# Struck Please Help!

[BIGSAGE136]

Lately I have been studying gas management much more than I did a few years ago.
I am trying to come up with a formula that will let me take the reading from my SPG, and using my SAC rate turn that into remaining minutes.

The Navy Dive Manual has helped a lot, but this has me stumped. That or I just cant see the forest for the trees.

Anybody? :06:

 

[The Kraken]

BigSage,
You'll probably want to use a relationship between the volume of the tank and the psi/min of gas usage.
SAC is good for planning, but not so good for diving because your volume of gas usage changes with depth. Your SAC, for the most part, remains constant.

Let's just say that you have a 100 CF tank at 2000 psi (just to keep the numbers easy to deal with)

1 cubic foot = 20 psi
So if your SAC is .50 you will use 10 psi per minute at the surface. At 99 feet you will use 40 psi per minute 4 ATA x 10 psi.

First find your average SAC and do the conversion. Learn you average psi usage per minute at the different ATA levels and you can compute you usage on the fly.

the K

 

[BIGSAGE136]

BigSage,
You'll probably want to use a relationship between the volume of the tank and the psi/min of gas usage.
SAC is good for planning, but not so good for diving because your volume of gas usage changes with depth. Your SAC, for the most part, remains constant.

Let's just say that you have a 100 CF tank at 2000 psi (just to keep the numbers easy to deal with)

1 cubic foot = 20 psi
So if your SAC is .50 you will use 10 psi per minute at the surface. At 99 feet you will use 40 psi per minute 4 ATA x 10 psi.

First find your average SAC and do the conversion. Learn you average psi usage per minute at the different ATA levels and you can compute you usage on the fly.

the K

Thank you!

 

[cummings66]

Look here for a post from lamont. Good info on how to manage it.

http://www.scubaboard.com/showthread.php?t=68203&page=4&pp=10&highlight=rock+bottom+time

 

[perpet1]

The posts here are good.

One thing to remember is that SAC rate is typically in terms of psi /min. That will help if you alwyas use the same tank but a more useful number is ft3/min. The previous post did a good job of explaining how to convert between ft3/min and psi/min. Understanding the relationship you can use any tank and figure out how long it will last.

As far as your initial question about being able to easily calculate this in the water... You have to understand that it is all relative to depth and as we seldome stay at nice depths like 33fsw or 66fsw for an entire dive this is not a calculation that can easily be done in the water with any accuracy (other then with experience and good knowledgable guestimates).

I recommend doing a good predive plan and diving your plan. Planning on the fly is a good way to get in trouble. You should plan your turn not based on time but on your ability to get back based on your current air supply.

Smile,
Pete

 

[BIGSAGE136]

The posts here are good.

One thing to remember is that SAC rate is typically in terms of psi /min. That will help if you alwyas use the same tank but a more useful number is ft3/min. The previous post did a good job of explaining how to convert between ft3/min and psi/min. Understanding the relationship you can use any tank and figure out how long it will last.

As far as your initial question about being able to easily calculate this in the water... You have to understand that it is all relative to depth and as we seldome stay at nice depths like 33fsw or 66fsw for an entire dive this is not a calculation that can easily be done in the water with any accuracy (other then with experience and good knowledgable guestimates).

I recommend doing a good predive plan and diving your plan. Planning on the fly is a good way to get in trouble. You should plan your turn not based on time but on your ability to get back based on your current air supply.

Smile,
Pete

I agree entirely. Some buddies and I were diiscussing rescue scenarios and the question came up.... If we turned at rock bottom and hit a CF, how much time to we have to fix it? Suddenly I was overcome with formulas. The Kraken cleared my head. Thanks to all!