Dive New Zealand No. 55

December/January 2000

During October Mark Olguin and Steve Hindman from Advanced Inspection Technology in the USA visited New Zealand to conduct seminars in Auckland, Wellington and Christchurch on the use of the Visual Plus method of visually inspecting cylinders for sustained load cracking.  Interview by Dave Moran.

How were the Visual Plus seminars received in New Zealand?

Enthusiastically, most that attended were open minded and eager to learn.

What were some of the concerns filling stations had with detecting cracks in Scuba cylinders?

The main concern was in regards to sustained load cracking, (SLC) mainly that these cracks were very hard to detect during the standard visual inspection and the hydro-test.  Another concern was referred to as 'Fast Crack Growth', where it is perceived that a crack has formed and grown to 20 cm in a period of less than one year.  We believe that these were cracks that had been there for a longer period, and simply not detected at the last hydro-test or visual inspection.  Sustained Load Cracking can take many years to propagate, for instance the Tairua incident cylinder showed that the crack had been evident in the neck for at least five previous inspection.  Visual Plus would have picked up the flaw several years ago.

How does Visual Plus detect cracks?

It uses eddy current technology, the same technology that is used in the aerospace industry to detect cracks in aeroplanes.  Electrical currents are introduced into the metal and fluctuations are measured when an imperfection is detected.

Is there any other crack detecting systems being used in the US besides Visual Plus?

There are systems available which use ultrasonic technology, but they tend to be cost prohibitive (from US$40,000 to US$500,000 to capture the entire cylinder) and labor intensive.

Does the Visual Plus system require expert training and is it necessary to periodically evaluate the skill level of the personal doing the testing?

It does require some training.  You will receive hands-on training from the selling agent.  A training video is supplied with each unit and our web-site is also a great source for information, www.visualplus.net

In your experience have you found that some cracks are not being detected due to the skill level of the tester both when doing a visual and with using the Visual Plus system?

All the knowledge in the world might be accompanied by a bad set of eyes.  Conversely the best set of eyes might not know what to look for.  Because there is always the human element to deal with, some cracks will slip by even the most experience and knowledgeable inspectors.  We know this to be true due to increased cylinder failure rate of facilities using Visual Plus and testimonials from around the world.  As with the hydrotest and any other piece of test equipment, the results from the Visual Plus test are only as good as the operator performing the test.  However when the Visual Plus is set up correctly and used properly the results are absolute, unlike the human eye.

How important is cleaning of the neck area before doing a visual or Visual Plus test?

It is much easier to detect anomalies or imperfections in a cylinder, which has been cleaned properly; it is a requirement of both Occupational Safety and Health (OSH) and Department of Transport (DOT).  Therefore, we believe it is paramount that the neck/thread area be as clean as possible.  Clean cylinder threads will also help to extend the life of the Visual Plus test probe.

In your experience what percentage of cracks are found by Visual Plus that were not detected by a normal visual using a light and a mirror?

In the US failure rates at facilities using Visual Plus have risen from 1% to 7%.   There is too much at stake and too many variables involved when evaluating the cylinder for neck cracks, such as bad lighting, poor eyesight, lack of knowledge, tooling smears, temperature, etc. not to take advantage of technology that will eliminate the guesswork and ensure safe cylinders.

Where are the majority of cracks found?

Approximately 95% of SLC in aluminum cylinders begins at the base of the threaded area that provides a radius point.  These cracks propagate up the threads and down into the shoulder simultaneously, propagating more rapidly as the crack elongates.

Have you experienced any particular make of scuba cylinder or series of cylinder that are more prone to cracking than others?

SLC in aluminum cylinders is more of an alloy problem than the problem of a particular manufacturer.  However, the 6351-T alloy, which was used mainly in the 70's and 80's does appear to show more of a propensity for SLC, but is not exclusive to this alloy.

Are filling stations in the US demanding that a scuba cylinder be Visual Plus tested before filling?

Just about all the test stations using the Visual Plus have experience what the machine is capable of finding and therefore will refuse to fill a cylinder unless it has proof of Visual Plus inspection.

What percentage of US test stations are using Visual Plus?

About half.

In your opinion what is more important, a visual or hydro?

Although both are important, for us the visual inspection is much more important than a hydro-test.  In excess of 90% of all cylinders removed from service fail the visual inspection before it even reaches the hydro-test.  The 'hydrostatic retest' alone will return a cylinder with one or more cracks to service, unless the cylinder actually leaks.  Keep in mind, water is denser than air, therefore a cylinder that is actually leaking air can and has passed the hydro-test.  A well-trained visual inspector, using the correct tools, should detect most of these problems, with the exception of heat exposure.

Have you any thoughts on the suggestion that Scuba cylinders be classified to have a finite life then pulled from service?  This has the obvious problem of determining what is regarded as moderate use compared with frequent use, such as hire cylinders.

That is a very good point, the problem is tracking fill cycles.  It can't be done.  My personal opinion is that I would prefer to fill an older cylinder that has recently been inspected with Visual Plus and had a proper visual inspection, than a newer cylinder that might be of the frequent use variety and not had these tests performed properly.

What is the current thought in the US regarding having scuba cylinders hydroed and the period of time between hydros?

As in New Zealand, many in the industry feel that the hydro test is simply a holdover from the steel cylinder test and serves little use, as they rarely fail due to expansion.  Still, many believe the hydro to be the be-all/end-all to cylinder testing, yet all agree that a proper visual inspection is essential.  Many are fearful that if the hydro-test were eliminated, the costs of new equipment would put them out of business, so it is hard to get the true feeling.  I don't think anyone would like to see the frequency of the hydro-test increased and yes, there are a few pushing to extend the test (on aluminum only).  Regardless of the frequency of the hydro-test the bottom line is; too many cylinders that need to be removed from service for other reasons are passing this test and being returned to service.  The most important thing is that this test is done in conjunction with a visual inspection performed by a professionally trained individual, using the proper equipment.

What is your opinion of the standard of the filling stations that you visited in New Zealand?

We only visited three stations, as most of our time was spent in meetings and seminars.  Of the three we did visit, all seemed to be very professional and safety conscious.  More containment stations wouldn't hurt, and 'all' were reaching over the cylinders being filled to operate the controls. 

Because most catastrophic failures occur during the filling process, the emphasis in the USA is moving rapidly toward separating the fill station operator from the cylinder during the fill cycle, while providing as mush safety shielding as possible.  The further you are from the cylinder while it is being filled, the safer you are.

Are there any recommendations that you consider would be beneficial to the way we test scuba cylinder here in New Zealand?

As President of Advanced Inspection Technology my answers are probably biased, but I feel the testing of any aluminum cylinder is incomplete without a Visual Plus inspection.  The return rate of aluminum cylinders since the introduction of Visual Plus is proof of this.  Obviously the safety awareness in New Zealand is very high in light of a couple of recent incidents and therefore inspectors are now doing a more diligent inspection and not relying on the hydro-test as heavily.

Education is probably the key issue.  Let's face it, 20 years ago a visual inspection was unheard of and today it is mandatory.  We must continue along these lines and learn as much as we can.  In the US there are companies like PSI, Inc. and IAST which teach comprehensive courses on cylinder safety.  These types of courses are invaluable.

My understanding is that the use of a product called 'Version' is now mandatory for cleaning cylinder threads.  My exposure to this product is limited, but the scuba cylinders that I have seen which have had Version used on them are among the cleanest one can expect to see.  I would certainly support any product that enhances the visual inspection process.

Do you feel that spot checks by OSH would be a positive step to ensuring that test stations are up to standard?

This probably won't make me too popular with the test stations, but the threat of these types of 'spot inspections' do tend to keep people on their toes.  If it is in the interest of safety I am all for it.  If the test stations are doing their job correctly, it should be of no concern.

What do you think of the steps being taken by OSH and Luxfer with the labeling of information on tanks for the cylinder owner?

Any type of literature that helps educate divers on how to care for and maintain a product that contains so much energy must be looked at as a positive.  I think both OSH and Luxfer are to be commended for taking the proper steps in training the owner of these very powerful cylinders.  Many of these individuals are unaware of the potential damage these cylinders can cause.  The cylinder owner must take the final responsibility to ensure these cylinders are maintained properly and these are very good guidelines to follow.

Do you have any recommendations for a diver regarding the way they look after their own scuba cylinders?

These cylinders harness immense power and must be treated accordingly.  Keep your cylinder clean and in test.   Your cylinder should never be stored without internal pressure.  Never overfill or fast-fill your cylinders.  Use only reputable fill and test stations.

I understand you will be coming back to New Zealand every year.  Can you expand on what will be the main objective of these visits?

Our main focus next year will be training, however we do hope to introduce some new safety devices and procedures.