. . .
turn = 1/([SACa/(SACa + SACb)]+1) * times the fill.
The way I see it, there are two numbers we can use for gas planning.
Your actual RMV, or a safe RMV number that is easily calculated (should be) and easily encompasses any diver (hoover or not).
Me personally, in warm water tropics, I'm a 11L/min kinda guy. Cold water with doubles and stages, I'm a 16L/min kinda guy.
If one were to round that number UP to 20L/Min, it covers me on a 'having a bad day underwater - ie, not floating along staring at whatever I see' RMV. In the tropics, that gives most a huge buffer in the amount of gas for a dive (>50%), and in colder waters, not as much (<25%).
Same can be said for emergencies, plan with say a 30L/Min across 2 people. Ideally before you start the ascent (or hopefully the first stop), everyone has calmed down to almost their normal RMV, so that number gives a very nice big buffer on Gas if /when SHTF.
So you will need to calculate how much volume of gas you need to bring yourself and a teammate/buddy in an emergency from the suet gear distance away from the next available gas source (usually SURFACE) to the next available Gas source (Deco/staged stage/SURFACE).
There have been quite a few excellent posts here, and in other threads and other members websites. (Along with formal training) that can further explain the calculations for determining the Volume of Gas required.
From there, you then derive if you have enough gas in a cylinder, and how much of that is "usable" during the dive (the rest is for emergency).
If everyone is diving the same size tank, it's pretty easy as everyone should have the same turn volume.
If someone has a different size tank, then it gets slightly more complicated as you have to compare how much volume:tank size for everyone.
_R
Hi
Thats great thanks for your help. Just a quick question, what if our sac rates where also different and the diver who is my buddy has a larger sac than me and he carries larger tanks?
I'll be interested in the replies also.
The intent of the rule of thirds is for the gas supply to provide:
* inbound gas for the diver +
* outbound gas for the diver +
* outbound gas for the buddy in the event of an emergency.
Right?
Applying this to unequal SACs and tanks, we could determine the volume of gas the higher-SAC diver requires for the outbound journey. Then we could subtract that from the other diver's available bottom gas, halve what's left, and use that for the rule-of-thirds turn point.
I don't know if people do that. It's easier to match tank sizes across the team.
I totally understand about the diver with the better sac bringing smaller tanks makes the dive shorter for you but what about a diver with the worst sac bringing bigger tanks, does this give him any benefit because his buddy still needs to leave the required amount for him to get out? Does it depend how bad his sac is as to whether he should take bigger tanks?
Ok, Here's a comprehensive gas plan/turn pressure exercise for a planned wreck penetration at 30 meters for 30 minutes.
In this case, the plan is complicated by dissimilar tank sizes and unequal breathing rates between the Buddy Pair.
Given Diver A:
12L twinset (total 24L/bar); 230bar fill; nominal SCR (Surface Consumption Rate; also known as SAC rate/RMV) of 9.9L/min;
Diver B:
15L twinset (total 30L/bar); 230 bar fill; nominal SCR of 19.8 L/min (twice as much as Diver A).
Solution:
First start with Minimum Gas Reserve -Rock Bottom Calculation (assume emergency stressed 30 L/min SCR for both divers) to reach Open Water O2 Deco Stop at 6 meters, after exiting wreck:
4.0 ATA x 1min x 30L/min = 120 L
3.7 ATA x 0.3 min x 30L/min = 33.3 L
3.4 ATA x 0.3 min x 30L/min = 30.6 L
3.1 ATA x 0.3 min x 30L/min = 27.9 L
2.8 ATA x 0.3 min x 30L/min = 25.2 L
2.5 ATA x 0.3 min x 30L/min = 22.5 L
2.2 ATA x 0.3 min x 30L/min = 19.3 L
1.9 ATA x 0.3 min x 30L/min = 17.1 L
1.6 ATA x 2 min x 30L/min = 96 L. [Two minutes to switch to O2 Deco bottles].
Total: 392 L. Total MGR for 2 Divers: 392 x 2 = 784 L;
Diver A Pressure MGR: 784 L divided-by 24 L per bar = 32 bar
Diver B Pressure MGR: 784 L divided-by 30 L per bar = 26 bar, (both actual SPG readings or remaining "Rock Bottom" minimum gas reserve in tanks).
Diver A Usable Gas: 230 - 32 = 198 bar (198bar x 24L/bar =
4752 Liters)
Diver B Usable Gas: 230 - 26 = 204 bar (204bar x 30L/bar = 6120 Liters).
Tank Match Starting Reference is to Diver A since he has the lowest and limiting amount of usable gas (
4752 Liters).
Since Diver B has higher SCR than Diver A, use SRF Formula and apply adjustment factor to Diver A:
SRF = 1/([SCRa/(SCRa+SCRb)]+ 1) = 1/([9.9/(9.9+19.8)]+ 1) = 0.75 adjustment factor.
Apply SRF of 0.75 adjustment to Diver A's Usable Gas: 4752 L x 0.75 = 3564 L (148 bar). Therefore
Diver A turns the dive at 148 bar remaining reading on the SPG (same as 50 bar consumed from 198 bar usable).
50 bar consumed of Diver A's 24L/bar twinset is 1200 Liters; tank match this reference volume to Diver B:
1200 Liters of Diver B's 30L/bar twinset is the consumed pressure for Diver B of 40 bar (or
Diver B turns the dive with 164 bar remaining reading on SPG).
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New exercise -->Diver B still has twice the SCR, but both Divers have the exact same 12L/bar twinsets or 24L/bar total and Total Usable Gas of 4752 Liters. What are the Turn Pressures?
Solution: Diver A is the same SRF adjusted value of 50 bar consumed with 148 bar remaining turn pressure, but Diver B just takes straight Third's of his Usable Gas of 4752 Liters (198 bar);
Therefore Diver B has 66 bar consumed and turns the dive with 132 bar remaining reading on the SPG.
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New exercise -->What if Diver A is the "Hoover Vacuum" compared to Diver B, with dissimilar tanks (Diver A with 12L/bar twinset or 24L/bar total; Diver B with 15L/bar twinset or 30L/bar total).
Assuming same dive, Rock Bottom & twinset parameters above, what if Diver A had 19.8L/min, and Diver B had 9.9L/min (i.e. Diver A now has the higher SCR)?
Solution: Since Diver A has the higher SCR, apply SRF adjustment to Diver B's Usable Gas: 6120 L x 0.75 = 4590 Liters (153 bar).
Now tank match 4590 Liters to Diver A:
Critical Check -->Is this amount less than Diver A's Usable Gas of 4752 Liters? Yes. 4590 <
4752.
So then we can use this amount 4590 L (153 bar);
in this case, Diver B turns the dive at 153 bar remaining reading on the SPG (or 51 bar consumed). 51 bar consumed of Diver B's 30L/bar twinset is 1530 Liters.
Apply the 1530 Liters turn volume to Diver A:
1530 L of Diver A's 24L/bar twinset is 64 bar consumed.
Diver A turns the dive at 134 bar remaining reading on the SPG.
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Typically on my Truk Trips, I'm usually twice the SCR of my Dive Guides (
they are all a phenomenal 7.5 to 5 L/min), all of us using open circuit AL80 11L twinsets. To "normalize" or nearly scale & match bottom time and gas turn pressures with them on the deep dive penetrations, I have to carry and exhaust an additional single AL80 stage cylinder of bottom mix first before switching to my backmount AL80 doubles.
