres
15 litre/bar x 102 bar = 1530 litres

So the 12 litre/bar tank is the smaller, hence we apply the Rule-of-Thirds to the usable pressure of 118 bar:
118/3 = 39 bar; so 39 bar delta down from 118 equals an actual turn pressure SPG reading of 79 bar.

39 bar delta in the 12 litre/bar tank equals 468 litres (39 x12 = 468);

Therefore 468 litres divided-by the other cylinder's 15 litre/bar tank rating equals 31 bar;
So 31 bar delta from 102 bar equals an actual turn pressure SPG reading of 71 bar with regards to the 15 litre/bar tank.

See also:
Post #15
http://www.scubaboard.com/forums/ba...ving-thirds-vs-rock-bottom-2.html#post6469083
Post #20
http://www.scubaboard.com/forums/ba...ving-thirds-vs-rock-bottom-2.html#post6469264

Gotta frickin love it , I would dive with Kevrumbo any day.....

 

[lowviz]

Gotta frickin love it , I would dive with Kevrumbo any day.....

Me too.