Advanced Gas Planning

[ajduplessis]

^^^^ Nicely articulated. May I use your definition (cited)?

 

[lowviz]

That is not correct.

turn = 1/[(SACa)/(SACa + SACb))+1)] * times the fill.

 

[DevonDiver]

^^^^ Nicely articulated. May I use your definition (cited)?

If you mean my post, of course.

 

[Kevrumbo]

Anybody utilizing the IANTD SRF Table for unequal SAC rates between teammates.?

http://wetlands.simplyaquatics.com/d/14074-1/sac_ratio.pdf

For example, you have a 0.35 cuft/min (9.9 l/min) SAC, your buddy has a 0.7 cuft/min (19.8 l/min) a SAC rate that's twice yours. Look at the table and find where the "0.70" column and the "0.35" row intersect --that number turns out to be "0.75".

You then take that number and multiply it by your Usable Gas Pressure: so if you have 2400psi (160bar) then multiply by 0.75, which equals 1800psi (120bar)-->this value is the SPG indicated Third's Turn Pressure (or Third's delta consumed of 600psi (40bar)).

So for an emergency gas share with your buddy and 1800psi (120bar) in your tank to donate, you'll have 600psi (40bar) plus 1200psi (80bar) for your buddy; or .25 plus .50 equals remaining .75 of Usable Gas.

 

[ajduplessis]

If you mean my post, of course.

Yes, Thank you.

 

[kensuf]

Am I the only one curious about how bad the "worst sac" could be in technical diving, and how different that actually could be from the average?

I mean, in your example, one has almost twice the amount of gas the other one carries... (I'll also add that I expect everyone to carry roughly the same amount of gas, though I've been using 2 al80s when my buddy had a d12 on his back, was close enough)

I have a friend that's 6'1", weighs 250#, and regularly dives with a woman that's 5'3" and 100#. Their SAC rates diverge quite a bit.

 

[Kevrumbo]

]Am I the only one curious about how bad the "worst sac" could be in technical diving, and how different that actually could be from the average?

I mean, in your example, one has almost twice the amount of gas the other one carries... (I'll also add that I expect everyone to carry roughly the same amount of gas, though I've been using 2 al80s when my buddy had a d12 on his back, was close enough)

Well . . .I found a mathematical derivation for the special case of a buddy team in which one teammate has twice the SAC rate of the other, and is a basis for IANTD's SRF Table.

Abridged below --Good luck in implementing it.
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Let's first look again at General Rule of Thirds, which assumes that each teammate will continue at the same rate of breathing and that both have the same SAC rate.

For teammates Diver 1 and Diver 2, let's use two variables D1 and D2 to represent the Volume of gas needed to get to the turn point.

At the turn around point, there has to be D1 + D2 gas in EACH teammates supply. If D1 = D2, then there is D1 + D1 gas in each, and Diver1 & Diver2 would have used D1 amount of gas to get to that point. Hence, total net Usable Gas (i.e. less Min Gas Reserve/Rock Bottom & Tank Matching/Dissimilar Tanks) is equal to D1 + D1 + D1, or UsableGas = 3 * D1. Since the turn around point was when they used D1 of gas, this equals the 1/3 consumed gas amount or third's turn around delta value.

For the case of D1 is NOT equal to D2: There still needs to be D1 + D2 amount of gas remaining each teammates tank at the third's turn around point.

Now volume D1 = SAC1 * Time, and volume D2 = SAC2 * Time.

Note that the ratio of D1/D2 is the same as the ratio of SAC1 / SAC2, so D1/D2 = (SAC1 * time)/(SAC2 * time), "time" cancels out, leaving SAC1/SAC2.

Given unequal SAC rates, assume that the SAC rate of Diver 1 is one-half of Diver 2's (or SAC2 = 2 * SAC1. For example: SAC1 =0.5, SAC2 = 1.0).

Again there needs to be D1 + D2 amount of remaining gas left in the tank for Diver 1 at the turn point.

Substituting 2*D1 for D2, we get D1 + 2*D1 in terms of Diver 1 left in the tank.

Again, to get to third's turn around, Diver 1 has to consume D1 amount of gas.

So summing and then solving for D1:

UsableGas = D1 + 2 * D1 + D1
UsableGas = 4 * D1
UsableGas/4 = D1

Therefore Diver 1 has to consume "D1" amount of gas to get to the third's turn around point or .25 of the Usable Gas, leaving .75 of the remaining gas in the tank. (For an emergency gas-sharing egress, Diver 1 will use .25 and Diver 2 will use .5)

Now from Diver 2's perspective (the "Hoover Vacuum Diver"):

D2 = 2 * D1 (or D1 = D2 / 2)

Diver 2 needs D1 + D2 to get back. Substituting, we get D2 + D2/2 gas to get back.

Diver 2 uses D2 gas to get to the turn around point, so,

UsableGas = D2 + D2 + D2/2
UsableGas = 2.5 * D2

Solving for D2:

D2 = UsableGas/2.5
D2 = 2 * UsableGas / 5
D2 = UsableGas * 2/5

So, Diver 2 can use 2/5s of Usable gas to get to the turn around point, leaving 3/5s of remaining gas to get back. 2/5ths same as .4 used, leaving .6 remaining in the tank at turn around.

(For an emergency gas-sharing egress, Diver 2 would use 2/5 and Diver 1 will use 1/5).
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-->Might be able to get more conservatism & margin by combining above with Min Gas Reserve/ Modified Third's/Tank matching Methodology.

 

[lowviz]

Anybody utilizing the IANTD SRF Table for unequal SAC rates between teammates.? ...//...

Yes. (And I know that you meant RMV.) I use SAC loosely too.

SRF = 1/([SACa/(SACa + SACb)]+1) First you need to know who is diver "A". Run the numbers, you can generate the whole IANTD table with that equation.

I just had to derive SAC ratioing for myself before I would trust my life to it. The careful reader will note that I screwed up the parentheses on my earlier post. This is why I don't do calcs underwater...

 

[Kevrumbo]

Hi Everyone
. . .
But i was thinking..... if my buddy was diving with a much higher SAC than myself and he had larger tanks then if he needed to share my gas for some reason how could i be sure that i have enough for him and me.

Please guys i would love for you to tell me how you plan gas requiremens for your dives?

Thanks
Daniel Dilley

Yes. (And I know that you meant RMV.) I use SAC loosely too.

SRF = 1/([SACa/(SACa + SACb)]+1) First you need to know who is diver "A". Run the numbers, you can generate the whole IANTD table with that equation.

I just had to derive SAC ratioing for myself before I would trust my life to it. The careful reader will note that I screwed up the parentheses on my earlier post. This is why I don't do calcs underwater...

Thanks . . .never was instructed or derived the use of the General Equation above, and only utilized the SRF Table a few times taking it for granted. I found just figuring MGRRock Bottom/Modified Third's/Tank Matching based on the teammate with the highest SAC was an easier & sufficient methodology.

Btw (and may we all never have to do it for real!) . . .you gotta learn to do the gas calcs underwater, quickly & decisively on the fly, for a Lost Buddy Search in an Overhead. . .

 

[lowviz]

... I found just figuring MGRRock Bottom/Modified Third's/Tank Matching based on the teammate with the highest SAC was an easier & sufficient methodology.

Ah Ha!
YOU taking the easier and sufficient route? :snicker:

... Btw (and may we all never have to do it for real!) . . .you gotta learn to do the gas calcs underwater, quickly & decisively on the fly, for a Lost Buddy Search in an Overhead. . .

Yes. That calc REALLY bites. Especially when it is someone close to you.............