Basic Gas Planning Help

[HybridDiver]

Hello Guys

So im planning a dive to 60 feet for tomorrow, With a buddie I can sort of trust. Anyways this is how I planned my BT gas, Im not sure if this is right so bare with me.
60ft/2.55x75ft3x35min and I got was So i go 60 Divide by 2.55 X 75 X 35 right? Thats for my BT but i got a messed up answer 61764.75.
What am i doing wrong here.
Chris

 

[Jim Lapenta]

What formula are you using? What is your sac rate? Using nitrox? How long do you plan to stay down? On a 60 ft dive if I take my lp 95 pumped to 3200 psi with air I'll run out of no deco time before I run out of gas. On the PADI tables for air your max NDL is 55 minutes for 60 ft, on the navy tables it's 60 minutes. What tables are you using and if you're not why? Sounds like you are trying to over think things here.

 

[TSandM]

Chris,

What's your SAC rate? Do you know? If not, .75 cu ft/min is probably a reasonable estimate, unless you are a very new diver.

If you are going to be at 66 fsw, you're at 3 ATA, right?

.75 cu ft/min (on the surface) x 3 ATA (at 66 feet) = 2.25 cu ft/min at depth. If you are planning to be there for 35 minutes, then you have 2.25 x 35 = 78.75 cu ft of gas used at depth. Then you have to account for your ascent, and we don't know what kind of tanks you're using, so we don't know how many psi will be used in the process of emptying 79 cubic feet out of them.

Make sense so far?

 

[HybridDiver]

K I am using padi tables and my sac rate is .75 Im not sure what my real sac rate is but lets use this because its a normal type of sac rate I think. Im just using 21 so air. Im going to stay down for 35 mins im thinking with a 3-8 minute saftey stop.
Chris

 

[HybridDiver]

This is making sense AWESOME, im using a jacked up Steel 100 to 3500psi. Does this help?
Chris

 

[Jim Lapenta]

Is that you crash? But yesterday you were selling all your gear. and so just how many cu ft do you have with 3500 psi in that tank?

 

[HybridDiver]

Jim tell your the truth I have no clue, But I would like to learn. ( I am writing all this down for future)
Thanks
Chris

 

[TSandM]

Well, if you have HP 100s, your tank factor (cu ft/100 psi) is 3, so 79 cu ft will be approximately 2600 psi. That leaves you 900 psi, and minimum gas (rock bottom) for 60 feet in HP 100s is 700 psi, so you have enough gas to do the contemplated dive.

 

[HybridDiver]

How would you plan out the accent to the surface? How do you know when to stop?
Chris

 

[Jim Lapenta]

Ok in order to figure out how much gas is in there you need to take the cylinder size in this case a 100 divide it by the working pressure which if it's a high pressure tank it's 3442psi for low pressure (2400) 2640 with the 10% overfill to achieve 100 cu ft. take that answer and multiply by the actual pressure. So for a hp it would be 100 divided by 3442= .029 x 3500 psi gives you 101cuft. for the low pressure it would be 100 divided by 2640= .037x3500= 132 cuft of gas. It's why I prefer lp to hp tanks.