Over time, the interior of a SCUBA tank becomes contaminated. This can manifests itself in rust and corrosion and other times, it is residual oil that has been pumped into that tank in trace amounts for years. This oil may be well below the acceptable limits for a compressor air test, but over multiple fills may begin to accumulate in the walls and metal of the tank. Whatever the contaminant, it has may have some dramatic effects on the quality of air the diver is breathing. A big threat is the ability of rust to absorb the oxygen in the air mixture. Consider, you fill your tanks to 32% eANX at the end of the dive season and then sit them in your garage for 6 months. You may assume that tank contains 32%....but it might be much less.
Tank owners know that their tanks require a visual inspection annually and a hydrostatic test every 5 years, but when do their tanks require cleaning? The answer depends on the type of metal in the tank, whether you use eANX or just air, and how often you dive, and other factors to include to quality of air being put into your tank or if it was emptied.
Let's talk rust first.
Steel tanks have a lot of advantages for certain divers and they are very popular in the Northeast but they have one major disadvantage to Aluminum and that is the possibility of internal rust. Rust forms when moisture gets into the tank either from the air or directly. This can happen from a compressor that is not drying the air properly, from a tank being drained too low, from a tiny bit of water sitting in the din valve when it is filled, or various other reasons. For example, let's say you drained your tank to zero. It now has outside air inside it and the moisture of that outside air. Now, you close the tank valve. As that air cools, because you bring the tank in your house, or put it in your air conditioned car to bring to the dive shop to get filled, that relative humidity increases inside the tank. Over time, that moisture will cause rust. It is very difficult to keep rust out of steel tanks forever and most will see some rust at some time but rust that is caught early is easier to deal with.
That rust will be removed by a tumbling or whipping process. It is labor intensive but very effective. The tank is filled with ceramic pellets (approx 25 pounds worth) and a cleaning solution and then is put on a roller for what could be hours. Once the inside of the tank has been cleaned, the solution and pellets are removed and the tank is rinsed. If it is used for eANX, it is then oxygen cleaned (more on that in a minute). A rust inhibitor is then put into the tank and it is rolled again to ensure an even coat of the inhibitor. I quick rinsing removes the access and the tanks are put onto a low heat air dryer. If done properly, rust is no longer present. However, if any rust is missed or the inhibitor is rinsed to much, rust will re-present itself the next visual.
What about Oxygen Cleaning?
Tanks that are used for eANX Nitrox or Oxygen must be oxygen cleaned. Hydrocarbons that are in the air, on o-rings and on valves have a much lower flashpoint when 100% oxygen is introduced to them. That means, there is a fire risk and fire near oxygen is not nearly as fun as fire in your fire pit.
Due to this risk, Dive Centers filling Nitrox will require a tank, regardless of the metal, to be oxygen cleaned before filling. The object of this cleaning is to remove the hydrocarbons in their entirety from the tank and valves. Any single hydrocarbon can present a fire risk so the object is remove them all during this process.
The process is similar to the rolling for rust process. The tank has glass beads poured inside and a hydrocarbon solvent added. The tank is then rolled for approximately 30 minutes before the solvent and beads are rinsed from the tank and if the tank is steel the rust inhibitor is added. During this time the valve is also cleaned. It is broken down and placed into an ultrasonic cleaner with the same hydrocarbon solvent. The ultrasonic helps to break the hydrocarbons out of all of the dents and dings found on a SCUBA tank valve. The valve than receives new Viton (oxygen safe) o-rings and new copper and teflon discs. Finally, the valve is reassembled, put back on the tank, and the tank is filled with whatever Nitrox mix was wanted by the customer.
How often do tanks need to be O2 Cleaned?
This is an interesting question. With no regulatory body for SCUBA, there is no regulation on how often this must be done. Dive Centers around the Country have different ways of dealing with this, but there has been a shift to a much easier "best practice" course of action. That is, all eANX tanks will be oxygen cleaned at a very minimum, anytime their valves are removed. This means that for most people, your eANX tank will need to go through this service at least annually at the time of the visual inspection.
There is a very good reason for this. Dive Centers are not equipped with true oxygen clean rooms and even if they were, the very second your tank is exposed to anything outside of that room, it is exposed to hydrocarbons. That means that we can never fully remove the threat of combustion. Every day and every fill that tank moves away from when it was oxygen cleaned, more hydrocarbons become present and the threat of combustion is continually increased. Thus, we have seen a shift to a more frequent cleaning requirement. Remember, a year is a minimum and if you are filling your tanks more often, they are emptied and stored, or anything less than Grade E air is introduced to them, the tanks should be cleaned before then.
At Underwater World we have spoken with many Dive Centers to make sure that our prices and our services are aligned with everyone else. We have recently changed the way tanks are inspected and cleaned and the related pricing structure. For years we required O2 cleaning to only be accomplished with a hydrostatic test, but recent studies and changes in the industry has taught us that isn't acceptable anymore. Underwater World now required all eANX tanks to be Oxygen Cleaned at every Visual Inspection.
Doesn't that get expensive?
Prior to this, Visual Inspection were $20 and O2 Cleaning was another $50 + $17 in parts. So, if you were a diver that wanted to follow industry best practices and clean your tanks with the visual, it would have cost you $87 a year, per tank. Today, we have actually lessoned the price to remove the burden of having to choose between safety and your budget. A diver who brings in a eANX tank for Visual Inspection will now receive an Oxygen Cleaning including parts for $50. That is a 47% savings and represents our commitment to keeping you safe at a reasonable price. That commitment also carried over to eANX tanks that need to be hydrostatically tested. Previously the cost of a eANX hydro with O2 cleaning was $112. Now, that price is $80. For a diver with 2 sets of double steel tanks, who was following industry best practices, the savings over 5 years is a substantial $720!
Speak with an Underwater World Team Member to see what options are best for you!