I never really learned this stuff until I took a Master Diver course.....although it's all really simple and based on you surface air consumption (SAC) rate. I think it's also covered in a mix-gas blending course that you can take.
To give a short version: You should begin logging your beginning PSI and ending PSI on several dives, along with your average depth and the time of the dive. You'll also need to know the rated capacity and pressure of your tank. To compute your SAC:
Numerator is the PSI used during the dive divided by the rated pressure of the cyclinder times the volume of the cyclinder (gives you cubic feet of air consumed).
Denominator is the dive time and the ATA at average depth (ATA = d/33 + 1)....this brings the consumption 'to the surfce' and turns it into a rate.
Most people have SAC's between 0.5 cubic feet a minute and 1 cubic feet/minute. Why does this matter?
Because you can then compute your air consumption for any depth by simply multiplying it times the time and ATA for the depth you'll be diving. Do this for all depths in a multi-level dive, include the time for deco. stops and a safety stop. Don't forget to include consumption when descending and ascending (use average depth between start and stop depths and calculate time assuming travel at 30' per minute).
You can do this easily in an excel spreadsheet where each row is SAC x ATA (at each depth) x time, to give you air consumed at each depth. Add them up and you've got your expected air consumption.
To be safe you should pad the SAC (mine is about 0.5 but I use 0.75 in calculations), and add a 50% safety margin....you'll find that 80 cubic feet of air doesn't give you much bottom time, which is why you see a lot of deep divers using 'doubles.'
Note that this doesn't do the table work for you; you'll still need to do tables to be sure you are in NDL limits or to know what your decompression stop obligations are. You'll also need to check to be sure that you don't violate the max depth limit for the gas you are breathing, etc.
To give a short version: You should begin logging your beginning PSI and ending PSI on several dives, along with your average depth and the time of the dive. You'll also need to know the rated capacity and pressure of your tank. To compute your SAC:
Numerator is the PSI used during the dive divided by the rated pressure of the cyclinder times the volume of the cyclinder (gives you cubic feet of air consumed).
Denominator is the dive time and the ATA at average depth (ATA = d/33 + 1)....this brings the consumption 'to the surfce' and turns it into a rate.
Most people have SAC's between 0.5 cubic feet a minute and 1 cubic feet/minute. Why does this matter?
Because you can then compute your air consumption for any depth by simply multiplying it times the time and ATA for the depth you'll be diving. Do this for all depths in a multi-level dive, include the time for deco. stops and a safety stop. Don't forget to include consumption when descending and ascending (use average depth between start and stop depths and calculate time assuming travel at 30' per minute).
You can do this easily in an excel spreadsheet where each row is SAC x ATA (at each depth) x time, to give you air consumed at each depth. Add them up and you've got your expected air consumption.
To be safe you should pad the SAC (mine is about 0.5 but I use 0.75 in calculations), and add a 50% safety margin....you'll find that 80 cubic feet of air doesn't give you much bottom time, which is why you see a lot of deep divers using 'doubles.'
Note that this doesn't do the table work for you; you'll still need to do tables to be sure you are in NDL limits or to know what your decompression stop obligations are. You'll also need to check to be sure that you don't violate the max depth limit for the gas you are breathing, etc.
