by Doug Pemberton

One of the key elements in diving safety is proper equipment maintenance. And one item that has a strict maintenance schedule is the scuba tank. In order to ensure that our tanks are up to standard, we take them to a dive shop once a year for a visual inspection. The VIP system for inspecting scuba tanks has been around for decades and until recently was the primary way to detect any problems. The VIP involves a detailed visual inspection both inside and outside the tank during which technicians look for a host of irregularities that can show up over time. It is not necessarily the age of a tank that is a problem but the abuse it receives over time.

Recently I took some tanks in for a VIP and followed them through the procedure. After removing the boot, each tank was laid on its side on a bench and a straight edge was laid lengthwise along the outside of the tank. As the tank was rotated beneath the edge I was surprised to see that one side of the tank was slightly concave and the other side slightly convex. Tanks are actually banana shaped, a product of the manufacturing process and quite normal. If anything more than a very slight banana shape is found then there may be a problem.

The outside of the tank was then checked for any excessive pitting. There is usually some oxidation, especially on tanks that have been painted. Water eventually gets under the paint and causes some bubbling which is usually not serious. Some oxidation sites were scraped or probed with a dental pick to ensure that they weren't too deep.

When the valve was removed we got a chance to look inside the tank and inspect the threads in the neck. The neck area is the thickest part of the tank and yet it is one of the most vulnerable, with nearly all serious problems originating there. The threads were cleaned and then checked as well as possible using a dental mirror, flashlight and the naked eye. Finally a light rod was inserted into the tank, illuminating the inside to show any irregularities. If there is any kind of corrosion then a tank may have to be tumbled or any dirt or oil residue would have to be washed out. If a tank was found to meet the criteria set out by the visual examination then the tank was passed for another year of use.

Until recently, besides the five-yearly hydro that was about all there was to it. But over time tiny cracks can develop in the neck area of a tank and lead to serious problems. Now there is a new sheriff in town. Called the VIP Plus system, it is becoming more common in dive shops and is used in conjunction with the regular VIP. The VIP Plus system uses electronic eddy waves to detect cracks and other anomalies in the neck area of a tank that are not always obvious to the naked eye.

In order to use the VIP Plus system properly, the threads in the neck of a tank must be thoroughly cleaned using a clean cloth and toothbrush. The threads must also be counted as the number in each tank differs, and the probe must go precisely to the end of the threads and no further, otherwise a faulty reading might be acquired. The VIP Plus machine also has to be properly calibrated to begin with. The probe is first screwed into a known-defective test standard in order to calibrate it to detect the proper criteria to indicate cracks.

Once the probe has been screwed into the neck of the tank, it is slowly unscrewed and any anomalies are registered on a display screen. Tooling marks, folds in the aluminum, corrosion and cracks are all picked up and the technician must be able to interpret the various signals, some of which have their own distinctive characteristics. As I watched the screen, the flat line occasionally jumped but never spiked over the tolerance threshold line that indicates a potential problem. The technician often went back over suspicious sites and double checked the distance the probe moved between the beginning and the end of a spike and the location of the anomaly was marked on the neck of the tank using a grease pencil. If the probe is moved 1/8 of an inch or less and a spike is produced then a crack or fold has been detected. If the spike re-occurs at the same spot on subsequent revolutions then there may be a serious problem. A larger patch of irregularity could be due to corrosion and spikes at evenly spaced intervals around the neck may indicate tooling marks.

If the tank passes, a small VIP Plus sticker is stuck to the tank in addition to the regular VIP sticker. If a tank has been determined to have a crack, or a fold that propagates into two or more threads, the tank is condemned and the customer is notified and has the option of sending the tank back to the manufacturer for replacement. The only thing the customer pays is shipping cost. The three tanks that I brought in passed.

DIVER Magazine would like to thank Geoff Grognet and the Diving Locker for their assistance in preparing this article.