Can Old Scuba tanks be used for CO2?

Attention SCUBA divers! Crush depth of scuba tanks? NOT burst strength of tanks from internal pressure, but?

• how much external pressure can tanks withstand before rupture? (please read rest of details! ) I KNOW it will be far deeper than a person could actually dive with scuba tanks. I am not looking for the maximum depth a person could reach. Go to the deepest part of the ocean, put weights on the tanks so they will sink instead of floating, and let them go. How deep do they reach before the water pressure crushes the tanks and releases the air? Also, will the answer vary depending on whether the tanks are full or empty? Does the internal pressure of the tank help to resist the external pressure of the water? Finally, can analogous values be found for propane tanks, CO2 tanks, etc? Which ones would provide the greatest buoyancy? I may not be expressing this properly. Please be patient with me. If you don't understand, ask for more details and I WILL provide them. Thanks to all who give serious replies!

• Answer:

  Hmmm. To answer the question you do have to know what you referred to as the "burst strength of tanks from internal pressure". This is when the tank will rupture (not a pretty picture) from overfilling. To the tank it does not matter if the pressure is inside or outside. But here's some information that will help you figure out your problem. Pressure is measured in Pounds per square inch (PSI). An 80 Cubic Foot scuba tank (a typical tank) is usually pressurized to 3000 PSI. And yes, the answer will vary depending on whether the tank is full or empty. If empty and open, then you are talking about when aluminum (what most scuba tanks are made from) will crush. That will be an enormous figure and the ocean is not deep enough. To calculate your depth a full tank will burst, you must know when it would burst if being filled at sea level. Most tanks are pressure tested to 2.5 times their fill rating. So if your tank is rated 3000 PSI they will be tested to 7500. At sea level, normal PSI of the air you and I breath is 1 atmostphere (ATM) which equals 14.6959488 pounds per square inch. For each 33 feet of ocean depth you decend, you have increased 1 atmosphere. So at 33 feet you are at 2 ATM, 66 feet = 3 ATM, and so on. Remember you are starting off at 1 AT sea level. So your scuba tank contains 3000 PSI. That equals just over 204 ATM. Right? 3000 / 14.6959 = 204.14. So at 204 ATM the pressure inside a full scuba tank equals the pressure outside the scuba tank! 204 ATM is approximately 6609 feet deep. This is called equalibrium. Remember 1 mile = 5280 feet. I have no idea when a tank would burst, but just say for instance it did at 10000 PSI. That is over 680 atmostpheres. But that is 680 atmospheres below the equalibrium we just reached at 6609 feet!!! So add 204 ATM and 680 ATM and get 884 ATM. Going back to 33 ft = 1 ATM. 884 - 1 (for sea level) = 883 * 33 = 29139 feet. If a tank is going to burst at 10000 PSI and it contains 3000 PSI, it will burst at around 29,139 feet. That's about 5 miles deep. The average sea depth is around 2 miles deep with the Mariana Trench being about 6.75 miles deep. With this information you should be able to figure out an empty tank with closed valve. Empty tank = 1 ATM because it not a vacum. Propane tanks, CO2 tanks all have different ratings so they are totally different. A scuba tank is the strongest that I know of.