Advanced Gas Planning

[kensuf]

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?

That's why you match. Let's continue with the examples I worked out above, but say your buddy is the hoover and he has the 2200L of gas, and you have the 1800L of gas. His gas turn-around pressure is 146 bar, yours is 100 bar.

Let's say you both get to your pre-determined gas turn-around at the same time -- that means you have 100 bar, he has 146 bar. It also means he used 600L to get there, you used 600L to get there. It also means he should have 1600L in his tanks, and you should have 1200L in your tanks.

If he has a complete failure right then (a bad day indeed), you should still have enough gas to get both of you home (barely). If you have a failure instead of him, he has enough gas to get the two of you home.

 

[boulderjohn]

I don't know much about technical diving, but it seems to me that this example may lead to exhausting the gas supply. If the buddy uses air at a faster rate, then at the turn point the buddy will need more than 50 cf to finish the dive. If an emergency were to cause the buddy to lose his or her gas supply right at the turn point, only 100 cf of gas would be available. "Diver 1" would need 50 cf and the buddy would need more than 50 cf, which would lead to an exhaustion of the supply.

I should have said in my original that they should have at least 100 cubic feet. In reality they should have more. Let me explain.

Diver A has 240 cubic feet, and Diver B has 150, so their thirds is 50. That means they will turn when either of them USES 50, not when they have 50 left. Diver A will turn at 190, OR Diver B will turn at 100, whichever comes first. If Diver A turns at 190 (after having used 50) and then has a catastrophic gas loss, they will be relying on Diver B's gas supply. Diver B has a better SAC rate, so let's say Diver B only used 40 cubic feet. That means Diver B has 110 cubic feet left. Since the two combined only used 90 going in, then 110 should be enough to get them out.

 

[Daniel Dilley]

Hi Guys

Thanks for all of your replies, especially Kensuf.

So from what i understand we take the smallest third to use, calculate the turn pressure and we should then both definately get back?

Ok, now lets apply this to Open Water diving just to where you need to get to a deco stop to switch gas and not out of a cave?

Thanks

Daniel Dilley

 

[boulderjohn]

Ok, now lets apply this to Open Water diving just to where you need to get to a deco stop to switch gas and not out of a cave?

If you do not have to get to a specific point and are using the rule of thirds (which is not the only way to plan it), you again plan on the thirds of the most compromised diver.

 

[PfcAJ]

Hi Guys

Thanks for all of your replies, especially Kensuf.

So from what i understand we take the smallest third to use, calculate the turn pressure and we should then both definately get back?

Ok, now lets apply this to Open Water diving just to where you need to get to a deco stop to switch gas and not out of a cave?

Thanks

Daniel Dilley

Same same, just with 'rock bottom', or the amount of gas needed to get to the first gas switch while sharing.

 

[boulderjohn]

The idea of having the one with the worst SAC rate on a dive use a bigger tank works nicely in recreational diving when you are in an "all usable" mode and ascending when there is a certain amount left, but it does not work so well in technical diving. When I was doing my cave training, on one of the days my instructor and I were accompanied by someone doing instructor training. He had an excellent SAC rate, and he brought doubles that were MUCH smaller than ours. When he got them out, I overheard the instructor say something quietly to him about that decision, and I could see he was annoyed. By bringing those small tanks to the dive, he had greatly cut down on the amount of gas I could use on the dive, and it limited what the instructor could do with me during the dive because I would have less time to do it.

Ironically, in the example I used above, if the diver with the worse SAC rate had used the smaller tanks, they might have a longer dive. No matter who uses which tanks, they will turn when one of them uses 50 cubic feet. If the diver with the worse SAC rate uses the smaller tanks, then he or she might have a better SAC rate, leading to a longer dive.

 

[rivers]

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?

There is a reason I wear the same size cylinders as everyone I dive with, even though my SAC rate is a lot smaller than most of them (i'm a 5'2" female, they are all 6' ish tall blokes). It makes gas planning easier!

 

[Patoux01]

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)

 

[Daniel Dilley]

The idea of having the one with the worst SAC rate on a dive use a bigger tank works nicely in recreational diving when you are in an "all usable" mode and ascending when there is a certain amount left, but it does not work so well in technical diving. When I was doing my cave training, on one of the days my instructor and I were accompanied by someone doing instructor training. He had an excellent SAC rate, and he brought doubles that were MUCH smaller than ours. When he got them out, I overheard the instructor say something quietly to him about that decision, and I could see he was annoyed. By bringing those small tanks to the dive, he had greatly cut down on the amount of gas I could use on the dive, and it limited what the instructor could do with me during the dive because I would have less time to do it.

Ironically, in the example I used above, if the diver with the worse SAC rate had used the smaller tanks, they might have a longer dive. No matter who uses which tanks, they will turn when one of them uses 50 cubic feet. If the diver with the worse SAC rate uses the smaller tanks, then he or she might have a better SAC rate, leading to a longer dive.

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 benifit 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 tke bigger tanks?

 

[DevonDiver]

A team member with a bad SAC carrying bigger tanks doesn't lead to longer dives, because the team still has to turn when they reach their capacity to sustain the exit of that diver from their reserves.

If the team all carries identical cylinders, then the turn-point is simply determined by the highest SAC diver reaching 2/3rds gas pressure. The end-point (1/3rd) or planned bottom-time is simply determined by the highest SAC diver's consumption.

If dissimilar tanks are used, then the gas planning needs to switch from pressure to volume. What matters is the highest SAC in the team and how much volume of gas they require to complete each phase of the dive. Every team member needs to preserve that volume of gas as their minimum. Beyond that minimum volume, the remaining gas volume is divided in 2 to create a turn-point (and converted back into pressure reading, so that the turn can be based on an SPG reading).