Advanced Gas Planning

[Daniel Dilley]

Hi Everyone

I was having a good think about gas planning and have stumbled upon some questions. I hope to get some help and understanding. Thanks.

So normally when i plan a dive i have my normal plan, lost gas plans and a deeper and longer plan. For now lets keep this to soft overhead diving, meaning just decompression, not ice, cave or wreck or mine etc.

So for my normal plan i make sure that i have enough bottom gas to leave a third spare and enough deco gas so if i was to loose any deco gas i could complete the dive using the remaining deco gasses only.

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

 

[boulderjohn]

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.

This is called tank matching, and it is all part of the planning. If you are using thirds, for example, you plan on thirds for the most gas compromised diver, and both divers turn when one of them reaches that volume. You do not each plan on the thirds of your supply independently.

 

[Daniel Dilley]

This is called tank matching, and it is all part of the planning. If you are using thirds, for example, you plan on thirds for the most gas compromised diver, and both divers turn when one of them reaches that volume. You do not each plan on the thirds of your supply independently.

Hi boulderjohn, thanks for your reply.

If we plan thirds using my buddy with higher sac rate's sac but he also has larger tanks than me so if he needs to share gas for me then i wouldn't have enough gas for both of us i think?

 

[boulderjohn]

You are correct, and that is why you don't plan that way. If you are planning your dive on thirds, then gas supply is your priority. SAC rate allows you to estimate how much time it will take you to reach your turn point, but you turn the dive when your gas supply hits the planned level, not when that estimated time is done.

Let's say your buddy is going to carry 240 cubic feet, and you are going to carry 150 cubic feet. Thirds for the dive is 50 cubic feet--for each of you. You will turn the dive either when you reach 100 cubic feet or when your buddy reaches 190 cubic feet. That way if he has a catastrophic gas loss at the worst possible time, your 100 cubic feet should get both of you home. If you have a catastrophic gas loss at that time, you will have oodles of gas on which to get home.

 

[kensuf]

You are correct, and that is why you don't plan that way. If you are planning your dive on thirds, then gas supply is your priority. SAC rate allows you to estimate how much time it will take you to reach your turn point, but you turn the dive when your gas supply hits the planned level, not when that estimated time is done.

Let's say your buddy is going to carry 240 cubic feet, and you are going to carry 150 cubic feet. Thirds for the dive is 50 cubic feet--for each of you. You will turn the dive either when you reach 100 cubic feet or when your buddy reaches 190 cubic feet. That way if he has a catastrophic gas loss at the worst possible time, your 100 cubic feet should get both of you home. If you have a catastrophic gas loss at that time, you will have oodles of gas on which to get home.

^^This.

The person with the largest volume of gas needs to use the turn of the person with the lowest volume of gas.
You can do it mathematically, or you can make and use a lookup table.

The other solution is to also just have everyone dive the same size tanks and then match based on pressure.

Jim Wyatt has a pretty good lookup table and explanation online, Cave Dive Florida Gas Matching Table & Example

 

[Daniel Dilley]

You are correct, and that is why you don't plan that way. If you are planning your dive on thirds, then gas supply is your priority. SAC rate allows you to estimate how much time it will take you to reach your turn point, but you turn the dive when your gas supply hits the planned level, not when that estimated time is done.

Let's say your buddy is going to carry 240 cubic feet, and you are going to carry 150 cubic feet. Thirds for the dive is 50 cubic feet--for each of you. You will turn the dive either when you reach 100 cubic feet or when your buddy reaches 190 cubic feet. That way if he has a catastrophic gas loss at the worst possible time, your 100 cubic feet should get both of you home. If you have a catastrophic gas loss at that time, you will have oodles of gas on which to get home.

Hi
Once again thanks for your reply. I am really sorry to ask but is there any chance that you could convert that to litres for me as i have not idea about cubic feet.

Thank you
Daniel

 

[2airishuman]

Let's say your buddy is going to carry 240 cubic feet, and you are going to carry 150 cubic feet. Thirds for the dive is 50 cubic feet--for each of you. You will turn the dive either when you reach 100 cubic feet or when your buddy reaches 190 cubic feet. That way if he has a catastrophic gas loss at the worst possible time, your 100 cubic feet should get both of you home. If you have a catastrophic gas loss at that time, you will have oodles of gas on which to get home.

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.

 

[kensuf]

Hi
Once again thanks for your reply. I am really sorry to ask but is there any chance that you could convert that to litres for me as i have not idea about cubic feet.

Thank you
Daniel

Metrics are easy.

Let's say you have an 11L tank filled to 200 bar. That's 200 x 11 = 2200L. Each third is roughly 730L.

Let's say your buddy has a 12L tank filled to 150 bar. That's 150 x 12 = 1800L. Each third is roughly 600L.

Your buddy is the controlling diver. You have to turn after using 600L. 600 / 11 = 54 bar. You have to turn at 146 bar.

 

[Daniel Dilley]

Metrics are easy.

Let's say you have an 11L tank filled to 200 bar. That's 200 x 11 = 2200L. Each third is roughly 730L.

Let's say your buddy has a 12L tank filled to 150 bar. That's 150 x 12 = 1800L. Each third is roughly 600L.

Your buddy is the controlling diver. You have to turn after using 600L. 600 / 11 = 54 bar. You have to turn at 146 bar.

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?

 

[2airishuman]

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.