Cylinders, valves and regulators threads story…
Without being exhaustive, as there are lot of different threads used in the world, here are the main existing/used standards for diving.
BETWEEN THE CYLINDER AND THE VALVE
In Europe, two metric threads are currently used:
- M25x2: the standard for most cylinders
- M18x1.5: generally used for small-volume cylinders , and some 300b cylinders,
Some standards were used in the past, but are now reserved for storage cylinders, or cylinders not used for diving.
- E17: small cone
- E25: large cone
In the imperial system :
- G3/4 : very similar to M25x2 but not compatible. Obviously it can be also found sometimes in Europe…
Note: it is common practice, but also extremely dangerous, to fit a G3/4 valve to an M25x2 cylinder (or vice versa), as this is not at all safe, there is frequent accident in the filling stations with hurt people, don’t play with this.
BETWEEN THE VALVE AND THE PRESSURE REDUCER
- INT / ETRIER: 230 only, mainly used in recreational diving, the defects of this connection are mainly the length of the element on the regulator, and the two seals, one of which extrudes quite frequently, including in diving…
- DIN G5/8: available in 230 or 300b, the standard most commonly used in technical diving.
- M26x2 (also often called M26): exists in 230 and 300b, created to differentiate use with over-oxygenated mixtures.