The other day we went through our own cylinders and saw that 24 of them needed to be tested. So I thought it appropriate to revisit it and help remove some questions on it.
As some of you know TDL / STT has a full industrial hydro test facility called Havau Hydrotest. We inspect and re certify cylinders for all industries. Those big trucks driving down the road with hundreds of cylinders in them from the little oxygen bottle grandma uses to the big 6000 psi nitrogen bottles flow through our facility.
Let's talk about a few things according to DOT here so that we are all on the same page.
Hydrostatic tests are required by DOT (department of transportation) every 5 years for portable cylinders. Some industrial cylinders (not underwater use) can be extended to 10 years and some cylinders that are in a permanent mounting system do not need to be tested at all.
Visual Inspection is required by DOT of all cylinders prior to the Hydrostatic Test. This allows the cylinder technician to determine if the cylinder even warrants hydrostatic testing. They will inspect for rust, pits, damage, threads, neck cracks etc.
A Visual Eddy Current test is required on certain aluminum cylinders on a DOT list that were manufactured 1990 and prior. These include tanks from Walter Kidde, Luxfer, Parker, etc. The VE test is not required on industrial CO2 cylinders. Most hydrostatic test facilities will just do a Visual Eddy Current test on all aluminum cylinders 1990 and earlier as a routine measure so as to not miss any that may have been on the list.
If a cylinder fails a Visual Inspection / Visual Eddy test the cylinder will NOT be hydrostatic tested and the cylinder will be taken out of service. A series of Xs will be stamped into the Service Pressure Area and the Hydrostatic Test Area or rendered incapable of being refilled (drill holes, cut etc. or Stamped CONDEMNED. Most just stamp Xs.
Hydrostatic Test Pressures are set by DOT for specific cylinders. There is a fairly complicated formula but most scuba cylinders fall under two methods 5/3rds service pressure or 3/2 service pressure. Most aluminum and steel cylinders will be tested to 5/3rds service pressure to determine the Elastic Expansion and the Permanent Elastic Expansion. For example a 3000 psi cylinder will be tested to 5000 psi. a 2400 psi cylinder will be tested to 4000 psi and so on. Certain "Exempt" cylinders get tested to 3/halves service pressure. These are typically steel cylinders either rated at 3500 psi or 3442 psi these get tested to 5250 psi and 5163 psi respectively.
If the cylinder fails to meet the tolerance of the Elastic Expansion and the Permanent Expansion test it fails and must be taken out of service.
Plus Rating on Steel Cylinders "+" The "+" rating allows for certain cylinders to be filled to service pressure plus 10%. IE: 2400 psi steel cylinder can be filled to 2400 psi + 240 psi for a total of 2640 psi. A 2015 psi cylinder can be filled to 2015 psi + 201 psi for a total of 2216 psi.
Plus ratings cannot be placed on Aluminum Cylinders or Steel Cylinders that fall under the "Exempt" rule (3500 or 3442). However the plus rating can be added to a cylinder at any time it is hydro statically tested. There is no special test for it. It's just a standard hydrostatic test within the DOT guidelines so long at it meets the REE (Reject Expansion Limit).
A question always comes up from divers about the + rating. If it had it before can it get it again ? Yes if it meets the test. A cylinder is not tested any differently for plus rating than non plus rating. If it fails a hydro test it fails period. The ability to do a plus rating is at the discretion of the Cylinder Re-inspection Technician. For example if it's an old cylinder with rust that passed visual they may not go for the plus rating for ongoing safety issues. If a cylinder had a plus on one hydro and then the next hydro did not have it, the next one can get a plus rating.
Scuba Visual Inspection was created as an industry standard but not one that is required by the DOT. Scuba divers and scuba refilling facilities had inherently had a bad track record of taking care of cylinders. Plus because cylinders are immersed in water they have the possibility of getting water inside and accelerating corrosion. As such the scuba industry instituted the Annual Visual Inspection. It's a good policy and has enhanced cylinder safety in the scuba industry for years.
Refinishing a Cylinder. Anytime a cylinder has been refinished it becomes suspect. The only refinishing that can really be done to a cylinder is to clean it and repaint it. Any cylinder that has been exposed to heat for curing a finish can damage the structural integrity of the cylinder and should not be done. It will probably fail an inspection and or hydro test. Careful maintenance on a cylinder will preserve its finish.
Use of A Cylinder. The cylinder that has gas in it can continue to hold that gas indefinitely. If you had a cylinder filled 4 months ago and today you see the hydrostatic test or visual inspection (scuba) has expired you can still use the cylinder. It however will not be refilled until it has met inspection.
All the rules and laws fall under a variety of agencies. DOT, OSHA, US Coast Guard etc. each will impose different types of fines should things not be in place properly. But for the most part if you adhere to proper hydrostatic test time frames and for scuba the annual visual inspection you will be just fine.
The scuba industry has strongly suggested behavior for cylinders and that's all set up for the safety of persons handing and using the cylinders.
The Professional Scuba Inspectors company in Washington state has done an outstanding job of training dive centers and individuals in how to properly visually inspect and handle cylinders. www.psicylinders.com However it is only DOT inspectors that can certify under federal code a Cylinder Re-inspector (hydro facility).
As a diver you need to consider that your cylinders are consumables and at times will fail a visual or hydrostatic test. This can come about from a variety of ways many of which you have zero control over some of which you have a lot of control over. Damage to the cylinder, water incursion, "cave fills" (illegal overfilling), leaving a cylinder overfilled with a sustained load (pressure).
If a cylinder fails a visual or hydrostatic test that's it. Take it out of service and move on. Try not to second guess the tester he or she knows what to do and will do it properly they test cylinders every day. A good technician does his job well.