Scuba tanks while a very important piece of equipment are often under appreciated. While many divers are taught about tanks in their open water certification the role of a tank is left as the container of air. But in many cases the choice of tank can be as important as choice of BCD or fins. For most divers tanks are something that they might not normally think about, you travel to your destination and the shop provides tanks for you. Some divers might be surprised to find out that there is as much variety in tanks as most other pieces of scuba equipment.
Many divers are unaware of the effects of tank choice has on our diving especially buoyancy and time. Tanks come in a variety of sizes, the size of the take is determined by the volume of air that it is able to hold. A common tank size is 80 cubic feet, but these sizes can vary from as small as 6 cubic feet for a backup pony bottle to as large as 149 cubic foot high pressure tank. It is pretty obvious that the larger tanks will hold more air than the smaller if they are filled to the same pressure, but with the use of different metals and high and low pressure tanks this can also vary. For the most part Aluminum tanks despite the size will be filled to 3000psi, steel tanks on the other hand have a fair amount of variance. Low pressure steel tanks are exactly what they sound like they are rated for a lower pressure, depending on the tank it can range from 1800psi to 2600psi. High pressure tanks (commonly using a DIN valve) fill on average to 3445psi, which is higher than the standard 3000psi of aluminum tanks. What this means is that tanks that fill to higher pressure have more air packed into them than tanks that are the same size that till to a lower pressure. So by choosing tanks that are larger and fill to a higher pressure (my favorite is HP80) you can increase your dive time compared to a smaller tank.
While size of tank might be an obvious way to increase dive time, tanks also greatly influence our buoyancy underwater. With different choices of metal aluminum being a weaker metal and steel being a stronger metal these weights contribute to our buoyancy underwater. Although aluminum is a lighter metal because it is weaker the walls of the tanks are much thicker. This does usually give tanks a greater overall weight when full, when the tank is emptied the is a drastic swing in its buoyancy characteristics. Aluminum tanks while they may vary slightly from manufacturer generally are about 4 lbs positively buoyant when empty or near empty. This means you will be lighter at the end of the dive making it more difficult to complete a safety stop. On the other hand while steel tanks are a stronger metal they don’t require as thick of walls and on average may be 1 lb positively buoyant to 2 lbs negatively buoyant depending on the manufacturer. This means with a steel tank that might be 3 to 4 pounds less lead you will have to add to your weights. The high pressure steel tanks can even be up to 4 lbs negatively buoyant when empty.
So weather or not you are buying a tank or renting a tank it is important to know how it is going to affect your dive, weather its is going to affect the duration or your buoyancy. Be prepared to make adjustments as necessary. If your unsure talk to the dive professionals to find out the buoyancy characteristics, and don’t forget to record in your logbook, your weights with each type of tank you use so you never have to second guess again.