Maybe you are a new technician who wants to learn how to test a scuba tank, or just a scuba enthusiast curious about the methods used to verify the safety of your scuba tank. Either way, the process to test a scuba tank starts with a visual inspection and ends with a hydrostatic test.
The way that an accredited technician tests a scuba tank can be broken down to 14 steps:
- Recording all the specifications
- Inspecting the outside of the tank
- Emptying the tank
- Removing the valve
- Inspecting the valve and O-ring
- Examining the neck of the tank
- Inspecting the inside of the tank
- Checking the threads on the inside
- Putting the O-ring back onto the valve
- Starting the hydrostatic test by filling up the tank with water
- Checking the O-ring around the hydrostatic test chamber
- Placing the cylinder in the hydrostatic test chamber
- Testing the tank
- Removing the tank and finishing up
The steps above are the simplified version of how scuba shops test their tanks. The process is quite detailed and requires a full understanding before someone attempts it. Below are the 14 steps on how to test a scuba tank with more details provided.
After that, you may want to know why you should test a scuba tank, how often to test it, and the most common reasons for scuba tank failure. Or you may decide, like me, that renting tanks is the better option. But it doesn’t hurt to know if the rental shop is doing the right things with their tanks.
What Are the Steps to Test a Scuba Tank?
Before starting any testing, you must first record all the necessary information. Then, you can move onto an exterior inspection and interior inspection. Lastly, you will do a hydrostatic test. Below are the 14 steps to test a scuba tank.
Step 1. Record All of the Necessary Information
The first step before starting any visual inspection or hydrostatic testing is to write down all the specifications on the tank. These include manufacturer, serial number, size of the cylinder, working pressure, test pressure, water capacity, weight, capacity, design specifications, and date of manufacturer. The scuba tank should also have a sticker with the last visual inspection date and last hydrostatic test date. Record those as well.
Step 2. Inspect the Outside of the Tank
Check the bottom and sides of the exterior of the tank. Check for any deep scratches, loose coating, corrosion, dents, cracks, bulges, cuts, excessive wear, heat damages, electric arc or torch burns, and any other indications of abuse or harm.
Also, check for illegible, inaccurate, or unauthorized permanent stamp marking on the exterior of the tank. If the tank exceeds the rejection criteria for the exterior inspection, condemn it as permanently unusable.
Lastly, check for any other materials on the surface of the tank. Any foreign substances should be removed by either brushing the tank, water-jet cleaning, controlled shot-blasting, chemical cleaning, or any other non-destructive method. Do not remove a significant amount of tank material.
Once the exterior inspection is done, and the scuba tank has passed, you can move onto the internal review, which requires you to remove the valve. Before you can remove the valve, you need to make sure that the tank is empty. Further details will continue in the steps below.
Step 3. Empty the Tank
Before removing the valve, depressurize the tank to empty it. You will not always need to do this, but a lot of the times, the tanks do come in full. You should slowly release the air and record if any leaks are detected. The release of the air should take longer than 10 minutes.
As for breathing gases with a high oxygen fraction (oxygen-enriched air), they need special precautions. High oxygen fraction breathing gases should not be released in an enclosed space because that can lead to a fire.
Step 4. Remove the Valve
Once you have emptied the tank, remove the valve. You should be able to twist it off with your hand because the valves are usually only hand tight.
If the valve is hard to remove, stop immediately. Check to see if the valve is working correctly by adding a small amount of air to the scuba tank. Doing so will prove whether air goes in and out of the valve. It might be necessary to release the pressure by removing the burst disc. You can also choose to drill into the valve body below the valve seat. After the valve check, if the valve is working, you can then proceed to remove the valve.
If the valve still is not working correctly, contact the manufacturer.
Step 5. Inspect the Valve and O-Ring
Inspect the valve to make sure the knob is working, that it is not deformed. Also, make sure the safety valve is not leaking.
Then, remove the O-ring and check for flat spots and fraying. Although you do not necessarily need to replace it if it is still in good condition, it is recommended that you do anyways. Additionally, remove and replace the valve neck O-ring as well.
Lastly, carefully clean the threads of the valve since they will be going back into the tank.
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Step 6. Examine the Neck of the Tank
Make sure the neck area and threads around it are clean and without imperfections so that when you put the valve back in, it will go in smoothly. Problems with the threads include metal loss, galling, corrosion, cracking, and abuse.
Abuse would include deep nicks, cross threads, broken threads, stripped threads, and threads without well-defined and sharp peaks.
If the neck area and threads are not clean, clean them of debris and lubricant before continuing.
Step 7. Inspect the Inside of the Tank
Put your tank inspection light inside the cylinder to inspect the inside. You should also use a dental mirror to see better. The tank inspection light is very bright, and while it is suitable for illuminating the tank, it can also be so bright that it does not cast shadows. This can make it hard to see little imperfections, so if you turn it to the side, you can throw shadows around the inside of the tank to pick up those imperfections.
Make sure to inspect all around the bottom and all around up and down the sides. The dental mirror can help to see around the corners of the neck better. You are looking for cracks, corrosion, and any other imperfections. If any of these imperfections exceed the test limits, you must condemn the cylinder.
Step 8. Check the Threads
Inspect the threads on the inside of the cylinder well because they are critical for holding the valve in place. Use a non-destructive testing device, such as Visual Plus, so you can better check the threads. Make sure they are sharp and clean.
Step 9. Put the O-Ring onto the Valve
Place the O-ring carefully back onto the valve without rolling or twisting it. Then, put a little bit of grease on the bottom of the valve threads. Do not put oil on the O-ring. The oil can cause the ring to slip out.
Step 10. Start the Hydrostatic Test: Fill Up the Tank with Water
Once the tank has passed both the exterior and interior inspection, you can move onto the hydrostatic test. You start the hydrostatic test by first filling up the tank with water. After filling the scuba cylinder with water, you will put a hydro test adaptor on top of the tank. The reason for filling the tank with water is to avoid the tank expanding and exploding if it does not pass the hydrostatic test. Because there are no air spaces, and the tank is placed in the hydrostatic test chamber that is also filled with water, the tank cannot expand and explode.
Step. 11 Check the O-Ring Around the Hydrotest Chamber
Before putting the tank into the test chamber, remove the test chamber O-ring, clean it, and make sure there is no film, dirt, or rust on it. Check for flat spots or fraying as well. Once you have inspected it and you have determined that the O-ring is up to par, put it back onto the test chamber.
Step 12. Place the Cylinder in the Hydrostatic Test Chamber
Use the hydrostatic testing equipment crane to pick up the tank and put it in the hydrostatic test chamber. The hydrostatic test chamber should also be filled with water. The water in the tank and inside the chamber should all be the same temperature, which should be room temperature. Then, place the O-ring covering on top of the chamber to close the chamber. Use a hydraulic clamp to keep it tightly in place.
Step 13. Test the Tank
For those of you who have the appropriate software, you can test the tank by simply clicking “test” on your computer screen using the software. The software will tell you the percentage expansion number. Depending on the tank, the percentage you will be looking at as the limit will change. It is usually 10% for aluminum and steel tanks and 5% for fiber wrapped tanks.
For those who do not have software to test the tank, you will manually need to pressurize the tank. You need to do this to squeeze out some of the air. Pressurize the tank to 5/3 of its normal pressure. You can find the normal pressure on the cylinder. Remember, you should have recorded this at the beginning before doing any testing.
Make sure to connect the burette (a type of measuring cylinder) to the sealed chamber to record the water displaced during the pressurization. Pressurize the tank for 30 seconds. The tank will inflate and displace water from the chamber into the burette. Record the total expansion shown by the amount spilled.
Then, release the pressure and let the water return to the chamber from the burette. Even though releasing the pressure from the tank will cause deflation, the cylinder will still have stretched a bit. Thus, not all the water from the burette will return to the chamber.
In this case, the amount the tank has stretched after deflation would be considered the permanent expansion. Subtract the permanent expansion from the total expansion to get the elastic expansion. If this value exceeds the limits set for your cylinder, then the tank has failed the test and should be discarded..
Step 14. Remove the Tank and Finish Up
Once you have completed the test, remove the tank from the chamber. After removing the tank, clean it and then dry it immediately. The interior should have no trace of free water or other contaminants. If the cylinder needs to be repainted or coated, do not heat steel cylinders above 300 degrees Celsius. Aluminum tanks have even more restrictions on permitted temperatures, so check the specifications given by the manufacturer.
Make sure to put the correct valve back on. Putting the wrong valve on a scuba tank can result in an explosion.
If the tank fails, mark with “X’s” over the DOT specifications. If the tank passes, stamp the tank with the date and ID number for the shop that carried out the testing.
What is the Point of Testing a Scuba Tank?
Now that you know how to test a scuba tank, you may be wondering why you need to test the scuba tank in the first place. Other than it being the law, you need to test a scuba tank for safety reasons. Testing scuba tanks started when aluminum cylinders had begun receiving a lot of attention in the United States due to an explosion in Riviera Beach, Florida, in February of 1998. Before that event, the dive industry generally ignored the occasional explosion of aluminum or steel cylinders.
With that said, visual inspections are done to make sure there has not been so much corrosion that the tank walls become weakened enough to explode. The hydrostatic test is to check the elasticity of the metal. Every time you fill up a scuba tank, it expands. Once the air is released, it contracts.
Over time the metal slowly becomes less elastic. It becomes more brittle and susceptible to cracks and breaks. The elasticity begins to break down. Therefore, the purpose of getting the tank tested is so that you don’t fill your tank one day, and it ends up expanding and exploding due to substantially reduced elasticity.
What Are the Certifications Needed to Test a Tank?
There are federal licensing requirements for hydrostatic testers in Canada and the United States. Test procedures are established by law and enforced by the Department of Transportation in the U.S. or Transport Canada in Canada. Tank testers must legally take training every three years and examined by federal enforcement personnel every five years to keep their license.
However, enforcement is lacking, and many inadequately trained technicians will certify improperly evaluated cylinders. Because there is no legal mandate stating that inspectors need training on visual inspections, many of them do not know what the visual inspection of the hydrostatic test entails.
Keep in mind, though, that untrained inspectors are at legal risk if a cylinder that they inspected ends up exploding. They have no legal defense in this case, so most dive industry entities such as PADI, NAUI, Luxfer, Catalina, Compair/Mako, and most dive stores accept the PSI training protocol as the industry standard.
How Much Does a Scuba Tank Inspection Cost?
The cost of an inspection will depend on the dive shop that conducts it. For the annual visual examinations, the fees usually range from $15 to $25. Hydrostatic testing can range from $20 to $70, depending on what kind of cylinder it is.
How Often Does a Tank Need to Be Tested?
How often you need to test a tank depends on your location. In the United States you are legally mandated to have your scuba tank(s) tested every five years. For some special tanks, you need to test every three years. As for the visual inspections, you need to do so once every year.
In the European Union, you need to do a hydrostatic test every five years, and a visual inspection every 2.5 years. In Norway, a hydrostatic test with visual inspection is needed three years after the tank production date and then every two years after that. Tanks are required to be hydrostatically tested yearly in Australia. As for those in South Africa, you need to do a visual inspection every year and a hydrostatic test every four years.
Check the regulations regarding when you need to get your tank-tested for your country if they were not mentioned above.
What are the Most Common Reasons for Tank Failure?
A common issue is weak valve threads. Also, a visual inspection is more likely to fail than a hydrostatic one. According to Luxfer gas cylinders, over 90% of scuba tanks fail the visual inspection before even getting to the pressure test. In a hydrostatic test, corrosion on the inside is typically the reason for failure. In terms of corrosion, steel tanks are more likely to damage due to rust.
Aluminum tanks are less likely to corrode. Yet, when they do, it is usually due to excessive corrosion in the thread area or beneath the boot, or from cracks found during the visual inspection.
What is the Most Common Cause of Corrosion?
Corrosion damage is typically the fault of the fill station or cylinder owner. Water of any kind, but especially salt water, can cause the metal to corrode. It isn’t uncommon for there to be several drops of water left in the valve after last use or rinsing by the owner. To avoid this, always open the valve and blow out the valve aperture before filling the tank.
As for the fill stations, poorly maintained filters and water separators can pump water into the cylinder during fills. Make sure to go to a fill station that upkeeps its filters and water separators well. Also, ask that the fill station operator blow out the fill whip before attaching it to your cylinder to avoid dirt, dust, or any other foreign matter getting inside.
Can a Tank Still Be Used if it Fails?
You cannot use a scuba tank if it fails. Due to DOT regulations, a tank that fails hydrostatic testing is not allowed to be filled again and will not be given a test stamp. Without a hydrostatic test stamp, no reputable dive shop will fill it. To ensure this happens, the tank serial number is stamped out with an “X,” and the cylinder threads scored.
What is the Safest Way to Fill Up a Scuba Tank?
The need for testing is to ensure that scuba tanks don’t explode. It may come as a surprise to some, but most scuba tank explosions occur during the filling process due to the power of the compressed air. Thus, the safest way to fill up the tank is to make sure there is a separation between the operator and the cylinder.
Separation can be accomplished by either creating a barrier or creating distance. Fill stations should only accept cylinders that have been inspected by a trained visual inspector and are within the retest period. Read more in my full step by step scuba tank fill guide.
What Are General Tips for Scuba Tank Maintenance and Safety?
Scuba tank maintenance is necessary to ensure the highest scuba diver safety. There are many tips for maintaining your scuba tank to ensure your safety:
- Ensure water never enters or forms inside of your tank. Keep the threads and cylinder interior dry and free from contamination of any sort.
- Before using any scuba tank, make sure it is within the retest period.
- Do not store tanks full of air for more than three months. Clean, dry aluminum cylinders may retain their quality slightly longer.
- For storing tanks for more than three months, the tank should have just enough pressure (200 psi) to keep moisture out. Some say it should be less than or equal to 3 bars (50 psi). Regardless, the higher the tank pressure, the greater chance for internal corrosion.
- Take your scuba tank for a visual inspection once a year. Have it inspected every three to six months if you use your tank frequently. The University of Rhode Island conducted a test in 1971 where they found that a small amount of saltwater inside a steel cylinder can damage it in 100 days to the point of imminent explosion. Getting a visual inspection is a top priority from a maintenance and safety perspective.
- Make sure to have a reputable hydro testing facility hydrostatic test your scuba tank. Reliable facilities will entirely service your tank, including testing, cleaning, drying, zinc coating, and painting.
- Before putting your tank away, always give it a fresh water rinse.
- Make sure the tank valve is closed when rinsing it.
- To avoid putting unnecessary stress on the metal of the tank, do not fill the tank past stamped ratings. Doing so would be considered overfilling it and can weaken the tank over time.
- Avoid rough handling of the tank that can cause corrosion to the metal or weaken the cylinder. Secure the cylinder during transport, so it does not move around and potentially get damaged.
- Always store your scuba tank in a vertical position unless a visual inspector recommends otherwise.
- To check for corrosion or contamination, see if you can see or smell anything coming outside of the tank valve.
- If you tap on the side of the tank and hear any rattling on the inside, get it visually inspected. Something is wrong.
- If you find water on the inside of your tank, but no corrosion has occurred, rinse the tank with freshwater or steam clean it. Then, dry it with warm air.
- Check with a qualified inspector to determine the extent of any hints of corrosion.
- Never leave a cylinder standing unattended.
- When cylinders are out of reach, always lay them down to prevent them from falling or being knocked over.
- Avoid exposing cylinders to extreme heat, so keep them in the shade during the summer. The ambient temperature should technically be less than 140 degrees Fahrenheit (60 degrees Celsius). Also, avoid storing them near heat sources such as hot lights, heaters, furnaces, freezers, electrical or combustion motors.
The Bottom Line
Whether you are a scuba diver or technician, each scuba tank must be tested regularly and accurately to maintain the safety of the divers and the sport, in general. If you are a technician, make sure you fully understand the procedures for testing the scuba tank because it is you who is legally at risk if the tank explodes. Divers: make sure to test your scuba tank according to the regulations of your country, at the very least. It also doesn’t hurt to get it checked more regularly if you are using the tank frequently. After all, it’s your life that is at stake.