SCUBA Valves

This article explains valves used in SCUBA cylinders.
How a regulator attaches to the valve and how the valve connects to the cylinder.

© Copyright 2002-2021 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 CGA-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 international standards considered it suitable up to 230 Bar at 15°C (3442 psi @ 70°F). The original 230 bar has since been superseded by the ISO 12209:2013 standard of 232 bar (3472 psi @ 70°F), although the Yoke A-clamp service pressure is rarely referenced as such. We'll use the original 230 bar value for this article, but you may see 232 bar written elsewhere.

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 specialized drysuit inflation regulators that sometimes have 200 bar connectors. We have 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 CGA-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 '232 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.

Can I use a 300 bar SCUBA valve with Nitrox? Probably not. We are not aware of any currently available 300 bar valve that is rated for anything other than Air Only. The problem is that a 300 bar SCUBA valve would have to be tested with pure oxygen at the hydro pressure of the cylinder, meaning the valve is very unlikely to pass an oxygen compatibility evaluation. If you want to dive with Nitrox then you should use a 200 bar valve.

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 diving air only and 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 exclusively for the North American SCUBA market in the last 50 years or so, look at the SCUBA Tank neck for the DOT stamp. If it reads DOT-3AL or DOT-3AA it almost certainly 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.

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.

In very rare circumstances, we see very small cylinders intended primarily for the European market but also qualified for the North American market in specialty inflation or rebreather diving applications. If your cylinder is stamped "M18x1.5" or "M25x2" then the threads are metric and not compatible with 7/8 UNF or 3/4 NPS imperial thread valves. 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. For example, some high pressure paintball cylinders look similar to SCUBA cylinders but have 0.625-18 UNF (aka 5/8 UNF) threads.

Valves should be installed in scuba cylinders with the least amount of force that will form a seal and retain the valve in place. Even in the absence of lubricant, the threads should not bind and force is NOT required to fully screw the valve home. One important caution regarding thread compatibility with the 3/4 NPS imperial thread form; outside the United States (particularly in Europe) a widely seen SCUBA cylinder neck metric thread form is M25x2 (BS 3643). The UK (and Australia) has an obsolete neck thread form BSP G3/4 (ISO 228) sometimes seen on older SCUBA cylinders. The US, EU and obsolete UK thread forms are very similar but not identical; with the use of excessive force it's possible to mate diving cylinders and valves with these different forms. However, the mismatched threads eventually separate with dangerously explosive force, often causing injury. See IMCA Guidance on Diving Cylinder and Valve Thread Compatibility for more information.

Lubrication with SCUBA Valves and Cylinders

The benefit of using lubrication during installation of valves in SCUBA cylinders is debatable. Generic cylinder and valve maintenance guides rarely address the specific nature of the SCUBA industry, especially in regards to Nitrox applications. Since the valve, O-ring and cylinder manufacturers in any assembled SCUBA tank are all different, it's commonplace to hear conflicting recommendations regarding use of lubricant. Our suppliers do agree that nearly all installation, sealing and removal problems are caused by improper technique and over-tightening the valve into the cylinder, not the absence of lubricant. Some local dive shops intentionally avoid all lubricant, some use it very sparingly and others use it liberally. The bottom line is that lubrication use in this SCUBA application seems to be a matter of opinion, not science.

While oxygen compatible lubricants might have higher temperatures of ignition than hydrocarbons, they can still serve as fuel in the kindling chain of an oxygen fire. Any lubricant will attract and retain contaminates, plus there is the significant risk that an incompatible lubricant might be applied. In our opinion, legitimate oxygen safety concerns outweigh the questionable benefits espoused by those recommending lubrication in Nitrox SCUBA cylinder and valve applications. Based on oxygen handing best practices and advice of industry experts, Dive Gear Express recommends NOT using any form of lubricant, including oxygen compatible lubricants, with SCUBA valves and cylinders that will be exposed to compressed gases containing greater than 23.5% oxygen.

SCUBA Regulator DIN Connector O-ring Sizes

The ISO 12209 standard specification for dimensions of the O-ring groove in the nose of the 232 bar and 300 bar DIN connectors for compressed air has an inside dimension of 12 mm and an outside dimension of 17 mm. In 2003, the standard depth for the groove was revised from the original 1.9 mm to a slightly deeper 2 mm. The specification for the sealing O-ring itself is an inside diameter of 11.2 mm with a cross section of 2.65 mm. From what we have been able to determine, not all SCUBA regulator manufacturers seem to use the current standard groove and O-ring in the universal DIN fittings found on their first stages.

The sealing O-ring in the nose of the DIN connector groove we find on most SCUBA regulator first stages is commonly an SAE AS568 and BS ISO 3601 imperial standard 112 size that is equivalent to 12.37 X 2.62 metric dimensions. In the case of a few regulators (notably Apeks, Atomic, and ScubaPro brands) their groove better accepts the next smaller 111 size O-ring that is equivalent to 10.77 X 2.62 metric dimensions. Both the 112 and 111 size O-rings are very common sizes available in a variety of choices for material and durometer. The groove in the Cressi and Poseidon brand regulator DIN connectors measures closest to the original standard, best accepting a BS ISO 3601 metric size O-ring with 11.3 X 2.4 dimensions for which there is no imperial standard equivalent. The M2.4X11.3 size O-ring is uncommon, especially in a material suitable for use with Nitrox. (FYI, the O-ring in standard CGA-850 Yoke K-valves is easy, it is always an 014 size. However, there are some very rarely seen older K-valves that pre-date the CGA-850 standard which might use other o-ring sizes. )

Compared to the 111 size, the M2.4X11.3 size is slightly smaller cross section and slightly larger diameter with a good fit for the O-ring groove of the DIN connectors on Cressi and Poseidon regulators. Because of availability, most Nitrox and technical divers make do with a 111 size O-ring for their Cressi and Poseidon regulators, but the 111 is not as good a fit for the connector of those two brands. However, Dive Gear Express does stock a generic Viton M2.4X11.3 O-ring and also the OEM Poseidon regulator DIN connector O-Ring.

In the European Union nations, the EN 144-3:2003 standard requires a valve outlet known as M26x2 to be used on dive cylinders containing oxygen or Nitrox mixtures with greater than 22% oxygen. The M26x2 outlet is similar in appearance but slightly larger than the standard DIN outlet, and thus incompatible with the standard DIN connector found on SCUBA regulators. The purpose of this incompatible valve outlet is to force the dedication of cylinders, regulators and fill stations to Nitrox use. The sealing O-ring in the nose of the M26x2 connector is 13.9 X 2.6 metric dimensions, equivalent to the 113 size. The M26x2 outlet and connector for Nitrox and oxygen is very rarely seen outside the EU.

For more about O-ring standards and materials used in diving applications, read our Sizing and Selecting O-rings for SCUBA TekTip.

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