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FAQ Graveyard Diver scuba cylinder visual inspections oxygen cleaning hydro testing valve servicing repair


Information regarding SCUBA cylinders, Nitrox, valves, oxygen cleaning, fill station operations, the cylinder inspection process, etc varies quite a bit from dive shop to dive shop.  Much of this information is not SCUBA industry standard or federal regulations, but is dive community folklore, dive shop policy, dive shop employee lack of training regarding SCUBA cylinder HAZMAT, misinterpretation of DOT regulations, or simply rumor passed on by cylinder "inspectors" with no formal training.

My goal in these Frequently Asked Questions is to clarify some popular misconceptions, and I provide reference to DOT, CGA, or accepted and approved industry standard to support my answers.   
This section of the Graveyard Diver website is updated frequently as new questions arise, or as regulations and standards change.  Please check back often.  If you have a question that you think belongs here, eMail or text Me at the phone number below.


1)  I was told that  if I repaint my SCUBA cylinder that it would fail a visual inspection (or that it is "illegal" to repaint a cylinder).
2)  
I was told that if I repaint my SCUBA cylinder that it would have to be re-hydro'd before it could be filled. 
3)  
The dive shop scraped off all of my cool dive stickers when they inspected my cylinder.
4)  I was told that tumbling my cylinder would require it to be requalified.
5)  The Inspector told me that my aluminum cylinder had corrosion inside.  Aluminum doesn't rust, so what's the deal?
6)  The dive shop told me that they can't fill my cylinder because it is too old.
7)  The dive shop told me that my cylinder had to be visually inspected again before they could fill it because I breathed it totally empty on my last dive.
8)  The dive shop told me that my cylinder had to be visually inspected again before they could fill it becuase the valve had been removed.
9)  The dive shop told me that my cylinder had to be visually inspected again even though my VIP decal isn't expired.
10)  My VIP decal expires at the end of this month but the dive shop told me my cylinder had to be inspected again becuase it was expired. 
11)  I
f the hydro guy visually inspected my cylinder when he requalified it last week, why do I have to pay to visually have it visually inspected again?
12)  You talk a lot about PSI-PCI.  What about Cylinder Training Services (CTS)?  Is their inspection program not valid?  
13)  
So if there is no law requiring an annual visual inspection, I can just inspect my own cylinder?  
14)  
My dive shop told me that it is illegal to fill old 6351 cylinders.  
15)  
My dive shop told me they had to Eddy Current test my steel cylinder.  
16)  
My dive shop told me they had to Eddy Current test my 6061 alloy aluminum cylinder.  
17)  
My dive shop refused to put Nitrox in my cylinder because I didn't have a big ugly yellow and green Nitrox sticker on it.  
18)  
My dive shop refused to put regular air in my cylinder unless I scraped the big ugly yellow and green Nitrox sticker off first.  
19)  
So is it a "cylinder" or a "tank"?  What's the difference?  
20)  
Can you visually inspect my old 6351 alloy aluminum cylinder?  
21)  
Why don't you get certified and buy a Visual Plus Eddy Current test machine then? 
22)  
Somebody told me it was illegal for you to condemn my cylinder.  
23)  
Why don't SCUBA cylinder visual inspectors get a "RIN" number from DOT then?  
24)  
Why do I have to have a visual inspection on my brand new, just-got-it-in-the-mail-yesterday SCUBA cylinder?  It's brand new from the factory!  
25)  
So whats "Sustained Load Cracking" and why does it only happen in 6351 alloy aluminum cylinders? 
26)  
The dive shop told me they couldn't fill my cylinder with nitrox because my cylinder wasn't oxygen clean.
27)   My hydrostatic requalifier told me that is is illegal for him to restamp a "+" on my cylinder after the first requalification.
28)   Can I request my aluminum cylinder to be stamped "+" so it can be filled to 10% over it's rated service pressure?
29)   How thick is the wall of my cylinder?   
30)  
What do you do with cylinders that do not pass a visual inspection?


  

I was told that  if I repaint my SCUBA cylinder that it would fail a visual inspection (or that it is "illegal" to repaint a cylinder).
This is false.  Repainting a SCUBA cylinder does not "automatically" condemn it IF it is done properly.  Cylinder inspectors are taught to be suspicious of a repainted cylinder as customers often repaint cylinders to try to hide repairs or defects.  Sometimes a customer paints a cylinder and uses methods that damage the cylinder and cause it to be condemned.  If you bring a repainted cylinder to me, I will inspect it according to PSI-PCI guidelines and it may pass a visual inspection or it may not, depending on how it was repainted and what chemicals and methods were used to repaint it.  Chapter 19, page 115 of the PSI-PCI "Inspecting Cylinders" manual explains how to properly repaint your SCUBA cylinder.  If you are considering repainting your cylinder, contact me and I will answer your questions about how to do it properly without damaging it.

I was told that if I repaint my SCUBA cylinder that it would have to be re-hydro'd before it could be filled.
This is false.  There is no requirement under DOT regulations or PSI-PCI standards to requalify a repainted cylinder.  A dive shop may refuse to fill your cylinder if the fill station operator feels that is some damage you have caused that might affect the integrity of the cylinder, but repainting alone does not require a cylinder to be requalified.  

The dive shop scraped off all of my cool dive stickers when they inspected my cylinder.
This is proper procedure for visually inspecting a cylinder.  Sometimes customers attempt to hide cylinder defects by covering them up with stickers.  Sometimes water gets trapped underneath stickers and accelerates corrosion.  Inspectors who properly inspect cylinders will remove stickers so that they can perform a thorough inspection.   For a proper cylinder visual inspection, the entire surface of the cylinder must be visible to the inspector.  Chapter 9, page 62 of the of the PSI-PCI "Inspecting Cylinders" manual addresses stickers...."The inspector is obligated to clean away whatever hinders his view."   and  "Cylinders shall be clean for inspection to permit the interior and exterior surfaces to be clearly observed. " ~ CGA, C-8 (5.1)

I was told that tumbling my cylinder would require it to be requalified.
This rumor is false and is widely circulated under the pretense that tumbling a cylinder "removes metal from the cylinder wall therefore weakening the cylinder wall." While it certainly is possible that your cylinder can be damaged by tumbling, a skilled cylinder technician knows how to properly tumble a cylinder.  Since the inspector cannot see inside the cylinder while it is being tumbled, skill and experience in tumbling cylinders is definatley helpful.  However, tumbling in itself does not require a cylinder by hydrostatically retested before it can be filled again.  Chapter 9, page 66 of the PSI-PCI "Inspecting Cylinders" manual discusses cylinder tumbling..."Proper tumbling does not remove significant quantities of base metal therefore it is not necessary to have a cylinder hydrostatically re-tested simply because it was tumbled."

The Inspector told me that my aluminum cylinder had corrosion inside.  Aluminum doesn't rust, so what's the deal?
Aluminum does not rust, but it does corrode.  Corrosion in steel is called "rust" and is usually a shade of orange.  Corrosion in aluminum is called "aluminum oxide" and is a white powdery substance usually seen on the exterior of an aluminum cylinder.  When aluminum corrosion is wet, it is called aluminum hydroxide and is usually found on the inside an aluminum cylinder in the form of a white paste-like substance.  Aluminum corrosion is a slower process than steel corrosion, but it is still a destructive process that can seriously damage your aluminum cylinder.

The dive shop told me that they can't fill my cylinder because it is too old.
This is a dive shop "policy" and has no foundation in DOT regulations or generally accepted industry standards.  The oldest known cylinder still in use is a steel cylinder manufactured in 1908 and it is still in use by a fire department in the northeast United States.  Cylinders do not have an "expiration date" simply due to age alone.  The oldest cylinder I have personally inspected was manufactured in 1961, and it was in better condition than many newer cylinders I have inspected.  Many dive shop owners concerned for safety often use the "Well, you got your moneys worth out of it" excuse to refuse to fill a cylinder.  While they are within their right to refuse to fill a cylinder for any reason, it is the owners discretion to determine when he has "gotten his moneys worth" out of a cylinder.  Oddly enough, one local dive shop I frequent still rents cylinders that, the oldest of which I found was manufactured 47 years ago!

The dive shop told me that my cylinder had to be visually inspected again before they could fill it because I breathed it totally empty on my last dive.
This is proper fill station proceedure.  Chapter 6, page 42 of the PSI-PCI "Inspecting Cylinders" manual discusses when a cylinder must be visually inspected.  "A visual inspection is appropriate when any of the following occurs... Cylinder completely emptied or burst disk fails."

The dive shop told me that my cylinder had to be visually inspected again before they could fill it becuase I had to remove the valve in order to bring it on the plane to the dive destination.
This is proper fill station proceedure.  Chapter 6, page 42 of the PSI-PCI "Inspecting Cylinders" manual discusses when a cylinder must be visually inspected.  "A visual inspection is appropriate when any of the following occurs...The valve is removed."

The dive shop told me that my cylinder had to be visually inspected again even though my VIP decal isn't expired.
This is proper fill station procedure.  Chapter 6, page 41 discusses when a visual inspection is required.  "The frequency of inspection is dependent upon usage."  A cylinder VIP decal indicates when your cylinder was last inspected.  Extinuating circumstances, suspicious conditions, and other factors may require your cylinder be inspected more often than once per year.

My VIP decal expires at the end of this month but the dive shop told me my cylinder had to be inspected again becuase it was expired.
I suspect this belief comes from motor vehicle inspections, which usually expire on the last day of the month of the date punched on the window sticker.  According to Hattie Mitchell, Chief DOT Examinations and Regulations Termination,  U.S. Department of Transportation, a VIP decal expires on the last day of the month in which it is punched.  Thus, if your VIP decal is punched 11/2019, your cylinder visual inspection expires on November 30th, 2019.
 
However, the SCUBA industry accepted standard is that a SCUBA cylinder VIP decal expires on the first day of the month in which it is punched.  The general reason for this is that since we don't punch the "day" on a VIP decal, only the month and year, the fill station guy has no way of knowing what day that cylinder was inspected absent a written report.  Most divers don't carry the written inspection report around with them in their pocket.   Thus, the SCUBA industry considers a visual inspection decal to expire on the first day of the month so as to not inadvertently exceed the 365-day validity of the cylinder visual inspection.


If the hydro guy visually inspected my cylinder when he requalified it last week, why do I have to pay to visually have it visually inspected again?
The scuba cylinder visual inspection program is not (no matter what the dive shop kid tells you), law.  DOT regulation requires that a cylinder be visually inspected at each 5-year hydrostatic requalification.  The annual visual inspection is a SCUBA industry standard that dive shops around the world have chosen to adopt voluntarily for the sake of increased safety.  There is no law in the United States that requires you to have your cylinder visually inspected annually.  However, a reputable dive shop will not fill a cylinder that has not been visually inspected within the last year.  The difference is that the hydro guy is peforming a federally mandated DOT visual inspection, whereas the SCUBA visual inspector is performing a SCUBA industry standards inspection.

Let me give you an example.  When I lived in Texas my car had to be inspected by a mechanic who was licensed in Texas.  He inspected my vehicle and put a decal on the windshield.  I moved to North Carolina with my car four months later.  The Texas vehicle inspection sticker was still valid for another eight months, but not in North Carolina.  When I registered my car in North Carolina, it had to be inspected again by a mechanic licensed in North Carolina and he put his North Carolina sticker on the windshield, even though it had just passed a safety inspection four months earlier.

The hydro guy does visually inspect your cylinder when he requalifies your cylinder every five years.  Both inspections are the same, but under different programs.  Just like the Texas and the North Carolina vehicle safety inspections are the same, but only valid in their own jurisdictions

A scuba shop certainly may, at their discretion, accept your hydrostatic inspectors requalification stamp as evidence that a formal cylinder visual inspection has been done.  I haven't found one as of yet that will.

You talk a lot about PSI-PCI.  What about Cylinder Training Services (CTS)?  Is their inspection program not valid?
Again, annual visual inspection of SCUBA cylinders is an industry safety program voluntary standard, not a mandated law.  The only visual inspection required by law is the visual inspection at each 5-year hydrostatic requalification.  Therefore a dive shop chooses what inspection agency they will honor and which they won't.  However, if  PSI and CTS are both teaching to DOT standards, then the result of a visual inspection done by a PSI visual inspector and visual inspection done by a CTS visual inspector should yield the same outcome.

So if there is no law requiring an annual visual inspection, I can just inspect my own cylinder?
I know I'm going to get some pushback from dive shop owners with this, but I do provide relevant federal law to support my answer...  
Technically yes, practically no.  The SCUBA industry standard requires that the annual visual inspection be performed by an inspector who has formal DOT approved training and who has had that training within the last three years.  Each CTS and PSI visual inspector is required to "renew" his training every three years by attending a training update class.  If you own your own compressor and fill your own cylinder, you don't have to have an annual visual inspection, only a visual inspection at the 5-year hydrostatic requalification as mandated by law.  However, no reputable dive shop will fill your cylinder if your cylinder has not been visually inspected by a formally trained inspector from a recognized DOT approved certification course within the last year.  And if you are filling your own cylinders and doing you own "visual inspection" without formal training, you are taking you life into your own hands and endangering others around you.   I certainly hope I'm not on the same cattle boat with you when your uninspected SCUBA bomb explodes.  
173.34(e)(3)
Code of Federal Regulations states, "The only requirement in the Code of Federal Regulations for visual internal and external examination is at the time of retest (once every 5 years).  Annual visual inspections are by mutual agreement... scuba cylinder owners submit their cylinders for annual inspections to individuals known to have been trained and certified to do annual inspections "    173.34(e)(3)  is very plainly worded and crystal clear.  

My dive shop told me that it is illegal to fill old 6351 cylinders.
This is false.  DOT regulations state that cylinders manufactured of 6351 alloy are legal to use and safe to fill IF they pass a hydrostatic test, pass an Eddy Current test at the time of requlification, and pass a visual inspection.  Most dive shops refuse to fill older 6351 alloy cylinders due to safety concerns over sustained load cracking. However this is a dive shop policy and not a law.  You can read the article regarding use of 6351 alloy cylinders here.

My dive shop told me they had to Eddy Current test my steel cylinder.
This is dive shop policy, not law.  The only cylinders required by federal regulation to be Eddy Current tested are aluminum cylinders made of 6351-T6 alloy.  Steel cylinders are not prone to sustained load cracking, however some visual inspectors choose to Eddy Current test steel cylinders as an added safety precaution.  While steel cylinders are not prone to sustained load cracking, all cylinders are susceptible to cracking.  The Eddy Current test may reveal a crack in the neck of a steel cylinder that is too small for even a careful visual inspector to see.  If your dive shop insists on Eddy Current testing your steel cylinder, they are erring on the side of caution.  It is your option, of course, to decline this additional expense and take your steel cylinder elsewhere for a visual inspection.

My dive shop told me they had to Eddy Current test my 6061 alloy aluminum cylinder.
Again, this is dive shop policy, not law.  The only cylinders required by federal regulations to be Eddy Current tested are aluminum cylinders made of 6351-T6 alloy.  Cylinders made from 6061 alloy are not prone to sustained load cracking and thus an Eddy Current test is not required by law.  However, the industry standard recommends that 6061 alloy cylinders be Eddy Current tested to detect possible cracks in the neck area that are so small that they may be overlooked by even a careful cylinder visual inspector.  So while 6061 alloy cylinders are not required by federal regulation to be Eddy Current tested, it is recomended by the SCUBA industry standard to be Eddy Current tested.   If your dive shop insists on Eddy Current testing your 6061 alloy aluminum cylinder, they are erring on the side of caution.  It is your option, of course, to decline this additional expense and take your 6061 alloy aluminum cylinder elsewhere for a visual inspection.
 "Eddy current inspection of 6061 aluminum alloy cylinders at the time of the 5 year hydrostatic retest or annual visual inspection in the case of SCUBA cylinders is not required by the U.S. Department of Transportation (DOT) or the Canadian agency Transport Canada (TC).  Catalina Cylinders does not require eddy current inspection of the threads of their 6061 aluminum alloy cylinders."  ~www.catalinacylinders.com/faqs/current-inspection.


Graveyard Diver Not Your Tank Nitrox cylinder wrap

My dive shop refused to put Nitrox in my cylinder because I didn't have a big ugly yellow and green Nitrox sticker on it.
This is standard dive industry safety practice.  Your dive shop is adhering to recognized and accepted industry safety standard.  The Compressed Gas Association states, "SCUBA cylinders shall not be filled with EANx unless they have been properly prepared for EANx use. This includes marking the cylinder for EANx service.  EANx cylinders must be conspicuously marked as to the specific gas mix (oxygen percentage) inside."

and...

My dive shop refused to put regular air in my cylinder unless I scraped the big ugly yellow and green Nitrox sticker off first.
This is standard dive industry safety practice and federal regulation.  Your dive shop is adhering to recognized and accepted industry safety standard.  The Compressed Gas Association states, "SCUBA cylinders shall not be filled with EANx unless they have been properly prepared for EANx use. This includes marking the cylinder for EANx service.  EANx cylinders must be conspicuously marked as to the specific gas mix (oxygen percentage) inside."   Furthermore,  49 CFR 178.65(i)(a) states "a stamped or marked cylinder can only contain (or be filled) with the gas for which it is stamped or marked. "   So your dive shop is correct when if refuse to fill your cylinder with air when you have a Nitrox decal on it.

So is it a "cylinder" or a "tank"?  What's the difference?
A "cylinder" is a portable vessel that can be removed and is transportable by the user.   A "tank" is a vessel that is permanently affixed and is not transportable by the user.  For example, the LP gas vessel used on most forklifts is a "cylinder".  It is mounted on the rear of the forklift with bands.  When it becomes empty it is removed from the forklift and refilled, or the empty LP cylinder is simply removed from the forklift and replaced with a full one.
My father has an RV that runs on LP gas.  The fuel "tank" is permanently mounted to the frame of the RV.  When it becomes empty, It is not removed  for filling or replaced.   He has to drive the RV to the LP station and have it filled with LP gas in the same manner that you fill your car with gasoline.
The pressurized vessels we wear on our backs for SCUBA are "cylinders", not "tanks".  
In the SCUBA industry, "cylinder" and "tank" are often used interchangeably. However, formally, there is a distinct difference.

Can you visually inspect my old 6351 alloy aluminum cylinder?
Unfortunately, no.  Under current CGA and accepted SCUBA industry safety standards, all 6351 are required to pass an Eddy Current test at each annual visual inspection.  Graveyard Diver does not have a Visual Plus Machine and is not certified to do Eddy Current testing.  If you have a 6351 aluminum alloy cylinder, we recommend Divin Dawgs SCUBA in New Bern. They will be happy to visually inspect and Eddy Current test your 6351 alloy cylinder.

Why don't you get certified and buy a Visual Plus Eddy Current test machine then?
Graveyard DIver feels that the costs associated with a $1,500+ machine, the cost of periodic recalibration of that machine required by DOT, and the expense of the certification class is not a good investment since 6351 cylinders are being gradually "phased out" of service.  While it is still legal and safe to use and fill 6351 cylinders, they are no longer manufactured due to sustained load cracking, and within a few years 6531 alloy cylinders will be very rare.  Graveyard Diver just feels it is not cost effective for us to invest thousands of dollars to inspect the few 6351 cylinders that occasionally show up and that will probably no longer exist in SCUBA service in a few years

Somebody told me it was illegal for you to condemn my cylinder.
Sort of - kind of - but no....173.34(e)(3) Code of Federal Regulations states that only a person licensed to do hydrostatic requalification (RIN) may condemn a cylinder..."
A person who only performs visual inspections on DOT or ICC specification cylinders is not required to obtain a retester identification number. However, a compressed gas cylinder may only be rejected [condemned] by a person who has obtained such a retester identification number."  Since SCUBA cylinder visual inspectors are not required to obtain a RIN from DOT, they can't reject (condemn) your cylinder under 172.34(e)(3).  But here's the catch... When you request  a cylinder visual inspection from Graveyard Diver, or any formally trained inspector, you will complete a form called a  "Visual  Inspection Release"  wherein you agree that should your cylinder fail to meet DOT safety regulations, you consent to Graveyard Diver condemning your cylinder.  In part, the release form states, "Should your cylinder fail our formal and documented visual safety inspection, it will be condemned.  After required codes and threads have been obliterated, as a conspicuous disabling measure, the cylinder and valve will be returned to you. "  Your signature on this release form gives the visual inspector written permission to condemn your cylinder.
So the truth of the matter is, it is illegal for a visual inspector who does not have a RIN number issued by DOT to condemn your cylinder unless you give him permission to.

Why don't SCUBA cylinder visual inspectors get a "RIN" number from DOT then?
Requalifier Identification Numbers (RINs) are issued to trained and certified individuals who hydrostatically test cylinders in a licensed test facility.   A RIN is not required under 173.34(e)(3) for simply visually inspecting a cylinder.  Of course, a visual inspector could qualify for and be issued a RIN from DOT,  but it is not required by DOT and the expense and recordkeeping requirements are very extensive.  Chances are very good that the cylinder visual inspector at your local dive shop does not have a RIN if your shop does not do 5-year hydrostatic requalifications on SCUBA cylinders.

Why do I have to have a visual inspection on my brand new, just-got-it-in-the-mail-yesterday SCUBA cylinder?  It's brand new from the factory!
Believe it or not, SCUBA cylinders do come from the factory sometimes defective.  In the United States, SCUBA cylinder manufacturers don't inspect each and every cylinder as it comes off the assembly line.  DOT permits manufacturers to do "sampling".   In sampling, a quality control employee inspects every ten cylinders.  If a defect is found, he then goes back and inspects the previous nine to make sure there are no defects.   It is simply not cost effective to take the time to inspect every single cylinder that rolls of the assembly line.  It is simply more cost effective for a manufacturer to possibly have to destroy 9 defective cylinders than to slow the production line down to impractically slow speeds.  So sometimes, albeit very very rarely, a  cylinder with a defect can elude the quality control inspector at the plant.   As a prudent safety precaution, your cylinder is visually inspected by a formally trained inspector even if it is brand new.  

Two years ago, I ordered a pony bottle for a customer.  FEDEX delivered it to me.  I opened the cardboard box, tore off the plastic bag, and inspected it.  The cylinder had a valley in the neck area causing the threads not to meet the safety requirements of DOT for that cylinder to be used in SCUBA service.  Had that cylinder been filled without a fornal inspection, the valve would have likely blown off during it's first fill, possibly seriously injuring or killing the fill station employee and bystanders.
This is the reason we inspect brand new cylinders before they are put into SCUBA service.  Every cylinder manufacturer in the U.S. has a warranty on their cylinders. Although the customer was disappointed in the delay of his pony bottle delivery, Catalina replaced the pony bottle at no charge and the customer received a cylinder that was perfect and safe to use.

So whats "Sustained Load Cracking" and why does it only happen in 6351 alloy aluminum cylinders?
Sustained load cracking is a
phenomenon where cracks form in the neck area of 6351 alloy aluminum cylinders because of the migration of metal in the alloy.  6351 alloy contains lead.  Lead  is a soft metal.  Steel cylinders do not contain lead and 6061 alloy aluminum does not contan lead, so they are not susceptible to sustained load cracking.  Sustained load cracking differs from stress cracking becuase sustained load cracking is a crack opening formed by movement of the metal whereas stress cracking is formed by seperation of the metal.  So while all cylinders are susceptible to stress cracking, only 6351 alloy aluminum cylinders are susceptible to sustained load cracking.  Stress cracks usually happen in a short time.  Sustained load cracking does not happen instantly.  It takes time, often years, for the migration of the lead in the cylinder alloy to move, or "creep" and form a sustained load crack.  The reason Eddy Current testing is federally mandated on 6351 alloy aluminum cylinders is becuase the Eddy Current test can detect the very slow creep of the metal before it becomes catastrophic.  With 6061 and steel cylinders, stress cracks can happen instantly, so there is no preventative measure to detect those cracks.  The diagram below may help you to better understand sustained load cracking.

Graveyard DIver Scuba cylinder sustained load cracking
Aluminum 6351-T6 alloy consists of nine different metals, the softest being lead.  At the time of manufacture, the molecules
in the Aluminum 6351-T6 alloy are fairly evenly distributed and the cylinder is able to hold the pressure of the compressed gas.


Graveyard DIver Scuba cylinder sustained load cracking
Over the span of time, as the immense pressure inside a SCUBA cylinder "pushes" against the molecules of the alloy, the
softer lead molecules migrate away from the force of the pressure, towards the exterior of the cylinder wall.  Since the other metals
are harder, they do not move as much, but where the lead molecules congregate, the cylinder wall  becomes weaker.


Graveyard DIver Scuba cylinder sustained load cracking
Over a period of years, more and more soft lead molecules are pushed away, or "creep", from the force of the pressure towards
the outside of the cylinder wall.  Eventually, the cylinder wall becomes too weak in that area to hold the pressure inside the cylinder
until BOOM!  an explosion occurs from the pressurized gas escaping through the opening.


The dive shop told me they couldn't fill my cylinder with nitrox because my cylinder wasn't oxygen clean.
Your dive shop was following accepted  industry safety standards   As you should have learned in your SCUBA certification class, "regular" air that we breathe from our SCUBA cylinders consists of 21% oxygen.  No special cleaning of your cylinder is required for regular air.  However, as the percentage of oxygen increases, the danger of ignition increases.  
The percentage of Nitrox requiring an oxygen clean cylinder varies among dive shops depending on which organization's threshold they choose to follow.  Most (but not all) dive shops follow the PADI recommended practice that any Nitrox mixture of 40% oxygen or greater requires your cylinder to be oxygen clean.  However, some dive shops may follow other agencies recomendations.   Here are some other agencies oxygen cleaning thresholds:

PSI-PCI:  23.5% Luxfer:  23.5% OSHA:  40%
NASA:  21% Catalina:  23.5% NOAA:  40%
US Navy: 25% NFPA:  21% PADI:  40%
CGA:  23.5% ASTM:  25%



Thus, if your dive shop follows PADI or NOAA standards for oxygen cleaning thresholds, they probably won't fill your cylinder with Nitrox over 40% oxygen content unless your cylinder is oxygen clean.  If they follow PSI, Luxfer, or Catalina oxygen cleaning thresholds, they may require your cylinder to be oxygen clean if you request a Nitrox blend over 23.5% oxygen.  Regardless of which organizations standard they follow, any reputable dive shop will require your cylinder to be oxygen clean if they fill your cylinder with Nitrox using the partial pressure blending method.

My steel cylinder has "+" stamped on it.  My hydrostatic requalifier told me that is is illegal for him to restamp a "+" on my cylinder after the first requalification.
This is false.  Steel cylinders may be requalified to be filled to 10% over their rated service pressure at any time during their service life and stamped with a "+" by a qualified hydrostatic inspector, assuming they pass required testing.  Your hydrostatic requalifier may choose not to do this extra step, but many do.   If you are unable to locate a hydrostatic requalifier in your area that will requalify and mark your steel cylinder for 10% over fill, and you can deliver your cylinder to PSI, they will requalify it and stamp it with the "+" authorizing it to be filled to 10% over it's rated service pressure.  Exception:  Pressed Steel cylinders manufactured under the Special Permit number 9791 do not use the "+" and may not be filled beyond service pressure under DOT regulations.

Can I request my aluminum cylinder to be stamped "+" so it can be filled to 10% over it's rated service pressure?
No.  Aluminum cylinders never qualify for the "+" stamp and may not be stamped "+" at requalification.  Any aluminum cylinder bearing a "+" must be condemned and removed from service.

How thick is the wall of my cylinder?
I get this question quite often from customers who are curious.  Cylinder wall thickness varies by manufacturer and type of material.  Steel cylinder walls are generally between 4 to 5 millimeters thick.  The walls of aluminum cylinders have to be thicker because aluminum is a softer metal.  Aluminum cylinder walls are generally 10 to 15 millimeters thick.
Graveyrad Diver cylinder cut in half
Wall thickness of a standard AL80 aluminum cylinder
With a penny used for comparrison.

What do you do with cylinders that do not pass a visual inspection?
Under DOT regulations and SCUBA industry safety standards, a cylinder that does not pass a visual inspection based on DOT standards is condemned and may no longer remain in service.  After your cylinder is stamped "CONDEMNED" and disabled so that it can no longer hold pressurized gas, it will be returned to you.  Customers with condemned cylinders often sell them for aluminum scrap.  Creative customer sometimes make cool things from them.

Graveyard Diver scuba cylinder art    Graveyard Diver scuba cylinder art    Graveyard Diver scuba cylinder art