Manta Divers November, 2009
It was a dark and stormy Halloween night. Lisa was tidying up the classroom, I was giving candy to the last of the trick-or-treaters, and a couple of customers were browsing around in the shop. Suddenly, there was a loud bang like a gunshot followed by what sounded like screaming. I rushed back into the shop find the customers hunkered down under cover but nothing seemed amiss. I then realized what had happened. The burst disc on the tank I was filling had blown. But why had this burst disc blown in spite of the fact that the tank was not even filled to its capacity? To figure out this mystery, it helps to understand what goes into a routine tank inspection. You probably remember from open water class that scuba tanks need to have a visual inspection annually and a hydro test every 5 years, but you probably don’t know exactly what goes into that inspection.
The hydro test is performed by taking the valve off the tank and filling it with water. The tank is then placed into another tank with water and the tank is sealed. The water inside the tank is then pressurized to five thirds of the tanks capacity. The tank will expand under this pressure and the increased volume will be measured. If it does not fall within a specified tolerance, it fails and the tank, by law, must be condemned.
I have a U.S. DOT RIN (re-inspection number) as well as a PSI certification. That means that I have been trained and passed the requirements to inspect breathing air tanks, such as SCUBA and SCBA. Prior to taking that class, I had no idea what went into a tank inspection. The purpose of the annual visual inspection is to discover problems before they become an actual hazard. During the visual inspection, the tank’s exterior and interior, threads and valve are examined. The outside of the tank is checked for scratches, dings, dents and gouges. It is necessary to remove any stickers from the exterior of the tank to ensure that no damage is covered up. Deep dents or gouges are measured, and if deep enough may cause the tank to be condemned. After checking the exterior of the tank, the valve is removed and the inspector tips the tank over to see if any debris is loose inside. It is not unusual to find a small amount of aluminum oxide inside a tank, but it is troublesome if oil, water or a large amount of debris is found. This is almost always a result of bad air fills due to some trouble with the fill stations compressor. Always look for proof that the air you are filling your tank with is checked at an air testing lab. If oil or a foul smell is found inside the tank, it will need to be thoroughly cleaned. The inspector will also look inside the tank with a special light and check for signs of corrosion, pitting, oxidation, or stress cracks.
After checking the interior and exterior of the tank, the inspector moves on to the threads at the top of the tank. It is very important that all the threads be in good condition so the valve stays in place when the tank is fully charged. They are checked for continuity and integrity. According to recent changes in the U.S. Department of Transportation regulation, tanks made prior to 1989 using the 6351 alloy must be also eddy current tested as part of their 5 year inspection, in other words, when they are hydro tested. This is because that particular alloy of aluminum that was used in those tanks has been shown to be prone to cracking. During the eddy current test, the inspector runs electrical current through the threaded neck of the tank. The instrument is attached to a computer and the continuity of the current is graphed. If there is an interruption in the current it could just be that there is some dirt built up in the threads, but it could also mean that the threads have a crack, which would result in condemnation of the tank. Although it is not required that all tanks older than 1989 be eddy current tested, at Manta Divers, we feel it is good practice and do it to be extra safe and assure that no cracks have developed just due to years of use.
The last part to be inspected is the valve. If the valve holds pressure, it is not necessary to break it down, but the threads, dip tube and O-ring are checked. The dip tube is a safety feature that prevents water from entering your regulator should any get into your tank. The O-ring should be flexible and not over lubricated. The last part of the valve to be inspected is the burst disc, which brings us to our dive shop mystery.
The burst disc is a safety feature that acts as a relief if the tank is over filled. This is decreases the chance of developing a stress crack because in the event of the tank being over filled, the burst disc will give way before the tank material will fail. The rating of the burst disc must match the pressure rating of the tank. In the case of our “run away” tank, the burst disc showed some signs of wear: slight dimpling, but no creasing. Upon filling the tank, however, it became evident that the disc had been stressed enough to cause it to fail before the tank reached its full pressure. The burst disc on the second tank was much worse. Needless to say, both tanks needed new burst valves. Divers often bring in tanks and ask that we over fill them, and of course we cannot do that, but even filling them right to their capacity can put added stress on their tank if it sits in their car for any amount of time on a hot summer day. These two tanks definitely showed signs of being over pressurized. This sets up a dangerous situation for the diver and an even more frightening situation for the tank filler.
Now you know more than you ever imagined about tank inspections. As with all gear intensive sports, the safety of the participant is directly impacted by the quality and frequency of gear maintenance. All of your scuba equipment including your tank should be cared for by you and at regular intervals by a trained professional.
In Other News..........
If you are interested in learning about gear maintenance and care beyond the introduction you got in open water class, consider taking the Equipment Specialist Class, December 2 & 3 6-9pm. While this class will not qualify you to rebuild your regulator or inspect a scuba tank, it will teach you to make minor repairs and gain a deeper understanding of how your gear works. You will also be treated to an introduction to other scuba activities that require specialized equipment , such as recovery and lift bag use and spearfishing.
November and December are the months you should be bringing in your regulators for service so they are in tip top shape for your next trip. To ensure that you are in tip top shape for your next trip, join the Scuba Review class Tues., Dec 8, 5-9pm
For your non-divng friends and relatives, we are offering our Into to Snorkeling Class Dec.8 6-9pm. this is a good way to start to cultivate a diving buddy, or just get a bit more confidence and comfort while snorkeling.
Need more bottom time? Learn to dive with Enriched Air! Join the Nitrox Class Dec. 10 6pm.
Finally, time is runnning out to sign up for the Grenada trip, April 17-24, 2010. Don't miss out on this fantastic trip to the Spice Island! Deposits due Dec. 8, final payment Feb. 1.