Tips on buying a SCUBA tank
October 1st, 2007 Filed Under Article
Making a purchase decision on your own SCUBA tank can be a confusing one. Aluminum or steel? High pressure or low pressure? What size? Here is what I learned in doing research for a purchase decision:
Aluminum versus steel:
As a primary/back cylinder, the common aluminum tank (AL - the Dept of Transportation code for an aluminum constructed compressed gas cylinder) is the 80 cu.ft. A 100 cu.ft. can also be found. There is one primary advantage of AL tanks - they cost less than steel tanks. Other than that, most people go steel over AL as a tank they wear on their back (a stage bottle or pony is a different story). In terms of buoyancy, AL tanks are not very negative when full and are positively buoyant by about 4 pounds when at 500 PSI. That means packing more lead in order to sink. One case where an AL can be beneficial is in the case of warm water diving where large capacity steel tank will have enough negative buoyancy that a diver in a thin wetsuit could still be negative even after dumping their weights. That is not a concern for me since I am a big guy and get cold easy, so I have more neoprene than most people and all that foam requires more weight to sink me; Plus where I live I will only be doing cold water diving with the tank. There are neutrally buoyant 80 cu.ft. AL tanks on the market, but they are more expensive than regular AL tanks. AL tanks have very poor resale value, and they tend to fail hydro long before a steel tank will. Most AL tanks are rated (will be full at) 3000 psi, with some rater at 3300 psi.
Steel (AA - the Dept of Transportation code for a steel constructed compressed gas cylinder) tanks have a thinner wall than AL tanks. Most AA tanks are more negative (for a given volume) at the beginning of the dive than AL tanks, and AA tanks end the dive still negative while weighing similar to an AL tank of same compressed volume on land. AA tanks come in a wider range of volumes and sizes than AL tanks do. AA tanks have good resale value because they last a long time and therefore hold their value well. It isn’t unheard of to buy a new one, dive it for a bit and sell it for 80% of what was paid for it. AA tanks cost anywhere from 2 to 3 times the cost of an AL tank.
High pressure or low pressure?
AA tanks pretty much come in two flavors, low pressure and high pressure. Low pressure (LP) is defined as 2400 psi. LP tanks on the market today have a plus (+) rating, which means they can be filled 10% over the 2400 psi, or to 2640 psi. High pressure (HP) tanks are rated to 3442 psi. HP tanks are relatively new to diving; they have been around since 2003 or so. Here are some fundamentals on LP or HP tanks before we move on:
- An advantage of a LP fill is that you know you will get a complete fill no matter where you go since all fill stations can fill to LP. Also, the tank doesn’t get as hot as a HP tank so there is less settling (lowing of tank pressure from drop in pressure) after a fill.
- HP tanks are great in that you get a lot of air in a compact cylinder, but they need to be filled slowly and preferably submerged in water to control heat so there will be minimal settling. There are probably some fill stations out there that cannot go higher than 3000 psi, but all that I have seen recently can give a HP fill. The problem is if you get a quick fill, like on a dive boat or in a shop at a dive location where the tank is not going to be in water during the fill, then you will end up with a short fill. I was talking to someone who recently went on a live-aboard where his fills would consistently settle to around 3100 psi.
- In the past many people would over fill their LP tanks to 3000 psi and above. The nickname for this is a “cave fill” since cave divers want the maximum amount of gas volume in the minimal amount of space. I think it is also because HP tanks haven’t been around very long, so what they were essentially doing was creating HP tanks from LP tanks. To do this they would need to add another burst disk to the tank. Over filling is a practice you can’t count on getting. Everyone is so safety paranoid today that even though there is a past record of the practice working, tank manufacturers and many fill stations will not condone it.
- Buoyancy characteristics of LP and HP tanks are about the same
- The price between LP and HP tanks are about the same for a given volume
- A HP tank does not need to be filled at HP all the time. It can be filled “short” to mid or low pressure, but the volume will be less. For example, according to a steel tank information page on the DiveRiteExpress.com web site, a HP 100 will be an 80 at 2640 psi, and an 89 cu.ft tank at 3000 psi.
- I read one belief that HP tanks strain equipment in the upper psi range of the fill, and that it is better to fill HP tanks to about 3000 psi.
Since a HP tank can also become a LP tank (with smaller volume), I decided to choose a HP tank of appropriate size so that the LP volume would meet my needs. That way I would always get enough of a fill that I need for most dives. My plan is to have it filled to around 3000 psi the majority of the time, and then occasionally take it up to 3442 psi if I need the extra volume.
One disadvantage of a larger tank is if the volume in it is more than you need (i.e. you come up with a bunch of air left) then you are lugging around the extra weight of that air in the water. For example, a 130 tank when full is about 11 pounds heavier than when at 500 psi. If a diver consistently only uses 5 pounds of that air, then they are carrying around an extra 6 pounds that has to be compensated by having more air in their BC…and the more air in the BC the more buoyancy is hard to control. Why not just drop 6 pounds off the weight belt in that scenario? Proper weighting means the diver can hold a safety stop with 500 psi, so if you don’t breath the air in the tank, then it is just extra weight. So….the biggest tank on the market is not necessarily the best for everyone. It is more like – get the tank that has physical dimensions that fit you and that has a LP volume that will work for you, and then choose the fill based on what you will probably need for the dive. This is why a diver may choose to short fill a high capacity tank and turn it in to a LP tank.
See the Variable Capacity of Cylinders section on this page from Dive Rite Express: http://www.diveriteexpress.com/gas/steel.shtml
Tank dimensions:
Different capacity tanks have different dimensions. Some are close in volume but different in shape. The diameter of the tank will be either approximately 7.25 or 8 inches, and the length varies by tank. As an example of two similar volumes with different sizes, the XS SCUBA 120 cu.ft. is 7.25 inches by 28 inches long. Their 119 tank is 8 inches by 24 inches long. For comparison, an AL tank is 7.25 by 26 inches. Length can play a part in comfort and trim. A shorter, fatter tank will put more weight high up on the diver’s torso. A tall diver probably wants to get a longer tank since they have a longer torso, and a shorter diver probably wants a shorter tank so they don’t feel like is it hanging between their legs in the water. I am 6’4”, but I chose not to get the 120 as I believe anything longer than an AL 80 will feel a tad long on me.
Which size did I choose? I dived a LP 95 and a HP 100 and felt those volumes were the minimum I want to be able to do the slow ascents that I want to do. In terms of volume, a 119 is perfect but it is a bit short for a tall person. I was torn between the 120 and 130 cu.ft. tanks as far as a purchase decision. I decided to go for the 130 since I was a little concerned about the length of the 120. So the 130 is three volumes in one tank – according to the Dive Rite Express web site regarding tank selection, a 130 is also a low pressure 104 and mid pressure 117. It all depends on how much air you put in it.
Additional reading:
Dive Rite Express page about choosing a tank
http://www.diveriteexpress.com/library/tanks.shtml
Dive Rite Express page about tank volumes
http://www.diveriteexpress.com/gas/steel.shtml
XS SCUBA specs page for their tanks
http://www.xsscuba.com/tank_steel_specs.html
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3 Responses to “Tips on buying a SCUBA tank”
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That’s a great post with a ton of information. Would have been really welcome when I was searching for new/used tanks!
Congrats, the 130s are an excellent choice. Excellent capacity and great buoyancy characteristics.
Its the same tank I went with now that I’ve moved beyond my AL80 and am heading towards tech diving.
The extra air available in the 130 over the 80 on a wreck dive at 80 feet is a beautiful and reassuring thing.
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