Is our Recreational breathing gas planning not safe enough? Part 2
Cozmo Neiger Dec 17, 2022
Is our Recreational breathing gas planning not safe enough? Part 2
A little recap from part 1 when doing a recreational dive to 60ft with a single Aluminum 80 and planing to be back to the surface with 500 psi in reserve all is good when there is no emergency, However when we had one of our divers have an out of air emergency at our planed end dive pressure both divers were out of air when they had reached the surface with only a one minute safety stop at 20ft. They would have had to orally inflate their BCDs due to not having any reserve. Even though both divers would have barely made it safely to the surface one of the divers could have easily gone into a full blown panic.
Now we will look at doing both dive scenarios but this time we will be planing our dives and diving our plan using the rule of thirds. The rule of thirds comes from the tech diving world and its practice is to use one third of your gas to start your dive and complete half of your bottom time and one the second third to complete the second half and return back to the surface with the last third as a unused reserve. We plan our total dive time with only two thirds of the gas available, so at the end of the dive we are planing to reach the surface with 1000psi of an AL 80.
So to compare dive floats to dive floats we will still use the Surface Consumption Rate as before in part 1 @ .8cu ft per minute breathing rate for moderate exertion and will be using the same aluminum 80 that = 77 cu ft. As always we will be doing a safety/deco-stop at 20’ for 3 minutes.
Time to Plan the dive, because we are going to use the rule of thirds we will be planing out dive with the 2000 psi of our AL 80 = 52 cu ft to use.
So let’s dive
Depth: 60ft Time: 20 minutes Gas used: 51 cu ft
We leave the bottom when our pressure in our tank is @ 1616psi
As before the total Ascent and 3 min deco-stop uses 6.87 cu ft
We will reach the surface with 1000psi or 26cu ft
So what is the total cu ft used?
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- Bottom Phase 45 cu ft
- 60’ to 20’ 2.35 cu ft
- 20’ stop for 3 min 3.84 cu ft
- 20’ to 0’ .68 cu ft
- Total for the dive 51 cu ft
- Reserve left in tank 26 cu ft
- Total gas needed 77 cu ft
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To do this dive by the rule of thirds we had to reduce our total bottom time by 5 minutes from the original 500 psi reserve dive in part 1 of this diving mini series. 26 cu ft of gas is a good amount of reserve for a dive like this and if you get your tanks filled by a shop that fills by the cubic foot (like mine) it’s like putting gas in your car, it’s no big deal to have this left over breathing gas because you are not paying for it again unlike places that charge a flat rate for a fill. (A good reason to own your own tanks)
And now for the emergency scenario just like before in part 1 of this novella we have an out of air diver at Murphys favorite time right at the end of the bottom phase and both divers breathing @ 1.4 SCR. Let the drama begin or non-emergency, emergency????
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- Dive @60’ for 20min 45 cu ft
- Link up @ 60 for 30 seconds 3.9 cu ft
- 60’ to 20’ 1.33 min 8.23 cu ft
- 20’ for 3 min. 13.41 cu ft
- 20’ to 0’ 2.4 cu ft
- Total for this emergency 72.9 cu ft
- Looks like we have a reserve of 4.06 cu ft or 156 psi
- Grand total Gas need w/Reserve 77 cu ft = Al 80
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Wait a second . . . both divers were able to do a 3 minute deco-stop and make it to the surface with gas left over huh that’s kinda neat. Unlike diving using the 500 psi for reserve at the surface we are able to do a safety/deco-stop for three minutes using the rule of thirds hedging our bets with decompression sickness and being more controlled and relaxed. When more relaxed and controlled we are less likely to be stressed and farther away from panic.
