by Gary Carlson
A typical scuba tank in use by recreational divers is the “Aluminum Eighty” meaning, as we learn in our Open Water class, that the tank is made of aluminum, and contains 80 cubic feet of air. I was taught that this is compared to the volume of a telephone booth, but just try to find one. We are also told that when full it contains 3000 lbs. of air. These “pounds” are just an abbreviated “pound per square inch (psi)” because certainly the tank does not weigh a ton and a half.
The square inch bit is easily explained; get an American ruler 12 inches long (300mm if it helps), holding it on edge with the numerals reading upright, an inch is the distance from the left end of the ruler to the mark designated by the number 1. Drawing a square on a piece of paper using this measurement for each of the four sides produces one square inch.
The “pound” in psi refers to the archaic unit of weight still used in unenlightened parts of the world, and is made up of sixteen separate units called ounces.
To understand the force of 1 psi get a tall can of beer (16oz.) which has a weight of one pound prior to opening, and balance it on the pad of your thumb (which is close enough to a square inch for this discussion) and you will feel 1 psi as weight. Consider 3000 of these and the resultant thickness of your thumb!
3000 pounds is the pressure on every square inch of the inside of a scuba tank, and although it may seem like a lot, we are just getting started. The typical scuba tank (aluminum 80) has exterior dimensions of 7.25 inches diameter, and a height of 26 inches without the valve. The tank walls are deceivingly robust being about a half an inch thick with the top and bottom at about an inch, giving us an estimated inside dimension of 6.25 inches diameter, and about 24 inches in height. Calculating the surface area of the inside we get 532 square inches, of which each and every one of them will have 3000 lbs. pressure against it when full. For those without a calculator handy, this equates to a total pressure against the interior of an aluminum 80 scuba tank when full to being 1,598,655 lbs. ONE MILLION FIVE HUNDRED NINETY EIGHT THOUSAND SIX HUNDRED FIFTY FIVE POUNDS!
Yikes! (You may exclaim) I routinely strap one of these on my back and go underwater! While the numbers make it seem like you are carrying a nuclear device, the chances for explosion are relatively small. In fact a diligent internet search has shown no cases of a tank exploding during a dive. (Whew)
In a responsible dive shop, every tank goes through regular scheduled visual inspections every year by a qualified technician, where the tank valve is removed and the tank is inspected on the inside for moisture and corrosion. Every five years scuba tanks are hydrostatically-tested at a testing facility where the tank is filled with water and pressurized while in water, and the tank expansion is calculated. The tanks passing this test are marked on the top with a metal stamp. All aluminum scuba tanks must be stamped “3AL” to be legal, and have the date of the last hydro test stamped on them as the month and year separated by the testing facility’s ID stamp. Those tanks not meeting defined criteria after either of these tests, are removed from service, and destroyed. Please recycle.
However Googleing “scuba tank explosion” will result in finding horrific examples of tank failure, all of which have happened at fill-stations. Most of these involve old tanks that have been discontinued made of the aluminum alloy 6351 by the company Luxfer. If mechanically abused, or overfilled the tank could become compromised with a chance, however small, of rupture. Becoming an issue around 2004, a special testing called Eddy Current Testing was required for these cylinders to remain in service. Passing this test along with the normal regular inspections deemed these cylinders acceptable for use, but most dive companies now refuse to have them, or fill them regardless of the testing.
Modern dive tanks have a safety device in the valve called a burst disc which is designed to rupture and release the tank pressure should it be filled to too high a pressure. This minimizes problems with compressor pressure switch failures resulting in the compressor pumping the tank far too full, or less than attentive tank filling employees (there can be a lot of distractions at a tropical dive destination).
As mentioned earlier scuba tanks are quite strong and tank failures so rare that the chance of a rupture is the same as winning the Mega-Lottery while being hit by lightning on your birthday. If you have seen the movie “Jaws”, a suspense flick from the last millennium featuring a giant shark eating most of the beach population of Martha’s Vineyard, you remember the last scene where the leading role is facing certain death and shoots the shark at the last minute right in a scuba tank lodged in its molar. The resultant explosion has pieces of shark raining down all over the ocean.
Certainly you can’t expect an aluminum tank to hold up against a .30-06! Actually you can, here is a video of Kyle and his friends shooting scuba tanks out on the range. Just a little spoiler; there is a decided lack of explosions.
When diving on vacation check with your resort or shop as to their tank inspection schedule, they should be willing to provide this information along with regular air testing certificates. It is comforting to know what is on your back. If you have your own tanks, be sure to exercise proper care and feeding, stay up on your inspections, be picky about where they are filled, and pack them carefully when traveling. Treat them as if they were an important part of your life support system.