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     Many divers, as well as other people that handle high-pressure cylinders, are concerned with their safety when around them. The United States Department of Transportation (DOT) licenses companies to test such cylinders in order to insure that they are safe to use by divers, firefighters, homeowners, etc. The way in which hydrostatic testing of cylinders is done will be explained in the rest of this Webpage.

 Click here to see a hydro picture    

   This is the beginning of testing a cylinder for structural integrity. The tank to the left is a Luxfer,100 cubic-foot, scuba tank. It has a working pressure of 3300 psi and is constructed of aluminum.

     The first thing that is done is to remove the tank valve after insuring it is empty. The tank is given a thorough visual inspection (VTI). The tank's interior is checked for corrosion, particulates, and any other abnormalities. The threads are checked for integrity and imperfections. If the tank passes the VTI, it is filled with water for the hydrostatic test. If a tank were to rupture the results would be minor since it is filled with water instead of gas.

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     A special valve is inserted in the top of the cylinder that has been filled with water. The valve is sealed with teflon tape or an O-ring. In this case, since this is an aluminum cylinder with an O-ring gland at the top of the threads, the valve you see has an O-ring just at the top of the threads.

     It is hand tightened so the seating is firm. This should insure that the water does not leak out of the neck of the tank when it is subjected to the testing pressure. Valves requiring teflon tape for a seal are driven in with a torque wrench to insure there is no leak when pressure is applied to the tank.

     The quick-release disconnect at the top of the valve will be mated to the top of the hydrostatic test tank. This is where the water will enter to insure the tank is full and to apply the pressure to the tank for the test.

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     Although it is hard to see in the picture top the left, the tank has been mated to the top of the hydro test chamber. The tank is suspended about 2 feet off the floor. The top of the chamber is just above the tank. The tank will be raised higher and then swung over the bottom of the chamber that is visible in the lower background.

     The entire test chamber is filled with water as well as the tank to be tested. As the tank is lowered into the chamber the water rise and possibly spill over into the waste well that surrounds the chamber.

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   The tank is being lowered into the test chamber. Notice the O-ring used to make the seal between the lid (holding the tank) and the rest of the chamber. As the tank is lowered, the water will rise and possibly spill over. If not, there is a pipe running into the chamber to insure it is filled completely eliminating the possibility of compressing gas instead of fluid. Also, if the tank had a tank boot it would have been removed. If the tank had a curved bottom that could trap air the air would have been paddled out. 

Click here to see another hydro picture

     You are looking at the top of the hydro chamber. The tank is suspended from this and is in water. So, the tank is completely filled with water and the chamber outside the tank is filled as well. The top is attached to the bottom with the three winged screw clamps. There is an O-ring between the them to insure that water cannot leak out between the two.

     There is a hose to the right. It supplies water to the interior of the cylinder insuring that there is no gas there. It is also the source of the high pressure that will be applied to the tank.

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     The photograph to the left shows the entire outer cylinder. The scuba tank is enclosed in this. Both are filled with water and pressure is about to be applied to the scuba cylinder through the hose in the upper right.

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     The burettes to the left are for measuring the expansion of the scuba cylinder when pressure is applied to it. The cylinder is going to be pressurized to 5/3 of the DOT pressure stamped on the neck of the tank. If the stamp read 1800 psi, then the tank would be pressurized to 3000 psi. Likewise, for a standard 3000 psi scuba tank, it would be pressurized to 5000 psi. The cylinder being tested is a 3300 psi tank, so it will be pressurized to 5500 psi.

     As this test pressure is applied the tank will expand. Remembering the tank is in a chamber of water, the expanding tank will push against the water surrounding it. That water will be forced through a small hose and into one of the burettes. The burette that is chosen for each cylinder would be the one that would have the water go near the top when the final pressure is attained. But, only 1 burette is used for each cylinder. In this case, the scuba cylinder is large and pushes a lot of water from the chamber so the 3rd burette from the right will be used.

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     After the scuba tank is inserted in the chamber and everything is closed up, the water outside the tank is added to until it fills the burette to the 0 mark. The photo to the left shows the middle burette with the water at the zero mark (just behind the red reading bar that goes across all the burettes. The water must stabilize at the zero mark before the pressure is applied to the tank. Sometimes this takes several minutes because of temperature changes in the water surrounding the scuba tank, the tank itself, water inside the tank, etc.

Click here to see another hydro picture

   The pressure has been applied to the interior, water-filled, scuba tank. The pressure is at 5500 psi on this super-calibrated gauge. It took about 30 seconds to go from atmospheric pressure to 5500 psi! The tank has expanded and being held at this pressure for a short time while the amount of expansion is read on the burette.

Click here to see another picture of the hydro test

    The second burette from the right now reads 67.4  ml. The water has been pushed up the burette by the tank expansion due to the pressure on it of 5500 psi. This must be a stable reading. In other words, the water level in the burette cannot change for at least 30 seconds. For example, if the water were to continue to rise after the final pressure was reached it could be due to a leak from the inside scuba tank water to the water in the chamber. Or, it could be due to a continual expansion of the metal of the tank which would be reasonable cause to fail the cylinder. Should the water continue to rise, the scuba tank would have to be extracted from the chamber, the seal checked, and the pressure applied again.

Click here to see another hydro picture    

     The pressure has been released from the scuba cylinder. The cylinder has contracted. Because of that the water has returned from the burette to the chamber. But, the cylinder did not return to the size it was before the pressure was applied. It was permanently stretched. The burette to the left reads 1.0 ml. That 1 ml represents the amount of stretch. Now, if the tank stretched and did not return close to its original size, it would mean the metal of the tank was not resilient enough to be safely used for scuba diving. The tank must return to at least 10% of the upper reading. In this case the upper reading was 67.4 ml, so a reading of less than 6.7 ml would allow the tank to pass the hydrostatic test.


     This scuba tank has passed the hydrostatic test. It is removed from the chamber. The water is drained from the interior and it is put on a drying stand to remove all moisture. A final VTI is performed to insure no damage was done by the high pressure the tank was subjected to. The hydro test date and federal hydro station number are stamped on the neck of the tank. The picture to the left shows the owner of Deep-Six applying the date and station number to the tank. The valve will now be inserted and the tank filled for the customer.

     A good hydrostatic operator can perform about six tank tests in one hour. The first tank takes longer because the chamber and the gauges have to be calibrated. The calibration verification is done using the Pressure Indication Device (PID) against the Expansion Indicating Device (EID).

     Hydrostatic testing should be done on scuba cylinders every 5 years unless something has happened to the tank in the meantime that would indicate a need to check the tank sooner. Since the hydro test stretches the metal of the tank beyond what a normal fill would do, it is not recommended that the test be done more often. In any case, most scuba tanks are guaranteed for 100,000 fills if they have been visually inspected once per year and hydrostatically tested once every five years.

     Deep-Six thanks Gordon Fire Equipment in Highland, NY for the use of their hydrostatic test station for this Webpage! Deep-Six uses this facility to hydro our customers' tanks.

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