Scuba Cylinders - Misconceptions & Maintenance

There are a few things I would like to cover in this installment of the equipment blog.  First would be the big misunderstanding between cubic feet and psi. Then I will go over the different types of bottles, (metals and sizes).  Finally, I will cover safety and maintenance as well.


CUBIC FEET vs PSI  

Which tank holds more air:  1) aluminum 80 at 3000 psi, or 2) steel 65 at 3442 psi?  Most people would pick the steel tank because of the higher pressure at 3442 psi.  Well, if you are one of them, I am sorry to say you have been misinformed.  So let’s cover the difference between cubic feet and psi.  I have heard people say they want a tank a greater psi because it means it holds more air.  In a way they are correct.  An aluminum 80 with 3000 psi has more air than an aluminum 80 with 2000 psi, but a steel low pressure 120 at 2400 psi has more air than the aluminum 80 at 3000 psi.  It comes down to this, the cubic feet (cu ft) tells you how much air the tank will hold or the capacity of the cylinder while the pounds per square inch (psi) tells you the pressure (how full the bottle is).  So when you talk about an aluminum 80, this tank has a storage capacity of 80 cu ft which is less than a steel 120 (a storage capacity of 120 cu ft) regardless of the pressure.

So when picking out a tank, you need to know how much air you want to take with you.   If you are someone who uses a lot of air while diving and are tired of being the one who makes the group turn around due to air consumption, then you may want a bigger tank.  When I say bigger I mean a bottle with a greater storage capacity (cu ft) and not worry about the psi.

ALUMINUM vs STEEL

So what bottle do you get?  You basically have two options aluminum cylinders or steel cylinders.  From there you can break down the steel ones into two more categories; low pressure (2400) or high pressure (3442 or 3500 psi).  Here is a chart on the differences on the tanks:

LOW PRESSURE Steel

In sea water with valve	LP77	LP85	LP95	LP108	LP121
Service Pressure	2400+ psi	2400+ psi	2400+ psi	2400+ psi	2400+ psi
True Capacity @ 2640 psi	77.0 cuft	82.9 cuft	93.3 cuft	108.6 cuft	120.0 cuft
Buoyancy *FULL	- 6.8 lbs	- 7.1 lbs	- 10.1 lbs	- 10.7 lbs	- 11.1 lbs
Buoyancy *EMPTY	-1.0 lbs	- 0.7 lbs	- 3.0 lbs	- 2.6 lbs	- 2.0 lbs
Weight Empty	32.5 lbs	36.8 lbs	41.9 lbs	45.95 lbs	49.9 lbs
Outside Diameter	7.25 in	7.25 in	8.0 in	8.0 in	8.0 in
Length +/- 0.125”	23.2 in	24.7 in	23.7 in	26.5 in	29.2 in

HIGH PRESSURE Steel

In sea water with valve	X7-65	X7-80	X7-100	X7-120	X8-119	X8-130
Service Pressure	3442 psi	3442 psi	3442 psi	3442 psi	3442 psi	3442 psi
True Capacity @ 2640 psi	66.4 cuft	81.0 cuft	99.5 cuft	120.6 cuft	123.0 cuft	131.4 cuft
Buoyancy *FULL	- 8.7 lbs	- 9.0 lbs	- 10.0 lbs	- 11.0 lbs	- 10.9 lbs	- 11.7 lbs
Buoyancy *EMPTY	- 3.9 lbs	- 3.0 lbs	- 2.5 lbs	- 2.0 lbs	- 2.0 lbs	- 2.0 lbs
Weight Empty	25.1 lbs	29.9 lbs	33.1 lbs	39.7 lbs	42.5 lbs	44.7 lbs
Outside Diameter	7.25 in	7.25 in	7.25 in	7.25 in	8.0 in	8.0 in
Length +/- 0.125”	16.7 in	19.8 in	22.7 in	27.7 in	23.9 in	25.3 in

Now the nice thing about steel tanks as compared to the aluminum, is that they are more negative buoyant (Most people can take off about 6-8 lbs of lead when diving) and the high pressure tanks are also a bit shorter.

ALUMINUM

In Sea water with valve	AL50	AL63	AL80	ALN80	AL100
Service Pressure	3000 psi	3000 psi	3000 psi	3300 psi	3300 psi
True Capacity	48.4 cu ft	63.0 cu ft	77.4 cu ft	77.4 cu ft	98.8 cu ft
Buoyancy* FULL	- 2.3 lbs.	- 2.2 lbs.	- 1.4 lbs.	- 5.7 lbs.	- 4.3 lbs.
Buoyancy* HALF	- 0.5 lbs.	+ 0.2 lbs.	+ 1.5 lbs.	- 2.8 lbs.	- 0.6 lbs.
Buoyancy* 500 psi	+ 0.8 lbs.	+ 1.7 lbs.	+ 3.4 lbs.	- 0.9 lbs.	+ 1.9 lbs.
Weight EMPTY	21.2 lbs.	26.7 lbs.	31.4 lbs.	35.4 lbs.	41.0 lbs.
Outside Diameter	6.90 in.	7.25 in.	7.25 in.	7.25 in.	8.00 in.
Cylinder Length	19.0 in.	21.9 in.	26.1 in.	25.8 in.	26.2 in.

As you compare the charts you can see which bottle is going to be the best for you.  Aluminum tanks are less expensive but the steels have more benefits.

MAINTENANCE & SAFETY

Both Rod & Chris are certified
by PSI to perform visual inspections 

Once you get your cylinder you need to make sure you are safe with it and you properly maintain it. It is fairly easy to take care of the bottles; you just need to make sure you follow through. You can rinse off the bottle - this will help keep the salt away from the coating.  If you do choose to rinse your tanks, make sure you do not get water into the valve.  Make sure you always leave some air in the tank as well.  Never bring an empty bottle to the shop for air. If you do, a visual inspection will have to be conducted prior to any air fill to assess the condition of the tank.

The big maintenance exams for scuba cylinders are what are called a visual inspection and a hydrostatic test.  "Visuals" should be performed on your tanks annually, while "hydros", at least here in the U.S., should be performed every 5 years.  There are other times this maintenance has to be preformed: if there looks to be damage to the bottle, as if it appears to have been dropped; if the air inside has an odor; noise is made from inside the tank; increased weight; or if the valve has been removed.  In most cases though, under normal usage, just the basic visual will be done once a year and the hydro every 5 years in order for your tanks to be filled by a certified air station.

Now here is a little bit on safety.  Everyone should know not to leave tanks standing, especially with gear is attached. "Why is this?" you ask. Because if the tank falls, it could damage the valve or other equipment like your regulator or mask, or worse, could roll down the driveway or into the street. Strange how round things tend to roll down hill.  Another nice little safety tip is how to carry your tank properly.  When you grab the valve you should have the air outlet pointing away from the palm of the hand.  This will make sure the pressurized stream of air does not blow into your hand, and holding the knob in this manner will turn it into the off position ensuring air will not leak from the valve.

So remember, when you are ready to buy a tank make sure you are getting what fits your diving. Do you want small and compact, or do you want all the air possible?  No matter what one you get, make sure you are safe with it and take the proper measures to ensure the bottle will last a lifetime.  That is about it for cylinders, maybe next time I will talk about the valves. Or is there something you want to know a bit more about? Just let me know.