How Long Can You Stay Down On A Tank

The other day while I was lurking around the store somebody asked me “How long can you stay down on a tank” and my standard reply to this is always “It depends”. It depends on you as an individual and your comfort level in the water and how much work you are doing or simply stated your consumption rate.
Knowing ones Surface Air Consumption (SAC) rate is not required but can be handy at times and are calculated in two ways; a ‘working’ rate and a ‘rested’ rate. SAC rates are big with the “tech” crowd . For them it’s important to know how much air they will likely use at depth. SAC rates are also gaining popularity with the recreational crowd. Most dive computers that are air integrated will give you a SAC rate if you have the software and the cable to download it. Well what about those who don’t have an integrated computer? You can do the math if you would like.
The calculation is performed by calculating the volume of gas used (PSI used * ( Tank volume rating / Tank pressure rating )), dividing this by the time at depth, then dividing this result by the pressure at the average depth ((D/33)+1). This result is the ‘rested’ Surface Air Consumption (SAC)
Well while I had some time this past weekend I decided to create a SAC Calculator (SACCalc.xls) along with a Depth/Time Calculator which will help answer the question without using a pad of paper.
Here is what you need the next time your at the ‘Crater’ or anywhere. Know your cylinder type. Most rentals are aluminum 80’s. Go to the platform or a fixed depth and then note your tank pressure go ahead and breathe for a while, 20 minutes or so, and note your ending tank pressure. Then download the calculator and plug in the information. Then at the bottom of the calculator plug in your tank type, planned depth and according to your SAC rate it will give you an estimated time at that depth.