How long does a steel tank usually retain its + rating in hydro?

[Tracy]

That's plenty for a single dive for me, but got me thinking about if I were to buy older tanks or keep new ones for a long time, if they'd likely lose their + rating at some point in the future and if so, how long if kept in good condition until they usually no longer earn it in hydro?

They will bsaically keep their plus rating forever. It is rare to have a tank pass test and not qualify for a plus. Not saying it doesn't happen, it does. It is just very rare.

Also, it I were to find some older tanks that were originally + rated, but hadn't received the stamp in the last cycle or two or something, would most shops have any issue trying to get the + rating back, or once they fail to stamp + is that it for future hydros/fills?

It comes down the person doing the testing. It has nothing to do with past testing. If it passes currently, it qualifies. I will say this warning, if the REE is not stamped on the tank, the odds of getting a plus stamp at test drop near 0%.
It isn't that the tank doesn't qualify, but almost nobody is going to dig out books of factory specs to look up what the REE should be.
Many older steel tanks did not have it stamped on them. Most from the last 20-25 years do.

The loss in gas capacity wouldn't necessarily be a deal-breaker for me, I would just prefer to know what I'm getting into beforehand since the rated volume seems to rely on this stamp.

In most places, it has nothing to do with the stamp and everything to do with the person holding the fill whip.

 

[Tracy]

It's been consistent across 3 separate hydro facilities in Canada. Doesn't bug me much after getting a compressor.

Transport Canada and USDOT use the same rules for testing and standards. That is why most modern tanks are dual stamped and tanks tested in either are legal for transport in the other.

 

[drummerc]

They will bsaically keep their plus rating forever. It is rare to have a tank pass test and not qualify for a plus. Not saying it doesn't happen, it does. It is just very rare.

Awesome, that makes me feel a lot more comfortable picking them up. I'll try and find some recent ones used, but if not I'll probably just buy new ones and I'll 100% make sure any used ones I find have the REE stamped on them.

 

[Eric Sedletzky]

I don't understand the whole + rating thing or why it even exists, or how it started. When a tank is called whatever it's called it should be that at it's rated pressure, not 10% over. That's why I like 3442's because you get what they are called.
I have many steel 72's. They shouldn't be called 72's, they should be called 64.8's or whatever they are at 2250 psi.
Fortunately my dive shop is great, they are one of us and thinks the whole + thing is stupid and fills my tanks to 2500.
I've tried to get my 72's plus rated but nobody has the REE numbers that go back that far so I don't bother to ask anymore.

 

[Tracy]

I don't understand the whole + rating thing or why it even exists, or how it started.

Do you want to know? I happen to know a bit about tanks and various histories of them.

 

[chaad]

I don't understand the whole + rating thing or why it even exists, or how it started.

Check out this snip from a much longer thread. Dates back to WW2.

As an aside, the plus rating allowing the 10% overfill of 3AA steel tanks was a WWII invention to offset a shortage of welding tanks needed for war production.

 

[Eric Sedletzky]

Do you want to know? I happen to know a bit about tanks and various histories of them.

Do tell.

 

[Tracy]

Chaad alluded to it above. During WW2, there weren't enough bottles to support the war effort. They needed more gas and the ICC worked with the war commission to maximize gas without jeopardizing safety. They also didn't want to just make more bottles as the steel was needed for the war effort. Gas bottles were built with a lot of safety margin, they did testing and determined that 10% was a safe overpressure across almost all of the bottles. They created a formula to establish a new reject number based on current expansion and if the bottle met the criteria, it received a plus for the 10% overfill. If you look at the picture below, you can see that the plus stamps start during WW2. It never went away as it never caused a safety concern in practice.

Not all bottles were able to qualify for the plus. Since the calculation was based on tank pressure, the bottle had to have a tank pressure stamped on it. This one was made before they were stamping in a service pressure so it never received a plus.

As for why scuba cylinders state the capacity including the plus, it sounds better to customers. Same reason an AL80 isn't called an AL77. More is better. That is the american way.
OMS made an entire tank business out of lying about capacity. They sold standard Faber steel cylinders and just used bigger numbers to make them sound better. Faber 95 was an OMS 98. 120 became a 125. The cylinders are all identical, but people bought the OMS because they were "bigger."