The demand valves found in scuba second stages can be either upstream
or downstream design. In a downstream valve, the moving part of the
valve opens in the same direction as the flow of gas and is kept closed
by a spring. In an upstream valve, the moving part works against the
pressure and opens in the opposite direction as the flow of gas. The
upstream design is less likely to experience problems caused by wear and
more resistant to free flow, but most scuba second stages are
downstream design. If the first stage fails and delivers excessive
pressure to the second stage, a downstream valve is forced open
resulting in a free flow and rapid loss of breathing gas. With an
upstream valve, the result of over pressurization usually means the
valve is forced closed. This will stop the free flow but unless the
over pressure is relieved it may cause another failure such as a hose
rupture or loss of buoyancy control. This means that regulators with
upstream second stages must always incorporate an over pressure relief
valve (OPV) in their design. The OPV allows the diver to continue to
breathe the regulator with a minimal loss of gas.
Historically, Poseidon incorporated the OPV for their upstream second
stages in a unique fitting on the end of their low-pressure second
stage hose. The special hose was not widely available, particularly in
the uncommon lengths preferred by technical divers. In 2008 Poseidon
updated their designs (the Mk3 designation) to no longer require a
Poseidon-specific low-pressure regulator hose. The OPV is now located in
the Poseidon first stage, enabling use of a standard hose of any length
and allowing use of a hose swivel or angle adapter if desired. We offer a M31-1.5mm = 9/16-Inch Adapter that will convert any length standard LP regulator hose for use with older (Pre-2008) Poseidon second stages, when also used with an add-on OPV.
