Cylinder threads?

[Akimbo]

To my surprise, I tried Search & Google with no luck. Does anyone have a list of cylinder threads (where the valve seals to the tank) used on Scuba cylinders here and in Europe?

Related questions:
1. A lot of people refer to early ½" tapered cylinder threads used on Scuba tanks as NPT (National Pipe Thread). Gas suppliers refer to what appears as same thread on industrial cylinders as CGA threads. I have also read somewhere that there was a lot of confusion during WWI with the British threads and the standard was changed around WWII. I still see industrial cylinders (Oxygen, Nitrogen, Helium, etc) in use with dates like 1909. I could only find gas-specific fitting connections used on the outlet side of the valve on the Compressed Gas Association's Web site.

So what is the scoop? Is it standard ½" NPT or slightly different?

2. I have never noticed O-ring sealed valves on cylinders outside of the Scuba industry. Is the design adopted from the gas industry or invented for diving? As near as I can tell, Aqualung/US Divers introduced the first O-ring connection in their 1960 Catalog, Page 6: ¾" STRAIGHT PIPE THREAD WITH "O" RING ATTACHMENT (Patent Pending) replaces outdated litharge seal, (See Page 7)...

3. Since then (assumption on my part) it appears there are now three O-ring sealed straight thread Scuba cylinder threads in use:

• ¾" NPS (American National Standard Free-Fitting Straight Mechanical Pipe Thread)
• 7/8"-14 UNF (Unified National Fine) introduced in the US with 3,500 PSI cylinders
• M25 (Metric, 25mm diameter) used in Europe. Anyone have any idea when this was introduced? A wild guess would be it is the European counterpart to ¾" NPS, perhaps introduced by Air Liquide or another one of their diving subsidiaries.

4. All this started because I need to hydro some AGA (now Interspiro) Divator 324 bottles. They are DOT stamped. The tapered thread is about 30% smaller than the old ½" cylinder threads and use an AGA made valve with a 300 Bar DIN outlet. Anyone know what thread they use? They are 300 Bar/4,351 PSI

Hopefully, someone has compiled all this and I have just not been able to find it.
Thanks

 

[captain]

Luis is the man you need. He has a couple of AGA rigs and can tell you what you need to know but I think the taper threads on them are not either the NPT or GCA but European taper.

 

[rigdiver]

There were also some 1/2" straight threads with O-ring seal.
Bill

 

[Luis H]

1) The 1/2" NPT and the CGA 1/2” tapered pipe threads are exactly the same, with (I believe) one minor exception. I think the specifications for the CGA threads are a bit shorter over all, but they are still perfectly interchangeable.

2) Aluminum O2 portable tanks also use O-rings. I am fairly sure there are other aluminum tanks for other gases that also use O-rings.

3) The thread below may have some useful tank thread information. I don’t know when the 25 mm thread became popular, but the idea that it is so close to the 3/4" standard US tank thread is very concerning.
SCUBA Valves: Regulator Fittings, 200 vs 300 Bar FAQ and Cylinder Neck Threads - Dive Gear Express

4) I don’t have AGA tanks; I have Poseidon and Drager tanks with tapered threads. The threads are smaller and they have a bigger taper angle. I had an adapter made about 35 years ago that I still use, but a good hydro station should have an adapter for them. If you are near a large marine port, you should be able to find a hydro station that services life raft tanks. Many life raft tanks use all kinds of metric threads.

Good luck.

 

[vintagediver]

Man, that was quick....
Way to go Luis...

 

[Sam Miller III]

Luis,

I recall that Sportsways introduced the O ring seal.

The frist Sportsways valve has a OD of about 5/8 --bigger that the 1/2 but smaller than the 3/4 valves. However I can not locate my documentation on this...Can you verify?
sdm

 

[Luis H]

Hi Sam

I think the Sportways (and a few White Stag also used the same size) used the 1/2” straight pipe thread, which just like the 1/2" NPT (tapered pipe thread) are actually larger than 1/2". The nominal size refers to the inside diameter of a standard pipe (I think schedule 40, but I am not sure).

The 3/4" NPS (straight pipe thread) is also referring to the pipe inside diameter. The actual thread size is just over one inch. I think it is about 1.06”, but I don’t have a reference chart handy. I do recall that it is an odd thread size, but that is not unusual on threads.

 

[Sam Miller III]

Luis,
I am certain that you are correct. I also recall they were only produced for about a year or two.

I some how recall in the 1970s SDM had an article on the threads some years ago written by the late Jack Mc Kinney. -- By the way I was with Jack's son John last week end

SDM

 

[Akimbo]

Here are US standard outside diameter pipe sizes for straight and tapered threads through 2"

Size (in) = Nom. OD
1/8 = 0.405
1/4 = 0.540
3/8 = 0.675
1/2 = 0.840
3/4 = 1.050
1 = 1.315
1¼ = 1.660
1½ = 1.900
2 = 2.375

 

[Akimbo]

I recently received a reply from Interspiro:

Depending on the age of the pack, the Divator 324 (300 bar, 2 cylinders, 4 liter water volume) can either have a tapered thread or a straight thread.

The older steel cylinders had a tapered thread (EN 144-1, E17 con) and Teflon tape. The outer diameter at the largest point is 19.8 mm.

The newer cylinders have a straight thread (like our SCBA cylinders) of either a M17x1.5 or M18x1.5 with an o-ring seal.​