Scuba cylinders (tanks) come in a variety of sizes. They
range in size from a little over 9" tall by 2" across to over 22" tall by
8" in diameter. Smaller tanks are usually for an emergency backup. You cannot
always tell from the size of the tank how much air supply it holds.
Sometimes smaller tanks contain more air than larger ones.
How do you find out if a tank is full? It is necessary
to find out what the maximum pressure a tank can hold, and then put a gauge
on the tank to see what pressure it actually has. The maximum pressure is
stamped near the top of the tank. On the neck of the scuba tank there is
a number that will be in the thousands such as 1800, 2150, 2250, 2475, 3000,
3300, 3500, 4500, etc. The number might be preceded by a code such as 3AA,
3AL, E9498, etc. That number is the maximum pressure rating for that cylinder.
Once that is known, a gauge can be put on the valve to see if the tank is
full. Also, there is a gauge attached to the regulator supplying the air
for scuba diving with allowing you to monitor the air pressure under water.
There are two major types of scuba cylinders. Some are
made of steel and some are aluminum. There are advantages and disadvantages
of each. If you had 2 cylinders of equal outside dimensions, one was steel
and the other was aluminum, and they had equal maximum pressure ratings,
the steel tank would hold more air. Steel is a much stronger metal than the
aluminum alloy used to make tanks. So the steel tank's walls are less than
1/2 the thickness of the other. The interior volume of the steel tank is
greater than the aluminum tank. A disadvantage to steel is that it may rust.
Iron oxide rust flakes off, and then more rusting takes place in the same
spot weakening the wall of the steel cylinder. When aluminum "rusts" it forms
aluminum oxide. Aluminum oxide stays in place and protects the metal
underneath. However, there should be no rust in any scuba tank if the air
that is used to fill it is properly dried, and the tank has not been emptied
with the valve left open.
The amount of air in a scuba cylinder is usually measured
in cubic feet. The most popular of these is the 80 cubic foot, 3000 psi,
aluminum cylinder. The quantity is rounded off and the tank really contains
77 cubic feet (cf) of air when it is filled to 3000 psi. (For your information,
the tank has an actual interior volume of 0.4 cf..) The small Spare Air (R)
contains 2.7 cf at a similar pressure. Typical pony bottles contain 6, 13,
19, or 30 cf. The "standard" 2250 psi steel tank contains 71.2 cf when it
is filled to 10% above the 2250, or 2475 psi. That is only permitted if the
tank has a "+" stamped into the metal after the hydro symbol. Without the
"+" the tank should not be filled above 2250 psi. That steel tank now would
contain only 64.7 cf at its rated 2250 psi!
The above chart was from Lynn Laymon's article, A Tank is a
Tank, Dive Training Magazine, November 2004
This discussion brings up the question of how long can
a diver stay under water before the air runs out. First of all, divers should
not stay down until the air runs out. That is quite unsafe and also invites
contamination if the tank valve is open and the surrounding air enters it
after the regulator is removed. It is best to reach the surface with
no less than 300 psi in the tank. People breathe
somewhat less than a cubic foot of air per minute at the surface. This is
a very rough estimate because it depends on several factors:
People are different. Some breathe faster than others. Some have a higher
metabolism which requires more oxygen for energy production. These people
are slow movers and do not have high breathing rates.
Depth is a major factor. If you did breathe 1 cf/minute at the surface, you
would breathe 2 cf/minute at 33' (34' fresh water). The air supply would
disappear in 1/2 the time at that depth. The deeper you go the faster the
air is breathed out of the tank.
Temperature is another factor. If a diver is cold more oxygen will be consumed.
Oxygen combines with digested food in the muscle cells to produce heat. The
colder the diver the more heat/oxygen that is needed.
Exercise enters into the equation. The more divers exert themselves, the
more oxygen needed for this increased energy production.
Diving experience is related to air consumption. Novice divers tend to breathe
the air out of a tank faster. Anxiety, increased breathing, lack of good
buoyancy control, poor kicking with the fins, and not being relaxed are the
causes of this phenomenon. As the diving career continues this air consumption
The author has had a tank last only 20 minutes while
working in a reservoir laying a community water pipe in Harrison, NY in the
winter. Due to cold, and a heavy work load, the tanks were emptied rapidly
even through the water was only 25 feet deep. Contrast that to having a tank
last almost 2 hours while examining a reef in the Caribbean (83 degrees)
in 7-10 feet of water.
Air has weight. The air in an 80 cubic foot tank weighs
about 6 pounds. As the air is breathed out the tank gets lighter. That changes
a diver's buoyancy as the dive progresses. For example, if you threw an 80
cf aluminum cylinder in the ocean its negative buoyancy is 2.5 pounds. (It
sinks with a force of 2.5 lbs.) When that tank is empty it floats with a
force of 3.5 pounds. During the dive there is a change of approximately 6
pounds in a diver's buoyancy! Toward the end of the dive the tank exerts
quite a bit of floating force on the diver. It is just as if the diver removed
6 pounds of weight from the weight belt and discarded it! An aluminum 80
has a weight when full slightly more than 33 pounds, and a volume of 0.5
cu ft. That puts the full tank's density at (33 pounds/0.5 cu ft) 66 pounds
per cu ft. Since seawater is 64 pounds per cubic foot, the tank will sink
when full with a force of 2 pounds.
The following is a chart of various tanks, the amount of air
in them, and their buoyancy in fresh and salt water:
The submersible pressure gauge (spg) attached to the
first stage of the regulator tells the diver the air pressure in the tank
during the dive. It is important to monitor it frequently to avoid running
low on air. Be sure to leave enough air in the tank to safely surface with
at least 300 psi in the tank when the dive is terminated. Novice divers need
to concentrate on checking the spg. Sometimes it is not done because there
are so many new experiences overwhelming the senses.
Air for breathing at depth must be less polluted than
what is found at the surface. If the air contains some pollution it will
be twice as polluted at 33'. Poisons in the air are magnified as the diver
descends. One way to tell if you are breathing pure air is to smell it. By
opening the tank valve slightly (and carefully) and smelling the air,
your nose should not be able to detect any odor.
If even the slightest odor exists, such as an oily smell, it is best
to not dive with that tank until it is dumped, cleaned, and refilled from
a good air source. Scuba diving requires Grade E air. Grade E air, according
to the Compressed Gas Association (CGA), has the following maximum limitations:
10 ppm carbon monoxide (CO)
1,000 ppm carbon dioxide (CO2)
25 ppm total hydrocarbons
5 mg/cubic meter of oil and particulate matter
Oxygen 20% to 22%
67 ppm of water vapor
In another chapter we will be discussing Nitrox for breathing
underwater. For Nitrox the air must be less contaminated than what is allowed
for Grade E air. The CO level must not exceed 2 ppm, the oil and particulate
matter must be less than 0.1 mg/cubic meter, and the carbon dioxide level
must be less than 500 ppm.
Compressors used for filling scuba tanks must be designed
for that purpose. They are expensive! They are not like those used to fill
tires at your local gasoline station. The main compressor at Deep-Six costs
over $14,000 new. The air going into it comes from the outside the building.
No smoke from vehicle exhaust, wood stoves, lawn mowers, etc. ishould be
allowed to enter the intake. The air is dust-filtered and then compressed
in 4 stages. Water and oil are drawn out of the air in the stages. The air
then passes through filters that remove carbon dioxide, carbon monoxide,
remaining water, oil vapors, and other contaminates. The filter is electronically
monitored to be sure it is functioning. If the filter gets to a point it
is not doing the maximum job the compressor will not start. Finally the air
enters the tank in a super-clean and super-dry state. This results in dry
throats during dives and no odor when the air is smelled.
If you were to take an aerosol container, such
as a hair spray, and discharge the contents rapidly, a cooling effect will
be noticed. As gases decompress (expand) they cool. The opposite is true
as well. If you compress a gas it will get hotter. This is according to Charle's
law. If an 80 cf tank is filled too quickly it will get hot to the touch.
If you were to totally open the valve of a filled tank, frost would
form on the valve even on a hot day. Charle's Law states that if you compress
a gas it will get hot, and if you decompress a gas it will get cold. If you
take a scuba cylinder to a dive shop and they fill it too fast you will pay
for air you did not get. The pressure may indicate the tank is full. When
the tank cools down the pressure will drop significantly. There will be more
cooling and pressure loss when in the water. The loss could lower the
tank's pressure from 3000 psi to less than 2500 psi! For every degree of
change there is a change of about 5 psi in the tank's pressure.
Although it should never happen, running out of air is
not what you might think. The end point is not abrupt. It's not like you
exhale and then find there is no air to inhale. When the pressure is low
the air becomes increasingly harder to draw out of the tank. The breathing
gets harder and harder until there is a long and tedious breath at the end.
From the point of first realizing there is a restriction, until the air is
hard to draw in, may last fifteen seconds or more. The time is not definite.
It's actually a good experience to draw a tank down to zero as long as you
are expecting to do it, and it is done in the shallow water of a swimming
pool. Remember: There will be no air to add to the BCD when the tank is empty
so this experience should not be done with an over-weighted diver or in open
water. Again, an empty tank can become contaminated if the valve is left
open after the regulator is removed.
There are other important markings on the neck of a tank.
Of major consideration is the hydrostatic test date. Looking again at the
top of a tank, there will be a place that shows a month, followed by a test
station ID, followed by a year. It will look something like: 6 X 98. The
X represents the federal identification number. There may be several dates
if the tank has been tested in other 5-year intervals. The latest date indicates
the last time the tank was tested to be sure the metal is able to withstand
the extreme pressure it receives. Scuba tanks must be tested every 5 years.
If the last hydrostatic test date is more than 5 years old, the tank will
not (and should not) be filled by most dive shops. This policy is for the
safety of the shop and the owner of the tank. Also, there is a heavy federal
fine for transporting a pressurized cylinder that does not have a current
When a cylinder is filled it expands due to the pressure
inside. When the air is breathed out of the tank it contracts. The
metal of the tank flexes as it is being used. If you take a piece of metal
and repeatedly bend it back and forth it will eventually break because the
metal gets fatigued. Although aluminum scuba tanks can withstand over 100,000
fills it is important to be sure the metal has not becomed fatigued or brittle.
That is why the federal government requires hydrostatic testing of high-pressure
cylinders every 5 years. Essentially a scuba tank is subjected to 5/3 of
the pressure rating found on the neck so the expansion and contraction may
be measured exactly. A 3000 psi cylinder would be hydrostatically tested
to a pressure of 5000 psi!
Because tanks contract when the air pressure drops, if
the BCD was not put on the tank tightly, the tank may come out and trail
behind the diver. Sometimes it will not come out until the diver leaves the
water and the tank gets heavy. It presents a problem that can be avoided
by simply insuring the tightness prior to the dive. Wetting nylon BC tank
straps has little affect on the tightness achieved. All it may do is to lubricate
the strap so it slips more easily through the buckle.
In addition to insuring the integrity of the metal every
5 years, it is important to have a tank visually inspected by trained personnel
every year. This is called the Visual Tank Inspection (VTI). During a VTI
the tank is opened and the interior is carefully examined for contamination,
corrosion, neck cracks, intact threads, and damage. The area where the treads
join the body of the tank must be checked for cracks if the tank is aluminum.
The exterior is examined for bows, bulges, corrosion, dents, and gouges.
If the tank is free from damage a decal is applied that states the inspection
was done and the tank is in satisfactory condition. The decal is dated by
month and year and expires after one year.
Neck cracks (Sustained Load Cracking - SLC), as explained
at the end of this page, is a problem with certain tanks that were made of
the 6351 aluminum alloy. Many of these cracks are not able to be seen with
the naked, and especially the untrained eye. They are detectable with a
sophisticated instrument called the VisualPlus 3. If you tap the next link
you will be able to see some interesting pictures and data on tanks with
and without neck cracks.
A few years ago a person decided to add a chrome finish to
an aluminum tank. To do this required heating the tank to 400+ degrees. After
the process was completed the tank exploded while it was being filled! This
unfortunate incident led to the discovery the aluminum alloy in scuba tanks
becomes more brittle and less flexible if it is heated to above 350 degrees.
Aluminum tanks that have been subjected to extreme heat (less than 350 degrees),
such as a house fire, must be hydrostatically tested prior to use. In cases
where the tank has been heated to over 350 degrees they must be condemned
and rendered usless so they will never be pressurized again!
That brings up another point: Charle's law relates heat
to pressure. If there was a house fire and scuba tanks were in it, would
there be an immediate threat to the firefighters? When scuba tanks are heated
the pressure rises. Would the tanks explode if they were full? The answer
is "No." In the valve of the tank there is a safety plug that is designed
to split when the pressure climbs near the hydrostatic test pressure. If
a 3000 psi tank were heated to the point the pressure climbed to 5000 psi
(about 400 degrees F) the safety plug would split and the air would quickly
leave the tank. Contrary to myth, there is no danger of tanks getting that
hot in the trunk or in the interior of a car!
At the bottom of the tank there is usually found a boot.
It is made out of plastic or rubber. It is there to keep the tank from getting
scratched and gouged. Some tanks have round bottoms. Tank boots conform to
the round bottom on the inside but have flat bottoms on the exterior. This
will allow a round-bottom tank to stand up. The boot should be self-draining.
It should allow water to run out of it when the tank is on land and the salt
water is rinsed off. Small grooves around the inside edge of the boot do
this. If the boot has a flat and smooth interior it will keep water permanently
against the tank's metal. If it is salt water corrosion may develop.
When tanks are transported in a vehicle the bottom of
the tank should be facing toward the front. The biggest danger in accidents
is a front-end collision. If that were to happen the bottom of the tank,
rather than the valve, would be subjected to the force of the impact. If
the vehicle is too small to allow the tank to be placed in that position
then it would be best to place them sideways. No matter what, the tanks must
be packaged so they cannot roll or shift around!
Many instructors insist on never letting a tank stand
upright without some support. This is especially true if the tank could fall
over with the valve hitting a rock, chair, etc. It's not that dangerous if
the floor is flat and there is nothing sticking up. But,
never allow a tank to be placed on a high object such
as a chair, bench, table, etc. unless it is supported at all times. Don't
even walk away for 1 second with a tank off the ground!
This author has seen, on more than one occasion, a tank fall off a bench
at a dive site, land on the regulator, bend the yolk that attaches the regulator
to the tank, causing the air come screaming out through the leak that developed.
One time the escaping air blew a trench in the shale on the ground that was
at least 10" deep before the valve could be closed! Needless to say, the
cost to repair the regulator was extensive.
the question, "How long is the air in a tank good for?" As
long as the air is not contaminated, especially with water, it should last
indefinitely. If a steel tank had been filled from a compressor that
did not remove water from the air, during storage the moisture may react
with the steel to create rust. Rust is iron oxide. The iron comes from the
tank. The oxide comes from the oxygen in the air. The oxygen is used to form
the rust, reducing the amount needed for sustaining the diver's life. Taken
to an extreme case, the diver could dive breathing almost pure nitrogen.
There would be no real warning of the appending asphyxiation because there
would be no carbon dioxide excess indicating a need to breathe.
The LUXFER 6351 Aluminum Tanks and Sustained Load
Cracking (all in red):
The Luxfer 6351 aluminum alloy
was used in tanks that predate 1989. In a very few of those tanks there has
been found sustained-load cracking in and around the neck and threads of
the tank. Out of 30,000,000 6351 tanks, 17 ruptured. That is 0.00006%! Out
of 6,100,000 6351 tanks, 0.37% were found to have sustained-load cracking.
It takes an average of approximately 6 years for a tank with sustained-load
cracking to begin to leak. The tanks that have developed sustained-load cracking
are ones that have been abused, damaged, or over-filled.
RIVERSIDE, Calif. (December 9, 2002)-Luxfer Gas Cylinders
has announced a new scuba tank trade-in program to enable owners of older
Luxfer scuba tanks manufactured in the U.S. from 6351 aluminum alloy to trade
in their tanks and receive a $50 credit for each tank toward the purchase
of new scuba tanks made from Luxfer's proven, proprietary 6061 alloy. Luxfer
implemented the simple four-step program outlined below on January 1, 2003.
The program ended on December 31, 2003. That was a limited one-year
To determine whether your Luxfer scuba tank is made
from 6351 aluminum alloy, check the original hydrostatic test date (the earliest
date) stamped on the crown. (Look for a month/year combination, such as 3/75.)
If the stamped year is 1972 through 1987, the tank is made from 6351 alloy.
A limited number of Luxfer scuba tanks were also made from 6351 alloy during
the first half of 1988. If you have a 1988 tank with an original hydrostatic
test date of 6/88 or earlier, Luxfer assumed that it was a 6351-alloy tank
and issued an RG number. Tanks with an original hydrostatic test date of
7/88 or later are made from 6061 alloy and were not eligible for the trade-in
Why did Luxfer sponsor this program? Is something wrong
with these older tanks made from 6351 aluminum alloy?
As long as these older tanks are properly inspected
and requalified as required by U.S. Department of Transportation (DOT) and
Luxfer, you may continue to use them. DOT requires that the tanks be requalified
every 5 years. In addition, Luxfer requires that the tanks receive a visual
inspection by a properly trained inspector and an eddy-current test at least
every 2½ years. (Please note as well that an annual visual inspection
is a standard practice in the U.S. scuba industry, and Luxfer endorses this
practice for all scuba tanks, not just those made from 6351 alloy.)
Luxfer sponsored this trade-in program for commercial
purposes. The program operates much like a coupon for a commercial product.
The intent is to increase demand and brand awareness in the marketplace by
offering a trade-in discount on a new Luxfer scuba tank. This trade-in program
also enables divers to upgrade tanks made from older- technology aluminum
alloy to newer tanks made from Luxfer's proprietary 6061 aluminum alloy.
How do I know if it is a Luxfer tank?
You can tell by several markings on the tank. The serial
number will begin with a prefix. (For example, in the case of an 80-cubic-foot
tank, the prefix will be a P.) The serial number line should also contain
TANK SIZE PREFIX TANK SIZE PREFIX TANK SIZE PREFIX
S030 UU S063 PP S808 UV
S040 LS S072 Y S092 US
S050 R S080 P S100 UT
The US Government has determined
that no recall is warranted for the Luxfer 6351 tanks. No scuba cylinders
have been recalled! The Luxfer 6351 tanks
are not to be condemned unless they show evidence warranting such. There
is no reason for a dive shop to refuse to fill them. There is the potential
to wrongly convince a customer they should be replacing the cylinder with
a new one.
Luxfer scuba cylinders were
manufactured from 6351 aluminum alloy during the following periods:
· United States
: 1972 through mid-1988
· England :
1967 through 1995
: 1975 through 1990
Luxfer requires that every Luxfer
6351-alloy all-aluminum scuba cylinder be visually inspected at least every
2.5 years by a properly trained inspector. As part of this inspection, Luxfer
further requires that the cylinder neck be tested with an eddy-current device
such as Visual Plus, Visual Plus 2, Visual Plus 3,Visual
Eddy or equivalent non-destructive testing equipment. When properly
used, eddy-current devices contribute significantly to early detection of
difficult-to-observe sustained-load cracks in the necks of 6351-alloy cylinders.
If the cylinder passes the inspection, the inspector will document that fact.
If the cylinder fails the inspection, it must be removed from service
immediately. Do not use Luxfer 6351-alloy scuba cylinders that have not been
both visually inspected and eddy-current tested and then properly
This Luxfer-required visual and
eddy-current inspection is in addition to periodic requalifications (including
retesting and inspections) required or recommended by various regulatory
agencies around the world. The intervals between retests and inspections
vary from country to country.
In addition to the required inspection
and testing described above and in keeping with U.S. scuba industry standards,
Luxfer recommends that all Luxfer 6351-alloy scuba cylinders be visually
inspected at least once each year by a properly trained inspector.
For cylinders in heavy use (for
example, those filled five or more times a week), Luxfer recommends visual
inspection every four months. For more information, refer to Luxfer's Scuba
Cylinder Inspection Guide, which is available by calling Luxfer at 800-764-0366
or by visiting the Luxfer web site at www.luxfercylinders.com.
Cylinders made from Luxfer's
proprietary 6061 alloy are not susceptible to sustained-load cracks. Therefore,
Luxfer does not recommend the use of earlier-generation eddy-current
devices-e.g., Visual Plus, Visual Plus 2, and Visual Eddy-with Luxfer 6061-alloy
cylinders because field experience has shown that using these devices can
result in misleading "false-positive" readings and erroneously condemned
However, Luxfer does approve the
use of the Visual Plus 3 eddy-current device with Luxfer 6061-alloy
cylinders. After laboratory testing, Luxfer has determined that properly
calibrated VisualPlus 3 equipment eliminates the "false positive" problem,
accurately measures features in the neck area of aluminum cylinders, and
generally provides correct pass or fail assessments with cylinders made from
Luxfer's 6061 alloy. As of the date of this Luxfer Policy Statement, VisualPlus
3 is the only eddy-current testing device suitable for use with Luxfer 6061-alloy
cylinders. (During a visual tank inspection (VTI) at
Deep-Six, the VisualPlus 3 is used on all aluminum
cylinders and a full report is produced for the owner!)
Luxfer emphasizes that the use
of eddy-current devices to improve the quality and accuracy of inspections
does not eliminate the need for a thorough visual inspection by a properly
trained and qualified inspector.
In keeping with U.S. scuba industry
standards, Luxfer recommends annual visual inspection of Luxfer 6061-alloy
cylinders by a properly trained inspector. For cylinders in heavy use (for
example, those filled five or more times a week), Luxfer recommends visual
inspection every four months. These recommended inspections are in addition
to periodic requalifications (including retesting and inspections) required
or recommended by various regulatory agencies around the world. The intervals
between retests and inspections vary from country to country.