Valves & Neck Threads
This article explains valves used in SCUBA cylinders.
How a regulator attaches to the valve and how the valve connects to the cylinder.
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 themselves!
Frequently Asked Questions About 200 vs. 300 Bar
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 outlet!
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. 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.
Is it safe to use a yoke adapter valve insert and yoke regulator on the new 3442 psi SCUBA cylinders? Yes, it's OK to adapt 200 bar outlet SCUBA valves for use with all modern Yoke regulators. However, we have seen a few older yoke regulators with the yoke assembly marked for 3000 psi max service (or unmarked), and we do not recommend using those yoke regulators on the 3442 psi cylinders.
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 valves.
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, 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. The current United States Department of Transportation rules prohibit the transport of metal scuba cylinders on public roads with pressures above 3442 psi (230 bar), even if the cylinders and gas delivery systems have been rated for higher pressures.
When Pressed Steel Tank Company began manufacturing their high-density steel 3500 psi service pressure SCUBA cylinder in 1987, they used 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 reverted 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 "K" insert.
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||Service
|Low Pressure Steels||2400+||4000|
|High Pressure Steels||3442||5250|
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.