A scuba cylinder is one of the crucial pieces of equipment that divers use. Great care is taken to ensure that it functions properly, providing clean, uncontaminated, breathable air to the diver when underwater. Filling a scuba tank involves many steps that are taken to ensure that the pressure in the tank is correct, the tanks do not malfunction, and that the air is suitable for breathing underwater.
A Brief Overview Of Filling A Scuba Tank
Dive shops around the world use similar processes to fill scuba cylinders of all sizes with different air blends. It is entirely possible for divers to safely fill their own tanks if they stick to the following steps.
- Scuba tank inspection, discarding tanks that are unfit for use.
- Draining the tank
- Compressing air
- Proper attachment of the tank and compressor
- Filling the scuba tank
- Release excess air and disconnect from the compressor
- Safe storage
Even if you do not intend on filling your own tanks, fully comprehending the different steps needed for proper scuba tank filling can help you choose a dive shop that complies with the best standards.
We are only focusing on filling a scuba tank with compressed air, not nitrox or trimix. Filling tanks with nitrogen requires that they are cleaned using a special solvent and is very dangerous as the process is highly flammable. For nitrox or trimix, it is better to visit a dive shop that can further assist you in creating a custom blend of gases according to your needs.
Equipment And Other Requirements
You may need to invest a little in high-quality equipment to enable you to fill scuba tanks. Most sellers will avoid selling such equipment to inexperienced and uncertified divers, so you will need to get all the scuba diving safety certifications first. These safety certifications are beneficial even outside of filling your tanks, so they are worth getting.
Garage air compressors are not suitable for properly filling scuba tanks because they can’t build up enough pressure. The highest pressure they can reach is 200 psi. For underwater use, scuba tanks need to have air pressure that is 3000 psi or more, depending on the type of tank and the intended diving depth.
The cost of an air compressor that can handle pressures suitable for filling scuba tanks is well over two grand, so keep in mind that this is a big investment. Of course, filling tanks at dive shops is relatively cheap, and it will take a long time for you to get a return on your investment. However, for some, it may be worthwhile.
The Correct Inspection Procedure
Inspection is a vital part of the filling process and will save you a lot of trouble ensuring your safety and keeping your tanks functioning properly. You should be able to guarantee that there will be no mishaps while filling, nor will there be any air leaks when the tanks go on a dive. Remember that just as divers are subject to abrasions and minor (or major) collisions when underwater, the same is true for scuba cylinders and other scuba gear.
Inspection can be divided into the following steps:
Search for dents, prominent markings, and scratches on the cylinders. Check every part of the tank, including the valves and gauges. Look for imperfections or anything that looks out of place. This does not require great skills but a keen eye to spot any visual imperfections.
Keep in mind that scuba tanks need to be filled with air and gases. This means that their proper functioning requires them to be completely dry on the inside. If water finds a way into the tank, it is not suitable to be used (or filled). The easiest way to check for any water inside the tank is to agitate it and listen to any sloshing sounds gently.
Inspecting Invisible Cracks
Some cracks may be too small to spot by the naked eye. Much like you would do with a car tire, submerge the scuba tank in cold water. There should be no visible bubbles exiting the cylinder. Any bubbles are indicative of air leakage from tiny invisible cracks that you might not have seen upon the first inspection. Cool water is also used after filling to avoid damaging the tank from the heated air used to fill it.
Do not underestimate a tiny crack. Remember that diving takes scuba tanks to a highly pressurized environment, which would easily cause smaller cracks to expand and lead to bursting the tank. To avoid any underwater accidents that are potentially fatal, be very thorough when inspecting a scuba tank.
Additionally, a regular professional inspection is a must, especially with frequent use. Professionals conduct a hydrostatic test on cylinders by visual inspection and pressure tests to check how the cylinder handles increased pressure. The service costs $50 on average and normally includes full servicing of the cylinder (cleaning also). Aim to take your cylinders for these tests at least once a year.
Dispose of any faulty tanks by taking them to a dive shop or a scrap metal dealer. Keep in mind that scrap dealers will not accept the valve or a half-full tank, but they will pay for the fault tank by weight (as opposed to diving shops that take it for free and in any condition).
Draining Excess Air
Before filling a tank, you should properly release any remaining air. This will serve two functions. Firstly, it helps avoid overpressurizing the tank (by filling too much air), which can be dangerous and damaging to the cylinder. Secondly, it keeps the new air free of contaminants. Draining is also required when storing scuba tanks for longer than three months.
Make sure you do not drain the tank completely, but leave about 200 psi of air inside, or else moisture may find its way in to fill the negative pressure. The last 200 psi can be released directly before refilling.
When opening the air valve to drain the cylinder, do it slowly and do not open it all the way. The best way is to open the valve only very slightly, allowing the air to be released and preventing moisture from entering. Use a pressure gauge (the same one used when diving) to know how much air is left to drain.
How To Compress Air
As mentioned above, you will need to purchase a specialized air compressor to fill scuba tanks. The machine will be able to compress air to the higher pressures required for scuba diving. Most air compressors come with an air filter to ensure the quality of the air being compressed. If the model you have is not equipped with air filtration, consider getting a separate air filter to connect to your air compressor. Even trace amounts of contaminants can have exponentially higher toxicity at high pressures.
Turn on the air compressor and check if it reaches the required pressure for your tank. The minimum requirement for most tanks is 3000 psi, but some may need even higher pressures (up to 4350 psi), so check your tank for guidelines and recommendations. If your compressor can be set to shut off at a preset pressure, make use of this feature to avoid overfilling (overpressurizing).
How To Attach Tanks The Air Compressor
Check that your air compressor and scuba tank are compatible, which means the hose from your air compressor is the same size as the hole in the tank. Using the wrong size will make it difficult to attach them securely. Dive shops normally have various sizes to solve this problem, but if you are only using tanks from one manufacturer or of one size, you’re likely only to need one air compressor hose. Be careful not to strip the metal from both ends when attaching the tank to the yoke. You want to connect them securely, but avoid breaking either end by fastening too fast or too tight.
Filling Your Scuba Tank (Finally)
After you’ve conducted all the safety inspections, ensuring that the tank is drained completely and securely connected to the air compressor, slowly turn the nozzle of the air compressor yoke. Don’t rush this process either, to avoid breaking the equipment from the sudden high pressure exerted.
Air compressors are noisy machines and use heat to pressurize the air. Many professionals keep the scuba tank in a cold water bath while filling them to keep its temperature relatively low. This will keep the tank from overheating and getting damaged. Also, try filling the tank away from any heat sources and keep some distance between the tank and compressor to avoid unnecessary heat transfer. If the compressor is too noisy, consider building (or purchasing) a small box-shaped structure to house it in and insulate some of the noise.
Do not leave the tank and compressor unattended. Keep your eye on the pressure gauge in case the automatic shut-off feature malfunctions and is not triggered at your desired pressure level. Do not fill up the tank too quickly to avoid damage to the valves or accidentally overfilling the cylinder.
Disconnecting The Tank
After you’ve confirmed that your scuba tank is filled to the recommended pressure, you will need to disconnect it from the air compressor. First, turn all the valves to the off position, ensuring they are safely closed. Taking care not to damage the valves, disconnect the tank from the air compressor. Avoid scraping the parts against each other or moving them too quickly.
After disconnecting, give the tank some time to cool down (it shouldn’t take more than a few minutes). This is to avoid moving overheated tanks, which is potentially dangerous.
Storing Your Filled Scuba Tank
More often than not, people tend to fill their tanks days before their scheduled dive. It is important to store the tanks in a safe manner to ensure that all the hard work hasn’t gone to waste. You must consider three factors when storing a scuba tank: position, stability, and temperature.
Scuba tanks should always be stored in an upright position and secured in place to keep them stable. Side storage is only suitable for transporting scuba tanks. The gas composition inside the tanks can change if they are kept at an angle or on their side. To keep tanks secure, they should not have much room to move, so try pushing heavy furniture against them.
As previously mentioned, overheating tanks is dangerous and can potentially lead to explosions and injuries. Always keep them in a cool, dry place. Also, maintaining your scuba tank is an important part of storage. Keep them clean at all times, and give them a freshwater rinse after diving to avoid salt buildup. Salt on the surface of a scuba tank makes visual inspection difficult.
Due to the potential dangers associated with filling a scuba tank, a review of the safety precautions is important. A malfunctioning scuba tank can cause fatal injuries in dives, and the process of filling the tank is dangerous in itself.
Do Not Allow The Tank To Fall
When filling a tank and during storage, make sure that it does not have the opportunity to be knocked over. If it lands on a sharp object, it can potentially burst and cause an explosion.
Do Not Overfill
Overpressurizing a tank weakens the structural integrity of the tank because of the excess force exerted from the air pressure. Always keep an eye on the pressure gauge when filling, regardless of whether you have an automatic shut-off mechanism or not.
Cleanliness And Valve Care
Keeping your tanks clean (especially after a dive) will avoid corroding the material of the tank. Also, keep the valves clean and replace any damaged valves. As previously mentioned, be cautious when tightening the valve to avoid damaging it. Fasten it slowly, in half-turns.
Filling a scuba tank is not a labor-intensive process, but attention to detail is important to avoid mistakes that could lead to injury. If you take your time and don’t rush, it can be a relatively simple task. If you find this process complex, you may consider using Mini Air tanks, but “Are Mini Air Tanks a Good Solution“? Check out our article to find it out.