With the introduction of the D420, SCUBAPRO engineers have combined the iconic design of the original D-Series second stages with the latest advances in breathing performance technology. They then teamed it with the MK19 EVO balanced diaphragm first stage to create a 21st century smooth, natural breathing regulator. The D420 features a balanced valve inside a housing supported by a full metal structure. Called the Progressive Flow Control Valve, it is able to deliver abundant airflow for the best breathing performance with a natural smoothness comparable to what you might experience breathing on the surface. The uniquely shaped fiberglass-reinforced nylon casing is ultra-durable and allows the LP hose to be connected on either the right or left side for maximum rigging flexibility. A dive/pre-dive system includes a switch on the top of the casing with a thumb tab for adjusting the direction of airflow right out of the valve.