Visual Inspection Protocol Checklist
If you perform professional inspections on scuba tanks, read this. It is
a list of points that are often overlooked while inspecting these tanks.
This list is not a complete list and it is not intended as a substitute for
proper training, but it is included here as a reference source.
-
New aluminum cylinders have a shiny stainless-steel-like inner surface. This
is un-oxidized aluminum. The inside may stay shiny for years or it may turn
into an even dull gray. This is aluminum with a thin, even layer of oxide.
This is no problem. Aluminum will oxidize in the same way as copper or brass
-- you lose the "fresh" look, but the surface corrosion will not go any further.
-
Aluminum that is exposed to fresh water (condensation) but never sees salt
water will turn dull gray eventually. With salt water, however, the pattern
of corrosion takes an entirely different direction. In this case you'll get
pitting -- spots where the corrosion eats into the material. Pitting will
show as white spots of oxide. Often the spots will appear as raised boils
on the otherwise clean surface. If you get rid of the aluminum oxide you'll
will reveal severe scarring. Pitting is an absolute contra-indication to
further service.
-
Old tanks may appear mottled rather than dull gray. This could be a combination
of the innocuous surface oxide and residues from poorly filtered air. Whenever
a cylinder appears mottled, try to rinse it out. Since the tank probably
is very old the mottling could hide more serious problems. If there's the
slightest indication of pitting, condemn it.
-
Cracks are most commonly found in the neck area. This is because the dome
is formed by pulling the extruded material together, which may lead to undue
tension. You may find stretch marks when checking the inside of the dome
with a mirror. When cracks form, this is where it will start. A hairline
crack may show as a line of a different hue or color. It could be darker
than the surface or it could be a white streak of oxide. While mottling in
itself is no direct indication of a problem, be very skeptical of stains
that follow a pattern. Lines and "geometrical" patterns may suggest cracks
or crazing.
-
Corrosion may give away a crack, and cracks may form as a result of corrosion
also. With severe pitting in the neck area -- which you may not see without
a mirror -- the loss of strength may cause cracking in the eroded surface.
However, I would assume this to be a combination of structural flaws (almost
inherent to the way the dome is formed) and further corrosion in a weaker
area. It's very important to check the cylinder with a mirror inside the
dome!
-
Over-tightening a valve may ruin threads and add to the inherent tension
of the dome. Old European cylinders with a conical valve stem may develop
cracks due to transverse neck strain.
-
Rolling a tank is a crude way to look for unbalance. If a tank wobbles this
means it's far from symmetrical along it's axis. The implications are uneven
thickness or folds in the metal.
-
Eddy current is used extensively in checking the integrity of airplanes.
Since there's a lot of aluminum requiring structural strength, cracks are
the thing to look for. Commercially made devices, the
Visual Plus, the Simple
Eddy, and the Visual Eddy Machines
are available for checking scuba tanks. Since aluminum tank manufacturers
are involved in the marketing of these devices, it's in their best interest
to make the price right. Eddy current will disclose cracks and uneven density
/ thickness of the material.
-
There's one thing we can't check for, namely cracks that haven't developed
yet. If the latest accident was caused by a spontaneously forming fatigue
there's only one thing to say -- let's get rid of that alloy!
-
Another thing, don't forget the external surface. General corrosion is no
problem on the outside but local spots and gouging may destroy a tank. Severe
corrosion may be present behind a tank boot. Sometimes the upper rim of the
boot causes a ring of corrosion. Anything that traps salt water may cause
outside pitting. If there's a backpack, take it off and check. Since aluminum
is soft, a poorly positioned bolt on a backpack or a slack steel cam band
may have gouged it. Further, galvanic corrosion may ensue when another metal
is attached to an aluminum tank permanently.
This list is incomplete and is not intended to be a substiture for proper
scuba tank inspection training. If you know of another important point that
you would like added, please send it to me,
Dennis Hawkins (Chris's Uncle) and I'll
see that it gets added.
The following experts have contributed thus far:
Hans Petter Roverud
BACK TO CHRIS'S SCUBA HOME PAGE
