The original specification for dimensions of the O-ring groove in the nose of the DIN connector for compressed air has an inside dimension of 12 mm and an outside dimension of 17 mm with a depth of 1.9 mm. The original 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, the groove and O-ring specifications for the DIN fittings found on first stages seem to vary among the SCUBA regulator manufacturers, perhaps because they have referenced different versions of different standards or to address practical issues that arose during usage by divers.
The sealing O-ring in the nose of the DIN connector groove we find on most SCUBA regulator first stages is typically an SAE AS568 and BS ISO 3601 imperial standard 112 size that is equivalent to 12.37 X 2.62 metric dimensions including our DGX Gears, Dive Rite, and Mares XR brands. In the case of Apeks (plus the 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 available in a variety of choices for material and durometer. The groove in the Poseidon (plus the Cressi 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 in North America, especially in a material recommended for use with Nitrox. (FYI, the O-ring in standard 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. ) Dive Gear Express does stock a generic fluorocarbon elastomer (aka Viton™, aka FKM, aka FPM) O-rings in all these sizes, and also the OEM Poseidon DIN connector O-Ring as well as the OEM Apeks 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.
