An O2 clean diaphragm reg, in the strictest sense of the word, is a bit of an oxymoron as the large diaphragm is a fuel source that is far larger than the sum total of o-rings (nitrile, EPDM or Viton) found in it or in anything else.
Also, a diaphragm reg is not the best choice for a deco bottle that is for the most part carried unpressurized as if it floods, it is more likely to suffer problems than a piston reg when re-pressurized. If it is sufficiently flooded, the non compressible water driven by the high pressure air entering the reg may rupture the diaphragm before the excess pressure can be vented through the second stage. What you get then is a large popping sound and a non functional regulator. In contrast, the internal portions of a piston first stage are equally incompressible and far less likely to be damaged.
My personal preference is to use unbalanced flow by piston first stages for deco applications. They are flood tolerant and by design have no high pressure areas past the seat and in general have very few o-rings making them relatively O2 freindly regardless of the o-ring material used. The two dynamic o-rings in the first stage are also in intermediate pressure areas and as such are not exposed to the extremely high PPO2's found at tank pressures.
Performance wise they are not exceptional, but are more than adequate for the depths where high pecentage O2 will be use (70' or less). One other advantage is that the inhalation effort goes up noticeable as the tank pressure falls below about 300 psi, so you are never surprised by a deco bottle going dry.
Scubapro uses EPDM o-rings and Christolube as standard in the Mk 2 Plus so it is already in the ballpark regarding having the potential to be O2 cleaned for use with 100% O2 in addition to being a "flow by" unbalanced piston design. Plus the design is about as simple and bullet proof as you can get, making it a good choice for a first stage that has to work every time.
Also, a diaphragm reg is not the best choice for a deco bottle that is for the most part carried unpressurized as if it floods, it is more likely to suffer problems than a piston reg when re-pressurized. If it is sufficiently flooded, the non compressible water driven by the high pressure air entering the reg may rupture the diaphragm before the excess pressure can be vented through the second stage. What you get then is a large popping sound and a non functional regulator. In contrast, the internal portions of a piston first stage are equally incompressible and far less likely to be damaged.
My personal preference is to use unbalanced flow by piston first stages for deco applications. They are flood tolerant and by design have no high pressure areas past the seat and in general have very few o-rings making them relatively O2 freindly regardless of the o-ring material used. The two dynamic o-rings in the first stage are also in intermediate pressure areas and as such are not exposed to the extremely high PPO2's found at tank pressures.
Performance wise they are not exceptional, but are more than adequate for the depths where high pecentage O2 will be use (70' or less). One other advantage is that the inhalation effort goes up noticeable as the tank pressure falls below about 300 psi, so you are never surprised by a deco bottle going dry.
Scubapro uses EPDM o-rings and Christolube as standard in the Mk 2 Plus so it is already in the ballpark regarding having the potential to be O2 cleaned for use with 100% O2 in addition to being a "flow by" unbalanced piston design. Plus the design is about as simple and bullet proof as you can get, making it a good choice for a first stage that has to work every time.
