2nd AL80 or Al40 - That is the question...

[Stone]

MikeS once bubbled...
I think the key to determining tank size is to calculate which you will run out of first gastime or no decompression time, so as to safely maximize the amount of bottom time. If you’re thinking about Nitrox don’t forget to use that in the equation.

For example:

No Deco Time

Depending on the table, diving EAN 32 at 100 fsw allows about 30 minutes of no decompression time.

Gas Time

30 minutes of a SAC rate of 0.7 at 100 fsw requires 84 cf of gas. And this does not include the gas required to ascend, for the safety stop, or any margin. This is well beyond the capability of an aluminum 80, so you would end up surfacing well before you reached the no decompression limit. Mike

Scuba446,

If you're interested, here's a link to a visual aid that relates to Mike's comments above. I also have a spreadsheet version that allows you to change tank volume and SAC. Be sure to read the assumptions by clicking on the "Assumptions" tab at the bottom of page.

Nitrox vs Air Charts