This article explains valves used in SCUBA cylinders.
How a regulator attaches to the valve and how the valve connects to the cylinder.
© Copyright 2002-2014 Dive Gear Express, LLC
SCUBA cylinder valves come in right-hand and left-hand designs,
referring to the side of the valve knob, not to which direction
the valve opens. In addition to the typical standalone versions,
cylinder valves are also available in modular
styles. The advantage of the modular valves is that they
may be connected using a manifold to construct a set of doubles,
or connected together to form what is known as an H-valve
for use on a single cylinder. The manifolded doubles are
sometimes disassembled, the manifold removed, and the left-hand
and right-hand modular valve manifold ports are plugged to make
two single cylinders. Note the following discussion primarily applies to
cylinders and valves found in the United States, subject to
US federal regulation.
Regulator Fittings and SCUBA Valves
The K-valve is by far the most common valve on SCUBA
cylinders in the U.S. and nearby areas, such as the Caribbean.
The regulator first stage attaches to this valve using a yoke
A-clamp fitting, and there is a sealing O-ring that is held in
the face of the valve outlet (formally described by the GGA-850 standard)
where it mates with the regulator.
Although this system has been in use a long time, it is
considered by many to be somewhat unreliable. The US standards
consider the Yoke A-clamp fitting suitable for a maximum
pressure of 3000 psi at 70°F, although European standards consider it
suitable up to 230 Bar at 15°C (3442 psi @ 70°F)
The Deutsches Institut Für Normung (DIN) is a German
standards setting organization similar to our American National
Standards Institute (ANSI) and Compressed Gas Association (CGA).
DIN 477 is a specification that recommends cylinder valve outlet
and connector designs for specific types of gases and pressures
based upon safety considerations. These various designs have
deliberate incompatibilities to preclude the possibility of
errors when handling different types of compressed gases at
differing working pressures. The two valve outlets and connectors
of interest for divers are the DIN 477 No. 13 and the DIN 477 No.
56 (formerly No. 50), both designated for use with compressed
air. The DIN 477 scuba valve outlet and regulator fittings are most widely
used outside the U.S. and their thread form is also sometimes referred to as G 5/8" BSP.
The regulator first stage DIN connector is
a male screw type, and instead of clamping on to the outside of
the valve as does the yoke, it screws directly into the female
DIN outlet of the valve. The sealing O-ring is held in the end of
the regulator connector rather than in the face of the outlet.
The DIN 477 system, with it's captured o-ring design, has proven
to be very reliable for use with SCUBA.
Outlet/Connector #13 is from DIN 477 part 1 - for cylinders
with test pressure ratings up to 300 bar and is commonly referred
to in the US SCUBA industry by the slang term "200
bar", because most European dive cylinders with 300 bar test
pressures have working pressures in the 200 bar range.
Outlet/Connector #56 is from DIN 477 part 5 - for cylinders with
test pressure ratings up to 450 bar and is commonly referred to
in the US SCUBA industry by the slang term "300 bar",
because most European dive cylinders with 450 bar test pressures
have working pressures in the 300 bar range. The two designs are
nearly identical, but the #56 valve outlet is deliberately deeper
so the shorter #13 connector will not be long enough to seat
properly. This is a safety feature to prevent connecting a low
pressure device to a high pressure supply. It's important to
understand that the "200 bar" or "300 bar"
descriptions are just slang terms that have nothing to do with
the pressure ratings of the outlets and connectors
Frequently Asked Questions About 200 vs. 300
Is a 300 bar SCUBA valve outlet stronger than 200 bar?
No, they are equally strong. Only the first few threads in both
the 200 bar and 300 bar outlet designs are doing the work, the
remaining threads on the 300 bar outlet are there simply to
create a deliberate incompatibility with a 200 bar connector. In
fact, in practice, the 300 bar valve outlet has proven itself to
be more fragile than the 200 bar valve outlet. The 300 bar valve
outlet is so long that the smallest 'ding' on the edge of the
opening can slightly warp the cylindrical opening, causing the
regulator to become difficult or impossible to completely seat.
This problem does not seem nearly as pronounced with the 200 bar
valve outlets, although it remains an issue and all DIN outlets
should be protected from dings.
If a SCUBA regulator has a 300 bar connector does it need a
300 bar SCUBA valve? No, the 300 bar connector of the
regulator is designed to work just fine with a 200 bar valve
outlet on a cylinder. The regulator connector will require fewer
turns to seat with a 200 bar outlet and is much more convenient
to use in that regard. Once fully seated in a 200 bar valve
outlet, two threads of the regulator 300 bar connector are
visible; this is completely harmless and normal -- two exposed
threads are not going to 'weaken' anything.
If I have 200 bar valve outlets on my cylinders, should I
purchase a regulator with a 200 bar connector? The DIN
connectors seen on modern regulators sold in the US are almost
always the 300 bar variety that fit a 200 bar outlet just fine.
There are specialised drysuit inflation regulators that sometimes
have 200 bar connectors. We have also (very rarely) seen primary
regulators, usually hand carried into the US by individuals from
Europe, that have 200 bar connectors. We also have seen a few
special order DIN 200 bar retrofit kits to change yoke regulators
to DIN. However, there is no reason to seek out and use a 200 bar
connector on your regulator.
Is the 300 bar SCUBA valve "better"? The 300 bar
valve is not better than a 200 bar valve for the SCUBA
applications for which they are being used. The 200 bar DIN
outlet actually has the advantage that it can accept an insert
that allows it to be used with the very common GGA-850 connector,
aka "Yoke" or "A-clamp", in wide use on
regulators in the US. Because 200 bar valves are more widely used
than 300 bar, they benefit from economies of scale in production
and distribution, meaning that 300 bar valves are more difficult
to find and more costly. Since 200 bar valves can be adapted to
Yoke, are more convenient to use and less susceptible to damage,
some feel the 200 bar outlet is better than the 300 bar
Would a 300 bar SCUBA valve make the regulator less likely
to shear off if the tank is dropped? No. We have seen several
damaged regulator/valve assemblies, but never has the damage been
such that 300 bar fittings made a difference. The weak point for
such force is always where the DIN connector is attached to the
body of the regulator. The DIN connector does not separate from
the valve, what happens is the regulator separates from the DIN
connector; the valve is irrelevant.
Is it safe to use a 200 bar SCUBA valve on the new 3442 psi
SCUBA cylinders? Yes, it's fine for use with DIN regulators
and modern Yoke regulators. We have seen a few yoke regulators
with the yoke assembly marked for 3000 psi max service (or
unmarked), and we do not recommend using the yoke adapter with
those yoke regulators on the 3442 psi cylinders. While 3442 psi
is approximately 230 bar, keep in mind the '200 bar' term is
meaningless so far as the pressure rating of the valve outlet
itself is concerned, it is a DIN 477 #13 valve outlet and the
outlet itself does not have a pressure rating. Because of
confusion about the 200 bar slang description for the valve and
the 3442 psi cylinder pressure, we have noticed that some
literature has taken to referring to the DIN 477 #13 valve outlet
as '230 bar', even though the valve outlet is unchanged.
Can I replace the 300 bar SCUBA valve on my 3500 psi
"Genesis" cylinder with a 200 bar valve? No. Many
divers would like to have the option of using yoke regulators on
their 3500 psi cylinders. However, the older design 3500 psi
"Genesis" steel SCUBA cylinders are forced to use a 300
bar valve because a 200 bar valve with the proper 7/8 UNF stem
threads to fit the cylinder is not available. (See the next
section for more details about neck threads.)
Does a yoke adapter insert exist for my 300 bar valve?
No. An adapter insert does not exist, and even if it were made,
we have never seen a yoke clamp long enough to fit over the 300
bar SCUBA valve. There is a yoke-to-din fill adapter device for
300 bar valves, but it is strictly for filling only.
Is there any reason I should use a 300 bar SCUBA valve
instead of 200 bar? For the sake of uniformity, some divers
who started with 300 bar valves continue to select them when
purchasing new cylinders, just so all their cylinder valves have
the same type of outlet. Some individuals in diving leadership
roles may insist their students and peers use 300 bar valves,
usually with the vague justification they are 'better' because
300 bar is a higher pressure than 200 bar. A common misconception
is that DIR philosophy or GUE training standards require 300 bar
valves, which is untrue. If using the 300 bar valve outlet will
provide uniformity, peace-of-mind or peer acceptance then do so,
because there is no major reason to avoid using 300 bar SCUBA
For those traveling to European Union nations, the European Commission (yet another
standards setting organization) has adopted EN 144-3:2003 "Outlet Connections For Diving Gases Nitrox And Oxygen" specifying the valve outlet/connector known as M26x2 must be used on dive cylinders containing those gases. The M26x2 outlet is similar in appearance but slightly larger than the
DIN outlet, and thus incompatible with DIN fitting regulators. The purpose of
this incompatible valve outlet is to force the dedication of cylinders, regulators and fill stations
to Nitrox use. There is controversy concerning the M26x2 outlet, with the popular opinion being it fails to address any significant safety or handling issue for Nitrox divers and implementation by the European recreational diving community has been slow.
"The nice thing about standards is that you have so many to choose from." - Andrew S. Tanenbaum
Cylinder Neck Threads and SCUBA Valves
Valves attach to the necks of SCUBA cylinders using one of
several types of screw threads. Screw threads date back to the
third century. For a few centuries everyone did their own thing
regarding screw threads. As you might imagine, by the nineteenth
century, interchangeability was non-existent. With the first
World War, the American National thread form was established so
that war materials could be more easily manufactured. With the
second World War, the American National thread form had
interchangeability problems with the Whitworth form of Great
Britain. Thus the United States and Great Britain (and Canada)
agreed upon a Unified thread form that would allow better
interchangeability. The point of this history lesson is there are
different thread form standards.
The first SCUBA cylinders originally introduced in the
early days of the sport were steel and used a 1/2-inch National Gas Taper Thread
form, often referred to as "1/2 NGT." The cylinder neck
does not use an o-ring seal with the valve, instead the tapered
valve threads are wrapped with Teflon or PTFE pipe tape before insertion. These cylinders
typically have relatively low service pressures of 2015, 2216 or
2250 psi. Keep in mind that these very early SCUBA cylinders,
typically from the late 1950's and early 1960's (sold under the
names US Divers, AMF Voit, and Healthways), are very uncommon.
Today, the most common thread form used with SCUBA cylinders
and valves is National Pipe Straight also known as NPS. The specific
thread form used with most SCUBA cylinders is named 0.750-14 NPSM,
often referred to as "3/4 NPS." This thread form is
seen on both steel and aluminum SCUBA cylinders with service
pressures of 2400+ (2640), 3000, 3180+ (3498), 3300 and 3442 psi.
When Pressed Steel Tank Company began manufacturing their high-density steel
3500 psi service pressure SCUBA cylinder in 1987, they elected to use a
more modern thread form for the neck opening known as Unified
National Standard Fine. The specific thread form used with the
steel 3500 psi SCUBA cylinder is named 0.875-14 UNF, often
referred to as "7/8 UNF."
It's very important to understand that the 3/4 and 7/8 trade
descriptions do NOT indicate relative sizes of the physical
dimensions of the opening! This is because the two thread forms
are from two different standards, and the names used to describe them
refer to only the trade size
and schedule. The 3/4 NPS opening is
visibly larger than the 7/8 UNF opening.
Until recently, it worked as follows:
"High-Pressure" steel SCUBA cylinders (i.e., 3500 psi,
sometimes referred to as "Sherwood Genesis"
cylinders... the brandname they were originally sold under) were
manufactured with the smaller 7/8 UNF threaded openings, and all
other modern steel and aluminum SCUBA Tanks were manufactured
with larger 3/4 NPS threaded openings. Then in 2003, Pressed
Steel Tank Company introduced their new high-pressure 3442 psi
E-series cylinders, and they elected to revert to the 3/4 NPS
threaded opening. This was followed by similar designs from Faber
and Worthington. So now there are two different high-pressure
steel SCUBA Tanks in wide use, the older 3500 psi cylinders with
7/8 UNF threaded opening and the newer 3442 psi cylinders with
3/4 NPS threaded opening.
Luxfer recently introduced a limited production run of 106 cubic
foot capacity SCUBA cylinders that are a composite design: aluminum
shell with fiberglass hoop wraps. This cylinder has a service pressure
of 4350 psi and 7/8 UNF neck threads.
So how do you figure out what you have? Assuming your cylinder
was manufactured in the last 45 years or so, 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-Xxxxx-3442
it's a steel cylinder, also with 3/4 NPS threaded
opening. If it reads DOT-Xxxxx-3500 or DOT-Xxxxx-4350,
it's a steel or composite cylinder with a
7/8 UNF threaded opening. (The Xxxxx is a manufacturer specific
four or five digit exemption number that starts with the letters
E or SP.) Another test, if the valve
is out of the cylinder, is that a U.S. quarter coin will fit
through the 3/4 NPS cylinder neck opening but will not fit
through the 7/8 UNF opening. By the way, if it's not a SCUBA
tank, all bets are off; it will probably have some other neck opening
threading which is incompatible with SCUBA valves.
Currently, SCUBA valves with 3/4 NPS threads are widely available from a
variety of different manufacturers. However,
because most cylinders with 7/8 UNF neck threads or 1/2 NGT neck threads have not been
produced in recent years, finding replacement valves for these less common
thread sizes can be a challenge.
Currently, XS Scuba is still manufacturing some 7/8 UNF valves with 300 BAR only
outlets and 1/2 NGT pillar valves with 200 BAR outlets that will accept the
Valves should be installed in scuba cylinders with the least amount of
force that will form a seal and retain the valve in place. For modern valves that capture an o-ring between the
neck and the valve, this usually means they can be installed by hand and just snugged up with a hand wrench across a flat at the base of the valve. The threads should not bind and force is NOT required to fully screw the valve home. Do not use the wrench across the face of the valve, do not over tighten and do NOT bang on the handwheel. One important caution regarding thread compatibility; outside
the United States, particularly in Europe, a widely used SCUBA
cylinder neck thread form is known as M25. The 3/4 NPS (ISO 228) and M25 (BS 3643)
threads are very similar but not identical; with the use of excessive force it's possible to
mate diving cylinders and valves with these two different
forms. However, the mismatched threads eventually separate with dangerously explosive force, often causing injury.
Cylinders, Valves and Pressure Release Devices
In the U.S. at the turn of the twentieth century, a series of boiler explosions caused significant loss of life. The most
famous of the accidents, the Grover Shoe Factory disaster in 1905 resulted in stringent safety laws and
a national code related to safe operation of boilers but is inclusive of most pressure vessels, including modern scuba cylinders. All U.S. cylinder valves are required by the Department of
Transportation (DOT) to have a pressure release
device (PRD) that operates if the pressure exceeds the cylinder test specifications. The design of the PRD used in scuba valves
is a non-reclosing rupture type, also called a burst disk assembly. The thickness of
a small metal (usually stainless steel or copper) disk in the assembly determines the pressure at which it will rupture,
releasing the cylinder pressure more slowly than if the cylinder itself were to rupture with explosive force.
Type of Scuba Cylinder
Low Pressure Steels
High Pressure Steels|
If the PRD is installed in the scuba valve too tightly it will rupture prematurely, and if installed not tightly enough it will leak.
The burst disk assembly should be installed with a very precise amount of force using a special tool known as a
torque wrench, calibrated in a unit of measurement known as inch-pounds. The typical recommended torque force is in the range of 100 to 120 inch-pounds. In addition to ruptures due to being over tightened, if a burst disk is ever removed from the
valve the disk must be discarded because reusing a disk will also cause it to rupture prematurely.
In practice, the presence of PRD's in scuba valves has proven to be problematic. Over time
the thin metal disk can weaken or corrode due to exposure to moisture, causing it to fail and release prematurely. Sometimes a valve with burst disk intended for a low pressure cylinder is inadvertently installed in a high pressure cylinder. If the PRD does not fail immediately upon filling, the mismatched PRD is almost certain to eventually fail prematurely. Mismatched PRDs and cylinders are common, in part because most older PRDs on scuba valves are not clearly marked or not marked at all as to their release pressure.
The safety of PRDs on scuba valves is debatable. While the PRD is considered a safety device required by law on all pressure vessels in the US, the PRD is not present on scuba valves in most other countries. Even where scuba cylinders are involved in fires and subsequently fail, in most cases before the PRD would have released the cylinder wall actually ruptured first due to the loss of the annealed strength of the cylinder material. The typical PRD failure typically occurs while the scuba cylinder is being filled, or often within a few minutes after being filled. The sudden and unexpected rupture of the PRD creates a powerful jet of high pressure gas which can propel the relatively small metal scuba cylinder like a rocket and cause it to careen about the immediate area with enormous force. PRDs could, in theory, fail during a dive, causing rapid loss of breathing gas for life support. For these reasons, Dive Gear
Express recommends that the cylinder valve be overhauled
and an appropriately matched new burst disk be installed by a qualified Valve Repair Technician whenever the cylinder is hydro tested
(i.e., every five years) and annually if the cylinder is known to
have been overfilled.