to make it simple and generic a bar is atmonpheric pressure or 14.7 psi or 15 psi for short. it is atmosphere pressure. instead of saying the tank holds 3000 units of 1#s it holds 200 unites of atm's which is 200 atms or 200 x 15 or 3000 psi. If yo are non us and on the metric system then to figure out who much air is in the tank you look at how much water the tank will hold. when full. (dont do this but take off the valve and pour water in it and measure it. Say this results in 12 liters. then you have a 12 liter tank. Again dont do this because they did this at the factory for you. that is why you buy a 12 liter tank or a 15 liter tank or 10 liter tank. now you multiply this by the bar (working pressure of the tank. So if the tank has 12 liters of air in it at surface pressure of 1 bar or 1 atm. (1 bar = 1 atm) then if it is a 200 bar tank you now have when full a 12 liter tank has 12*200 liters or 2400 liters of air pumped into it . if the tank was a was a 15 L/200 then it would have 15*200 or 3000 Ltrs in it.
Now you use this to see how long the tank will last you.
If you breath 14 liters a minute (1/2 cu ft a minute) then you can do this 2 ways. using a 15L/200 tank...... divide 14/ min into 3000liters and you 214 minutes of breathing sitting in your car until the tank goes dry. we will call that 200 for math sake. now you go to 100 ft or 30m depth 30m is 3 bar so you ad one for the atmosphere and you get 4 bar ,,,your depth pressure is 4 times that of the surface so you consume 4 times as much gas per minute so instead of using 14 now are using 4*14 or 56 ltrs a minute cal it 60 for easy math. 3000 available and you are using 60 a minute. So you have 3000/60 or 50 minutes untill the tank goes empty. the other way is that if you have 3000 in the tank and you breath 14 a min call it 15 a minute then you have 200 minutes on the surface and at 4 atmospheres you use it 4 times faster so 4 into 200 dives you 50 minutes till it runs dry.
Next say you are at 100 ft and you read you tank and it says 180 bar , 10 minutes later it ways 120 bar. you used 60 bar in 10 minutes or 6 bar per minute that is 6 * the 15L SIZE OF THE TANK) and you get 90 ltr's used in a minute at 4 absolute atm depth or 3 bar or 30 meters. divide the 90 by 4 atm and you get 22 liter a minute at 1 atm. your sac for that 10 minutes was 22 liters a minute. Also if you used 60 bar n 10 minutes and you have 120 still inn the tank then you have 2 more 10 minute periods of breathing 60 bar per 10 min before the tank goes dry. so you have 20 min till you are out of air.
The metric method is a very nice way to label a tank you now by its marking its volumn and pressure so you can make all the calculations based on knowing the tank you are looking at is a 10L/300 or a 12L/180 etc. Imperial tanks is a night mare they are marked as working pressure capacity not liquid volumn and pressure..... IE AL80 cuft 3000 psi.
For an AL 80 tank it is really 77 cuft and we have to do all the math to figure things like converting depth in ft to atm 100 ft /33 = x plus one. to get atm. you guys just look at your depth gage in meters and divide by 10 and then add one. so at 66 ft we have to say 66/33 = 2 then 2 plus 1 = 3atm.. you guys look at your gage and see 20m ,,,,divide by 10 and add 1 to get to 3 atm. we need a calculator and you do it by moving a decimal place and adding 1.
That math was easy but what if it was 80 ft imperial. your metric would be 25 meters to figure out absolute pressure in depth it is simple 25/10 then ad 1 or 3.5.
Metric and Imperial Conversion Charts and Tables
btw THER IS ABOUT 28.5 LITERS IN A CU FT.
ONE ATMOSPHERE AT SEA LEVEL IS IS ABOUT 14.7 PSI
A BAR IS EQUAL TO ATMOSPHERE PRESSURE 1 BAR IS 14.7 PSI
33/34 FEET DEPENDING ON FRESH WATER OR SALT WATER IS EQUAL TO 1 ATM OR BAR
PRESSURE INCREASES ABOUT 1/2 PSI PER FOOT OF DEPTH
100 ' IS 3 ATMOSPHERES OF WATER +THE ATMOSPHERE ITSELF FOR A TOTAL OF 4ATM
FOR GRINS OUR s80 AL TANK IN METRIC WOULD BE
77*28.5 = 2197 LITERS FULL AT
3000 PSI / 15 PSI PER BAR OR 200 BAR TANK
THE TANK VOLUMN WOULD BE 2197/200 OR 10.97 LITERS
Our AL S80 would be called in your land an AL 11L /200 tank
