Why High Pressure Steel Tanks Beat Aluminum Tanks

In dive shops and dive boats around the US, the standard 80 cubic-ft aluminum tank is still the most common.  Many women who dive, especially those who, like me, could be described as vertically challenged, find AL-80's to be too long to find a comfortable placement of the tank in their BCDs.  If you place the top of the BC in the typical height relative to the top of the tank (many divers use the curved, "shoulder" of the tank as a guide to where the top of the BCD should be,) the tank will then hang down way too low on height challenged divers like myself.  The bottom of the tank extends too far down the body.  In very short people, an aluminum-80 with that placement in the BC can reach almost to the back of their knees, interfering with trim, and even with kicking properly.  If you hike the tank up relative to the BCD, the regulator ends up right behind the diver's head, which means hitting their head every time they tilt their head "up" or "back" (depending on their orientation in the water when they do it.)


So what's the best course of action for female scuba divers (or shorter male divers, for that matter)?  Don't dive an aluminum scuba tank.


High pressure steel cylinders, which are steel scuba tanks manufactured to be filled to a pressure of 3000 psi or more, will hold the same amount of air as an aluminum tank in a smaller overall package.  This is because steel is much stronger than aluminum, and so the walls of a steel tank to do not have to be as thick as the walls in an aluminum tank.  What that means is that if you were to compare a steel tank at 3000 psi with an aluminum tank at 3000 psi that's exactly the same size on the outside, on the inside of the steel tank there's more space available, so the tank holds more air.  So, to get the same amount of air as in an aluminum tank, the steel tank does not have to be as big externally.  (Note: standard, low-pressure steel tanks do not have this advantage, because they are designed to be filled to a maximum pressure of 2250-2500 psi.)

There are two ways you can take advantage of this property of high pressure steel tanks:  You can increase the amount of air you're carrying for a given external volume, or you can decrease the external volume of the tank you carry for a given amount of air.

High pressure steel 80's (HP-80s) are much shorter than AL-80s, making them a much more comfortable and appropriate 80 cu-ft cylinder for shorter divers.  There are also HP-100s, which hold even more air, and although they're bigger than HP-80's, are still shorter than an aluminum-80.

What about AL-63's?

Many women divers solve the tank-length problem by using AL-63's instead of AL-80's.  The 63 cu. ft. aluminum tanks are significantly shorter than the AL-80's, eliminating the difficulties of having a tank that's too long for their body.  Unfortunately, using an AL-63 instead of an AL-80 means that you're bringing less air with you on the dive.  Although women do tend to have a lower air consumption rate, on average, than men do, this doesn't always hold true, and when air consumption is the limiting factor in your bottom time, diving an 63-cubic foot tank can often result in shorter dive times than with an 80-cubic foot tank.  On the other hand, struggling to maintain proper trim and buoyancy control with an AL-80 that's too big will increase your air consumption rate as well, shortening your available bottom time.

There are other advantages to using a steel tank as well.  Since aluminum tanks actually become positively buoyant towards the end of a dive, using an aluminum tank requires wearing extra weight to compensate for this, so you don't go floating up to the surface during your safety stop.  Steel tanks remain negatively buoyant even when they get down to 500 psi, so you don't have to wear as much weight when diving with a steel tank as you would with an aluminum one.

The main disadvantage to steel scuba tanks is primarily that they are more expensive than aluminum tanks.  That's why the AL-80 is pretty much the industry standard today.  Most dive shops and dive boats do not want to spend the extra money to buy HP-steel tanks instead of aluminum ones.  But if you're a diver that finds the AL-80s to be too long, investing in your own steel tank is well-worth it. You'll have more air, be more comfortable, and wear less weight on your belt if you do.