PS - the only thing I might stress is that you always ’minus 500 PSI’ and the corresponding CF that would remain in your tank for contingencies before starting out. That way you are using a volume equal to starting PSI minus 500 PSI. Example: Most AL-80s actually hold 77.4 CF when exactly full at 3000 PSI. If you divide 77.4CF by 3000 PSI you get 0.0258, meaning 1 PSI = 0.0258 CF. If you multiple 0.0258 by 500 (ending PSI) you get 12.9 CF. Starting volume 77.4 CF minus ending volume at 500 PSI of 12.9 CF = usable volume on the dive of 64.5 CF out of an AL-80 (another reason to use HP S-100s). So, instead of having ’80’ CF available for the dive, you really only have 64.5 CF available for the dive. Divide that by your past/average SAC rate(s) and factor in the depth in ATA and you should be good to go. See ya U/W :)