How is the REE number on a tank used

[NAM001]

I like the post. When i first started diving int he 60's the general rule as told was that the 72's had a pluss rating for the first 5 years and it was lost at the 5 year point. Your post seems to explain how/why that position was taken back then.

Its a maximum allowable expansion in mL (cubic cm)
My lp cylinders have REEs on them, < than REE = pass for a +
My bank bottles all have REEs on them too, some of those are lp cylinders eligible for +s some are 4500psi bank bottles which aren't.
All my 3442 and 3500 psi tanks have REEs on them too, that is the max allowable expansion in CCs. If they are > the REE they fail

Basically the REE is a mega cheat so the hydro shop doesn't have to do a lick of math at all. < REE = pass (and + eligible if manufactured under 3AA specs)

If a 3AA tank is over the REE it can potentially still pass under the 90% rule. This is actually pretty rare. LP72s in hard to plus rate without the REE because you need the wall thickness and a whole bunch of other variables, plus math and more math. Hence they are typically only tested against the 90% rule which doesn't mean they aren't good to 2450...

AL3000 tanks are only evaluated under the 90% rule.

 

[captain]

The REE stamped on the tank is a volume in cubic centimeters. This is the maximum total expansion of the tank when subjected to test pressure. The standard hydrotest does not have a maximum; the tank simply has to return close enough to its original volume so that it 90% of the expansion is lost when the tank is depressurized. I'm not sure I'm explaining it exactly correctly, but I think that's the gist.

For example, lets say one of LP95s has a REE of 95. That means to get the plus rating the tank can't expand more than 95 cc when the tester fills it with water to 4000 PSI. (2400 x 5/3). That's how freakin simple it is to give a tank the plus rating; you'd really think that more professional hydro shops would be able to do this.


Getting back to the example, lets say the tank expanded 90 cc, then, when depressurized, contracted back down 82 cc. So the permanent expansion was 8 cc. 8 is less than 10% of 90, so the tank passes.

The REE (Rejection Elastic Expansion limit) is based on elastic expansion not total, Using your example, total expansion at 4000 psi 96cc, elastic expansion 93 cc after pressure is removed ( permanent expansion = 96 - 93 = 3 cc, ) Passes hydro, permanent expansion (3.1%) less than 10%, also qualifies for plus, elastic expansion (93 cc) is less than REE of 95

 

[Eric Sedletzky]

Plus ratings really don't mean much in my world.
I must be lucky, my LDS routinely fills my 72's to 2500 - 2800 - sometimes as high as 3000. No plus ratings on any of them from the second hydro date.

The one thing that still makes me wonder though, a steel 72 is shorter and skinnier than an aluminum 80, yet the 72 filled to 3000 I am told it puts them up to 86 cf. An AL80 filled to 3000 holds 78.5 cf and is just called an 80 to round things up and simplify the name.
Is the wall thickness in the 80 that thick?

 

[spectrum]

Is the wall thickness in the 80 that thick?

AL80 wall ~1/2"

LP72 Wall ~3/16" if I remember right. In any case significantly thinner. Of course the OD is also a little bigger and the bottom is not rounded.

+ ratings usually don't mean much in my world but to the opposite effect!

 

[NAM001]

I have no problems getting a fill to what ever i want. I fill my self when i can. The underlying purpose of the OP was to figure out why hydro shops insist that will not do + hydros when the info is all there. After getting the explanation of the REE from posts here, it just proves that hydro shops should have no issues with doing the + hydros. My local hydro guy does not do scuba tanks as a main business. he does other tanks and says they do scuba tanks if brought to them. Their explanations for why to not do + hydros just dont hold water. They say they have to look up information to do the + hydro. My thing with the + is that there are shops that will not fill beyond the stamped rating. the + does a lot for comping for cooling. I occasionally get a cooled lp tank psi of 2100 after hitting the 80-90 degree water. Getiing this when you have a 2400 and a + tank it is only a 3/4 fill. They say it is acceptable to be 10-15% short of rated psi when cooled. Really more of an annoyance than any thing else.

Plus ratings really don't mean much in my world.
I must be lucky, my LDS routinely fills my 72's to 2500 - 2800 - sometimes as high as 3000. No plus ratings on any of them from the second hydro date.

The one thing that still makes me wonder though, a steel 72 is shorter and skinnier than an aluminum 80, yet the 72 filled to 3000 I am told it puts them up to 86 cf. An AL80 filled to 3000 holds 78.5 cf and is just called an 80 to round things up and simplify the name.
Is the wall thickness in the 80 that thick?

 

[Luis H]

In general, industrial gas places have no interest on the plus stamp. Industrial gases are sold by the pound, not by the tank full. If you need more welding oxygen, you just get another cylinder. There is very little to no advantage to fill a cylinder to the maximum possible. It is not like they are carrying it on their back.

If you know the history of the + stamp, it was to carry more gas with less containers during the war effort (WW-II) when there was shortage of many raw materials. We do not have a shortage of pressure vessels at the moment. So there is very little incentive to + stamp cylinders for other applications outside of Scuba.


We had an LDS with their own hydro equipment and license. The DOT inspector would occasionally drop by unannounced. She would inspect the records and for every mistake she found, there could be a fine (mostly minor copying of numbers).

For a hydro station that is not doing + stamping on a regular bases, it could add the risk writing something wrong, just because it is a bit out of their daily routine.
This is not a very good excuse, but I can see why some may think it is not worth the trouble. Personally, I avoid doing business with that type of hydro facility.



Captain statement in post 12 is correct.


If you want more information on REE, you may want to take a look at this thread:
vintagedoublehose.com/forum/viewtopic.php?f=12&t=4737



Also look at post number 26 (and some of my other posts) on this thread:
www.scubaboard.com/forums/tanks-valves-and-bands/513209-questions-about-lp72-tanks.html



I have calculated REE numbers for over a dozen steel 72. To perform precise calculations I actually measured the wall thickness of my cylinders using ultrasound equipment.

The REE number is stamped on a lot of the newer cylinders, but not always. The hydro station sometimes has to actually try to find it, if you provide it to them.

 

[duckbill]

( permanent expansion = 96 - 93 = 3 cc, ) Passes hydro, permanent expansion (2.88%) less than 10%

Not 3.125%?

 

[Luis H]

I can&#8217;t figure out how Captain came up with 2.88%. I am going to guess a key punch error in a calculator. To get that number, you have to divide the 3cc by 104cc.

It is interesting to notice that a mistake like that, in the recording data sheet, could have cost the hydro station about $100 in fines per line item (if caught by the inspector, and they catch those mistakes).

I didn&#8217;t notice the mistake because it was close enough to 3% that it didn&#8217;t register. I was not in a checker frame of mind (looking for errors), like an inspector would be.

If it was rounded to 3% it would probably not have been questioned, but it should have actually been rounded to 2.9%, just like the actual answer should be rounded to 3.1%. It is appropriate to use 2 significant digits. The accuracy of the data collected does not support more digits.

 

[captain]

Interesting. I used the calculator in Windows 10. I went back and checked after Luis's post and divided 3 by 96 and hit the percent key and got 2.88, more than once. If I just divide 3 by 96 I get .03125. If I use my handheld calculator and divide 3 by 96 and hit the percent key I get 3.125. What up with that ????

 

[Charles2]

Interesting. I used the calculator in Windows 10. I went back and checked after Luis's post and divided 3 by 96 and hit the percent key and got 2.88, more than once. If I just divide 3 by 96 I get .03125. If I use my handheld calculator and divide 3 by 96 and hit the percent key I get 3.125. What up with that ????

Wow! I just tried what you described and got the same answers. That is bizarre! I will now forever doubt the calculator in Windows 10. I do hope somebody can explain that.