Hydrostatic testing, or "hydro" for short, is a mandatory inspection and
must be performed at least every five years. Although you'll take your cylinder
to the local dive center for the hydro, few can conduct the test on the
premises due to the expensive and highly regulated equipment and licensing
that is required by the government.
Hydrostatic tests are supervised by the Department of Transportation (DOT), the government agency which determines safety standards for
manufacturing, transporting and testing of pressure cylinders of all types.
The DOT controls the setup and operation of hydrostatic machines and maintains records concerning manufacturers, selling agents, and testing facilities.
Hydrostatic testing and its required internal and external inspections are performed by certified professionals trained and tested by the DOT, The Bureau
of Explosives, and The Compressed Gas Association (CGA).
Hydrostatic testing uses water under pressure to measure the strength and flexibility of the cylinder's walls. Even though the test is required
every five years, it should be performed when the cylinder is filled beyond its working pressure, isn't used for more than two years, or has been structurally weakened
through impact damage or tumbling. Cylinders should be hydro-tested or even condemned if exposed to high heat.
To test a cylinder, the inspector first gives it a thorough internal and external visual inspection. Then the cylinder is filled with water. Next, the
cylinder is placed inside a water-filled, high-pressure container. (Filling the cylinder with water and sheathing it in a water-filled chamber is much safer
in the event of a rupture. Water dissipates the energy rapidly, where air would expand explosively.)
The next step is to inject high-pressure water into the cylinder and overfill it to five-thirds of its working pressure. (In an aluminum 3,000 psi
cylinder, the test pressure is 5,000 psi.) This causes the cylinder to expand slightly. Pressure is held steady for 30 seconds to make sure the cylinder has
reached, and can maintain, full expansion.
As the cylinder expands, the water in the surrounding vat is displaced. The
displaced water is measured in a calibrated collecting tube called a burette.
Any permanent cylinder expansion from the test is also measured by the burette.
A cylinder passes the hydro test if its permanent expansion after the test
is less than 10 percent of that measured during the test. (If the expansion of
the cylinder was 60 cc during the test, it would take a permanent expansion of 6 cc
for it to fail. A typical cylinder shows an expansion under 1 cc.)
If the cylinder passes the hydrostatic test, it is stamped with the current month and year, and the inspector's initials or a special marking that is registered with the Department of Transportation. The marking is stamped in between the numbers representing the month and the year. Hydrostatic test results are kept on file, and the tester is legally obligated to provide a copy of the results.
After many inflation/deflation cycles, the metal loses some of its resiliency and becomes work hardened. When this condition reaches a certain threshold, and the metal can no longer contract to its unpressured diameter as indicated in a test specification for the cylinder, it is condemned. In the trade, this is called a "failed hydro", and is one reason that buying used equipment is not always the most economical choice.
Cylinders that pass the hydro are dried with a modified hair dryer or special hot air jets. Hot air and circulation are necessary to rapidly dry
the inside so corrosion won't start. An internal inspection is completed to
make sure the cylinder is dry and free of contaminants. A new visual inspection
sticker is applied and the cylinder is filled.
Do cylinders ever explode during a hydro test? Rarely, but it is not unheard of. Usually it is the neck of the cylinder that fails from being work hardened during the manufacturing process, or from improper threads that hold the pressure valve. Sometimes the bottom of the cylinder can fail if condensation has run there from gravity and weakened the metal.
If the cylinder fails, it may not be refilled. When a cylinder is condemned by a hydro
inspector, the DOT markings are stamped out or the cylinder is destroyed.
Visual inspections and hydrostatic testing are an important part of a
diving cylinder's life. The procedures are designed with the safety of the
people handling the cylinder in mind. Knowing when your cylinder needs service will
prevent disappointment when you're ready for your next weekend excursion and
the store employee tells you, "Your cylinder's out of hydro." Don't try to plead,
don't get mad, and don't tell us to "Fill it anyway and
I'll bring it in on Monday." You are asking us to break the law.
Just laugh at your blunder, rent a cylinder, and have the proper inspections
completed on your cylinder.
THINGS TO REMEMBER
The contact for both the visual and hydrostatic cylinder inspections is Sandia Snorkel and Scuba, Inc.
Have a visual inspection completed at least once a year.
A hydrostatic cylinder test must be completed at least every five years.
A hydrostatic cylinder test is not a substitute for a visual inspection. You need BOTH.
Before using a cylinder, check the markings around the neck to ensure that
it has had a "hydro" within the past five years.
Before using a cylinder, look for a visual inspection sticker. Make sure
the inspection has been done within the preceding 12 months.
When storing a cylinder, always leave at least 500 psi in it to prevent
water from entering the interior. Never drain a cylinder before storage.
Avoid exposing cylinders to temperatures in excess of 150'F (66' C).
