Understanding the Visual Inspection Process

By Capt. Joe Coleman

It’s not as simple as removing the valve and taking a peek inside the tank…

As tank owners, we get a visual inspection every 12 months (usually when we take the tank in for a fill and a shop employee notices that the tank is out of inspection).  It may be something that we rarely think about, but it’s an important part of being a safe diver. We are going to break down the visual inspection process and talk about the three different types of visual inspection certifications and how not knowing the difference could affect you. 

The Inspection Process 

The tank technician begins the inspection on the outside of the tank, before the valve is even removed.  The tech will start by looking at the regulatory marks on the tank. What is the born-on date of the tank?  This matters for aluminum tanks of a certain age. What is the DOT or TC regulatory license for the tank? What’s the working pressure?  Does the tank have a current hydrostatic test date stamped on the shoulder? These are all important questions that the technician must answer before the actual inspection begins.  

With these questions answered, the inspection begins in earnest.  Still outside the tank, the technician will look for obvious signs of rust, corrosion (especially under the tank boot, if equipped), and damage from falls or burning.  Next, the technician, with a straight-edge, should check the tank for “banana-ing”; over time, aluminum tanks can take on a very slight banana shape. Believe it or not, a certain small amount of “banana-ing” is allowable. 

Getting inside the tank 

Next, the technician will remove the valve, and here’s where the real fun begins!  Often, the first thing that happens is a sniff test- yes, we smell the inside of your tank.  The technician will smell for anything that may be “off”; the smell of dead sea life could mean water intrusion, the smell of hydrocarbons or a burnt odor could indicate a previous fill from a compressor with failing piston rings or a bad filter.  These are things that you, the tank owner, need to know. The inspector will also check the tank valve, to determine if it needs service or a cleaning.  

Once the cylinder passes the sniff test, the technician will move on to a detailed inspection of the threads on the neck of the cylinder.  The technician will use a small, lighted periscope device, like an Optical Plus, to magnify and closely examine each thread. Thread issues tend to be the most common reason for visual inspection failures. Bad threads, or more specifically, cracks in the neck of the tank between the threads, are responsible for the majority of visual inspection fails.  Cracks in the neck can be caused by slight congenital irregularities that may not show themselves for several years, or, some technicians argue more commonly, by chronic over-filling that leads to sustained load cracking, or SLC. Those years of 3500 psi fills from your local dive shop (in an aluminum tank) may give you a little more bottom time, or more safety margin, but they’re not doing your tanks any favors!  

Checking for rust 

If the threads are up to snuff, the tech will now take a closer look at the inside of the tank, with a long, flexible LED light.  The tech is looking primarily for iron oxide in steel tanks and aluminum oxide in aluminum tanks. Iron oxide is, of course, more commonly known as rust.  Aluminum oxide is simply the kind of “rust” that forms on aluminum, and it has a white, somewhat chalky appearance. Too much of either of these may mean that your tank needs a tumble, where the tank is partially filled with abrasive material that sands off the iron or aluminum oxide.  Alternatively, the technician may whip the tank. A whip is a long aluminum rod with medusa hair-like lengths of abrasive cable on one end. The rod attaches to a drill, and the whip is moved up and down inside the cylinder to remove light oxides. If the tank needs to be tumbled or whipped, it is then rinsed and dried.  

A note on aluminum types 

Aluminum tanks manufactured before 1989 were produced from an aluminum alloy known as 6351 aluminum.  After 1988, the industry changed to a different aluminum alloy, called 6061. The 6351 alloy presented some SLC issues after years of use, so much so that it spurred the scuba cylinder industry to change to the new 6061 alloy.  Many shops will tell you that they cannot fill older 6351 aluminum cylinders, but this is not the full truth of the matter. Even if there are no visible cracks in the threads of the tank, which are easily seen with the Optical Plus tool, hairline cracks may be developing just under the surface.  An eddy current machine screws into the tank and sends electrical current through the threads and can detect cracks that are under the surface and invisible to the eye, even with the aid of magnification. The Visual Plus eddy current machine is the most commonly used tool. 6351 alloy tanks must have an eddy current test performed at each inspection, and some shops even perform this test on all visual inspections.  You can never be too safe when it comes to scuba cylinders.  

If you are an air-only diver, congratulations!  Your tank, having now passed its visual inspection, is filled with air and is ready to dive.  But if you are a Nitrox diver, your tank may undergo another level of scrutiny, depending on the dive shop that’s performing the inspection.  This is the point at which things get a little more complicated, and, if you’re a Nitrox diver, or you’re considering becoming nitrox certified, you have to pay close attention.  There are multiple ways to create Enriched Air Nitrox. The two most common are banked nitrox and partial pressure filling. We will discuss the difference and the implications for your visual inspection sticker.   

Banked Nitrox

This form of nitrox is purchased by dive shops from industrial gas suppliers in large cylinders.  EAN 32, EAN 36, and EAN 40 are common. This is an easy way for a dive shop to sell nitrox. No special equipment is needed, aside from a booster, which helps move gas from the large bottle to your cylinder.  If you order nitrox fills from your local dive shop, and they ask if you want 32 or 40, it’s likely that this is the method they’re using.  

Partial Pressure Blending 

This method is more complicated, takes more equipment, and requires that higher level of visual inspection scrutiny we mentioned before.  In this process, the customer can order any EAN percentage they want. You want EAN 35.7? no problem! In partial pressure blending, your empty cylinder is first filled with a certain quantity of 100% oxygen, and then is topped with oxygen compatible air, or OCA, to create or blend a particular EAN percentage.  For example, EAN 32 is created (in a 3000psi tank) by filling an empty tank with 418psi of pure oxygen, then topping with OCA to 3000psi. Partial pressure blending gives a dive shop the flexibility to mix EAN to any percentage the customer wants. Shops that teach technical diving and offer technical fills almost invariably use this method.  

But here’s where some issues may creep up- in order to partial pressure blend a cylinder, that cylinder must first be O2 clean.  This means that any potentially flammable materials must be cleaned out of the tank and valve. Many tanks use rubber (Buna, or nitrile) O-rings, and silicone grease, both of which are flammable.  Mixing 100% oxygen with high pressure, elevated temperature (from the filling process,) and flammable materials is a recipe for disaster. Just Google “dive shop explosions,” you’ll see what I mean.  To check for flammable materials in the tank, the tech will drop a UV light into the tank. Any organic or flammable material will glow bright green, indicating the need for oxygen cleaning.  

Back to banked gas 

By contrast, any gas mixture with an FO2 below 40% is, according to the Compressed Gas Association, not at risk of explosion in the presence of silicone grease or rubber O-rings.  So, the shop that gives you banked nitrox needs to take no more special steps to ensure safety. None of this is to say that one method is better than the other, and either method can make sense, depending upon the location of the dive shop.  For a dive shop on an island where all of the diving is either a 20’ deep reef or a 100’ wreck, there is really no reason to sell anything other than air and EAN 32.  

Visual Inspection Stickers are Not Created Equally 

I have worked in the dive Industry for nearly 25 years, and I have seen this happen a thousand times.  I work at Dive Shop A, and we are a shop that partial pressure blends our nitrox. A customer walks in for a nitrox fill.  He or she is a regular customer at Dive Shop B, who sells banked nitrox and performed the latest visual inspection on this customer’s tank.  I cannot fill this customer’s tank with nitrox. Why? Because the visual inspection was performed by a shop that only certifies tanks to 40% pre-mixed EAN, and I use 100% O2 to blend my nitrox.  This tank likely has rubber O-rings and silicone grease on the valve. Unless I want to risk blowing up my shop, I cannot fill his tank, unless he wants to have the tank visually inspected and O2 cleaned, meaning more time and money for the customer.  And all he or she wanted was a tank fill. It can be frustrating to encounter this as a customer. This is why it is important to recognize the difference between VIP stickers.

There are three types of scuba cylinder visual inspection certification levels:

  1. Air Service Only
  2. EAN up to 40%
  3. Cleaned for Oxygen Service

Air Service Only is pretty self-explanatory.  

EAN up to 40% means you can get air fills, and pre-mixed nitrox fills up to 40%.

Cleaned for Oxygen Service means you can get up to 100% O2, with the appropriate Tech Diving certification, pre-mixed nitrox up to 40%, and air.  But guess what? Here’s another issue- if you get air fills in your Oxygen clean cylinder, the air should be OCA, or oxygen compatible air. This is air that has been highly filtered, typically with an extra bank of hyper-clean filters on the shop’s compressor system.  If you fill an oxygen clean tank with non-OCA, it’s definitely not the end of the world, you just risk the possibility of having to have your cylinder oxygen cleaned again. OCA is definitely not universally available, so it is not uncommon to have to take that small risk when diving out of town or on the road.  

The Long and Short of it for You, the Tank Owner

As divers, many of us are creatures of habit.  We have our favorite (or maybe only) local dive shop that fills our tanks and gets all of our loyal business.  It’s when you dive and travel with your tanks, or if your local dive shop is closed and you need a last-minute fill, that problems with the wrong visual certification level can creep up.  If you need to get a nitrox fill at a shop other than the shop that inspected your tank, just make sure that your level of visual inspection will work with the dive shop that will fill your tanks when you’re doing that weekend dive road-trip to the coast or springs.  A little pre-planning can save the headache of delays and added expense to your dive trip.

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