Have you ever considered diving with a different size SCUBA tank than the one you normally dive with? Maybe you want to stay down longer so you figure a bigger tank will buy you some more bottom time? Perhaps the tank you always use just seems too big and bulky and you want a smaller, lighter tank that will be easier to handle? A bigger tank equals more gas. More gas may potentially get you more bottom time. How much more? Check out our nifty little calculator below (one of many throughout this article) to find out how much more or less gas you will have.
If you look at how much more gas you get by switching from an aluminum 80 (11 liter) to a steel 120 (15 liter) tank and what you want is more gas on your dives then you'd think the decision is a no-brainer. After all, you get 55% more breathing gas from making that switch. You can get even more out of even bigger tanks! However, there are other things to consider. Every SCUBA cylinder has its positives and negatives. You can see a full breakdown of the characteristics of different tanks in our chart of common diving tanks later in this article.
There’s much more to choosing a SCUBA tank than simply getting a bigger or smaller tank for your needs. By reading this article you'll know more about SCUBA tanks than most divers do and how to choose the best one for you.
What to consider when purchasing a SCUBA tank?
Many factors go into choosing the perfect SCUBA diving tank for you. If you choose poorly you could end up with a tank you hate or one that simply sits around and gathers dust because you'll realize after a few dives that you'd rather not use it. . Some of the things you'll need to consider when choosing the tank that's right for you will include the size/capacity of the tank, the material (aluminum vs. steel), the physical weight of the tank, the buoyancy characteristics of the tank in the water, how the tank affects your trim, and, of course, the cost of the tank.
Tank Terminology - Getting To Know The Parts That Make Up Your Tank
Before you learn how to choose the right tank, you must understand the terminology used for tanks and what it all means so that you can fully understand the choices that you are going to make when choosing a cylinder.
Tanks, Botttles, Or Cylinders, What's The Difference?
Depending on where you took your SCUBA classes and maybe where your instructor is from, you may have heard either the term SCUBA tank, SCUBA cylinder or maybe even SCUBA bottle. They are all the same thing. In this article, we use the terms interchangeably. Just to be clear, if you hear someone mentioning any of the terms above, they are talking about the same thing.
Steel vs. Aluminum
All SCUBA diving tanks are made out of either aluminum or steel. Each material has pros and cons which we will explain later in this article. Generally speaking, steel cylinders tend to be more expensive, they also tend to hold more gas than a similarly sized aluminum tank. Steel tanks also generally tend to have better buoyancy characteristics in the water, but more on that later.
Pressure ratings
All SCUBA cylinders have a maximum pressure that they can be filled to. In the United States, we use pounds per square inch (PSI) to measure the maximum pressure. In many parts of the world, BAR is the unit of measurement used. One BAR is roughly equal to 14.5 PSI.
Tanks come in three varieties when it comes to pressure.
Low pressure (2400 to 2540 PSI – 165 to 182 bar)
Standard (3000 PSI – 207 bar)
High Pressure (3300 to 3500 PSI -227 to 241 bar)
Again, each pressure has pros and cons which you will learn about in this article.
Capacity and Internal Volume
In the United States, all SCUBA tanks have a rated capacity. The capacity of a tank is the amount of gas a cylinder will hold when is full. In the Imperial system we use cubic feet to measure the capacity of a tank. The capacity of a tank is how many cubic feet of gas is inside the tank when filled to its rated pressure. So, as an example, an aluminum 100 cubic foot tank that is rated to 3300 PSI will hold 100 cubic feet of gas when the pressure inside is at 3300 PSI.
In other parts of the world capacity for tanks is expressed as its internal volume. The internal volume is expressed as the amount of water a tank can hold. This is mainly used in countries that use the metric system. So, a 9 liter tank holds 9 liters of water if it were filled with fluid. To determine the amount of gas a tank can hold you simply multiply the size of the tank in liters by its service pressure. So, as an example, an 9-liter tank that is filled to 207 bar will hold 1863 liters of gas when full (9 x 207 =1863).
To get a better understanding of how pressure relates to volume when it comes to SCUBA cylinders check out our SCUBA tank volume calculator below. Notice on the calculator how, when the tank is full, you have the full capacity of the tank. As you move the slider to the left, the pressure decreases as does the amount of available breathing gas.
SCUBA Tank Volume Calculator
The K-Valve
Close up of a K-valve on a tank
SCUBA tanks can be purchased with or without the tank valve. If you are purchasing a SCUBA tank with a tank valve you'll most likely be purchasing it with a K-Valve. Most all modern SCUBA cylinders use a K-valve. There are other types of valves, but their use is a topic for another article. Just know that if someone refers to the K-valve they are talking about the SCUBA tank valve.
DIN vs. Yoke
When you choose a SCUBA tank often the tank will come with a K-valve. That valve comes in two varieties, a DIN fitting or a yoke fitting. Your choice is often determined by the type of regulator that you are using. Yoke fittings are for the clamp-on type of regulator and DIN fittings are for the screw-on type. You can tell the difference because the DIN valves have a larger opening and threading on the inside in order to screw on a DIN regulator.
Notice how the DIN valve on the left has a large threaded opening for a DIN regulator to screw into. The Yoke on the right does not as yoke regulators are clamped on.
The bolt in the center is the burst disk. It can be unscrewed and easily replaced.
The burst disc is a valve that screws into the K-Valve itself. It looks similar to a bolt and is there for safety. If a tank is filled to an unsafe pressure above the tank's maximum rated fill pressure, the burst disk will break and safely expel all the gas contained in the cylinder. The purpose is to have the burst disc break before the tank explodes.
Burst discs can rupture due to age or even temperature changes when the burst disk is faulty. When a burst disc breaks, it is LOUD! All of the gas will escape the tank in a matter of minutes. It can be pretty scary if one goes unexpectedly.
Gas Mixtures
Because SCUBA tanks can hold air, nitrox, trimix, argon or, even pure oxygen when used for decompression in technical diving, it’s common for divers to refer to what’s inside as gas. Saying a cylinder is filled with something like air or oxygen is usually only done so when you know what the content of the gas inside the tank is, and you are referring to it specifically.
Inspections
SCUBA diving tanks must be inspected periodically to ensure that the metal is structurally sound and can be filled safely by a SCUBA compressor. Because SCUBA cylinders contain pressurized gas inside, if the integrity of the metal were compromised, the tank can explode when being filled.
In the United States tanks are visually inspected yearly and hydrostatically inspected once every five years. After the visual inspection is passed, the tank is usually marked with a sticker indicating when the visual inspection was performed. When Hydrostatic inspections are passed the tank, is physically engraved with the date of the hydrostatic test. The hydrostatic test is frequently abbreviated to just hydro. It's common to refer to this exam as the hydro test or the hydro date. .
Tank Engravings
All tanks have markings engraved on the shoulder of the cylinder. These markings indicate things like the manufacturer, material of the tank (aluminum or steel), serial number, working pressure, and the original hydrostatic test. For more information on how to interpret these markings check out this article.
Other than the visual inspection sticker, it’s common practice to label tanks that are designated for use with Nitrox or other gasses such as Trimix. It’s also a common practice in the technical diving community to place labels on tanks indicating the mixture of gas that’s inside and its maximum operating depth.
Tank Coatings
In the presence of moisture, aluminum will oxidize and steel will form rust.
For this reason, steel tanks have a zinc coating. Depending on the manufacturer, the coating may either be a zinc dipped coating that is applied during the galvanization process of the tank, or a hot zinc spray coating that is then protected by epoxy paint. This is done to protect the tank from rust.
Aluminum cylinders are either painted, left natural, shot blasted, or brushed. Many people believe aluminum tanks are best left unpainted because once the paint begins to chip, moisture can get under the paint and oxidize faster than if no paint had ever been applied in the first place. Also, because chipped paint can end up in the ocean, many manufacturers have moved towards using paints that are more environmentally friendly. Many believe these paints do not adhere as well as the paints used on tanks of yesteryear.
Whether or not the paint helps the exterior of aluminum tanks to avoid rusting is a topic for debate. My personal preference for aluminum cylinders is no paint at all since, sooner or later, the paint will chip and scratch no matter what.
Tank Boots
Both Aluminum and Steel SCUBA tanks can have tank boots. A tank boot is a rubber or plastic sleeve that goes around the bottom of the tank. The boot's purpose is to protect the surfaces that the tank is placed on.
Because a tank is a heavy piece of metal, a tank without a boot can damage floors, the fiberglass on a boat, and even the cement inside of a swimming pool. Also, because the bottom of most steel cylinders are curved, tank boots make it so that you can stand up the tank. Notice how the steel cylinder in the image is curved. If it were not for the tank boot, you would not be able to stand this tank up.
The drawback of tank books is that they can accumulate water and accelerate the oxidation process. For this reason, many divers choose to not have boots on their tanks.
Steel Vs. Aluminum
Now that you understand the tank terminology you can begin to figure out what tank(s) will be best for you. One of the first things you need to consider is whether to get a Steel SCUBA tank or an Aluminum SCUBA tank. Each has its pros and cons.
Aluminum Tanks
Most people begin diving with an aluminum tank. The most common tank is the aluminum 80 (11 liter) cylinder. Many dive boat operators and dive shops use this tank because 80 cubic feet of gas is enough to do most recreational dives safely, this cylinder is widely available and it’s also one of the cheapest tanks you can buy.
As a side note, the aluminum 80 does not actually contain 80 cubic feet of gas. Technically it contains 77.4 cubic feet of gas when filled to 3000 PSI. The number is rounded up to 80 by almost everyone. Two of the most popular manufacturers of these tanks are Luxfer and Catalina.
Just because you began diving with an aluminum cylinder does not mean that it is the best option for you. The main positive of using aluminum tanks is that they are cheaper and lighter than steel tanks.
For some specialized types of diving such as sidemount and technical diving, this is often the preferred option, but that is beyond the scope of this article.
Because aluminum is less dense than steel, many aluminum tanks will float when they are empty. You have to remember that the gas in the tank also has weight to it. As you consume the gas the tank will become lighter throughout the dive. All tanks will become lighter throughout the dive as you consume the gas contained inside. In the case of aluminum tanks, the shift in buoyancy from the start of the dive to the end of the dive is enough that most divers need to add weight to their BCD or weight belt to offset the positive buoyancy.
The slider below illustrates how an aluminum 80 goes from being negatively buoyant when full to being positively buoyant when empty. The concept is the same in the metric system even though the units of measurement are different.
Keep in mind that many divers need ballast to be able to descend in salt water. If a diver were to rely on the negative buoyancy provided by the gas inside a tank at the start of a dive, at some point during the dive, as the gas is consumed, the diver may become too positively buoyant to be able to stay underwater. This can be a dangerous situation. You can see in the video below how the same aluminum cylinder will have completely different buoyancy characteristics depending on the amount of gas that's inside.
Something else to consider about aluminum tanks is that they hold less gas than a similarly sized steel cylinder.
Finally, because aluminum is a weaker material than steel, many dive shops will not fill older aluminum tanks.
The good thing about aluminum tanks is that they are cheaper and lighter on the surface than steel tanks.
Pros: Familiarity, Cheap, lighter to carry around on the surface
Cons: Unfavorable buoyancy characteristics, you need to add ballast to offset its buoyancy, they hold less gas than steel cylinders and tend to have a shorter lifespan.
Steel SCUBA Tanks
Many experienced SCUBA divers as well as technical divers prefer steel cylinders. The main reason is that they hold more gas than a comparably sized aluminum tank and they also usually have more favorable buoyancy characteristics.
Because steel is a denser material than aluminum, steel SCUBA tanks tend to remain negative even when they are empty. This means you can use less ballast on your SCUBA rig. Also, because the weight that you take off of your rig is being distributed across your back, you will usually trim out better in the water than you would with a comparable aluminum cylinder (more on this later).
Also, steel tanks are available in larger capacities than aluminum tanks. If you need more than 100 cubic feet of gas in a single cylinder, then steel is the only choice.
As mentioned previously, steel SCUBA tanks also tend to have a longer lifespan than aluminum tanks. For this reason, many divers believe that, when you look at the true cost of owning steel, it isn’t more expensive when you consider the longer life.
The downside to steel SCUBA tanks is that they are more expensive to purchase. Also, you need to be careful to make sure you rinse off your tanks if diving in saltwater and pay attention to any water that may collect in the tank boot if you have one. This can lead to corrosion over time. You also should, as with all cylinders, never drain them completely to avoid the possibility of moisture entering the tank. Finally, steel cylinders tend to be heavier out of the water than aluminum cylinders.
Pros: Hold more gas than a comparable aluminum tank, better buoyancy characteristics, better trim properties, only choice for larger capacities
Cons: More expensive, can corrode if not maintained, heavier to carry around on the surface
Chart of common SCUBA cylinders
Below is a chart of some of the common SCUBA cylinders used by divers. This list is not comprehensive. However, if you compare the cylinders you should begin to get an idea of how different tanks have different buoyancy characteristics. For a more comprehensive list, check out our article dedicated to tank specifications.
High-Pressure Vs. Low-Pressure
As you look at the chart above you should notice that different tanks have different service pressures. The service pressure is the maximum pressure a tank can be filled to. The capacity of the tank is measured at its service pressure in the imperial system. In the metric system you multiply the capacity of the cylinder by its fill pressure in bar.
Below are the definitions for low, standard, and high-pressure:
Low-pressure (2400 to 2540 PSI – 165 to 182 bar)
Standard (3000 PSI – 207 bar)
High-Pressure (3300 to 3500 PSI -227 to 241 bar)
This brings up the question, is there any advantage to low pressure vs. high-pressure tanks?
In general, high-pressure tanks put more stress on O-rings and the internals of your SCUBA regulator. With modern regulators and if you are regularly servicing your equipment as recommended, this should not be a problem.
The only disadvantage high-pressure tanks may happen if you are filling your tanks with Nitrox from a provider that uses a partial pressure filling system and does not have a gas booster. If they are using this system and they are running low on oxygen, they may not be able to get the oxygen level high enough in your tank for you to reach your desired mix.
This can also happen with standard pressure and low-pressure tanks it’s just that because you need to reach a higher pressure with the high-pressure tanks, they are the most affected.
Another disadvantage of high-pressure tanks is that, if the dive shop you go to does not regularly fill high-pressure tanks, they may mistakenly under-fill your tank. This is easily remedied by checking your tanks before you leave the shop.
The big advantage of course is that you get more gas with high-pressure tanks.
Some people prefer low-pressure tanks because they are not as negatively buoyant as high-pressure tanks. Here, your gear configuration and your buoyancy requirements will come into play. Another reason for choosing low pressure tanks for some, is because they place less stress on your regulator.
One thing to be aware of if purchasing a low-pressure tank is the plus rating. When many low-pressure tanks are first sold, they are technically rated to 2400 PSI. The original hydrostatic test then has a plus rating that allows them to be over-filled by 10 percent. This plus rating is indicated by a “+” after the original hydrostatic date. This additional 10 percent brings the working pressure to 2640 PSI. When they are sold, they are marketed at the capacity reached at 2640 PSI (with the plus rating).
The image to the right shows the original hydrostatic exam performed on a steel cylinder. This hydro exam was performed on January of 2003. This is indicated by the 01 and the 03 respectively. The "+" sign after the date lets the person filling the tank know that this cylinder can be overfilled by 10 percent.
Sometimes when you have the hydrostatic testing done on the tank years later the operator will not conduct the test for the plus rating. If the subsequent hydrostatic test does not have this plus rating, then the tank may not get filled to its original capacity by some dive shops.
DIN Vs. Yoke
When purchasing a SCUBA cylinder, you’ll often have a choice as to the type of fitting you want for your K valve. The choice will most likely be determined by the type of regulator that you own.
As mentioned previously, the K-valve that comes on a SCUBA cylinder can be either a DIN or Yoke fitting. You can see in the image below the difference in how yoke regulators are clamped on and DIN regulators are screwed on.
The Yoke Valve
The Yoke Valve is a clamp-on type of valve. It is the earliest type of valve used by the original Aqua Lung SCUBA regulators. The yoke style regulator is also sometimes called an A-Clamp. The only real advantage to this type of valve is that clamping on your regulator is slightly faster and easier than screwing it on. It’s not a big deal however, we’re only talking about a few seconds difference when setting up your rig.
SCUBA DIN Valve
The DIN valve is a screw-on type of valve. They are a newer type of fitting and are gaining popularity over time. The DIN fitting is more secure which means it is less likely to leak. Also, because there isn’t a clamp sticking out of the back it’s less likely to get knocked loose during the dive. DIN valves are preferred for this reason by technical divers.
DIN Valves come in 2 varieties: 200 BAR and 300 BAR. The difference is that the threading on the 300 bar valves are a bit deeper to accommodate more threading and hence higher pressures. You can fit a DIN regulator made for 300 BAR into a 200 bar valve without issue. You can see in the image below how the same regulator fits more snugly on the 300 bar valve but has some threading exposed when placed on a 200 bar valve.
If you have the choice as to what kind of valve to get with your SCUBA TANK our recommendation is to get a 200 bar DIN valve with a donut insert. This is sometimes referred to as a convertible DIN valve. You can see an image of one below.
The donut insert adapter converts a 200 bar DIN valve into a yoke valve. This way you can have the flexibility of using either regulator. This is always convenient if you need to lend your tanks to a dive buddy who doesn’t have the same type of regulator set up that you do.
Choosing a Tank Capacity
Now that you know about all the parts of a tank, the materials it is made of, and the types of valves you can get with one, it’s time to begin to figure out what the best tank size is for you.
The capacity of the cylinder you choose is determined by your breathing rate, the depth you plan to dive to, and how long you plan to dive to a particular depth. You also need to factor in the amount of gas you’d like to have in reserve in addition to the gas you’ll consume throughout the dive.
Your Surface Air Consumption (SAC) Rate or Respiratory Minute Volume (RMV)
The most precise way for you to figure out how much gas you will consume is by knowing your Surface Air Consumption (SAC) rate or Respiratory Minute Volume (RMV). Both measurements are used to measure the rate at which you consume the gas inside your tank. We wrote an in depth article explaining the difference between the two here. The main difference between the two measurements is that SAC measures your breathing rate in PSI per minute (BAR per minute), and RMV measures your breathing rate in cubic feet per minute (liters per minute). Your SAC rate is tied to the tank you measured your breathing rate on. For this reason, we prefer to measure breathing rate in RMV. Most people in the dive industry use both terms interchangeably.
Your RMV rate is the amount of gas you consume per minute while diving. The number is expressed at the surface. So, as an example, a diver with an RMV rate of 1 Cubic foot per minute can expect to consume 2 cubic feet per minute at 33 feet, 3 cubic feet at 66 feet, and so on…
If you know this number, you can use it to estimate how much gas you will need for any given dive.
Planned Depth
To determine the correct tank size, you need to know what depth you plan to dive to. The deeper you go the faster you will consume your gas due to the increase in atmospheric pressure. This is why it is common for experienced divers to use bigger tanks for deeper dives.
Time at Depth
More time underwater equals more gas consumed. This is especially true on deep dives.
If you are at 99 feet (10 meters) for example, you can expect to be breathing 4 times the amount of gas that you would be at the surface with each breath because of the increased atmospheric pressure.
One way to plan for time at depth in recreational diving is by looking up the no-decompression limits at the depth you plan to dive to on a decompression table. You can then use the maximum time allowed at the given depth. This does NOT mean you should dive to the absolute limit, it’s just a way of estimating the amount of gas you might need conservatively.
Because most dives end up being multi-level dives another way to estimate the gas you’ll need on your dives would be to use your dive data from a dive computer and estimate how long you tend to dive at a particular depth.
Either way, it’s best to over-estimate the amount of gas that you will need since having more gas than what you need will only make your diving safer.
Gas Needed For A Dive Calculator
Reserve Gas
Once you know the gas you will need at depth, you should add in additional gas as a reserve. We all learned in our entry-level SCUBA course that you never dive until a tank is empty. It’s common for dive operators to tell divers they should be back on the boat with no less than 500 PSI or 35 Bar. This is usually assuming an 80 cubic foot (11 liter) tank. As stated earlier, it’s best to over-estimate the gas you’ll need. It’s also a good idea to try to have more reserves than the minimum.
Estimating Gas Without Your SAC or RMV Rate
If you do not know your SAC or RMV rate, a less precise way to estimate your gas requirements would be to look back at your dive log and estimate the amount of gas you would need to complete the type of dive you want to do.
Let's say as an example that you have done a dozen or so dives to a shipwreck at 100 feet (30.5 meters) using Nitrox and an aluminum 80. Your average dive was around 24 minutes returning to the boat with 500 PSI (35 BAR). You would like to have more bottom time, so you know you will need a bigger tank.
Following our example, the NDL at 100 feet (30.5 meters) for 32% Nitrox is 30 minutes (this number varies depending on which table you look at). This means you’d like to get 6 additional minutes or 25 percent more time. So, you’ll need a tank that has at least 100 cubic feet of gas to accomplish this.
Round-Up the Estimated Amount of Gas
It’s important to realize that you’re better off having more gas than what you need for a dive than not having enough. For this reason, when in doubt about choosing two similar tanks you’ll usually be better off choosing the larger size.
Tank Buoyancy, Trim & Size Characteristics
Tank Buoyancy
Once you know how much gas you need to be able to do your desired dives, you need to consider the buoyancy characteristics of the tanks that hold the amount of gas that you need. Different tanks have different buoyancy characteristics.
In the video below you can see how I was able to add just enough weight to an aluminum 80 to get it to be neutrally buoyant in fresh water. This is an exercise I suggest you do for yourself with the tank that you dive. You want to know exactly how much weight (if any) you might need to offset the buoyancy of your tank at the end of the dive. Do not rely on manufacturer specifications or SCUBA charts! We've found that those numbers can be off in the real world when testing for ourselves.
Note that the buoyancy characteristics of tanks on most graphs are usually for saltwater diving. Fresh Water will affect your buoyancy and the buoyancy of the tank you use. You'll need less lead in fresh water than you would in saltwater. Tables, like the one on this page should only be used to compare similar tanks and get an idea of the cylinder you want to use or purchase. You should perform your own buoyancy check for your tank as shown in the video above.
If you’ve done a proper buoyancy check and know how much lead you need to be properly weighted then you might want to consider using a steel tank to offset some of the ballast that you’re using either in your weight belt, integrated weights, or even in the form of a stainless steel backplate. Doing so has three advantages. First, by spreading the weight across your back you'll trim out better (in a more horizontal position) in the water. Second, by shedding some of the ballast you need to dive, it'll be easier to carry your dive gear. Finally, if you move to a steel tank you'll usually be able to get a tank with a higher capacity (more gas) in a similar size as an aluminum tank.
Something else to consider is if you know you need little to no weight, or if you are diving in freshwater, then you may need to consider the amount of lift your BCD or Wing provides and whether it’s enough to offset the negative buoyancy of the tank that you are considering.
Similarly, if you are using a thick wetsuit or a drysuit when you go diving then you need to consider the buoyancy characteristics of the suit and undergarments you are using. Here it may be advantageous to choose a cylinder that's more on the negative side to help offset the positive buoyancy created by your suit.
Proper weighting is a completely different topic. For more information, you may want to see this in article.
Tank trim Considerations
Something else to consider when choosing your tank is how it will affect the way your body trims out in the water.
Consider the diagram below. The diver on the left is using aluminum 80 (11 liter) tank while the tank is still full and negative. Notice where their center of gravity is. The diver on the right is using a high pressure steel 80 (10 liter) tank. Both tanks hold the same amount of gas when full. Notice however, that because the high pressure steel tank on the right is shorter, the center of gravity shifts. It helps the diver maintain a more horizontal and streamlined position during the dive. This does not mean that choosing the right tank is the only way to properly trim out a diver but it is a factor to consider.
How large or small a Cylinder is will affect how easy it is to carry and store your tank. A high-pressure steel 133 cubic foot tank for example, is both larger heavier, and bulkier than a Aluminum 80. The 133 cubic foot tank will however give you 55 additional cubic feet of gas during your dive. You can see the size difference in the image to the left.
If you are considering getting two different tanks that each have a different diameter you should know that you will have to adjust the tank strap on your BCD when you switch from one tank to the other. It’s not a big deal, but it is a small inconvenience you should be aware of. You can see that difference in diameter in the picture above.
Hidden Tank Costs
Overall if you dive enough it’s usually more economical to own your tanks. It’s also best because you can have the tank size and weight that best suites you.
You should, however, be aware of the costs associated with owning a tank. Over the life of the cylinder, you’ll have to pay for more than just the gas fills.
Visual & Hydrostatic Inspections
Every country has regulations regarding how compressed cylinders need to be maintained. In the United States, the Department of Transportation (DOT) regulates SCUBA cylinders. In the US, SCUBA cylinders need to be visually inspected yearly, and hydrostatically inspected every 5 years. This is a cost you should factor into owning a tank. Prices for this varies, so check with your local dive shop.
The visual inspection is performed by dropping a special light into the tank to check for any signs of corrosion, cracking, pitting or anything else that may affect the structural integrity of the cylinder.
Some service technicians may also perform a visual eddy as part of the visual inspection. This is an additional test where an electrical current is run through the neck of the tank in order to further check the structural integrity of the tank.
When a visual exam is passed a sticker is placed on the tank to indicate the date when the visual inspection was performed and passed. You can see an example of a visual inspection sticker from a local dive shop in the photo to the right.
A hydrostatic exam requires a special machine. There are several ways to perform this test. The most common is a machine where the tank is placed into a steel container that is surrounded by water. The machine then pressurizes the SCUBA cylinder to five thirds of its working pressure. The technician will measure the expansion of the metal under pressure. If the tank is within the allowed tolerances for the exam, it will pass the test.
When a hydrostatic test is passed the tank is permanently engraved with the date of the hydrostatic test.
Both the visual and hydrostatic tests are essential to make sure SCUBA cylinders are safe to fill. Dive operations can deny filling any SCUBA cylinder they feel may be unsafe to fill. This is because, if a SCUBA cylinder bursts while being filled, it’s like a bomb exploding. It can kill or injure anyone who is unfortunate enough to be nearby as well as cause significant property damage.
Valve maintenance
Over time the O-rings and other internal parts of a SCUBA valve can become worn and even corrode. Your local dive shop will inspect the valves at a minimum when you do the visual inspection. From time to time, the valves will need to be rebuilt since, just like your regulator, they are mechanical and need to handle 3000 PSI (207 BAR) or more regularly.
Tank & Valve Cleaning for Nitrox
If your SCUBA tank will be filled with Nitrox, you’ll most likely need to have the tank and valve cleaned for Nitrox. Most SCUBA cylinders come cleaned for Nitrox from the factory, but once the first hydrostatic test is done, you’ll need to clean the tank afterward.
Additionally, many dive shops will require you to label your tank with a “Nitrox” sticker. This is so that everyone knows that the tank contains a mixed gas and to avoid anyone confusing your tank for an air tank.
Renting Vs. Buying
When to Rent
Should you even buy a tank at all? If you have to fly to get to all of your diving destinations, then owning a tank doesn’t make sense. Also, if you live far enough from a dive shop that driving there to fill your tanks is a major inconvenience, then you’ll probably also be better off renting your tanks when you dive.
Another thing to consider is how often you dive. If you only dive several times a year all the hassle of filling and maintaining the tank may not be worth it to you.
When to Buy
If you dive fairly regularly and live close enough to a dive shop that can fill your tanks, you’ll probably want to own your tank.
Below are just some of the advantages of owning your tanks:
- You can match the size of the tank to the gas that you need for a dive
- You can choose a tank that has better buoyancy characteristics
- Your tank can help you achieve better trim in the water
- You can have more control over the mix of gas inside the tank and not be limited to what may be available for rent
- You can choose the type of tank valve that you prefer
What to Do If You Can't Buy
If buying a tank doesn't make sense because you travel to dive or it's simply inconvenient, it doesn't necessarily mean you need to settle for whatever the dive operator gives you. Many dive operators rent out different size tanks and steel tanks as well as aluminum ones. If you want to dive something other than the standard aluminum 80 (11 liter) you should ask. If the operator you ask doesn't carry a variety of different tanks then you might want to ask a different dive operation that goes to the same sites. It might be as simple as making a phone call or browsing their web site.
How Many Tanks Do You Need?
How many tanks you’ll need will depend a lot on how often you dive, what types of dives you do, and what you can afford.
If most of your diving is going to be using commercial dive operators, you’ll probably want to start with 2 tanks since most dive trips consist of 2 separate dives.
Your first 2 tanks should be able to hold enough gas for the deepest dives you plan on doing. Since you’ll only have 2 tanks, you’ll want to be able to do both deep and shallow dives. It’s better to have bigger tanks that you can also use on shallow dives than to get tanks that are too small for you to use on deeper dives.
If you do a lot of diving in both shallow water and deep water, you may want to consider owning 4 tanks. One set of tanks for the shallow dives and another for deeper dives. This is a personal choice if you want to go this route. The advantage of having smaller tanks for shallow dives is that they are easier to handle and may trim you out better in the water.
For most recreational divers 4 tanks are more than enough to cover just about anything they might want to do.
Tank Maintenance
A SCUBA diving tank is a long-term investment. Regardless of which cylinder you decide is best for you, you’ll want to take care of it so that it can last as long as possible. Below are some suggestions to help get the most life out of your SCUBA tank.
Rinse after every use
If you are diving in salt water, you’ll want to rinse the tank off after every use. Saltwater left on tanks can accelerate the oxidation process. Don’t forget to rinse off the tank valve thoroughly since it contains moving parts.
Never completely drain your tank
The inside of your SCUBA diving tank is particularly vulnerable to oxidization if moisture gets in. Luckily the easy way to avoid this is to never completely drain your SCUBA tank.
Even if you never completely breathe down your tank during the dive it’s still possible for the tank to drain after the dive. A small regulator leak or even a low-pressure inflator that’s leaking ever so slightly can slowly drain the tank if it’s not disconnected or at least turned off. So, make sure you close your SCUBA tank valve after the dive is complete.
Periodically remove your boots
As mentioned earlier tank boots tend to collect water inside of them. If you choose to have boots on your tank, consider removing them when you are rinsing your tank and allowing the tanks to dry without them. If this is too much of a hassle to do every time at least check periodically to ensure there is no oxidation forming under the boots. Better to catch it early and fix it than to allow the oxidation to grow.
If the valve got wet, open and close it before connecting a regulator to it
This one isn’t strictly for your SCUBA tank, but it’s worth mentioning. If your tank gets wet because of rain, because it was dunked in a rinse bucket, or any other reason, you want to make sure the moisture that may be in the valve doesn’t get into your regulator and damage it. A quick opening and closing of the valve should shoot any moisture that’s trapped inside the valve out so that it doesn’t escape under pressure into your regulator’s first stage.
Don’t forget your visual and hydrostatic inspections
Dive shops won’t fill your tanks if they are not current on their inspections. There’s nothing worse than going in to get your tank filled a few days before a planned dive only to realize you can’t get your fill because it’s due for a hydro. It’s a good idea to set a calendar reminder for when your visual and hydros are due so you don’t get caught by surprise
If you suspect water entered a tank, take it to your dive shop as soon as possible
As mentioned before, moisture inside a SCUBA diving tank can ruin the tank. Your dive shop can inspect the inside of your cylinder to see if any damage was caused by moisture inside the tank. If there is oxidation there is a process called tumbling which can be performed to remove the oxidation and restore the tanks for continued service.
If a Cylinder is involved in a fire, a significant car accident, or anything else that may affect its material strength, have it tested
It’s always better to be safe than sorry. All of the above can weaken the material of a SCUBA tank. If your tank was involved in a house fire or anything that may weaken it, then you should let your local dive shop know and have the tank tested before having it filled. Anything that affects the structural integrity of the tank's metal can be extremely dangerous if the tank explodes while it is being filled.
Tank Under-fills - Why doesn’t my dive shop fill my tank to its service pressure?
One of the complaints many divers have is that when they check their tank at the dive site it isn’t full. Below are several reasons why this may happen.
Hot Fills
When SCUBA tanks are filled with compressed gas they tend to heat up. Ideally SCUBA tanks should be filled slowly to avoid tanks getting too hot. However, no matter how slowly they’re filled, SCUBA tanks will usually heat up somewhat while being filled. This heat causes the molecules of gas inside the tank to expand somewhat producing a higher pressure. Once the tank cools down, the pressure drops a bit and can cause you to have a fill that is less than the pressure that was read when it was hot.
The best way to avoid this is to drop your tanks off with plenty of time so that your tanks can be filled properly and so that they can cool off. Then make sure you check your tanks before leaving. If they are under the rated service pressure, you can always ask to have your tank topped off.
Cold Temperature Outside
Just as hot air expands cold air will contract. A tank that was filled at service pressure in a warm environment can appear to have less gas when taken outside if it’s cold. This is also common if you are diving in cold water. It’s something to keep in mind when selecting your SCUBA tank. If you frequently dive in cold water you should account for the fact that your tank will most likely be under its full service pressure when you enter the water because of the cold environment.
The compressor operator is unfamiliar with your SCUBA tank
From time to time you may have a situation where the person filling the SCUBA tanks incorrectly assumed the service pressure of your tank. They may be used to filling all tanks to 3000 PSI and your tank has a service pressure of 3300. Maybe they saw that your tank was a low-pressure tank but missed the “+” rating on it and only filled it to 2400 PSI instead of 2640. Whatever the reason, this is why you should always check the pressure in your tanks before leaving the dive shop.
How to Store and Transport Tanks
Always lay your tank down
Because SCUBA tanks contain compressed gas, it’s always best to lay them on their sides to avoid a tank tipping over and possibly damaging the valve. It is possible that if a tank fell and damaged the valve on the way down that the damaged valve now leaking compressed gas could cause the tank to move violently and cause further damage or even injury.
Always leave some gas inside the tank
As mentioned previously, SCUBA tanks should never be completely drained. This is to avoid the possibility of moisture entering the tank. If SCUBA gear is attached to a SCUBA cylinder, make sure the valve is completely closed.
Always secure your SCUBA tanks when transporting them
You should never allow tanks to roll around in the trunk of a car or the bed of a truck. An unsecured tank’s valve can easily get hit and begin to drain. There’s nothing worse than coming to a stop in your car hearing the tanks in your trunk shift around and then suddenly hearing the hissing of gas leaving your filled SCUBA tank.
Securing your tanks can be accomplished by using a ratchet tie-down, using a portable cylinder rack (usually made out of foam or PVC), or simply wedging the tanks with your dive bag or other gear.
Conclusion
There are a lot of factors that go into choosing the tank that’s right for you. Some of the factors that you need to consider when choosing a tank are the amount of gas you’ll need for the dives you’ll be doing, the buoyancy characteristics of the tank you choose, and how the tank will trim you out in the water. Additionally, you should also consider the type of SCUBA tank valve you’ll need, whether or not to dedicate the tank for Nitrox use, and of course price. Choosing the right SCUBA cylinder can help you have longer more enjoyable dives. It’s one of the pieces of dive gear that most influences your diving.
What SCUBA tank do you dive with? Are you looking to change to a bigger or smaller tank? If so, why? Let us know in the comments below.
Additional Resources on SCUBA Tanks
