The + Rating Myth And Some Facts, For A Change

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Posted by Eins on January 29, 2001 at 10:33:21:

Bob shared some facts in his post at
that I thought are worth bringing up to the board before they get lost in the archives...

Bob wrote:

I read the article you posted, along with some others on Diverlink about tanks. I tend to trust what they say and have read more detail on many subjects that no one else covers.

Considering the conflict and contradiction in many statements made in this thread, and because I wanted to know the truth, I decided to do what Diverlink suggests and called the DOT directly on their toll free number.

Here's what I found:

First, Diverlink is correct. They were really helpful and eager to answer my questions. I didn't get the feeling like I was keeping them from something more important.

Next, I asked them questions brought up here. It turns out that everybody taht made a statement was wrong on something, even when they were right on a single point.

I'm sorry of the truth bothers anybody, but here it is:

Bill Johnson said:
"When you send your cylinder in for a retest, it is normally only hydrostatically tested. In order to qualify for the + designation, "wall stress" must also be determined."

A hydro measures stress and structural integrity. There is no difference in the testing procedure for a steel LP to get a + than for any other tank.

They did say that the 5/3 test pressure should be based on a service pressure that includes the 10%.

That means that a tank stamped at 2250, which can be filled to 2475, should be hydroed with a TP of 4125 (5/3 of 2475) instead of 3750 (5/3 of 2250). A hydro is the only test required by the DOT for these cylinders.

There is nothing different or additional (other than the TP) in the hydro of a steel tank to give it a +.

In fact, like one of the tank articles on Diverlink said, when they revise their rules, the new ones will just make the LP tank pressure 10% higher and do away with the + altogether. That will eventually end the confusion over the matter.

Chris said:

"It takes a special certification for the person doing the hydro to give the tank another +. Not all hydro shops have the certification."

That's pure fiction. The DOT never heard of it. Again, the procedure is the same regardless of the test pressure being tested to.

They also never heard of the 35 year limit for these cylinders. It is not part of the specs. With care, the cylinders can last a lifetime and get + stamped every time.

Some cylinders do have service life limits, but none used by scuba divers.

It is easy to see why there is so much confusion about the + rating. No one seems to know what they are talking about, even when they are partly right.

I'll bet there are damn few dive shops that do their own hydros that will get it right."

Anybody thinking this is not correct is advised to call the DOT Info Center Hotline 1-800-467-4922 and find out from the source.

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