Current situation with PST tanks

[swimlikethefish]

It is reverse logic, the 7/8 valve diameter is smaller than the newer 3/4 valves. That is the easy way to tell them apart.

 

[Bob DBF]

The difference in sizes has to do with the type of thread involved. Pipe threads are measured by the inside diameter of the pipe, and bolt threads are measured by the outside diameter of the bolt. This means a NPT (P for pipe) will be larger than a bolt thread (UNF) for the same numerical diameter.

I have heard that a quarter will fit in the neck hole, it's 3/4"-14 NPS. If the hole is too small, it's 7/8"-16 UNF, since I don't run across the 7/8 thread I haven't checked personally.

HP 3500 tanks have used both 3/4 and 7/8 threads dependant on when they were made.

Some older 72's had 1/2 valves.

Two types, 1/2" NPT ( pipe thread ) and 1/2" NPS (straight thread). I have seen very few 1/2" NPS, I have one old valve but no tank to use it on.


Bob

 

[tbone1004]

@Bob DBF haven't seen a 3500psi tank with 3/4" threads. Any info on that? I know PST went to 3442 when they changed to the 3/4" threads and the only other company I know of that sold 3500psi tanks in the US was Asahi

 

[Bob DBF]

Second hand info off a thread here. Someone had a set of 3500# tanks that came with 3/4 thread and 300 bar DIN manifold. If I can find it again I'll link.


Bob

 

[2airishuman]

I have a few steel 72's, and one of them has a thinner neck. I'm assuming this is the special manifold you're both talking about. Maybe I'll make sure to hang on to that one so I have one as a spare.

If/when the time comes for hydro, I'll make sure the places follow the PST standard for testing. Theres a few pretty knowledgeable dive shops nearby that do in house hydro's.

LP72s don't have a problem passing hydro. I'm not sure what precipitated the problem, but it's not specific to exemption cylinders since I had an HP40 (3AA-3130) fail so badly that the hydro shop retested it because they thought there was something wrong with their test rig. Never had a problem with LP72s.

Regarding neck threads, older LP72s may be any of:
* 1/2" tapered (1/2" NGT or 1/2" NPT)
* M18, that is, metric 18mm straight thread with an O ring. This thread is still widely used outside the USA and valves are readily available on the Chinese export web sites.
* The now-standard 3/4" NPSM.