Throughout the 1950's and early 1960's several steel cylinder sizes were
marketed having pressures ranging from 2,150 to 3,000 psig. Between 1958 and
1960, valves changed from mostly 1/2" tapered thread to 3/4", 14 turns to the
inch straight thread with an o-ring. However, there were some 1/2" straight threaded
cylinders as well. Only steel cylinders were authorized for commercial service until
late 1971.
Most early commercial steel scuba cylinders ranged in volume from about 65
cu ft to 95 cu ft. However, many surplus World War II era 38 cu ft cylinders with
a service pressure of 1,800 psig were used, commonly as doubles. Some "38's"
still pass their quality assurance tests and remain in service today.
The U.S. manufacturers of steel scuba cylinders included Pressed Steel Tanks
Co. (PST), Norris Industries, and Walter Kidde Co. The Italian firm, Faber, makes
DOT approved cylinders sold by the brand names Scuba Pro and OMS.
By the mid 1950's, the PST Co. and perhaps one other company began
making aluminum cylinders from 6061 alloy for the U.S. Navy. The fabrication
process was very different from the way aluminum cylinders are made today.
Although many of those round bottom cylinders found their way into civilian
service, they are illegal, may not be hydrostatic retested and may not be filled at
commercial air stations.
Warning: There are NO legal round bottom aluminum scuba cylinders even though
many of the old navy cylinders are used today, are illegally re-tested by careless re-testers and filled by thoughtless fill station operators.
Newly formed Luxfer USA, Ltd., with DOT special permit SP6498, began
producing aluminum cylinders in late 1971. Using 6351 alloy in a cold extrusion
process, the cylinder did not require a bottom plug like the former military type and
the approved cylinder had a FLAT bottom. In order to be equivalent to the then
popular steel cylinder, Luxfer made its cylinder 6.8" in diameter to fit existing non-adjustable back packs and with a similar 2,475 psig service pressure. To achieve
the 72 cu ft capacity, the cylinder was longer but, as a consequence, it was about
11 pounds buoyant when near empty. Divers quickly named it the "floater."
Both Walter Kidde (with special permit SP7042) and Norris
Industries(SP6688), using the 6351 alloy, followed Luxfer into the aluminum
cylinder business during the early 1970's. Kaiser Aluminum (SP6576) joined in but
chose 7039 alloy which proved to be unsuitable for high pressure cylinders. That
cylinder (brand name AMF) has a 2700 psig service pressure and a somewhat
rounded (beveled) bottom.
Luxfer and Walter Kidde continued production under their DOT SP6498,
E6498 or SP7042, E7042 until the DOT formalized the aluminum cylinder category
3AL in July of 1982. Unfortunately for owners of Norris Industries SP6688 and
Kaiser SP6576, both cylinders became illegal when their permits expired by 1979.
Like the illegal navy surplus aluminum cylinder, many SP6688 and SP6576
cylinders are used today because hydro re-testers fill station operators and owners
ignore or are ignorant of their illegal status.
The Catalina Tank Co. (now called Catalina Cylinders) began manufacturing
3AL cylinders in 1986 using the 6061 navy type alloy. Luxfer switched to the
6061 alloy in late 1987 while Walter Kidde continued using alloy 6351 until
production ceased in July, 1989. Today, only Luxfer Gas Cylinders and Catalina
Cylinders produce 3AL scuba cylinders for sale in the U.S. and much of the world.
PST created renewed interest in steel cylinders by introducing high density
3,500 psig steel E9791 cylinders in 1987. Coyne joined the steel scuba cylinder
business in 1993 producing 2,400 psig cylinders in several sizes.
Readers interested in more scuba cylinder history should read INSPECTING
CYLINDERS- 3RD EDITION available from PSI, Inc. 6531 NE 198th St., Seattle,
WA 98155-2031 $18.50 including U.S. shipping.
Walter Kidde Co. was located in California. Their cylinders were very popular in
the scuba diving community. That is where the name "K"-valves came into the
picture. This may not be the story that you have heard, but truly that is how it
came about. The first cylinders were really oxygen bottles. The oxygen bottles had
rounded bottoms, so the first scuba cylinders were also manufactured with rounded
bottoms. They needed bottom boots to stand up. Now you don't need the boots,
but we still have them. Mostly for protection of the cylinders, but really not.
Scubapro came up with the idea of galvanized steel cylinders with an epoxy coating.
U.S. Divers was the last company to pick up the flat bottom. It seems that U.S. Divers
always waited 5-6 years before coming up with the same thing as everybody else.
The cylinders have been a good extra piece of business for the diving stores.
