Posted by Chris on December 12, 2001 at 11:16:03:
The highest flow rate first stage design is that of the venturi assisted balanced diaphragm type.
The venturi assist works by using the pressure reduction of the air flow into the second stage to assist in opening the first stage.
From a control systems perspective it can be considered an application of mild regenerative feedback.
For optimum performance with this system the second stage must be attached to a particular port (usually marked "R") since the internal direction of the air flow is a significant.
Other LP ports can be used, but do not get the optimum benefit from the venturi assist.
The two presently being made are the Poseidon and the Oceanic DX4.
The Poseidon, which is also used on the Abyss regulator because of it's superior performance, is the smaller of the two.
The DX4 now comes standard in an environmentally sealed unit.
The DX4 is much larger than the Poseidon, and in my experience has a more stable output pressure.
The Poseidon only comes in a DIN model; the DX4 comes as either DIN or yoke.
I ordered my DX4 with a DIN because I prefer the robustness and reliability of the DIN to a yoke.
Also you can always use a DIN to yoke adaptor if only a yoke tank is available.
In my opinion none of the adaptors made for using a yoke regulator on a DIN tank are not safe.
The output pressure of both of these first stages is externally adjustable.
In the case of the DX4 the environmental cover must be removed.
The next level down in flow rate performance from the venturi assisted balanced diaphragm is the balanced piston.
Although almost all manufactures now make a balanced piston first stage, the ScubaPro, Atomic, and Oceanic are generally acknowledged the best of this type.
One advantage of this type over the venturi assisted balanced diaphragm type is that there is no favored port for the second stage, thus giving the user a bit more freedom in how his/her regulator is assembled.
The output pressure of most balanced piston first stages in only adjustable through the use of shims during assembly and is not externally adjustable.