Test stand, test vessel, vessel cap and pneumatic
Tank attached to test vessel cap, with quick release and
Inc. is the proud owner and operator of a Cylinder Requalification facility.
Certified by the Department of Transportation (DOT), Sound Dive Center/Hydro Test NW
performs "Hydrostatic testing" on PAINTBALL, SCUBA, SCBA (fire) and other "small" bottles that require cylinder
requalifications (a hydrotest). To get your paintball tank tested go to this
The United States Department of Transportation (DOT) licenses companies to test cylinders
in order to insure that they are safe to use by divers, firefighters, homeowners, etc.
This is the beginning of testing a cylinder for structural integrity. The red tank is a 90
cubic-foot, aluminum scuba cylinder . It has a working pressure of 3300 psi, the
test pressure is 5500 psi.
After insuring the tank is empty, we remove the tank valve. The tank is then
given a thorough visual inspection (VIP). The tank's interior is checked for corrosion,
particulate, and any other abnormalities. The threads are checked for integrity and
imperfections. On aluminum tanks, a special electronic device is used to check the neck
threads for cracks. See the pictures on the right. We use Visual Plus to test
for stress cracking.
If the tank passes the VIP, it is filled with water for the
hydrostatic test. If a tank were to rupture the results would be minor because it is
filled with water instead of gas! Remember, water does not compress, unlike
air. So if the tank were to rupture, even at 5000psi, only a couple of gallons of
water would escape and be far less destructive than 90 cubic feet of air. Water has
little kenetic energy.
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 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 tightened with a 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.
The entire test chamber is kept full of 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.
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 vessel to insure it is filled completely eliminating the possibility of compressing
gas instead of fluid.
You are looking at the tank attached to the test vessel top. The tank is suspended
from this and is lowered into the 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
vessel lip with the pneumatic clamps. There is an O-ring between the them to insure that
water cannot leak out between the two.
Any air is purged from the test vessel. There is a hose to the back left. It
supplies water to the interior of the cylinder. It is also the source of the
high pressure that will be applied to the tank.
The scuba tank is enclosed in this vessel. Both are filled with water and pressure
is about to be applied to the scuba cylinder. The air that might be in the SCUBA
tank is being purged.
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 2250 psi, then the
tank would be pressurized to 3750 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 up 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 at a time. 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.
The water pressure is applied to the interior of the 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 at least 30 seconds(we use 40 seconds for good measure). The amount of
expansion is read on the burette.
The second burette from the right now reads 65ml. The water has been pushed up the burette
by the tank expansion due to the pressure on it of 5000 psi. This must be a stable
reading. In other words, the water level in the burette cannot change for at least 30
seconds. This could be due to a continual expansion of the metal of the tank which
would be reasonable cause to fail the cylinder.
After the burette level is recorded. The pressure is released from the cylinder. The
cylinder will not return to the size it was before the pressure was applied because it was
permanently stretched. The burette to the left reads 2.4 ml. That 2.4 ml represents the
amount of permanent stretch. 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 to contain high pressure. This SCUBA tank must return to at least 10% of the upper
reading. SCBA bottles must return to 5% usually.
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 VIP 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
crown of the tank.
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. Paintball and SCBA bottles are tested
in a different manor and have a totally different marking procedure.
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tools include a bright light, reflective tools and good eyes.
Plus" used for eddy current/non distructive inspecting