Talking Tanks

Scuba Tanks - The Essentials

Have you ever been curious about the reliability of scuba tanks and the air they hold? Do you have unanswered questions about the ways they differ and how to care for them?

Cylinders are obviously one of the pieces of gear central to scuba diving. But for such a vital part, why is there such diversity? Scuba tanks come in many different sizes and each have a certain maximum pressure rating. And not only that, but they can also come in aluminum and steel versions, as well as a few other minor types.

So, what type of tanks should you use? The smaller sizes are generally used for alternate air sources in an emergency. Larger sizes hold more air, but you can't always tell how much air a tank will hold just by looking at its size. Steel is a stronger alloy than aluminum, so the walls of steel tanks are thinner, enabling it to hold more air. Aluminum tanks are lighter and are not at the same risk of rusting.

The never ending discussion of what types of scuba tanks are best to use often brings up the question of how long a diver can stay under water. One thing to remember is that a diver should never drain the tank completely while diving. It is best to leave at least 300 psi in the tank to prevent out of air situations and prevent the tank from being contaminated. Some of the factors that affect how quickly a diver may use up air are:

Diversity - people are all different and will breathe at different rates;

Exercise - the more divers exert themselves, the more air they will breathe - performing strenuous activities and being out of shape will require more oxygen for the increased energy production;

Depth - the deeper you go, the faster the air is breathed out of the tank;

Temperature - the colder a diver is the more oxygen is required in order to assist the body in heating itself, and

Diving experience - anxiety, lack of good buoyancy control, poor kicking with the fins, increased breathing and not being relaxed are often attributes of novice divers. As a diver becomes more skilled and comfortable in the water, air consumption usually decreases.

Compressors used for filling scuba tanks are designed to follow a strict list of guidelines and maximum limitations for the specific make-up of the air the tanks will be filled with. The air is drawn from a clean, outdoor source and is dust-filtered and compressed in 4 different stages. Water and oil are drawn out of the air and the air is then filtered to remove carbon dioxide, carbon monoxide, remaining water, oil vapors, and other contaminants. Finally, the air enters the tank in a super-clean and super-dry state. This will cause dry throats when breathed during dives and no odor when the air is smelled.

Poisons in air are magnified as a diver descends. One way to tell if you are breathing pure air is to smell it. If even the slightest odor exists, such as an oily smell, it is best to not dive with that tank until it is dumped, cleaned, and refilled from a good air source.

Running out of air is not what you may think - the end point is not abrupt. The pressure in the tank will be low, so each breath will be increasingly difficult to draw. However, it will not suddenly be empty - the breathing will continue to be harder and harder until the air is depleted. The time is not definite, but there should be enough for a diver to stop and think, and then approach his buddy for an alternate or make an emergency ascent.

At the top of the scuba tank, there will be a place that shows a month, followed by a test station ID, followed by a year. It will look something like: 8 X 04. That is the hydrostatic test date. Scuba tanks must be given a hydrostatic test every 5 years. If the last hydrostatic test date is more than 5 years old, the tank will not be filled by most dive shops. This policy is for both the safety of the shop and the owner of the tank.

When a cylinder is filled it expands due to the pressure inside and contracts as the air is used up. If you take a piece of metal and repeatedly bend it back and forth it will eventually break because the metal gets fatigued. Although scuba tanks can withstand many, many fills, it is important to be sure the metal has not weakened or become brittle. Hydrostatic tests involve filling the tank with water and subjecting it to 5/3 of the pressure rating found on the tank's neck. This is so that expansion and contraction of the tank may be measured exactly and any cracks or weaknesses can be detected. It is also important to have the tank visually inspected inside and out every year for contamination, corrosion, neck cracks, intact threads, bows, bulges, corrosion, dents, and gouges. If the tank is free from damage a decal is applied that states the inspection was done and the tank is in satisfactory condition.

Tanks are fragile, so they should be well cared for. Making use of tank boots, covers and protective sleeves will ensure the tank has a long life and prevent it from becoming damaged. Pack tanks safely while travelling by car so that they don't roll around and bang into each other. Never leave a tank standing unattended where it can be easily knocked over, or on a high object unless it is supported at all times.

Finally, the question of how long air in a scuba tank is good for. If the air is not contaminated, it should last for years. If a steel tank was filled with air that did not have the water removed, the tank may rust on the inside. If in doubt, you can get a local dive shop to check the tank for you. If you haven't been diving for a long time, it's a good idea to get the tank checked - while you take a scuba review course!

Tanks for reading and happy diving!

by Marie