Posted by Eins on January 24, 2001 at 12:56:52:
There are obviously two schools of thought on this issue: the hydro and filling side who say that + can not be given again from the second hydro, and the DOT, who explains how the + rating came to being and who confirm that there is no reason why it can't be given again as long as the tank is serviceable.
For a detailed coverage of this context, please read the long text below which I copied from http://diverlink.com/gear/tankfills.htm
In essence, it says, that due to shortage of pressure vessels, the DOT generally allowed an overfill of 10% and this permit has not been revoked. Nowhere does it imlpy a limit to the first in-hydro cycle.
I, as a diving consumer, would feel duped by a hydro place that refuses to give the + rating or by a dive shop that refuses to fill to 10% over because of a missing + mark, and I would boycott such places.
Eins
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Following is the full background text:
Most any diver who has had an LP steel tank for any length of time knows what the + rating on the neck signifies. It means the tank can be filled to ten percent over the rated pressure. That means a tank with an operating pressure of 2250 can be filled to 2475. That is 2250 + 225 (!0%), or 2475.
The misunderstanding about the plus rating is that some people think it can only be done on a new tank, not when it is hydroed (hydrostatically tested) every 5 years. This is an erroneous belief. To know why, it helps to understand how the plus rating came about in the first place.
The plus rating came about during World War II when there were material shortages, including shortages of high pressure cylinders. Some movies of that time, or reflecting that era, make reference to scrap drives, which were common place at the time and indicated the shortages. The plus rating came about simply because there was a need to conserve materials. Upon re-evaluation, the operating pressure of certain steel tanks was deemed to still be within safe limits if raised by 10 percent, hence the + stamped on the neck. This was easier than revising the whole regulation, which takes a considerable amount of time, and was done a decade later
You can read a quotation from the DOT's web site that explains this.
"In 1930, the ICC implemented regulations for periodic inspection and testing of cylinders; the regulations, as amended, were first published in the Federal Register on December 12, 1940 (5 FR 4908). During the 1930's and 1940's, the Compressed Gas Association (CGA) developed and refined the water jacket test method for determining the serviceability of a cylinder. During World War II, there was a shortage of high pressure gas cylinders. Because of CGA's work on steel wall stress limitations, the ICC granted "temporary'' regulatory relief to increase the gas carrying capacity of existing cylinders by allowing the cylinders to be filled 10% over their marked service pressures, and by marking those cylinders with a plus, "+'', mark. Ten years later, the regulations were codified into the Code of Federal Regulations (15 FR 8261; Dec. 2, 1950). In 1967, pursuant to the Department of Transportation Act, Pub. L. 89-670, 80 Stat. 931, regulatory responsibility for the transportation of dangerous articles in commerce was transferred from the ICC to DOT."
Nearly everyone I spoke with at the DOT, and everyone I spoke with at length, either was a diver or had been, which helped a great deal. There is no justifiable reason why a steel tank cannot be filled to the operating pressure indicated by the + as long as it is serviceable. The fact that there are dive shops who think the + only applies to the first hydro indicates that there are people who just don't understand the regulations, or have taken the time to make a phone call or send an e-mail for clarification.
Even if an LP steel tank has been hydroed since its original date and is lacking the plus sign, there is no reason it cannot be filled to ten percent over the stamped service pressure. There is no sense in compounding one mistake with another. Just because a dive shop or other retester was ignorant of the regulations concerning the plus mark and omitted it, is no reason to continue to penalize divers with such tanks by only filling them to the stamped service pressure.
In fact, in the revised regulations being proposed by the DOT, the service pressure of LP steel tanks will be raised to incorporate the 10% increase and do away with the confusion over the plus rating altogether on new tanks. However, it will be a while before the new regulations are adopted, perhaps years, and there will still be older tanks in service for many years, so a knowledge of the regulations is certainly helpful. See Proposed Changes to DOT regulations concerning cylinders (including scuba tanks) and Update to Proposed Changes to DOT regulations concerning cylinders (including scuba tanks) for more on this.
For all those with steel tanks, it would be a good idea to talk with the shop or other authorized retester and ask what their understanding of the plus rating is before leaving your tanks with them. If a steel tank (that had a + initially) passes hydro, it should get the + rating every time. If it doesn't pass hydro, it's no good at any pressure. There is no in-between. Authorized retesters other than dive shops are probably more aware of this overall, though it is best not to assume it. Ask. That ten percent (technically, not an overfill) can cost you precious minutes underwater if it is missing.
What to do if you have steel tanks missing the plus sign
If you are the victim of a dive shop or other authorized retester's ignorance, you can politely insist they correct their mistake. You can show them this page or have them contact the DOT directly for confirmation. If your LP steel tanks passed hydro and the plus sign was omitted at the time of testing, there is nothing fraudulent or wrong about them adding the plus sign at a later date. It is simply correcting an earlier mistake they made, one of omission.
You can also make the dive shop or other air station personnel aware of this page and the regulation. Once they understand it, they may fill your tanks to ten percent over even without the plus sign, This is as it should be. They would be staying within the parameters established by the DOT and simply ignoring a mistake made by a retester who wasn't as aware of the regulations as they should be.
U.S. Department of Transportation (DOT) Info Center Hotline 1-800-467-4922.
Office of Hazardous Materials Safety
phone number (202)366-4535
Disclaimer
We are not advocating overfilling of tanks beyond their proper service pressure. The difference in the case of LP (lower pressure) steel tanks is that their proper service pressure is ten percent over the stamped amount, so it is not really an overfill to exceed the stamped amount by ten percent. However, that does not apply to aluminum tanks and HP (higher pressure) steel tanks, which should only be filled to their stamped pressure. There is some flexibility in the tolerances to allow for increased pressure due to higher temperatures without deliberately exceeding the stamped limits and straining those tolerances to the limit or beyond.