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SCUBA Valves FAQ

Products - Gas - Valves

ABOUT SCUBA VALVES

Thermo™ SCUBA valves are manufactured in Taiwan and are distributed in the US by XS Scuba. The Thermo Pro DIN/K valve has been independently tested as meeting the CGA V9-2009 Compressed Gas Association Standard for Compressed Gas Cylinder Valves. The Thermo valves have a polished heavy chrome cosmetic finish and include a pre-installed DIN-to-Yoke adapter.

DGX Premium SCUBA valves are also manufactured in Taiwan. DGX Premium valves are available in an exceptionally wide variety of specialized versions. The DGX Premium valves have a satin chrome cosmetic finish. Our unbranded generic valves have a lower retail price as a result of being sourced directly from the manufacturer. Although the DIN-to-Yoke adapter is included with most cylinder and valve packages, when sold individually our DGX brand valves do not include a DIN-to-Yoke adapter. The adapter is always available separately as a low cost accessory.

There are internal differences (for example the bonnet nut seal on the Thermo uses a copper crush washer and the DGX uses an O-ring), but oxygen compatible materials and construction of the valves are similar for both Thermo and DGX. Both are described as Nitrox Ready, although Thermo valves are explicitly labeled "40% MAX OXYGEN PREMIXED" on the valve body. Our long experience with the branded Thermo and generic DGX Premium valves has been both have similar reliability and durability in the field. Regardless of brand, every product we sell is backed by Dive Gear Express 5-star quality customer service.

Our DGX Long Neck modular valves are about { 0.4 in | 1 cm } taller than our DGX Pro modular valves, but internal parts are identical. The design of the area between the outlet and the base, the 'neck', is more accessible in the Long Neck version. This advantage makes the Long Neck version more popular with sidemount divers who find the extra neck space more convenient to use with the sidemount bungee. The squat nature of the Pro modular valves offers a lower profile that is preferred by most overhead divers to reduce chance of impact on the valves and regulator first stages when wearing manifolded doubles. However, individuals who have restricted shoulder motion may find advantage in the extra height of the Long Neck valves making it a little easier to reach back to manipulate the knobs during S-drills. The disadvantage of the Long Neck version when worn with back mount single tanks is the divers head is more likely to touch the regulator first stage. As a general rule, if neither of the Long Neck advantages apply to your style of diving then there is no reason to prefer the Long Neck versions.

Probably. All of our replacement Nitrox Ready SCUBA valves have an inlet designed to fit 0.750-14 NPSM cylinder neck threads. This fits nearly all North American market SCUBA cylinders of recent manufacture, but there are some rare exceptions. Fortunately, you can use the cylinder neck stamp to determine the answer for yourself.

Assuming your cylinder was manufactured exclusively for the North American SCUBA market in the last 50 years or less, look at the cylinder neck for markings that begin "DOT". If it reads "DOT-3AL-xxxx", "DOT-3AA-xxxx", or "DOT-Xxxxx-3442" it almost certainly has an imperial 0.750-14 NPSM threaded neck opening, referred to in the trade as 3/4 NPS. SCUBA cylinders recently manufactured for other areas may have the name of their thread form stamped on the neck. If the neck stamp includes "3/4 NPSM" or "0.750-14" then the cylinder has a 0.750-14 NPSM threaded opening. Our replacement Nitrox Ready SCUBA valves will fit any of these cylinders.

There are some rare exceptions, especially for cylinders with very high service pressures or manufactured before 1970; for more information please read our article SCUBA Valves: Regulator Fittings, 200 vs 300 Bar FAQ and Cylinder Neck Threads.

The most common technical objections to the use of ordinary SCUBA valves with pure oxygen are: (1) too rapid a speed of opening and (2) chrome plating of the surfaces in contact with high pressure oxygen that are subject to friction or impact. The DGX Premium O2 valves are designed to answer these two specific objections. but otherwise similar to Nitrox Ready valves. For more information please read DGX Premium O2 Valve Differences.

The inlet and outlet on our oxygen compatible DGX Premium O2 Valves are oxygen clean copper metal alloy (aka raw "brass") which will very quickly exhibit some discoloration and spotting after cleaning that that is not present with electroplated chrome finishes typical of other SCUBA valves. Raw brass that is not coated with some kind of sealer (which would defeat the purpose of the valve) will discolor very rapidly, thus appearing aged or used.

If you consider the cosmetics to be important, and maximum compatibility with pure oxygen is less of a concern, then we recommend choosing our standard Nitrox Ready DGX Premium Valves over the DGX Premium O2 Valves.

The Thermo brand valves have a little sticker on them (below the outlet) with a "service pressure" value. However, that value can be misleading to the point of being dangerous in some cases. It can be misleading because nothing prevents someone from inadvertently or deliberately installing a mismatched valve in a cylinder. A trained fill station operator following best practice will be guided by the markings on the crown of the cylinder not the label on the valve. Those permanently stamped markings on the cylinder itself determine the maximum fill pressure of the cylinder, not the service pressure label on the valve. The markings on the nut of the pressure relief device (PRD) installed in modern valves indicates release pressure of the PRD and not service pressure of the cylinder.

All of our standard Nitrox Ready SCUBA valves will work with SCUBA regulators having yoke (aka A-clamp), 200 bar DIN or 300 bar DIN fittings. This is virtually all SCUBA regulators, with the rare exception of oxygen service M26 regulators sold in the European Union. The valves have a DIN outlet that requires an DIN-to-Yoke adapter insert for use with yoke regulators. The small DIN-to-Yoke adapter screws in to the DIN valve outlet and may remain permanently if you wish. Our Thermo brand valves include a pre-installed DIN-to-Yoke adapter at no charge. Our DGX house brand valves do not include the adapter, but a DIN-to-Yoke insert is available separately as a low cost accessory.

For more information please read our article SCUBA Valves: Regulator Fittings, 200 vs 300 Bar FAQ and Cylinder Neck Threads.

All the DIN-to-Yoke valve insert adapters for valves sold by Dive Gear Express use two #112 DIN O-rings, the same size as commonly found in DIN regulators. Regardless, if the adapter remains permanently in the valve, the rear O-ring rarely if ever needs replacing and only the face O-ring where the yoke regulator attaches needs to be changed. While the standard O-ring in yoke valves is a #014, the slightly 'fatter' #112 actually seals better and is less likely to extrude. For more information, see our O-rings page

We strongly recommend purchasing our valves ONLY for cylinders used in North America and the Caribbean. All cylinders in the United States and Canada have Imperial neck thread forms and the valves have over pressure relief devices (aka "burst disks"). Neck thread forms and over pressure relief device requirements in many other areas of the globe are different than North American standards.

One important caution regarding thread compatibility; outside the United States, particularly in Europe, a widely used SCUBA cylinder neck thread form is M25. The 3/4 NPSM (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.

Yes, you can completely open our SCUBA cylinder valves and eliminate the archaic quarter-turn-back practice you probably learned in your open-water class. All SCUBA cylinder valves from Dive Gear Express are modern designs that protect both the valve stem seal and the valve seat when fully opened; the valves are not prone to leaking or sticking open. The dive industry is moving away from the quarter-turn-back practice because it has been proven to cause dive accidents. The Alert Diver article titled Old Habits Die Hard has a detailed explanation from DAN.
Detailed assembly and installation instructions, along with torque specifications and lubrication recommendations, can be found in our Installing the Valve in the Cylinder TekTip.

The handwheel shearing off the valve stem upon a right-angle impact is a design safety feature present on nearly all modern SCUBA valves, in order reduce the risk that the flow of gas could be interrupted or gas could be lost. Without even having to drain the tank, it's a simple process to replace the valve stem. This is a simple equipment maintenance task many divers can do themselves. Watch our YouTube video titled Valve Stem Replacement.

Nominal Pipe Size (abbreviated NPS) is a term for American standard pipe sizes widely used in the North American industrial trades, but does not describe specific thread forms. Confusingly, National Pipe Straight (also abbreviated NPS) is the name of a specific type of thread form described in American National Standard Pipe Thread standards. Just to be clear they are not the same, one NPS describes a standard for specific sizes of pipe and the other NPS describes a specific type of screw threads used on the ends of pipe.

Historically, many different screw threads have been used on SCUBA cylinder outlets and valves. Using the national pipe thread standards, the proper technical name of the parallel straight threads commonly used to mate modern SCUBA cylinders and valves for the North American market is 0.750-14 National Pipe Straight Mechanical, typically abbreviated as 0.750-14 NPSM. It is frequently also written as 3/4 NPS or 3/4-14; for the purposes of description with SCUBA cylinder and valve applications the terms may all be considered to reference the same threads: 0.750-14 NPSM has a 60° thread angle, a pitch diameter of 0.9820 to 0.9873 in (24.94 to 25.08 mm), and a thread pitch of 14 threads per inch (1.814 mm). The same thread form is also described in a variety of other standards such as ANSI, ASTM, and ISO but those other standards are very rarely referenced in the North American SCUBA market.

See our article SCUBA Valves for more information.

Because we located in the United States , we do not sell SCUBA cylinder valves with the metric M25 inlet thread form. We only sell SCUBA valves for use with US DOT cylinders, which all have imperial neck thread forms and are required to incorporate an over pressure relief device not found on M25 valves.

The ISO3601 standard specification of the o-ring for the metric M25 scuba valve inlet threads is a slightly smaller (25mm) inside diameter, and a slightly smaller (3mm) cross section than the AS568 standard specification for the o-ring of the imperial 0.750-14 NPSM scuba valve inlet threads, which is 1 inch (25.4mm) inside diameter and 1/8 inch (3.175mm) cross section. However, the AS568-214 imperial o-ring is close enough in size to the M3.0X25.0 metric o-ring that most divers consider the imperial #214 o-ring a suitable substitute.

One important safety caution regarding thread compatibility: The 0.750-14 NPSM threads most commonly seen on SCUBA cylinders in the US and M25 threads most commonly seen outside the US (particularly European Union) are very similar but not identical; with the use of 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.

No compatibility testing has been performed by the different manufacturers and neither Dive Gear Express, Dive Rite, nor XS Scuba (who also supplies the Thermo brand) recommend mixing components from different brands of valve modules. However, our customers tell us these valves have similar modular port threads and we have no reports of problems with crossbars, H-adapters and port plugs in that regard. The DIN-to-yoke adapter inserts along with the standard knobs, but not the Vindicator or XS Safety knobs, should be interchangeable without issues. Beyond cosmetics, we can also say for certain that some other internal parts of all four brands do have slight differences and are not interchangeable.

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