Purchasing your first set of Scuba Cylinders

by TDC Instructor, Jim Vafeas

When new divers finally decide to begin purchasing their own scuba equipment, cylinders rate very low in priority.  Cylinders are easily rented and generally not considered a vital contribution to a divers personal comfort.

Purchasing your own set of cylinders is also a costly proposition. Regardless of how you care for your other scuba equipment, cylinders are required to be visually inspected annually and hydrostatically inspected every five years. And if you have Nitrox cylinders you have additional maintenance expenses.

However, there are several benefits to owning your own scuba cylinders. Having your own cylinders filled and ready to dive makes it easier to plan dives on a whim. You no longer have to reserve rentals in advance, make special trips to the dive shop to pick up or return the cylinders. You also donít risk the shop being out of cylinders when you do want to dive. Owning cylinders makes it easier to anticipate your weighting and improves your neutral buoyancy. So when the diver finally makes the decision to purchase their own cylinders, itís important to make the right cylinder choice.

Here's a brief history on scuba cylinders...  During the infancy of recreational diving, the most popular cylinder was the Steel 72.  Then in the 70's, Aluminum cylinders came along. Aluminum cylinders were cheaper to mass produce, had greater corrosion resistance and offered a little more gas.  The Aluminum 80 now became the standard.  In the 80's, high pressure steel cylinders became available with a fill pressure to 3500 psi.  These cylinders were work horses whose drawbacks were realized when some operators could not fill them fully, causing these high-pressure Steels to carry less gas then their rated capacity.  Then in the 90's, Steel cylinders once again reinvented themselves using a chrome/molybedenum/steel composite that produced lighter cylinders with a much lower fill pressure (2640 psi) and improved buoyancy characteristics.  These then became the preferred steel cylinder.  A few years ago, the same manufactures who offered these low pressure steel cylinders, came up with a process to make cylinders of similar specifications, but to higher fill pressures (3442 psi) resulting in increased gas capacities.  These cylinders are gaining popularity among local divers.  While the Aluminum 80 is still the most popular and widely used cylinder worldwide, several size choices exist for cylinders made from either Steel or Aluminum.

Tiedemannís Diving Center sells tanks from Sea Pearls, who distribute Luxfer Aluminum and Worthington Steel cylinders. Because of their favorable buoyancy characteristics, I always recommend Steel cylinders over Aluminum cylinders (even though Aluminum cylinders are roughly half the cost) for local diving.  Unlike Aluminum cylinders, Steel cylinders do not become positively buoyant towards the end of the dive.

Letís take a look at your common Aluminum 80 cylinder and how their specifications stack up against similar capacity Steel cylinders:

Cylinder pressure and capacity

Model

Capacity
(Cubic Feet)
Service Pressure
(PSI)
Buoyancy
(Pounds)
   Full      Empty
Weight empty
(Pounds)
Outside
diameter
(Inches)
Cylinder
height
(Inches)
Aluminum 80
77.4 3000 -1.40 4.40 31.4 7.25 26.1
High-Pressure 80
X7-80
80 3442
-9.00
-3.00 28.0 7.25 19.7

High-Pressure 100
X7-100

100 3442 -10.00 -2.50 33.0 7.25 24.0

The new high-pressure Steel 80's are more than 6 inches shorter than a standard Aluminum 80, which I think is too small to be comfortable for all but shorter divers.  A tank this short might make it difficult to don in a seated position.  The high-pressure 100 on the other hand, is less than 2 pounds heavier and 2 inches shorter, but carries 20 cubic feet more gas!

Now, let's compare the 3 most common sizes of low-pressure Steel cylinders (85s, 95s and 108s) to their newer, high-pressure counterparts:

Cylinder pressure and capacity

Model

Capacity
(Cubic Feet)
Service Pressure
(PSI)
Buoyancy
(Pounds)
   Full      Empty
Weight empty
(Pounds)
Outside
diameter
(Inches)
Cylinder
height
(Inches)
Low-Pressure 85
85 2640
-6.00
Neutral 34.0 7.25 25.0

High-Pressure 100
X7-100

100 3442 -10.00 -2.50 33.0 7.25 24.0

Low-Pressure 95
95 2640
-8.00
-1.00 40.0 8.00 24.0

High-Pressure 119
X8-119

119 3442 -10.90 -2.00 42.0 8.00 24.0

Low-Pressure 108
108 2640
-8.00
Neutral 43.0 8.00 26.0

High-Pressure 130
X8-130

130 3442 -11.70 -2.00 43.0 8.00 25.5

Across the board, the high-pressure cylinders offer several advantages over their low-pressure counterparts.  Specifically, greater negative buoyancy and increased gas capacity while weighing about the same.

However, the correct cylinder choice should take into account the type of diving you plan on using it for (shore, shallow wrecks, deep wrecks, technical), if you plan on filling them with Nitrox or if you plan and doubling them up.  It should also take into consideration your size, gas consumption rate, any physical limitations and your budget.  At Tiedemann's, we guide you through the decision making process to ensure the cylinder you purchase, is right for you!

Finally, a caution about purchasing new or used cylinders online.  While it's impossible to ignore the potential "great deals" that can be found through the Internet, when it comes to cylinders, additional costs are involved in making the cylinder "dive ready" that the buyer may not fully realize.

Cylinders weigh between 30 and 50 pounds so the cost of shipping alone can quickly consume any savings.  Cylinders also cannot be shipped full or with the valve in place, so all cylinders would have to be VIP'd before filling, regardless of the existing VIP date of the cylinder.  Additionally, once the valve is removed, all used cylinders to be used for Nitrox would have to undergo a complete re-cleaning - another costly service.

Other questions you should ask include: Why are the cylinders being sold?  What does the inside of the cylinder look like?  Are there any existing conditions that may prevent the cylinder from passing their next VIP or Hyrdo?  How can you be sure?  Some older Steel cylinders are prone to rust while some Aluminum cylinders made from a specific alloy have been banned from service.  Do you know which these are?  Are you purchasing one of these cylinders?

Even though there are some safeguards in place to protect the buyer if they have purchased equipment from unscrupulous sellers, those procedures are time consuming, drawn out and a lot of unnecessary headaches.  In the end, you may be able to recoup some of your expenses but you may still be without any cylinders.  Then what?  At Tiedemann's, the price we quote you for cylinders will include all the costs that are necessary to make your cylinder dive ready upon delivery.

So remember, even though purchasing your own cylinders is not an inexpensive proposition, with proper care and maintenance, we have seen old cylinders passing their 4th and 5th hydros!  Not much of your other scuba equipment can claim such a useful lifespan!  So examine all the options to help you make the best choice possible.  If you need additional guidance in your cylinder purchases, feel free to contact the shop at 516-796-6560.

Good Luck!
Jim Vafeas