Your scuba tanks are prone to marine and weather elements that wear the tank down both from the outside and inside. By the end of their dive certification, all divers are made aware of the necessity of periodic inspection of scuba tanks if they choose to own one (or multiple) instead of renting them.
Now, scuba tank inspection generally goes through two stages: the visual cylinder inspection (VCI) and the hydrostatic inspection. In this article, I’ll gloss over the ins and outs of the first stage of inspection i.e. VCI. If you’re a new tank inspector intern or a self-sufficient diver looking for a condensed scuba cylinder visual inspection resource, this is the article for you.
Table of Contents
- 1 The Scuba Tank Visual Inspection Checklist
- 2 Some Advanced Inspection Techniques
- 3 The Limitations of Inspections
- 4 Step-by-Step Tank Inspection Process
- 5 Best Practices for Visual Inspection
- 6 That’s All About Visual Inspection
The Scuba Tank Visual Inspection Checklist
The primary purpose of VCI is to determine the structural integrity of your scuba tank without performing any invasive or degrading procedures on it.
So, let’s jump into the intricate details without further ado.
Types of Corrosion to Lookout For
First things first, corrosion is the main culprit behind the degradation of your tank’s structural integrity. So your first course of action would be to determine the type of corrosion (if any) your tank is suffering from.
Brand new aluminum scuba tanks (which is the norm nowadays) have a shiny, stainless steel-like surface on the interior. This is unoxidized aluminum. However, over time this inside surface will change. The tank may gradually turn an even dull gray color. This dull gray appearance is caused by aluminum oxidizing, which is normal and not a problem. Aluminum oxidizes similarly to other metals like copper and brass.
While the shiny “new” look disappears as oxidation occurs, the corrosion does not go any deeper. An even layer of dull gray aluminum oxide does not affect the tank’s structural integrity. Freshwater exposure will generally cause this benign surface to dull over time.
Saltwater Pitting Is a Devil to Keep an Eye out For
Compared to Steel Tanks, aluminum tanks that are routinely exposed to saltwater show a different and more alarming corrosion pattern – pitting. This type of corrosion “eats” into the metal surface in spots, forming little white boils and marks.
The white marks are oxide formations. Pitting reflects pockets where the aluminum is degrading, leaving scars behind. This uneven corrosion is dangerous and means the tank should be taken out of service.
In some older tanks, the inner surface may have a mottled look versus a uniform dull gray. This mottling could simply be a mix of superficial aluminum oxide and residue from unfiltered air in the tank. Clean the tank interior to remove any debris or deposits. Examine the surface closely for any pitting or cracks disguised by the mottling. If there’s any sign of subsurface pitting corrosion, condemn the tank even if the damage is localized.
Check for Cracks in The Neck Region
Cracks represent another commonly encountered problem when inspecting scuba cylinders. But why do cracks tend to form predominantly in the neck region compared to other parts of the tank?
It comes down to how the neck dome is manufactured. The dome shape is created by pulling the extruded aluminum together. This process puts the metal under tension, which can later cause cracks to develop in this area over repeated pressurization cycles.
Look for any hairline cracks in the neck, which may appear darker or lighter than the surrounding metal. White lines could be cracks where aluminum oxide has formed. Carefully inspect the interior dome curve using a light and mirror. While harmless stretch marks can also occur in the dome from manufacturing, any crack indicates the tank must be retired.
Another reason cracks crop up in the neck region is corrosion. Severe pitting corrosion around the neck can lead to cracking as the integrity degrades. So cracks and corrosion often go hand in hand when it comes to cylinder neck failure.
Be Careful Not to Over-Tighten the Valves
Besides manufacturing flaws, human error during tank handling can also spell disaster. One prime example is over-tightening the tank valve. Excessive force can ruin the valve threads and create further strain in the already vulnerable dome section.
Using wrenches and tools improperly to open or close valves is an easy mistake that leads to premature cracking or tearing. On old European cylinders with conical threaded valve stems, the strain of over-tightening can be even worse as it pulls laterally on the neck.
The Risks of Uneven Wear
While corrosion eats away at the tank’s thickness, another type of wear can thin the walls unevenly: abrasion. If certain spots of the tank’s surface suffer abrasion damage, this makes the cylinder walls thinner in those areas.
During your inspections, look for any signs that the tank is unbalanced or wobbles when rolled on a flat surface. This can indicate abrasion damage or folds in the metal that create uneven thickness around the circumference. Such asymmetry means the tank walls have been weakened and lost structural integrity.
Other External Damage Signs to Watch Out For
While we’ve focused on the cylinder interior, don’t forget to check the outside surface too. General external corrosion usually isn’t a reason to condemn a tank. However, localized corrosion spots, deep gouges, and pitting on the exterior could be a problem.
Pay very close attention to the external neck and base regions, as these are common trouble spots. For example, salt water may collect behind the neoprene boot around the bottom of the tank, causing hidden pitting. Take boots off to inspect underneath them.
On tanks with backpack attachments, improperly positioned or loose bolts can grind into the cylinder surface, concentrating abrasion in one spot. Similarly, slack cam bands made of steel on a cylinder can gouge the softer aluminum. This damage then acts as a focal point for intensified corrosion.
Also, look out for any galvanic corrosion if the tank has permanent direct contact with another type of metal. The interaction of dissimilar metals through a conductive solution like salt water accelerates corrosion.
Some Advanced Inspection Techniques
Visual and hands-on inspection can catch many issues with scuba cylinders. However, some types of damage and flaws require more advanced non-destructive testing methods. Two main technologies stand out: eddy current testing and ultrasonic inspection.
Using Eddy Current Tools
Eddy current tools are commonly used to examine airplane fuselages for cracks and weaknesses since flight safety depends on aircraft structural integrity. This same technology is very valuable for thoroughly checking aluminum scuba tanks.
Special commercial eddy current instruments like the Visual Plus, Simple Eddy, and Visual Eddy machines are made for inspecting diving cylinders. These devices use electromagnetic fields to detect cracks and wall thickness variations that may not be visible externally.
Since scuba cylinder manufacturers also make and market these advanced eddy current devices, they are reasonably priced for dive operators to purchase. Investing in such professional equipment takes your tank inspections to the next level.
The Limitations of Inspections
Adopting techniques like eddy current testing makes tank inspections highly rigorous. However, some failure modes can’t be predicted even with thorough checks. For example, spontaneous fatigue cracks can arise suddenly from an inherent material defect.
If an accident occurs after a recent inspection finds no issues, this may indicate an unpredictable material failure at play. In those rare cases, preventing future issues may require switching alloy types entirely to remove that flaw risk.
Beyond Visual Checks, It’s Time to Go Electronic
While the inner and outer tank surfaces should be examined visually, some problem areas can be hidden from view. It’s essential not to rely solely on visual and tactile inspection.
For instance, remove any backpack or carrier assembly so you can check the outside surface underneath. Use a mirror to inspect the upper interior dome region. With the valve detached, a borescope camera can be inserted for interior visual access.
Step-by-Step Tank Inspection Process
Now that we’ve covered what flaws and damage to look for, let’s walk through the inspection flowchart from start to finish. Follow these key steps:
1. Start With a Clean Tank
It’s impossible to properly inspect a dirty tank. Any mud, oil, grease, salt, debris, or deposits on the surface could conceal cracks, pitting, or gouges.
Always start by thoroughly washing and rinsing the tank. Use a tank cleaning brush with a mild detergent to scrub away grime. This will expose any underlying issues.
2. Examine the Cylinder Interior
With the tank cleaned, conduct a close visual examination of the entire interior surface. Use a bright flashlight or shop light. A dental mirror makes it easy to view the upper dome section.
Look for any pitting, cracks, gouges, bulges, or variations in surface color and finish. Mark any problem areas. Compare to past inspection records. Even minor changes over time can be important.
3. Check the Exterior
After the interior review, switch to the outside inspection. Roll the tank and scan the full surface, noting any dents, scrapes, bulges, or gouges. Pay extra attention to the critical neck and base regions.
Also check the foot ring for cracks, as these high-stress areas are prone to failure. Mark any suspect spots for further testing like eddy current.
4. Assess the Valve
Open and close the tank valve while feeling for smooth operation. Any sticking, binding, or rough sensation indicates potential issues.
Inspect valve threads for signs of wear, galling, or damage. Gently try tightening the valve by hand to check for leaks. Put a bit of soapy water around the valve neck and watch for bubbles.
Any leak indicates the valve needs replacement. The valve seat should form a complete air-tight seal.
5. Do a Roll Test
After the visual review, conduct a roll test to check for balance issues. Make sure the tank is empty! Slowly roll the cylinder on a flat surface.
If it wobbles or rocks more than slightly, this suggests asymmetry and uneven wear. Marked wobbling is a red flag for wall weakness. An unbalanced tank must be set aside for proper re-certification.
6. Consider Advanced Testing
For tanks passing the initial inspection, further non-destructive assessments can provide assurance. Eddy current, and ultrasonic testing each detect flaws in different ways.
Specialized equipment like ultrasonic thickness gauges can measure metal loss from pitting corrosion or abrasion. Make use of these advanced techniques for critical tanks.
7. Document Any Damage Found
Keeping detailed records is critical for tracking tank life and hazards. Carefully document any damage found during the inspection. Note the flaw type (crack, gouge, pitting) and location on the cylinder.
Photos and video are great for records. If you find an issue, share documentation with the cylinder owner when explaining repair/replacement needs.
8. Determine Next Course of Action from The Report
Skim through the report you created to arrive at a sound conclusion about the fate of the tank(s). If damage is minor, the tank may be repairable based on manufacturer guidelines. For example, small dents can sometimes be worked out mechanically or small interior corrosion spots can be taken care of by tumbling the tank.
However, some degradation like deep cracks or severe pitting corrosion will unfortunately require the old tank to be retired and replaced.
Best Practices for Visual Inspection
To be an effective scuba tank inspector, there are several best practices to drill into your brain:
Get Proper Training
Make sure you receive formal training in scuba cylinder inspection methods. There are courses dedicated to correctly using tools like eddy current testers. Proper qualifications reinforce diving safety.
For each inspection, go through a complete checklist that covers every step. This ensures you don’t miss any part of the process. Checklists also aid consistency between inspections.
Don’t be a lazy bum. At a minimum, scuba tanks should be inspected annually by a qualified professional. Heavy usage requires more frequent inspections, such as every 6-9 months. Staying on top of inspections prevents dangers.
Know When to Condemn
This is not a technical point but is of utmost importance. Don’t hesitate one bit! Err on the side of caution – if you find any damage that could compromise safety, don’t let the tank stay in use. Tell them the truth for what it is and you’ll inadvertently be saving a diver’s (or your) life.
Communicate with Customers
This one is aimed at tank inspectors. Explain inspection findings clearly to tank owners, whether good or bad. Show them any damage and provide repair/replacement recommendations. Your advice keeps them diving safely.
That’s All About Visual Inspection
This guide covers corrosion patterns, flaws, and damages to look for, techniques to employ, the inspection sequence, and best practices to follow. That was my 2 cents on the visual inspection of your dive tanks from my 15+ years of experience in the field of scuba diving and seeing all sorts of unfortunate tank-related tragedies unfold before my eyes.