Question 1950s WK Twinset and Navy Double Hose

[Luis H]

I saw this comment, which is interesting, but DOT RIN numbers are relatively new. I am not sure when they were introduced, but we didn't have them in the 70's.

It looks as though they spent a lot of time overseas. The hydro stamps don't conform to standard DOT RIN. Shouldn't be a problem to get them tested though as they are DOT 3AA.

 

[n1023]

They are 3AA which is the code for chrome molybdenum steel. I don't know the exact dates when chrome moly steel was introduced to ICC pressure vessels, but I think it was during WWII. They could not be WWI.

Older tanks were typically stamped 3A which is the code for carbon steel cylinders.

The regulatory agency was the Interstate Commerce Commission or ICC (as stamped on the cylinder). The ICC is the predecessor to the DOT (Department of Transportation). The documents covering the material alloys and technical cylinder specifications are the CFR (Codes of Federal Regulations). You can search and download them on line. Make sure to have a lot of coffee on hand if you want to start reading them.

You may notice the + stamp next to the 2/70 + re-qualification date. The plus ( + ) was introduced during WWII to allow the filling and transportation of 10% extra pressure gas in portable cylinders. That was a big material and cost savings during the war effort, to be able to transport an extra 10% in the same container (pressure vessel). After the war the regulations stayed unchanged.

In order to qualify for the + stamp (and the extra 10% pressure) the original hydro date has to contain the + stamp next to it. And it can be re-qualified and re-stamped with the + during any of the later hydro test.

The hydro test procedure (and test pressure) is the same. the only difference is the criteria used for passing. The typical hydro qualification (for a 3AA) cylinder uses as failing criteria that the plastic (or permanent) deformation has to be greater than 10% of the total elastic deformation. I am sure this requires a lot more explanation, but it can all be found in the CFR and there are many videos on line (some of them are accurate ).

The pass/ fail criteria for the hydro test doesn't change, but to qualify for the + stamp, the total elastic expansion has to be less then the REE (Reject-able Elastic Expansion) number for that cylinder type/ design. The REE number for older cylinders is hard or impossible to find, but it can be calculated with some data and measurements from the cylinder. I have measured and calculated REE numbers for most of my old steel cylinders, but it is not something that the casual observer would care to do...


This is probably more information than most people cared for (and more than many dive shops and even some hydro stations are aware of), but it is all in the CFR and CGA (Compress Gas Association) manuals. Just be aware that there is a lot of wrong information shared in the scuba industry. If you have any question, you can go to the source.

Actually one of the tanks is 3A, the other is 3AA. I did bring the same hydro shop an old Norris steel tank from the 60s and they couldn’t give the plus because the REE was unavailable. I didn’t know you could calculate yourself. Wonder if the shop would be okay with using the number I provide?

 

[Still Kicking]

I saw this comment, which is interesting, but DOT RIN numbers are relatively new. I am not sure when they were introduced, but we didn't have them in the 70's.

Yes I was thinking about it. I have seem some old ones and wondered when they were introduced so I deleted my comment.

 

[Luis H]

Actually one of the tanks is 3A, the other is 3AA. I did bring the same hydro shop an old Norris steel tank from the 60s and they couldn’t give the plus because the REE was unavailable. I didn’t know you could calculate yourself. Wonder if the shop would be okay with using the number I provide?

To calculate the REE number for a cylinder you need some information/ dimensions from the cylinder.

Some are easy to measure, like the diameter and even an accurate volume.

To measure the volume I weighted the empty tank and then weighted it full of water. Subtracted the two numbers to get the weight of the water and calculate the volume.

The average wall thickness used to be the hardest to measure. I had co-workers who are certified NDT (non-destructive testing) inspectors with calibrated ultrasound measuring equipment. Measuring wall accurate thickness with ultrasound equipment is relatively easy. I marked the cylinder with evenly spaced dots to get over 20 data points for a good average.

Since then, I have seen ultrasonic thickness gauge at Amazon for as little as $90. I don't know how good they are, but it is probably good enough for this purpose.