How to Tell the Size of Your Scuba Tank

Scuba tanks look very similar to each other and it can be very difficult to determine the size of the cylinder you’re using. However, it is vital to know the size of your Scuba tank in order to safely plan dives.

How can you tell the size of your Scuba tank? You can determine the size of your tank by looking at the markings on the crown of the tank. A standard Scuba tank usually holds 80 ft3 of air at 3,000 psi. However, you should never assume the size of your tank.

There are a lot of different types of tanks that can vary greatly regarding pressure and capacity. Pressure and size determine the volume of air a tank can hold – its capacity. It is important to check the markings on the crown of your Scuba tank to determine the capacity of your tank. 

Without knowing how much air your tank holds, you cannot plan a dive accurately or safely. You may end up running out of air during a dive which can be disastrous.

Understanding the Size Markings on Your Scuba Tank

There are two major things to consider when trying to determine the capacity of your tank: pressure and size. Temperature can also be a factor, but we will discuss that later.

  • Pressure: The pressure listed on your tank is the working or service pressure. It’s the amount of pressure required for your tank to be filled to capacity.
  • Size: This is the physical size of your tank. Tank size is typically measured in cubic feet (ft.3)

The size and working pressure of your Scuba tank can be determined by looking at the markings on the crown of your tank. 

There is a lot of information listed on the crown of the tank and the order in which the information is listed varies from tank to tank, but the codes are standardized and easy to read if you know what to look for. Some of the codes are required by law, while additional codes are included depending on the manufacturer.

Example 1

A Faber Steel Cylinder may have the following printed on the crown of the tank:

FABER MADE IN ITALY M8303 17/0228/170 01C14 TC-SU7694-237 BAR

DOT-SP13488-3442 PSI REE XX TP 5250 PSI XS SCUBA HP100

These markings can be deciphered in order of appearance:

CodeWhat it Stands forRequired by law?Additional Notes
FABERManufacturerOptional
M8303Department of Transportation (DOT) Manufacturing Facility IDRequired if the company has M-numberIssued by the US DOT and recognized by the Canadian TC
17/0228/170Serial NumberOptional
01C14Original Hydrostatic Test DateRequiredWritten as month (01), symbol of Independent Inspection Agency (C), year (14).
TC-SU7694Transport Canada (TC) Special Permit NumberRequiredmade in compliance with Canadian regulations
237 BARService/Working Pressure in BARRequireddenotes working pressure of tank in metric units (BAR)
DOT-SP13488US Department of Transportation Special Permit NumberRequiredmade in compliance with US regulations
3442 PSIService/Working pressure in PSIRequireddenotes working pressure of tank in pounds per square inch (psi).
REE XXRejection elastic expansionOptional
TP 5250 PSITest pressure for hydrostatic retest in PSIOptional
XS ScubaDistributorOptional
HP100Distributor part number/cylinder sizeOptional, but TypicalDescribes the size of the tank

Example 2

A Luxfer Aluminum Cylinder may have the following:

TC-3ALM207

DOT-3AL3000 P845927 LUXFER 05A08 SO80

These markings can be deciphered in order of appearance:

CodeWhat it Stands forRequired by law?Additional Notes
TCTransport CanadaRequiredmade in compliance with Canadian regulations
3ALMAluminum specification (Canada)Required
207Service/Working Pressure in BARRequireddenotes working pressure of tank in metric units (BAR)
DOTUS Department of TransportationRequiredmade in compliance with US regulations
3ALAluminum Specification (USA)Required
3000Service/Working Pressure in PSIRequireddenotes working pressure of tank in pounds per square inch (psi).
P845927Serial NumberOptional
LUXFERManufacturerOptional
05A08Original Hydrostatic Test DateRequiredWritten as month (05), symbol of Independent Inspection Agency (A), year (08).
SO80Manufacturer part number/cylinder sizeOptional, but TypicalDescribes the size of the tank

For the purpose of determining the capacity of your tank, you need to be concerned with two of the above codes: the manufacturer part number (if it is available) and the service/working pressure code (in either BARS or psi depending on your preference).

The capacity of your tank is found in the manufacturer part number. In example 1, the code was HP100, and the capacity of the tank was 100 ft.3. In example 2, the code was SO80, and the capacity of the tank was 80 ft.3. These capacities are calculated when the tank is pressurized to the working/service pressure. 

So, in example 1, the tank’s capacity was 100 ft.3 when pressurized at 3442 PSI. In example 2, the tank’s capacity was 80 ft.3 when pressurized at 3000 psi.

Once you know how much air is in your tank, you can figure out approximately how long your air will last while diving.

Transport scuba tanks upright if braced or on side to prevent accidents

How Long Will the Air in My Scuba Tank Last?

On average, a standard 80 ft.3 tank at 3,000 psi will last about an hour when diving at 10 meters, but how long your air lasts can vary greatly depending on a lot of factors:

  • Physical Fitness. The more physically active you are, the more efficiently you breathe. Very physically active divers will use less air over time than those who don’t exercise regularly.
  • Diver Size: Generally, smaller divers tend to use less air over time than larger divers.
  • Diver Experience: Experienced divers can make a single tank last for up to double the time due to better breathing techniques and efficient movement.
  • Tank Capacity: As discussed, each tank has a different tank capacity that is determined by the working pressure and physical size of the tank. 
  • Temperature: In warm temperatures, pressure will be greater. In cool temperatures, pressure will be lower. If you get your tank filled at room temperature and then go ice diving, you’re going to experience a loss in air pressure and therefore, a loss in usable air for your dive. 

You can have the same experience if your tank has a “hot fill.” A hot fill occurs when a tank is filled too quickly which causes the air inside your tank to be excessively warm. During a hot fill, the pressure gauge may show that the tank is at capacity, but as the air inside cools to room temperature, the gauge would display a lower tank pressure.

  • Dive Depth: The deeper you dive, the more the air is pressurized, the more air is necessary to fill your lungs, the more you breathe, the faster your air gets used. An average diver should be able to dive for about 45 minutes at 40 feet on a standard 80 ft3 tank.
  • Gas Blend: Whether you are using normal air or Nitrox can change the length of time you can spend diving. Nitrox should only be used by those who are Nitrox certified. Normal air is filtered air pumped into your tank. Make sure you only get air fills from a Certified Pure Air Station.

Nitrox has a higher percentage of oxygen than normal air and is available in multiple blends to suit a variety of needs. Pure oxygen is pumped into your tank and then filtered normal air is added to fill the tank to capacity. Nitrox is generally considered safer than normal air because it has lower levels of Nitrogen than normal air. Less nitrogen decreases your risk of decompression sickness. Using Nitrox also allows you to dive deeper and longer. Only purchase your nitrox fills from an authorized nitrox blender.

Calculating the Air in Your Tank

Planning your dive requires calculating the amount of air you actually have in your tank. This can be done using a simple ratio:

For example: If you know that your tank holds a capacity of 80 ft.3 of air at 3000 psi (working pressure), then…

  1. Check your pressure gauge for the current pressure reading. (For this example, we’ll say it reads 1800 psi)
  2. Use a ratio to determine what the current capacity of air is based on the current pressure reading.
  • Cross multiply 3000X = (1800)(80) 

Answer: 3000X = 144,000

  • Divide both sides by 3000 to isolate the X.

                                    Answer: 144,000/3,000 = 48 ft.3

  • The tank in question has 48 ft.3 of air remaining.
Flying after diving is dangerous. These tanks are done for the day, the divers that used them need to wait 24 hours before flying home

Planning Your Dive

It is important that when diving with multiple divers, the diver with the lowest amount of air in their tank is the controlling diver. This means that you should plan the length of your dive using the amount of air the controlling diver has.

The easiest way to plan the length and depth of your dive is by using a dive chart. You can find them online or in your SCUBA certification text. You can use a dive chart to see how long (on average) your volume of air will last at different depths. It is important to remember that this calculation is an approximation. Your actual air usage may vary depending on the factors stated above. Always monitor your air pressure throughout the dive.

Another thing you must ensure when planning a dive is that you have enough air to return to your boat or dive location. If you calculate that your air is going to last an hour, make sure you turn around after thirty minutes to ensure you have enough air in your tank to make it back. 

If you’re a technical diver and you’re diving in an extreme location like a shipwreck or a cave, make sure you abide by the rule of thirds. 1/3 of your air should be for the beginning of your dive. Another 1/3 of your air should be used for your return, and the last 1/3 should be kept as a reserve in case of any emergencies or if you need more time to decompress. The rule of thirds is also a great idea for non-technical divers to consider as it ensures you’re covered in case of an emergency.

You should never let your tank become fully empty. Plan your dives so you always have a small amount of air left in the tank. If you allow your tank to completely run out of air, it leaves the inside of your tank vulnerable to moisture and if moisture gets inside of your tank, your tank may become corroded. Most dive guides insist that you have a minimum of 500 lbs when you reach the boat. I’ve seen guides refuse to allow divers a second dive when breaking this rule.

Some divers dive with either a second tank or a mini tank to either dive for longer periods of time or to provide extra air in case of emergencies. These mini tanks can be purchased by themselves or as kits that include air pumps and a secondary regulator. Some great options include:

What Factors Should I Consider When Purchasing a Scuba tank?

When purchasing a Scuba tank, be sure to do your research. There are a lot of factors to consider. First, make sure the tank meets the regulations for any country you plan on diving in. Different countries have different regulations. Check the markings. Other factors include:

Used or New

Used: 

  • Less expensive
  • Make sure the tank passes both visual inspection and a hydrostatic test

New:

  • Less likely to be faulty or damaged
  • More expensive
  • More reliable

Capacity

Again, this is calculated using your tank’s size and working pressure. 

  • A greater capacity means longer dives but can also mean more weight and/or more stress on your gear.
    • Higher pressures can damage your regulator and valve components more over time than lower pressures. This is less of a problem with more modern tanks.
  • Standard pressure is 3000 psi, high pressure is 3300-3500 psi, and low pressure is 2400-2600 psi. 

Steel or Aluminum

            Steel:

  • Stronger metal (damage resistant)
  • Lasts longer (with proper maintenance)
  • Prone to corrosion
    • May require occasional tumbling to remove rust from the inside.
  • Available in higher or lower pressurizations
    • Higher pressure means more air for longer dives
    • Lower pressure exerts less stress on gear over time
  • Smaller and lighter
  • Negatively buoyant throughout the dive
  • Tend to be more expensive

            Aluminum:

  • Most popular (especially for recreational divers)
  • Heavier due to thicker walls
  • Weaker metal (more likely to dent or crack)
  • Available with standard air pressure: 3000 psi
  • Easy to maintain
  • Start negatively buoyant, but become positively buoyant as air is used
  • Less expensive

Tank Valve (Yoke, DIN, Convertible):

            Yoke:

  • More common
  • Almost always used on aluminum 80 ft3 tanks.

            DIN:

  • Considered safer 
  • Usually used on high-pressure tanks. 

Convertible:

  • A DIN/Yoke Pro Valve allows you to frequently switch between the two styles of valves as needed. 
  • Either remove or insert a plug to make the conversion. 
  • Convertible valves are only available on newer tanks.

If you already have a regulator, check what kind of valve your regulator is compatible with.

Adapters:

  • You can place an adaptor on your regulator to be able to use a yoke-compatible regulator with a DIN valve. 
  • Consult a qualified Scuba repair technician to ensure it is properly installed.

Accessories

Cylinder Boots (plastic, vinyl, or rubber): Allow tanks with rounded bottoms to be stored vertically and prevent tanks from damaging surfaces.

Mesh Protectors: prevent scratches on your tank

Valve Covers: keep water and dust out of the valve opening.

Handles/Carriers: make carrying your tank easier

Extra O-Rings: O-rings get damaged easily and need to be replaced often, so keep a couple on hand to prevent your dives from getting canceled or delayed.

When Should I Replace My Scuba Tank?

A Scuba tank can last for decades if properly maintained and only needs to be replaced when it fails a hydrostatic test or a visual inspection. If it does fail an inspection, it should be replaced as soon as possible and cannot be used for a dive. For information on keeping your tank in good condition and some videos on tank testing, check out the “can you paint a scuba tank” post.

To maintain your tank, follow the following steps:

  • Handle your tank with care to avoid damaging it
  • Never overfill it (if you’re unsure of how to fill a tank, get it done professionally)
  • Don’t ever let your tank fully empty of air (moisture can get inside of the tank)
  • Rinse your tank with fresh water after dives
  • Don’t let a full tank of air sit for three months or more, get it refilled.
  • Store your tank in a vertical position
  • Get a Professional visual inspection (VIP) once a year
    • Not required by US regulations, but it is a diving industry standard.
    • When you get your tank filled professionally, the filler will check for your annual VIP sticker.
  • Get a hydrostatic test at least every five years.
    • This is a US Department of Transportation requirement for all Scuba tanks. 
    • When you get your tank filled professionally, the filler will check that your tank has had a hydrostatic test within the last five years.

Conclusion

Calculating the size of your tank and the amount of air in your tank is imperative for safe diving. Always make sure you read the markings on the crown of any tank you use, whether it’s rented or owned, to ensure you have the right measurements to use in your calculations.


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Tim Conner, M.D.

Tim Conner, M.D. started boating in 1974. He has been involved in recreational boating continuously since then. Dr. Conner has been active in boating and watersports safety education for decades. He rode his first jet ski in 1997, and rejoined the personal watercraft arena in 2012 with a Sea-Doo GTX 155, followed by 2 supercharged SeaDoos. Scuba certification came in 1988, and he and the family have traveled the world snorkeling and scuba diving for decades. The family has recently taken up paddle boarding. Click the photo for a lot more.

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