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[jswantek]

Nice post Andy, I just learned something. Thanks!!!

 

[argos59]

This site is the best, you guys know everything. Thanks for the info

 

[H2Andy]

I think its 7/8

and you'd be right

Tank is 4 years old DOT E13483-3500 3216355y TP5250 HP100

So how do you figure out what you have? Look at the SCUBA Tank neck for the DOT stamp. If it reads DOT-3AL or DOT-3AA it has a 3/4 NPS threaded neck opening. (3AL means aluminum, 3AA means steel.) If it reads DOT-Exxxx-3442 it's a steel cylinder, also with 3/4 NPS threaded opening. If it reads DOT-Exxxx-3500, it's a steel cylinder with a 7/8 UNF threaded opening. (The xxxx is a manufacturer specific four or five digit exemption number.)

http://www.divegearexpress.com/library/valves.shtml

 

[H2Andy]

Nice post Andy, I just learned something. Thanks!!!

thanks man ... thanks to the question, i learned something too

 

[Scared Silly]

Though the question has pretty much been answered - anything 3500psi and above are 7/8 UNF. Further, the valve must be DIN. Although, I have heard of some folks with yoke valves but these are about as rare as hen's teeth.

For 3442 PSI cylinders which are 3/4" NPS the valve can be either DIN or Yoke. So the 3500psi is a threshold for more than just the threads.

 

[WD8CDH]

When the HP 3500 psi tanks first came out, they selected the 7/8" UNF thread both because it was different from the rest of the SCUBA tanks but also because it was already in current use for SCBA tanks so hydro stations could test them. There is no real technical reason to go to a smaller neck thread at 3500 psi. There are many higher pressure tanks used in other industries that have larger threads than the 3/4 NPS SCUBA threads. Many European tanks even at 4500psi use threads very similar (but not interchangeable) to 3/4" SCUBA threads.