calculating air usage at depth

[emttim]

Walter,

I want to make sure I really understand this
ATA= (D+33)/33 for freshwater
ATA= (D+30)/30 for saltwater

Is that right?

Those numbers are wrong.

The form of your equation is correct, but you have the wrong numbers.

A column of freshwater 34' high has a pressure of 1 atm. 33' is 1 atm in saltwater.

So divide depth in saltwater by 33' to get how many atms you are below the surface, and then add 1 for sea level air pressure. Expressed as a formula, that's ATA = (Depth/33) + 1, which is also ATA = (D+33)/33. I find the first version of the equation works better for mental arithmetic, the 2nd one when using a calculator.

These numbers are right.

You guys are making it way harder than it needs to be to answer the OP's question, however.

OP, use this calculator SAC Calculator and play around with it. It has the mathematical formula on the page so you can verify the calculations it gives you to make sure they are correct...that way you can choose to do it manually or just use the calculator.

If you want to find out your air consumption at depth, find the pressure of that depth and multiply it by your SAC. Say you're diving to 99 feet, 3 atm at depth plus 1 atm at the surface for a total of 4 ata multiplied by say for example a 0.6 CF/min SAC would yield a 2.4 CF/min air consumption rate at 99 feet. Hope it helps.