Litigation has caused manufacturers to leave the market, and be replaced by others, who then left, and then others come back. From year to year, there's no telling what brand of tanks may be available, but the specs and recommendations below should apply to any.
Surprisingly, of all the devices that make up an underwater diving kit, the last to be developed was a suitable air reservoir. Demand air valves ( regulators to you and me, ) masks, fins, snorkels, weights, even drysuits may be found in the historical record up to several hundred years ago, but because of material and manufacturing requirements, the high-capacity high-pressure portable air cylinder is a relatively recent development. ( Jacques Cousteau often gets credit for something he did not invent - the demand-valve regulator predates him considerably, although he did much to perfect its application to diving. )
Your choice of tanks, in terms of size, number, and mounting hardware, should be closely related to your choice of BC and weight system, so if you have not already, you should take a look at that section.
Tanks are the most commonly rented of all scuba equipment, and it is often not much more expensive to rent full tanks than to have your own filled. For this reason, tank purchase can often be delayed to the end of your equipment buying period, especially if you are an infrequent diver. Returning rental tanks by the due date can be a pain in the butt though, so you might want to get your own anyway.
Scuba diving cylinders must undergo several types of periodic inspections. The annual visual inspection, or "VIP", is not a legal requirement, but is a standard maintained by the dive industry. Without a current VIP sticker from an accepted source ( such as your local dive shop, ) no respectable operator will fill your tanks, although it would be perfectly legal for you to fill them yourself if you have your own compressor.
Every five years, a tank must undergo a DOT-mandated "hydro" test. This results not in a sticker, but in a marking being permanently stamped on the tank. If a tank fails a VIP test, it may still be salvaged ( for example, internal corrosion may be correctable, ) but when it finally fails hydro, it is condemned - scrap metal - they usually drill a hole in it to make the determination final. The hydro test involves pressurizing the tank to 5/3 of its working pressure with water ( not air. ) Since water is incompressible, if the tank fails, the water will quietly leak out. If air were used, a rupturing tank would destroy a large part of the building it was in.
Much like the annual servicing of regulators, the industry-sponsored VIP inspection is more of a profit center than a safety enhancement. It depends entirely on the person doing the inspection, who may do a good job, or an indifferent job, or a completely incompetent job. Even when done properly, the completeness and effectiveness of a VIP inspection is questionable. The government-required hydro test is much more realistic, but unless you own your own compressor you are not going to get around paying your annual VIP "tank tax."
Main Tanks & Bands
Scuba tanks are made out of steel or aluminum. Aluminum is cheaper and has somewhat better corrosion resistance, but steel is stronger. The "standard" modern scuba cylinder is the aluminum "80", which actually holds about 77 cubic feet of air at its rated pressure of 3000 psi. This type of tank has probably caught on because for an average-sized person with reasonably good breathing skills, this size is just big enough to cover your allowable no-decompression bottom time, and no more. For smaller people, 63 cubic foot aluminum tanks are available, while for larger folks there are choices as big as 120 cft.
The most common alternative to the aluminum 80 is the steel 72, which holds about 72 cubic feet of air at about 2400 psi. The advantage of a steel tank is in its buoyancy characteristics. An aluminum tank becomes buoyant as it empties, up to +4 lbs., while a steel tank starts out around -3 lbs. and finishes near neutral. This means you can reduce your weighting by this much. ( Honestly, this is not much of an issue with a single tank. ) At the other end of the spectrum, high pressure aluminum ( 3300 psi ) and steel ( 3500 psi ) tanks are available, but these require special "DIN"-type regulators ( regulator conversion kits are widely available. ) Lower pressure tanks are faster and easier to fill; you may have problems getting a complete fill with high pressure tanks in some areas. A minor nit with steel tanks is that their negative buoyancy makes them somewhat tipsy in the water, as your tank and suit try to trade places, but you get used to this pretty quickly. With proper maintenance and handling, a steel tank can be expected to far outlast an aluminum one, which offsets the price difference somewhat. There are many 20 or even 30 year old steel tanks still in service, whereas an aluminum tank could be considered old at ten years.
One thing every diver should carry is a separate reserve air supply. This can be accomplished either with a small pony bottle that has its own separate regulator, or by doubling up two full-sized tanks, each with its own regulator. The latter system has the advantage of also doubling your air supply, giving longer bottom times on deeper dives, if you are willing and experienced enough to do some decompression. The advantages of a pony bottle system are lower cost and lighter weight.
There is only one way to set up a single tank, and you learned it in your open-water course, so there is really nothing more to discuss. There are a number of ways to set up double tanks, and each has its pros and cons. The fact that there is simply more to say about doubles than singles does not mean that it is a more valid or superior configuration, just more complicated, and not strictly necessary either.
The methods for doubling up tanks fall into two categories - metal bands that hold the tanks together semi-permanently, and Velcro bands that easily disassemble. Metal bands are often used with an air manifold that connects the two tanks together atthe valves. Without an extra isolation valve on the manifold, the two tanks function as one large one, and redundancy is lost. With an isolation valve they can be cut off from each other in the event of a regulator or valve failure and function as independent units. The isolation valve itself is a critical failure point, although in practice these are highly reliable. Since these bands do not easily break down, you must carry the entire bulk in one piece whenever moving it around, often an unpleasant proposition. The metal bands attach to your BC by means of bolts.
A more recent development is Velcro doubles bands. By far the best of these that I have seen (and use) comes from AquaExplorers - the
Ultimate Velcro Doubles Bands ( pictured below. ) these consist of a pair of central stainless steel brackets that bolt to your BC, which support two sets of regular Velcro cam-type tank bands. The brackets may be left permanently on the BC, or quickly removed if single-tank use is desired, as for an inlet dive. The tanks may be easily and independently swapped out of the bands, allowing for great flexibility, and any size of tank, including rentals, and even two different sized tanks, may be used. Scenarios include:
Redundant singles - use one tank on the fist dive, and the second tank on the second dive. ( In practice - use half the air on the first dive, and the remainder on the second. ) Effectively, this gives you a very large pony bottle, and spares you the trouble of breaking down your gear between dives.
One-and-a-halfs - use all of one tank and half of the second on the first dive, then swap out the empty tank and do it again on the second dive.
Independent doubles - empty both tanks on the first dive, then swap out both for fresh tanks for the second dive.
If these bands are set up very tightly, they are just as strong as steel bands, and could conceivably be used with a manifold, although this would be asking for trouble. If the tanks ever shifted, the expensive manifold would break. To tell the truth, I have endured many a lecture on the safety and superiority of manifolds over independent tanks, and in the end every one of them sounded to me more like an argument against manifolds than for them. One thing that is true is that with independent tanks, there is a need to periodically switch regulators in the water, and that simple but conscious gas management is required - it is best to draw down both tanks simultaneously in small increments, say 500 psi, so that in the event of the failure of the more-full tank, the other will not be empty. Manifolded tanks do away with these problems, but also lose the aspects of true redundancy, flexibility, and low cost. Another point in favor of independent doubles over conventional doubles is cost. Isolation manifolds and valves are dreadfully expensive.
A point against any kind of doubles is weight. Even the lightest set of doubled-up 80's is going to tip the scales at almost 100 lbs. with regs installed. This is about as light as it gets. Bigger folks who naturally use more air will want bigger tanks, but then they should be capable of hefting the extra weight. Of course, bigger tanks weigh even more, and that is why I say you should plan ahead for this when selecting your BC and weight system. Use of double tanks also makes a weight-integrated BC desirable, since you will now need to carry enough weight to sink two tanks. This was once the big attraction of steel tanks over aluminum, but if you no longer need to wear an uncomfortable weight belt, the price advantages of aluminum make them more attractive again.
It has been pointed out to me that I seem to be recommending double-tank setups. In fact, I am not. What I am recommending is the ability to flexibly configure your equipment for whatever type of dive you are doing right now - whether it be deep, shallow, long, short, boat, shore, currents, waves, etc. The discussion of temporary doubles bands is motivated by the fact that manifolded doubles are anything but flexible ( and are also ridiculously expensive ) and that it is relatively rare to actually need double tanks for a particular dive. See
Pony & Stage Bottles & Mounts
Having said all that about doubles, there are still times when it makes more sense to dive "lite". Out of the water, double tanks are very heavy and cumbersome, which makes them practical only for boat-entries, where you simply have to stagger across the deck and fall overboard. At other times, and in less deep situations, you can maintain a safe redundant air supply using a much lighter "pony bottle" - a small tank of 20-40 cubic feet, weighing only about 10-20 lbs, with its own regulator that is used only in an emergency. Some of the situations where this configuration is useful include medium-depth dives, 40-80 ft, training and practice dives ( especially in the quarry, where it is possible to reach quite deep water from the shore ), and excursions to the tropics.
Larger pony bottles ( 40-50 cf ) are also useful as "swing bottles" for carrying special decompression mixes, while the smaller ones are small enough to actually take with you when you travel. Most airlines will allow it in checked baggage if you remove the valve. This is very useful, since many tropical destinations are not equipped to provide the kind of safety equipment that we take for granted here. The smallest pony bottles are really not big enough to get you out of trouble on a deep Jersey dive, however.
The pony bottle is usually attached to your main tank by some sort of bracket or harness. The best one on the market is another ingenious AquaExplorers design, the
Ultimate Pony Bracket, pictured above. This compact and simple design is solid and secure, yet allows you to easily transfer the pony bottle from one tank to another between dives. All of the hardware attaches semi-permanently to the pony bottle, with nothing attached to the main tank. For air travel, simply slip out your pony bottle and leave it home, and install a rental at your destination, or take the whole thing along. This bracket is also available from Dive-Rite and Aqualung.
The one situation in which I would not recommend using a pony bottle is shallow shore diving. For slogging down a sandy beach or climbing rocks in full regalia, the lightest possible configuration is not just desirable but essential. Air redundancy is not really necessary in water that is only 15-20 ft deep - in the extremely unlikely event that you run out of air, there is an infinite supply just a few feet up. Shore dives like this also usually present many opportunities for entanglement, from fishing line to your own flag line, as well as currents and surges. All of these are good reasons to strip your configuration down to a streamlined lightweight working minimum for this kind of diving.
One consideration that seems to be seldom discussed is what to put in your pony bottle. There is no question in my mind that the only thing that belongs in a pony bottle is AIR. Filling your emergency air source with any of the typical depth-limited Nitrox mixes, 50/50 deco mix, or (worst of all) pure oxygen is simply courting disaster. This is because, depending on the mix, the pony bottle itself could become lethal at depth, either to yourself or to someone else, and is therefore no longer a safe and usable backup under all circumstances. People have died from making the mistake of using a pony bottle with the wrong mix at the wrong depth. If you want to use it as an emergency air source, then the only safe thing to fill a pony bottle with is air. If you fill it with something else, for example as a stage or "swing" bottle with a decompression mix that is not breathable at your intended depth, then it can no longer be considered a redundant backup, and you should make other arrangements.
Pony bottles of different sizes vary little in price, and are often not much cheaper than full-sized tanks - clearly the cost of the manufacturing process far outweighs the cost of the materials. Steel pony bottles are available, but at twice the price of aluminum, the tiny advantage in buoyancy is simply not worth it. Even aluminum pony bottles are marginally negatively buoyant.
A useful item is a small 'button' pressure gauge for your dedicated pony regulator. This lets you verify your pony bottle pressure quickly without having to swap regulators around.
A final word here is on a product called "Spare Air", often seen on television shows like Baywatch. This is a very small pony bottle of less than 3cf, with an integrated regulator. This is simply not enough capacity to get you out of trouble should you ever really need it. If anything, this device merely gives its owner a false sense of security that is more dangerous than having nothing at all !
Pony bottles are forbidden, except as stages. Senseless.
Scuba Cylinder Specifications
Here are some dimensions and specifications for scuba cylinders. This is by no means an exhaustive list. These numbers should be taken as approximate, since equivalent tanks from different manufacturers will differ slightly. The two most common models - the Catalina aluminum 80 and Luxfer aluminum 80 - are highlighted, along with my favorite, the OMS LP steel 85.
Weight, full - this is what you will have to be able to lift, in and out of your car, around the dock, and up the boat ladder with all your other gear. Times two for doubles.
Weight, empty - this is pretty much irrelevant
Buoyancy, empty - this is what you need to weight yourself for, so that you can do a safe free stop at 20 feet at the end of your dive
Buoyancy, full - this is what your BC needs to be able to support when you jump in the water, otherwise you are a "dirt dart" heading straight for the bottom and disaster
1 with valve, boot, and a good fill - 42 lbs
HP Steel: ( DIN valve only )
Pony & Stage Bottles: ( Aluminum )
Pony & Stage Bottles:
( Steel )
2 with valve & mounting bracket - 11 lbs 3 with valve, boot, and a good fill - 23 lbs
+ at rated pressure
* Faber ** Pressed Steel
density of dry air = 0.076 lb / cu-ft
Weights and buoyancies that I would consider to be excessive are highlighted in red. In particular, the Faber 72 is ridiculously heavy for its size, and excessively negative, and aluminum 80s are far too buoyant when empty.
Notes on the hydro test
The hydro test requires a tank to pass certain dimensional criteria when filled to 5/3 of its rated pressure. For a low pressure steel tank, this is 5/3 x 2400 = 4000 psi, which, for an OMS/Faber, is the manufacturer's rated pressure outside the United States. In other words, it is really no test at all, or, you basically can't fail hydro. ( You can still let it rust, though ! ) For an aluminum tank, this is 5/3 x 3000 = 5000 psi, and aluminum tanks fail quite commonly. For a high pressure steel tank, hydro pressure is 5833 psi. Good luck.
How to read specifications and inspection data - aluminum tanks.
How to read specifications and inspection data - steel tanks.
Thanks again to diver Art Greenberg for scrupulously checking these numbers ( even though he's still not quite satisfied with them. )!
For their resistance to corrosion, rough handling and general abuse, and their low price, aluminum tanks are very attractive to rental operations. That doesn't mean you should buy them for yourself.
The best tanks on the market are from OMS. These low-pressure ( 2640 psi ) steel tanks are costly, but have the best weight and buoyancy characteristics of anything available. The OMS 85's are my favorite because they are almost the exact same weight and dimensions as AL80's, with 10% more rated capacity. They also make a nice slim package when doubled-up. For shorter folks and ladies, the OMS 66 is a shorter version of the 85.
These same Faber / OMS tanks are used in Europe at rated pressures up to 3500 psi with no ill effects. They are rated to 4000 psi by the manufacturer, but derated to 2640 by the US DOT. Of course, overfilling a tank like is against the law, and you should never ever do it. ( If their products were not needlessly derated like this, Faber ( Italy ) would put the domestic tank manufacturers right out of business ! )
Apart from the extra capacity for deep diving, steel tanks are also superior for shallow shore diving, where the reduced total weight of tank + lead makes crossing a wide beach or climbing up and down rocks much easier. If you do a lot of shore diving, a steel 45 stage bottle would be a good investment. When used as a main, a tank like this has more than enough capacity for a shallow, time-constrained dive, and has virtually the same in-water buoyancy characteristics as a full-sized tank, with nearly half the out-of-water weight.
Not all steel tanks are created equal. I have heard repeatedly of other manufacturers' high-pressure steel tanks that fail their first hydro ! That's an expensive piece of junk. Based on what I hear from 'old-timers', OMS cylinders have no such problems, and will last forever if properly cared for. More and more of the local shops are carrying OMS tanks, but if you can't get them locally, you can mail-order them at very good rates from Leisure Pro. Don't settle for second-best.
June 2007 - this page has gotten rather dated. The OMS LP85s are still theoretically in production, but in practice unavailable. The LP line of tanks has been supplanted by a new line of HP tanks. The new HP100 is nearly identical in all specifications, but rated to 3440 psi. For all intents and purposes, the LP85 and the HP100 are equivalent. The HP tank is DIN-only, but is finally rated for its true working pressure, whereas the LP tank required some wink-wink-nudge-nudge to get a real fill.
I see a lot of shops nowadays selling manifolded doubles to beginners, claiming that this way they won't have to "upgrade" later. In fact, this is a very dangerous setup for an inexperienced diver, and has resulted in a number of accidents and more than a few fatalities. There is only one purpose for double tanks - decompression - the one thing an inexperienced diver should avoid. Beginners should instead start out with single tanks ( and a pony bottle, of course ) and build up their abilities by doing a great many safe, conservative, no-decompression dives before they even think about doing the kind of diving that would actually require the use of double tanks.
Most recreational diving in New Jersey or anywhere in the world can be accomplished with a single tank, and with experience, most people will find that they never want or need to "upgrade" to doubles. In fact, I know a number of people who bought expensive doubles rigs on the recommendation of their LDS, only to break them down into singles after one season.
It is unforgivably irresponsible for a dive shop to recommend and sell advanced equipment to inexperienced divers, and it is foolishly dangerous for beginners to buy equipment that is beyond their abilities and experience to safely use.
The only allowed DIR setup is double steel tanks with permanent stainless-steel bands and isolation manifold. There are other details, and some variance is allowed in tank size to suit the individual diver, but otherwise, it is implicitly assumed that this setup will cover all situations. It does not.
It is almost comical to see someone trying to climb down the rocks at Shark River with double tanks on, or shlep a huge set of doubles down the hill at Dutch Springs ( well, ok, most of those are Trimix students. ) Even in the open ocean, there are many situations where double tanks are unsuitable. The extra drag slows you down tremendously in the water, and the extra weight can turn reboarding the boat in rough seas into a real adventure.
For recreational diving, double tanks should be considered a last resort for big people who use a lot of air, and can easily manage the extra weight. I have seen many smaller people and women staggering under the weight of even a small set of DIR doubles that they don't need, and scratch dives on rough days because they don't think they can make it back into the boat ! In most situations, you are better off with a single tank and pony bottle.
I have presented a great many flexible options in tank selection and rigging above. DIR presents you with one option. It may be the one for you, but more likely it is not. Think about where you want to dive, what you want to achieve, and what you need, rather than mindlessly jumping into an expensive and limiting DIR rig.
DIR also forbids tank boots. All I can say about that is don't even think about bringing a set of round-bottomed tanks on a dive boat. For the real world - use tank boots.
The real hard-core DIR material states that steel tanks should only be used for cave diving, and only aluminum tanks should be used for open-water, because their buoyancy assures that in an emergency you will be able to reach the surface with your regulation steel backplate on. A better and much cheaper idea would be to lose the steel backplate and pick up some ditchable lead weights.
The proper way to fill a scuba cylinder is very slowly, so that the heat generated by the filling process can be bled away as it happens, and the tank does not become overheated. That's a great theory, but in real life, nobody does it that way. Most tanks are "hot filled", that is to say that they are filled too fast to prevent them from heating up during the process. If they are filled directly off a compressor, then they will receive a very hot fill. You would be surprised how warm the surface of a tank can get. This is not much of a safety issue, though, as it is well within the thermal limits of the material, either aluminum or steel.
The real issue here is that eventually ( after a few hours ) the tank will cool down, and as it does, the air inside will contract, and the pressure will drop. This typically results in about a 10% loss of pressure. At this point, you could top it off to the rated pressure again, and have a truly full tank. Most of the time that's not an option. Therefore, if the shop hot-fills your tank to 3000, you will end up with 2700. To get 3000, you need to fill the tank to about 3300. While this is verboten, it is well within the 5/3 overpressure of the hydro test, and realistically there is no danger. The slight overpressure may wear out your o-rings a little faster, but o-rings are cheap - get a dozen of them at the hardware store for a buck.
Some shops will refuse to do such hot-overfills, after all, they're not supposed to. If you are willing to leave your tanks and pick them up another day, you can get a top-off and walk out with a good fill. If that is not possible or practical, and your shop will not give you a good fill that will cool down to 3000 ( or whatever ) - then find another shop that will.
Another issue with air fills is the quality of the air you are getting. What you want is pure, clean, dry air. What you may get is water, compressor oil, carbon monoxide, pollen, and other contaminants. This depends on the setup of the facility and the quality and maintenance of the compressor and filters. Air intakes should be away from sources of chemical contamination such as parking lots. Filters should be changed regularly, and the compressor itself should be modern and well-maintained. Ask around and verify the shop's reputation with regard to their air quality. Most are good.
Here is the actual Federal regulation regarding pressure rating of gas cylinders, courtesy of Kenneth Adelman:
[Code of Federal Regulations]
[Title 49, Volume 2]
[Revised as of October 1, 2006]
From the U.S. Government Printing Office via GPO Access
TITLE 49 -- TRANSPORTATION
CHAPTER I -- PIPELINE AND HAZARDOUS MATERIALS SAFETY ADMINISTRATION, DEPARTMENT OF TRANSPORTATION
PART 173 SHIPPERS GENERAL REQUIREMENTS FOR SHIPMENTS AND PACKAGINGS -- Subpart G_Gases; Preparation and Packaging
Sec. 173.301a Additional general requirements for shipment of specification cylinders.
(a) General. The requirements in this section are in addition to the requirements in Sec. 173.301 and apply to the shipment of gases in specification cylinders.
(b) Authorized cylinders not marked with a service pressure. For authorized cylinders not marked with a service pressure, the service pressure is designated as follows:
Service Specification Marking
(c) Cylinder pressure at 21°C (70°F). The pressure in a cylinder at 21°C (70°F) may not exceed the service pressure for which the cylinder is marked or designated, except as provided in Sec. 173.302a(b). For certain liquefied gases, the pressure at 21°C (70°F) must be lower than the marked service pressure to avoid having a pressure at a temperature of 55°C (131°F) that is greater than permitted.
(d) Cylinder pressure at 55°C (131°F). The pressure in a cylinder at 55°C (131°F) may not exceed 5/4 times the service pressure, except:
(1) For a cylinder filled with acetylene, liquefied nitrous oxide, or carbon dioxide.
(2) For a cylinder filled in accordance with Sec. 173.302a(b), the pressure in the cylinder at 55°C (131°F) may not exceed 5/4 times the filling pressure.
(3) The pressure at 55°C (131°F) of Hazard Zone A and, after December 31, 2003, Hazard Zone B materials, may not exceed the service pressure of the cylinder. Sufficient outage must be provided so that the cylinder will not be liquid full at 55°C (131°F).
(e) Grandfather clause. A cylinder in domestic use prior to the date on which the specification for the cylinder was first made effective may be used if the cylinder has been properly tested and otherwise conforms to the requirements applicable to the gas with which it is charged.
[67 FR 51645, Aug. 8, 2002, as amended at 67 FR 61289, Sept. 30, 2002; 68 FR 24661, May 8, 2003]
In summary, a tank may be legally overfilled to up to 5/4 its rated pressure provided the temperature does not exceed 131°F, and the final cooled-down pressure at 70°F is at or below the rated pressure.
I make no claim as to the accuracy, validity, or appropriateness of any information found in this website. I will not be responsible for the consequences of any action that is based upon information found here. Scuba diving is an adventure sport, and as always, you alone are responsible for your own safety and well being.