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.
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.
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:
| Code | What it Stands for | Required by law? | Additional Notes |
| FABER | Manufacturer | Optional | — |
| M8303 | Department of Transportation (DOT) Manufacturing Facility ID | Required if the company has M-number | Issued by the US DOT and recognized by the Canadian TC |
| 17/0228/170 | Serial Number | Optional | — |
| 01C14 | Original Hydrostatic Test Date | Required | Written as month (01), symbol of Independent Inspection Agency (C), year (14). |
| TC-SU7694 | Transport Canada (TC) Special Permit Number | Required | made in compliance with Canadian regulations |
| 237 BAR | Service/Working Pressure in BAR | Required | denotes working pressure of tank in metric units (BAR) |
| DOT-SP13488 | US Department of Transportation Special Permit Number | Required | made in compliance with US regulations |
| 3442 PSI | Service/Working pressure in PSI | Required | denotes working pressure of tank in pounds per square inch (psi). |
| REE XX | Rejection elastic expansion | Optional | — |
| TP 5250 PSI | Test pressure for hydrostatic retest in PSI | Optional | — |
| XS Scuba | Distributor | Optional | — |
| HP100 | Distributor part number/cylinder size | Optional, but Typical | Describes 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:
| Code | What it Stands for | Required by law? | Additional Notes |
| TC | Transport Canada | Required | made in compliance with Canadian regulations |
| 3ALM | Aluminum specification (Canada) | Required | — |
| 207 | Service/Working Pressure in BAR | Required | denotes working pressure of tank in metric units (BAR) |
| DOT | US Department of Transportation | Required | made in compliance with US regulations |
| 3AL | Aluminum Specification (USA) | Required | — |
| 3000 | Service/Working Pressure in PSI | Required | denotes working pressure of tank in pounds per square inch (psi). |
| P845927 | Serial Number | Optional | — |
| LUXFER | Manufacturer | Optional | — |
| 05A08 | Original Hydrostatic Test Date | Required | Written as month (05), symbol of Independent Inspection Agency (A), year (08). |
| SO80 | Manufacturer part number/cylinder size | Optional, but Typical | Describes 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.
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…
- Check your pressure gauge for the current pressure reading. (For this example, we’ll say it reads 1800 psi)
- 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.
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.
